The Renal Sympathetic Baroreflex in the Rabbit

Transcription

1 618 The Renal Sympathetic Barreflex in the Rabbit Arterial and Cardiac Barreceptr Influences, Resetting, and Effect f Anesthesia Patricia K. Drward, Walter Riedel, Sandra L. Burke, Judith Gipps, and Paul I. Kmer Frm the Baker Medical Research Institute, Melburne, Australia SUMMARY. Curves relating renal sympathetic nerve activity and mean arterial pressure were derived in cnscius rabbits during ramp changes in mean arterial pressure, elicited by perivascular balln inflatin. The renal sympathetic nerve activity-mean arterial pressure relatinship cnsisted f a high-gain sigmidal regin abut resting, where renal sympathetic nerve activity rse r fell in respnse t mderate falls and rises f mean arterial pressure. With larger pressure rises, renal sympathetic nerve activity first fell t a lwer plateau and then reversed at even higher mean arterial pressure. When mean arterial pressure was lwered belw resting, renal sympathetic nerve activity rse t an upper plateau and then reversed abruptly tward resting at lw mean arterial pressure. Bth arterial and cardiac barreceptrs exerted substantial inhibitry influences n renal sympathetic nerve activity at all pressure levels. These effects appeared additive ver the central high gain regin f the curve, but beynd this regin there were nn-additive interactins. The latter were affected cnsiderably by alfathesin anesthesia. In ther experiments, we studied the effects f sustained alteratins in resting mean arterial pressure induced by infusing nitrprusside and phenylephrine, which prduced rapid resetting f the renal barreflex. The latter culd be accunted fr, in part, by resetting f the threshld f the arterial barreceptrs and in part by cntributins frm ther afferents, prbably the cardiac receptrs. During resetting assciated with nitrprusside-induced falls in resting bld pressure, high-gain reflex adjustments in renal sympathetic nerve activity t mderate changes in mean arterial pressure were preserved, but during resetting assciated with phenylephrine-induced rises in resting mean arterial pressure, the resting renal sympathetic nerve activity lay n the lwer curve plateau, resulting in reductin in the apparent gain f the reflex renal sympathetic nerve activity respnse t mderate changes in mean arterial pressure. (Circ Res 57: , 1985) THERE has been much recent interest in the rle f cardipulmnary barreceptr reflexes and their interrelatinship with reflexes arising frm the arterial barreceptrs (Oberg and White, 1970a, 1070b; Pelletier et al., 1971; Krner et al., 1973; Mancia et al., 1973, 1975, 1976; Mark et al., 1973; Kike et al., 1975; Chen, 1978; Chen et al., 1978, 1979; Iriki et al., 1979; Gu et al., 1982; Thames et al., 1982; Abbud, 1982; Abbud and Thames, 1983; Ludbrk and Graham, 1984). These studies have shwn that the magnitude f reflex effects n heart rate and sympathetic cnstrictr tne evked frm each f the abve inputs is affected by the level f activity f the ther receptr grup. Hwever, there still is uncertainty abut the rle f the cardipulmnary barreceptrs in the integrative autnmic adjustments in intact animals. Opinins differ as t whether they cntribute t the reflex respnses during relatively small intravascular pressure changes near resting levels, r whether they mainly affect the respnses at very high bld pressures (Mancia et al., 1973, 1975, 1976, Gu et al., 1982). One factr that may have cntributed t the uncertainty is that mst studies have investigated nly the rle f afferents traveling in the vagus. The rle f the sympathetic afferents has nt been cnsidered, althugh we nw knw that the cardiac receptrs which give rise t these afferents becme activated by mderate pressure changes and can mediate pwerful reflex effects (Brwn 1979; Thren, 1979; Malliani, 1982). In additin, the vagal afferents eliminated by cervical vagtmy r cld blck include nt nly thse arising frm barreceptrs in the heart and pulmnary vessels, but als thse cming frm a variety f receptrs frm the gastrintestinal tract and lung, sme f which can influence autnmic activity [e.g., lung inflatin receptrs (Daly and Rbinsn, 1968)]. In circulatry disturbances, the rle f these gastrintestinal and pulmnary afferents is f less interest than that f afferents arising frm the heart and pulmnary vessels. Last, mst previus analysis n the rle f the cardipulmnary afferents has been perfrmed under anesthesia. Anesthetics ften prduce selective depressin f central synapses, s that the autnmic respnses t particular stimuli are distrted, cmpared with thse f cnscius animals (Kmer et al., 1968; White and McRitchie, 1973; Zimpfer et al., 1974). Our first bjective in these experiments was t

2 Drward et al. /Renal Sympathetic Barreflex characterize the renal sympathetic barreflex and t examine the rle f the cardiac and arterial barreceptrs in the reflex regulatin f renal sympathetic nerve activity (RSNA) in cnscius rabbits. We recrded RSNA during balln-induced rises and falls f intravascular pressure, and derived mean arterial pressure (MAP)-RSNA functin curves. We then cmpared the barreflex respnses befre and after selective denervatin f the cardiac and arterial barreceptrs. We used prcaine instilled thrugh a pericardial catheter t blck reversibly cardiac afferents and efferents (Amdt et al., 1981; Drward et al., 1983; Ludbrk and Graham, 1984) in rabbits with functining arterial barreceptrs and again after sinartic denervatin. Since mst previus wrk has been perfrmed in anesthetized animals, we investigated the extent t which this altered the varius cmpnents f the renal sympathetic barreflex by cmparing the respnses befre and after alfathesin anesthesia in anther grup f rabbits. This agent was chsen because it is less depressant n autnmic functin than ther agents in cmmn use (Timms, 1976; Blake and Krner, 1981). Our secnd bjective was t examine whether sustained changes in resting MAP altered the prperties f the renal sympathetic barreflex. Such changes have been shwn t prduce rapid resetting f the arterial barreceptrs (Cleridge et al., 1981, 1984; Kunze, 1981; Drward et al., 1982; Munch et al., 1983). We prduced graded alteratins in resting MAP by cntinuus infusins f nitrprusside and phenylephrine, and derived functin curves in the usual way at each level f resting bld pressure. 619 prevent mvement between nerve and electrde (Fig. 1). The intact nerve was slipped int the bare wire spirals, taking great care t maintain its bld supply, and the entire preparatin was embedded in a silicne gel (Silgel 604, Wacker Chemie, Munich) (Krner et al., 1980). On day 7, after electrde implantatin, we perfrmed sinartic denervatin (SAD) under halthane-prpanidid anesthesia in the rabbits used fr the afferent analysis (Chalmers et al., 1967a, 1967b, 1967c; Blmbery and Kmer, 1979). Recvery was rapid frm this peratin, with the animals eating and drinking within hurs f its cmpletin. At the time f the experiment, 24 hurs after SAD, the animals appeared t be in gd cnditin and were grming themselves nrmally. Average weight lss ver the 24 hurs was 0.05 kg (range kg). After SAD, balln-induced changes in MAP f mm Hg always elicited heart rate changes f <5 beats/min (Blmbery and Kmer, 1979) (see Results fr RSNA changes). Bth resting MAP and RSNA were mre labile than in nrmal rabbits, with the frmer variable shwing the characteristic fluctuatins that we have previusly bserved in SAD rabbits studied abut 7 and 14 days after denervatin (Chalmers et al., 1967a; Blmbery and Krner, 1979; Krner, Bader, and Head, unpublished data). In nrmal rabbits with intact arterial barreceptrs, we did bserve significant changes in sme f the renal barreflex curve parameters 24 hurs after surgery, but they were relatively small (see Results). Ludbrk and Graham (1985) fund that, in SAD rabbits, the MAP fell during exercise, instead f rising as in nrmal rabbits. The falls tefln-cated wires Methds Operatins and Prcedures We used 20 crssbred rabbits (between 2.0 and 3.0 kg) in which three preliminary peratins were perfrmed under halthane anesthesia after inductin with prpanidid (Epntl, Bayer) (Drward et al., 1982). First, we implanted a Silastic perivascular balln arund the thracic arta thrugh a left thractmy. One week later we perfrmed a right thractmy fr placing a balln arund the inferir vena cava. In sme rabbits we inserted a Silastic catheter int the pericardial sac at this peratin (Drward et al., 1983) and stitched pacing electrdes t the left atrium at the left thractmy peratin. The pacing electrdes were tw lengths f Tefln-cated stainless steel wire, each flded in half, with the wire at the bend expsed and twisted t frm a lp which was secured t the atrium with several stitches. The wire leads were prtected by Silastic tubing, and the pericardium was stitched ver the electrdes; the sutures were cvered with a small piece f nyln fabric t prmte fibrsis and t ensure against subsequent leakage f prcaine frm the pericardial sac. At least 2 weeks later, we implanted the renal sympathetic nerve electrde. This cnsisted f tw lengths f either multistrand platinum indium wire (Medwire, Inc.) r single-strand stainless steel Tefln-cated wire (Gre, Inc.). The expsed ends were wund int shrt spirals, and the electrde was stitched t the renal artery t silastic sheath * fr wires tie t artery artery FIGURE 1. Diagram shwing electrde placed arund the renal nerve, as explained in text.

3 620 were similar 1 hur, 1 day, and 7 days after denervatin, despite sme time-related changes in resting MAP (Ludbrk et al., 1985). We have therefre assumed that the RSNA respnses t balln inflatin 24 hurs after SAD represent respnses that are characteristic f this preparatin, and that they wuld nt differ greatly ver a similar time span. Minr perative prcedures were perfrmed n the day f each experiment under lcal 0.5% lidcaine anesthesia. These invlved cannulating the central ear artery and vein, and retrieving the tubing frm the perivascular ballns and the leads frm the renal nerve and pacing electrdes. Animals then rested in their bxes fr at least minutes befre the start f cntrl measurements (fr prtcls, see Results). Pulsatile arterial pressure, MAP, heart rate, and integrated RSNA were recrded cntinuusly during the experiment n a Grass mdel 7 plygraph; in sme experiments we als measured right atrial pressure (RAP). The balln-induced bld pressure changes and assciated RSNA activity, tgether with the resting values ver 60 secnds preceding inflatin, als were recrded n magnetic analg tape (Hewlett-Packard, mdel 3968A FM tape recrder). Fr pacing the heart, the pacing electrdes were cnnected t a Grass SD9 stimulatr fr stimulatin at a pulsewidth f 1 msec and 7-10 V. In each rabbit, the heart was paced at a cnstant rate, which varied frm beats/min between rabbits, i.e., in the upper range f heart rate. Renal Nerve Electrde RSNA was recrded by a lw nise differential amplifier, using a bandwidth f Hz t 1 khz. It was calibrated by passing a 10-/xV, 1-kHz signal thrugh the recrding circuit at the beginning f each experiment. Amplified ptentials were rectified and integrated ver 1-secnd perids abve an adjustable vltage level which excluded baseline nise. We set the integratr t give zer reading during the shrt silent perids seen between bursts f RSNA by viewing bth raw and rectified signals n a strage scillscpe. A rapid rise in arterial pressure prduced by brief balln cclusin f the arta, r rapid release f the caval balln after previus cclusin, bth greatly extended the duratin f the silent perids. Hwever, in cnscius rabbits, even these rapid MAP rises did nt cmpletely reduce RSNA t zer in all the accmpanying 1-secnd integratin cycles, althugh they did s under anesthesia. We used the minimum 1-secnd integrated RSNA value recrded during these rapid MAP changes as an estimate f nise level in each rabbit. This averaged 1.6 ± 0.28% f the upper plateau RSNA value btained during the standard slw venus balln-induced fall in MAP (see belw, and Figs. 2 and 3). We administered the ganglinic blcking drug trimetaphan camphrsulfnate t sme cnscius rabbits (see Drugs), which cmpletely ablished RSNA ver the entire range f MAP values tested, cnfirming the pstganglinic nature f the discharge. After trimetaphan, nise level was 2.3 ± 0.25% f the upper plateau level, i.e., clsely similar t the nise level btained by the rutine methd. In all rabbits, the pattern f resting RSNA in relatin t the arterial pressure pulse, and the effects f balln inflatin were similar t thse bserved previusly when recrding efferent activity frm the severed nerve in anesthetized rabbits (Iriki et al., 1977; Drward and Krner, Circulatin Research/V/. 57, N. 4, Octber ). This, tgether with the results with trimetaphan, suggests that in the intact nerve we were recrding predminantly efferent, rather than renal afferent, activity (e.g., Recrdati et al., 1978). MAP-RSNA Curves Artic balln inflatin raises MAP and pulse pressure, left ventricular pressure and RAP, and, presumably, ther intracardiac and pulmnary circulatry pressures (Krner et al., 1972; Ludbrk, 1984). Similarly, inflating the venus balln lwers all the abve pressures. Kmer et al. (1972) fund that during square-wave changes in MAP, pulse pressure and RAP were highly crrelated. Similarly, in the present experiments, artic and venus balln inflatin prduced crrespnding changes in RAP, althugh the relatinship was nt linear (see Results, Fig. 2, lwer). We have nt previusly bserved any differences in RAP respnses between nrmal, SAD, and autnmi- > t, UJ cr UJ z I- LU I *r MEAN ARTERIAL PRESSURE - mmhg ARAP mmhg L_ 4 FIGURE 2. Upper. MAP-RSNA curve frm a cnscius rabbit with all afferents intact. Data pints are cnsecutive 1-secnd average values fr MAP and integrated RSNA (pv) during artic and venus pressure ramps. The large pen circle is the cmmn resting value, and the line shws the fitted lgistic curve. The arrw n the left indicates RSNA at the lwest MAP during maximum venus balln inflatin, which was taken as the hyptensive value. The arrw n the right shws RSNA at the highest MAP during maximum artic balln inflatin, which was taken as the hypertensive value. Lwer changes in RAP assciated with changes in MAP during inflatin f the artic and venus ballns. The fine lines shw results frm individual rabbits; the circles give the average relatinship btained frm five rabbits.

4 Dnvard et al. /Renal Sympathetic Barreflex 621 cally blcked rabbits subjected t graded Valsalva-like maneuvers (Blmbery and Krner, 1982). We have used the MAP changes as an index f the intravascular pressure changes accmpanying balln inflatin. When studying the effects f different treatments n the MAP-RSNA relatinship, we have always examined the parameter changes in the middle and bth ends f the MAP range, which will encmpass a wide spectrum f cardiac pressure changes. We btained MAP-RSNA curves by slwly inflating each balln, t prduce ramp changes in MAP at the rate f 1-2 mm Hg/sec (Drward et al., 1982). After deflatin, MAP and RSNA returned t cntrl levels in under a minute. Each respnse curve was derived frm a pair f artic and caval balln inflatins, 2 minutes apart. Pulsatile arterial pressure and integrated RSNA were digitized frm the data previusly recrded n magnetic tape, tgether with the resting values btained ver the preceding 30 secnds. We als recrded pressure and RSNA calibratins. When the arterial barreceptrs were intact, bth average MAP and integrated RSNA (jiv/sec) were determined ver 2-secnd intervals. MAP-RSNA data pints were displayed graphically t specify the end f the resting perid and t edit ccasinal utlying pints during the pressure ramps (Fig. 2, upper). These smetimes ccurred in assciatin with small pressure reversals, r in cnjunctin with mvements f the rabbit. We averaged the resting MAP and RSNA values fr 30 secnds befre each balln inflatin and expressed the data pints during the pressure ramp as changes frm these values. Fr each pair f artic and venus balln inflatins, we averaged the individual resting value befre each inflatin and adjusted the data fr each ramp t the average resting value by adding the difference between the actual and cmmn resting values t each pint f the ramp. In rabbits with functining arterial barreceptrs, we fitted a sigmidal lgistic functin ver the MAP range mm Hg: RSNA decreased frm a maximum value at the upper plateau t a minimum value at the lwer plateau (Fig. 3, upper). We used a general nnlinear regressin prgram t fit this part f the curve (Marquardt, 1963). The equatin used was as fllws: RSNA = P i + P2/(l + exp P 4 [MAP - P 3 ] ) where Pi = lwer plateau; P 2 = RSNA range between upper and lwer plateaus; P3 = BPM, i.e., MAP at half RSNA range; P 4 = a cefficient t calculate the average gain, G, f the curve, which is given by G = P 2 X P 4 / 4.56, and equals the slpe between the tw inflectin pints f the curve. The upper plateau equaled Pj + RSNA range, hi additin, we defined tw parameters at the extremes f the curve: (1) the hyptensive RSNA value at the lwest MAP attained with maximal caval balln inflatin, and (2) the hypertensive RSNA value at the highest MAP during maximum artic balln inflatins (Figs. 2 and 3, upper). In curves btained after SAD, we used anther averaging prcedure, since RSNA pints were mre scattered at a given MAP than when the arterial barreceptrs were intact and the fit f a lgistic functin was unsatisfactry. The data were divided int 5 mm Hg MAP bins abve and belw resting MAP in each rabbit, and the average RSNA was determined fr each bin. As befre, resting MAP and RSNA were averaged fr 30 secnds befre each pressure ramp and the data fr artic and venus balln inflatin were adjusted t a cmmn resting value. ~E ina- nrmalised u w z u cc 0 0 L Upper / Hyptensive value plateau \ / Ra nge \ i y Ganglin blck^ \ I ipe = Av. gain BP kresting V Hypertensive value ^ Lwer pjateau Spntaneus HR MEAN ARTERIAL PRESSURE - mmhg FIGURE 3. Upper, average curve in cnscius rabbits with all afferents intact relating MAP (mm Hg) t RSNA (nrmalized units), shwing the varius parameters discussed in the text. Apart frm the values shwn, the hyptensive reversal respnse was the upper plateau minus the hyptensive value; the hypertensive reversal respnse was the hypertensive value minus the lwer plateau. The dashed line is the nise level recrded during ganglinic blckade. Lwer, average MAP-RSNA curves btained in seven cnscius rabbits with afferents intact al the animals' spntaneus heart rate and during left atrial pacing at cnstant heart rate. Upper and lwer plateaus were determined by inspectin f the curves and the average gain by linear regressin between inflectin pints. Hypertensive and hyptensive RSNA values were determined as in rabbits with intact arterial barreceptrs. There was cnsiderable variatin between rabbits in the amplitude f the upper RSNA plateau values recrded n day 6 (range 3-35 nv/sec). This apprximately 10-fld difference was cnsiderably greater than the 1.3-fld difference bserved in the upper heart rate plateau values ( beats/min). Hence, the difference in RSNA plateau between animals was prbably due in part t nnbilgical factrs, e.g., the psitin f the electrde in relatin t the nerve, hi additin, we als fund evidence f time-related signal attenuatin within rabbits (see Results). We therefre nrmalized the MAP-RSNA curve in each experiment in each rabbit, by expressing RSNA in terms f the upper plateau level f the rabbit's cntrl curve which was taken t equal nrmalized units (NU).

5 622 Barreceptr-Heart Rate and Naspharyngeal Reflexes MAP-heart rate curves wete derived in sme experiments frm the same bld pressure ramps used t btain MAP-RSNA curves, using the same iterative lgistic prgram. The parameters defined included the upper and lwer heart rate plateaus, the heart rate respnse range between plateaus, the average gain, and BP (Kmer et al., 1972). We als lked fr hypertensive and hyptensive heart rate respnses, crrespnding t analgus parameters f the renal sympathetic barreflex. The naspharyngeal reflex was evked while the rabbit rested quietly in its bx. After resting MAP, RSNA, and heart rate were recrded, a puff f cigarette smke was blwn clse t its nse thrugh a fine plastic catheter ver a perid f 1-2 secnds, which is a strng stimulus fr this reflex; recrding cntinued fr anther secnds (White and McRitchie, 1973; McRitchie and White, 1974). The integratr was set at 1 secnd and the naspharyngeal respnse was taken as the average RSNA change frm resting ver the 6 secnds f maximum activity. Blckade f Cardiac Nerves with Prcaine We instilled 5% prcaine intrapericardially t blck bth efferent and afferent cardiac nerves, using an initial dse f 0.4 ml, fllwed by bster injectins f ml at 7-minute intervals (Drward et al., 1983). We used brief artic balln inflatin t test the blckade f cardiac efferents by checking that the reflex reductin in heart rate was ablished. In additin, after SAD prcaine blcked the reflex reductin in RSNA with large elevatins f MAP, in agreement with previus findings (Drward et al., 1983; Ludbrk and Graham, 1984). Arndt et al. (1981) and Samdelv et al. (1982) have recrded bth efferent and afferent activity frm cardiac vagus and sympathetic nerves and shwed that the same cncentratins f prcaine blcked bth kinds f activity. Nitrprusside and Phenylephrine Sdium nitrprusside was administered by intravenus infusin in dses f 0 (5% dextrse), 2.5, 5, and 10 /igag per min, each lasting 0.5 hur. The rder f administratin f the different dses was in accrdance with a Latin Square experimental design, t eliminate bias (Snedecr and Cchran, 1980). Reflex measurements began 15 minutes after the start f each dse, when resting MAP and RSNA had stabilized. Phenylephrine was administered similarly in dses f 0, 1, 2, and 4 MgAg per rnin - Trimetaphan Camphrsulfnate This ganglinic blcking agent was administered at a rate f Mg/kg per min, iv, in cnjunctin with MgAg per min nrepinephrine t maintain bld pressure clse t the previus resting value. Alfathesin Anesthesia The rabbits were anesthetized with Alfathesin (Glax, Australia) (0.24 mg/kg per min) and were paralyzed with succinyl chline (initial dse, mg, fllwed by 6 mg after 7-10 minutes) and artificially ventilated with xygen-enriched air at a respiratry minute vlume f liter/min. This maintained arterial Pcj at 29.7 ± 0.8 mm Hg and P02 at 98.9 ± 5.2 mm Hg, i.e., in the range f nrmal rabbits. We used a cmbinatin f anesthesia, paralysis, and artificial ventilatin t simulate cnditins Circulatin Research/VJ. 57, N. 4, Octber 1985 used in mst previus studies n arterial-cardipulmnary interrelatinships. Statistical Analysis Significant differences in curve parameters during treatment and recvery perids were assessed by tw- r threeway analysis f variance (ANOVA) (Snedecr and Cchran, 1980). Linear regressin was used t determine gain and BPM f the MAP-RSNA curves after SAD. In sme cmparisns between mean parameter values, we used the f-statistic mdified accrding t the Bnferrni prcedure, t assess significance (Wallenstein et al., 1980). Results We perfrmed ne series f experiments in which the MAP-RSNA and MAP-heart rate relatinships and the naspharyngeal RSNA respnse were determined n days 1, 6, and 8 after electrde implantatin. All ther experiments were perfrmed n day 6 t day 8 after the electrde was implanted. MAP-RSNA Relatinship in Nrmal Rabbits In cnscius rabbits studied n days 6-8 after electrde implantatin, a small rise in MAP during artic balln inflatin was sufficient t lwer RSNA frm the resting value t the lwer plateau (Figs. 2 and 3, upper). The latter ranged frm 6-12 NU (i.e percent f the upper plateau f the cntrl curve) in different grups, which was significantly abve the nise level f abut 1-2 NU (P fr significance f difference <0.025). The significant elevatin abve nise level f the cnscius rabbit's lwer RSNA plateau was characteristically prduced by the slw ramp rises in MAP used in ur experiments. As seen in Figure 2, the lwer plateau in a given animal was maintained ver abut 10 secnds f gradual balln inflatin. The differences in individual RSNA values making up the lwer plateau were due t differences in the duratin f the silent perids ccurring within each 1-secnd integratin cycle (see Methds: Renal Nerve Electrde). The lwer plateau was maintained ver the range f MAP between 85 and 105 mm Hg and then increased t the hypertensive value (Fig. 3). With falls in MAP due t venus balln inflatin, RSNA increased t the upper plateau, which was well maintained between MAP f abut 65 and mm Hg and was fllwed by an abrupt fall t the hyptensive RSNA value (Fig. 3). RAP had already fallen t its minimum value by the time the upper RSNA plateau had been reached (Fig. 2). It remained clse t the minimum with further reductin in MAP, and then returned tward resting in cnjunctin with the hyptensive reversal respnse (Fig. 2). We bserved n differences in MAP-RSNA curves in seven rabbits btained during spntaneus reflex heart rate changes, and during pacing at cnstant heart rate (Fig. 3, LOWER). The MAP-RSNA relatinship f nrmal rabbits thus included (1) a sigmid cmpnent between

6 Drward et al. /Renal Sympathetic Barreflex 623 upper and lwer plateaus with a central high-gain prtin, (2) the hyptensive reversal respnse bserved during large falls in bld pressure, and (3) a smaller hypertensive reversal respnse evked by large rises in pressure. The relatinship between MAP and heart rate was als sigmid (Fig. 4C). During artic balln inflatin, heart rate fell t a lwer (bradycardia) plateau until MAP exceeded abut 110 mm Hg, when there was diminutin f bradycardia, analgus t the hypertensive RSNA reversal respnse. Hwever, during pressure reductin prduced by venus balln inflatin, the upper tachycardia plateau was well maintained, and there was n heart rate change crrespnding t the hyptensive RSNA reversal respnse OT 10 a. B S.Day 1 Time-Related Effects There were n significant differences in curve parameters between duplicates btained during the cntrl, treatment, and recvery perids. Reprducibility was gd; fr example, the standard errr between duplicates f the upper RSNA plateau averaged 6% f the mean value ( NU). The preparatin als remained stable ver a 3-hur perid, as assessed by cmparing the cntrl curves with the recvery curves btained abut 2 hurs after prcaine (Fig. 5; Table 1). We bserved cmplete recvery in RSNA range, upper and lwer plateau levels, gain, and in the hypertensive reversal respnse. Hwever, BP M and the sigmid cmpnent f the recvery curve were shifted t a lwer MAP range, abut 5 mm Hg belw cntrl, in parallel with the change in resting MAP (see Resetting). In additin, there was sme attenuatin f the hyptensive reversal respnse during the recvery perid, which averaged 70% f the cntrl value (P < 0.05). We examined lnger term time-related effects in anther series f five nrmal rabbits, in which measurements were btained n days 1, 6, and 8 after implanting the electrde. There was prgressive attenuatin f the abslute magnitude f the signal, which affected all cmpnents f the curve (Figs. 4A and 6). The relative sizes f the upper RSNA plateau ver the three experimental days averaged 2.3:1:0.6, and crrespnding values fr the lwer plateau and RSNA range were similar (Fig. 6). The abslute rate f signal attenuatin expressed in terms f the day 6 upper plateau averaged 1 NU/hr ver the entire perid f these experiments. We als determined the magnitude f the naspharyngeal RSNA reflex in these animals ver the same perid (Fig. 6). With this reflex, the rise in RSNA was maximal abut 4-8 secnds after smke stimulatin and returned t resting after secnds; it was nt assciated with signs f distress r discmfrt. The same smke stimulus has previusly been shwn t prduce cmplete cessatin f renal bld flw (White et al., 1973; McRitchie and White, T3 ID j? Z E a. 400 a. t- tr c Day 1 Day 6 40 R MEAN ARTERIAL PRESSURE - mmhg FIGURE 4. Panel A- average MAP-RSNA curves frm five cnscius rabbits studied n days 1, 6, and 8 after electrde implantatin. RSNA is expressed in pv/sec. Large symbls equal resting values. Panel B: average MAP-RSNA curve frm the same rabbits with RSNA nrmalized each day, s that the upper plateau equaled units. Large symbls equal resting values. Panel C average relatinship between MAP and heart rate (beats/min) derived simultaneusly frm the same bld pressure ramps. Large symbls equal resting values. 1974). On each day, the naspharyngeal RSNA respnse was abut duble the upper RSNA plateau (Fig. 6). The relative magnitude f the naspharyngeal RSNA respnses n days 1, 6, and 8 were, respectively, 1.95:1:0.52, which was clsely similar t that bserved with the upper plateau f the renal barreflex. After each MAP-RSNA curve was nrmalized in terms f its upper plateau (= NU see Methds), the differences between the nrmalized curves btained n the three experimental days were rela-

7 624 Circulatin Research/Vl. 57, N. 4, Octber 1985 ARTERIAL BARORECEPTORS INTACT CD 400 x> ^* r Naspharyngeal RSNA reflex O > tr. LU I ARTERIAL BARORECEPTORS DENERVATED ID 15 I 200 c Lwer plateau Time MEAN ARTERIAL PRESSURE - mmhg FIGURE 5. Upper average MAP-RSNA curves btained frm seven cnscius rabbits during the initial cntrl perid (time 1) and after recvery (time 2) frm intrapericardial prcaine, apprximately 25 hurs later, btained n day 6 after electrde implantatin when the arterial barreceptrs were intact. Lwer cntrl and recvery curves in the same rabbits in experiment n day 8, after arterial barreceptr denervatin. tively small (Fig. 4B). They affected mainly nrmalized resting RSNA, which was higher n day 1 after implantatin (29 NU) than n days 6 and 8 (18 and 19 NU; P fr difference frm day 1 < 0.05), and the hyptensive reversal respnse, which was smaller n day 1 (34 NU) than n days 6 and 8 (51 and 54 NU; P < 0.025). The MAP-heart rate curves were clsely similar n each experimental day (Fig. 4C). The changes in resting heart rate were clsely parallel t the changes in nrmalized resting RSNA, with heart rate n day 1 after electrde implantatin (274 beats/min) significantly higher than n days 6 and 8 (245 and 230 beats/min; P = 0.025). The changes in resting RSNA and heart rate between day 1 n the ne hand and days 6 and 8 n the ther, were assciated with a small rise in resting MAP (day 1, 82 mm Hg; days 6 and 8, 86.9 mm Hg, i.e., 85.8, 86.0 mm Hg; P < 0.05). We have nt investigated the mechanisms respnsible fr the prgressive signal attenuatin, but have DAYS AFTER IMPLANT FIGURE 6. Average naspharyngeal RSNA respnses and the upper and lwer plateau values btained in five cnscius rabbits n days 1, 6, and 8 after electrde implantatin. Results are expressed as a percentage f the upper plateau n day 6. The bar n the day 6 value f each variable is ± 1 SEW based n the within-animal variance as determined by ANOVA. assumed that it was due t physical rather than bilgical factrs in view f (1) the restratin f the neural signal in tw rabbits with n recrdable RSNA n day 6, by remval f excess fluid arund the electrde gel-cmplex after expsing the electrde under anesthesia; (2) the small differences in prperties f the nrmalized MAP-RSNA curves, which paralleled the absence f differences in MAPheart rate curves (Fig. 4, B and C); (3) the similarity f pstperative changes in nrmalized resting RSNA and resting heart rate (Fig. 4, B and C). Our findings shw that the signal attenuatin can be neglected in experiments lasting 2-3 hurs, but that it must be allwed fr in lnger experiments. Rle f Arterial and Cardiac Barreceptrs We perfrmed tw experiments1' in seven rabbits n days 6 and 8 after electrde implantatin. On day 6, the arterial barreceptrs were intact, and we examined the effects f blcking the cardiac nerves with prcaine. On day 8, we again studied the effects f blckade f the cardiac nerves in the same rabbits, ne day after SAD. We have assumed that the average rate f electrde signal attenuatin between days 6 and 8 was similar in these rabbits t that bserved in the parallel series in Figures 4 and 6.

8 Drward et al. /Renal Sympathetic Barreflex 625 Arterial Barreceptrs Intact Prcaine lwered resting heart rate frm 268 beats/min (cntrl) t 252 beats/min (SED 7 beats/ min; P = 0.05). Hwever, pacing made n difference t the MAP-RSNA relatinship during cntrl and treatment perids s that we have pled the results btained at the rabbits' spntaneus heart rate and during pacing fr assessing the effect f treatment. After blcking the cardiac nerves, RSNA was significantly abve cntrl at resting RSNA and during balln-induced falls in MAP (Fig. 7, curves ac and a; Table 1). The greatest difference between curves ac and a was at the hyptensive RSNA value (69 NU), fllwed by the difference at the upper plateaus (28 NU) and the difference at resting RSNA ARTERIAL BARORECEPTORS INTACT a = arterial barreceptrs intact, 125 c = cardiac receptrs intact \ - cmbined denervatin 75 (9.1 NU); (P < fr all differences). By cntrast, during rises in bld pressure abve resting, the differences between the curves were small and nt statistically significant (Fig. 7). After prcaine, there were n significant changes in gain and BP (Table 1). Gu et al. (1982) have bserved large rises in MAP and in vascular resistance immediately after cutting the vagi, with cnsiderable attenuatin f these respnses by abut minutes after vagtmy. We btained fur MAP-RSNA curves between 3 and 45 minutes after giving prcaine. There were n significant changes ver this perid either in resting MAP r in resting RSNA. Hwever, in the first curve, the elevatin f the upper plateau was less marked than in the ther three (Fig. 8). This suggests that the actin f prcaine was gradual at nset, with maximum blckade f cardiac nerves attained at abut 5-10 minutes after starting the drugthus, in the presence f the arterial barreceptrs, the cardiac barreceptrs had an inhibitry effect n RSNA during venus balln inflatin. This inhibitin increased prgressively with increasing falls in MAP and became maximal at the hyptensive reversal respnse (Fig. 7, cf. differences between curves ac and a). Sinartic Denervatin > CO ARTERIAL BARORECEPTORS DENERVATED UJ < O. 1 LU MEAN ARTERIAL PRESSURE - mmhg FIGURE 7. Upper graphs: average MAP-RSNA curves btained 6 days after electrde implantatin in seven cnscius rabbits. In curve ac (slid line), bth the arterial barreceptrs and cardiac receptrs were functining. In curve a, nly the arterial barreceptrs were functining after instillatin f the intrapericardial prcaine (dashed line). Shaded area estimates the inhibitry influence f the cardiac receptrs, lwer graphs: average results in the same rabbits perfrmed n day 8, after sinartic denervatin. In curve c (slid line), the cardiac receptrs were functining, whereas, in curve (dashed line), these receptrs had been blcked by intrapericardial prcaine. See Table 1 fr significance f varius changes. R = resting values. Nrmalizatin f the curves was perfrmed, as usual, n the upper plateau f the cntrl curve after SAD, when the cardiac receptrs were still functining (Fig. 7, curve c). When the abslute value f the plateau was expressed in ftv/sec, it averaged 52 ± 3.7% f the day 6 value in the same rabbits, which was similar t the signal attenuatin between days 6 and 8 in the parallel grup f sham-perated rabbits (Figs. 4 and 6). If we assume a similar rate f signal attenuatin in bth grups f rabbits, then the abslute level f upper plateau after SAD wuld crrespnd apprximately t the upper plateau in the intact rabbit after maximum unlading f the arterial barreceptrs when all afferents were intact, i.e., NU in each experiment can be cnsidered apprximately equivalent. The pattern f RSNA respnses after SAD (but with cardiac receptrs intact) was prfundly altered (Fig. 7, curve c). Resting RSNA nw was n the upper plateau and shwed n further increase with falls in MAP t abut 40- mm Hg. There still was a significant hyptensive reversal respnse f 19.1 ± 6.8 NU (P < 0.05) which was abut ne-third f that bserved befre SAD (Table 1). With artic balln inflatin, elevatin f MAP abve resting prduced n alteratin in RSNA until MAP f abut 90 mm Hg. Hwever, with greater pressure rises, RSNA decreased gradually t a smewhat ill-defined lwer plateau f abut -60 NU, befre rising t the hypertensive value at MAP > abut 120 mm Hg. The resulting reversal respnse was abut three

9 626 Circulatin Research/Vl. 57, N. 4, Octber 1985 TABLE 1 Parameters and Resting Values* in Seven Cnscius Rabbits befre and after Sinaitic Denervatin Intact SAD Hyptensive value (NU) Upper plateau (NU) Resting RSNA (NU) Lwer plateau (NU) Hypertensive value (NU) RSNA range (NU) Average gain (NU/mm Hg) BP M (mm Hg) Resting MAP (mm Hg) Hyptensive reversal (NU) Hypertensive reversal (NU) C P 109.0f 128.0t 41.9f f f 19.0 R 65.4f f 76.2f 38.7f 15.6 SED ±8.4 ±8.6 ±2.2 ±1.6 ±6.0 ±5.6 ±0.6 ±1.5 ±1.3 ±8.6 ±5.9 C P 139.7f 151.6f f 211.0f lt f R SED ±19.1 ±23.5 ± 8.7 ±10.6 ±25.6 ±23.0 ± 0.4 ± 6.3 ± 1.6 ±11.6 ±21.1 * During cntrl (C), intrapericardial prcaine (cardiac nerve blck) (P), and recvery (R) perids. Results in each rabbit based n quadruplicate determinatin; RSNA values nrmalized n each experimental day as explained in text. f P fr difference frm cntrl < 0.05; SED = standard errr f difference within rabbits based n ANOVA. times that bserved in nrmal rabbits (Table 1; Fig. 7). The gain f the RSNA respnse ver the pressure-sensitive part f the curve was abut ne-fifth f that in nrmal rabbits and the BP (102 mm Hg) was significantly increased (Table 1). In this preparatin, the pressure-related inhibitin f RSNA ccurred ver a higher range f bld pressures than in nrmal rabbits. After prcaine, resting heart rate fell frm the cntrl value f 289 beats/min t 230 beats/min (SED 8 beats/min; P < 0.001). Pacing again did nt alter the MAP-RSNA relatinship, and we have pled the data at the rabbits' spntaneus and paced heart rates fr assessing the effects f prcaine.. After cardiac nerve blck, resting RSNA was clse t the middle f the pressure-sensitive RSNA range. The latter was abut half that f rabbits with functining arterial barreceptrs. With venus balln 'E T3 <D.2 "5 E c CO 1 Pr4 inflatin, a small rise in RSNA t an upper plateau was evked by falls in MAP. Hwever, the hyptensive reversal respnse f 11.9 ± 12.6 NU was n lnger statistically significant. After prcaine, RSNA was significantly higher in curve than in curve c during falls in MAP, with differences greatest at lw bld pressures and least at MAP mm Hg (Fig. 7). Thus, the cardiac barreceptrs had an inhibitry effect n RSNA, which was similar in the presence r absence f the arterial barreceptrs (Fig. 7, see differences between curves ac and a, and between curves c and. With rises in MAP prduced by artic balln inflatin, RSNA declined t an ill-defined plateau (Fig. 7, curve ). The mst striking feature f curve was the magnitude f the hypertensive value (211 NU), which was abut duble the upper plateau (Fig. 7, lwer). Hence, the reversal respnse was twice the value bserved in these rabbits when the FIGURE 8. Average MAP-RSNA curves derived frm the seven rabbits in Figure 7 during the cntrl perid and at varius time intervals after prcaine was infused int the pericardium. Curves Prl, PrZ Pr3, and Pr4 were btained, n average, 8, 14, 28, and 37 minutes, respectively, after instillatin f prcaine. Pr2 J Pr MEAN ARTERIAL PRESSURE - mmhg

10 Drward et al. /Renal Sympathetic Barreflex 627 cardiac receptrs alne were functining (Fig. 7; Table 1). During rises in MAP > 90 mm Hg the difference between curves and c again increased prgressively. This suggests a marked inhibitry effect frm the cardiac receptrs during elevatin in bld pressure ver an MAP range encmpassed by the lwer plateau and hypertensive value in nrmal rabbits. Alfathesin Anesthesia This was studied in five rabbits in which cntrl measurements were btained withut anesthesia. The rabbits then received alfathesin plus succdnyl chline and artificial ventilatin, but were nt subjected t surgery. During anesthesia, the resting MAP fell by 7.9 ± 2.9 mm Hg (P < 0.025); resting RSNA increased significantly frm a value f 17.6 NU befre anesthesia t 28.1 NU during anesthesia (SED 1.2; P < 0.001). The MAP-RSNA relatinship was shifted during anesthesia in the directin f lwer resting MAP (see Resetting) but there were n significant changes in upper plateau, RSNA range, r gain (Fig. 9). During artic balln inflatin, the lwer plateau fell t nise level, frm 6.7 NU befre anesthesia t 0.8 NU during anesthesia (SED 1.0; P < 0.01). The mst striking change was the marked attenuatin f the hyptensive reversal respnse, which equaled.5 NU befre anesthesia and was nly 8.5 NU c CD a E c Z CO cr E n UJ 10C w 200 anesthetized anesthetized cnscius nsclus MEAN ARTERIAL PRESSURE - mmhg FIGURE 9. Upper, average MAP-RSNA curves btained frm five rabbits while cnscius and during alfathesin anesthesia. Lwer average MAP-heart rate relatinship derived frm the same rabbits. Large symbls are the resting values. during anesthesia (P < 0.001). The effects f alfathesin anesthesia n the barreceptr-heart rate reflex were much greater than n the renal barreflex (Fig. 9). The upper plateau was depressed by abut 30 beats/min, while the lwer plateau was raised by 97 beats/min (P < 0.001) frm the value bserved befre anesthesia (Fig. 9). Gain and heart rate respnse range were bth reduced t abut ne-third the value befre anesthesia, and resting heart rate was significantly increased. Barreflex Resetting Assciated with Changes in Resting MAP Changes in resting MAP were induced by infusing vasactive drugs in tw experiments t alter the threshld f the arterial barreceptrs as described by Drward et al. (1982). In eight rabbits, we studied the changes prduced by nitrprusside-induced falls in MAP. hi five f these animals, the effects f phenylephrine-induced rises in MAP were studied in anther experiment. With each dse f drug, the new level resting MAP was reached within 7-10 minutes frm the start f infusin, s that determinatin f renal barreflex prperties starting at 15 minutes was within the time previusly shwn t prduce stable resetting f the barreceptrs (Drward et al., 1982). Nitrprusside Infusins Nitrprusside was given in dses f 0, 2.5, 5, and 10 MgAg per min (see Methds) and prduced an almst parallel resetting f the renal barreflex functin curves (Fig. 10). The falls in BP were linearly related t the falls in resting MAP with the regressin cefficient 0.74 ± mm Hg/mm Hg A resting MAP (P < 0.001). There were n significant changes frm cntrl in upper and lwer plateaus, in RSNA range, and in the hypertensive reversal respnse (Fig. 10; Table 2). Hwever, the hyptensive reversal respnse was significantly reduced at the tw highest dses f nitrprusside (P < 0.05). The sustained dse-related falls in resting MAP, were assciated with small increases in resting RSNA (P < 0.01, Table 2). The rises in resting RSNA were significantly less than wuld have ccurred if the MAP-RSNA relatinship had remained in the cntrl lcatin. Phenylephrine Infusins Phenylephrine was given in dses f 0, 1, 2, and 4 ^g/kg P et nun ( see Methds) and induced dserelated rises in resting MAP, with assciated reductin f resting RSNA (Fig. 11; Table 3). Over the entire dse range, the lwer RSNA plateau was depressed prgressively frm 11.7 NU (cntrl) t 5.6 NU at the highest dse (SED = 2.2 NU; P < 0.01). At the highest dse, the hypertensive value was indistinguishable frm the lwer plateau, s that the reversal respnse was absent.

11 628 Circulatin Research/V/. 57, N. 4, Octber 1985 Its C all E RSNA n 1 r\n 1 UU «v in _ -? / I /// NP 10y /// /' t NP 2.5 NP 0 C * \ \ \ v> \ l n a E ISNA ni SO. PE 1 0 NP MEAN ARTERIAL PRESSURE - mmhfl 120 n MEAN ARTERIAL PRESSURE - mmhg 120 r O z< X -10 L-20 CHANGES IN RESTING MAP - mmhg FIGURE 10. Upper average MAP-RSNA curves frm eight cnscius rabbits btained in cntrl perid (NP 0) and during infusins f nitrprusside at 2.5, 5, and 10 iig/kg per min. Large symbls are resting values. Lwer, relatinship between the change in resting MAP and the shift in BPxi each pint was derived frm individual measurements in a given rabbit. Only at the tw lwest dses was there parallel resetting f curves (Fig. 11). With thse dses, the upper plateaus and RSNA ranges were enhanced cmpared with cntrl (Table 3). In additin, BP M increased by 0.51 ± 0.10 mm Hg/mm Hg A resting MAP (Fig. 11, lwer), which was significantly less than crrespnding changes bserved with nitrprusside (P < 0.025). Furthermre, at the highest < I L +10, CHANGES IN RESTING MAP - mmh FIGURE 11. Upper, average MAP-RSNA curves frm five cnscius rabbits btained during cntrl perid (PE 0) and during phenylephrine infusins at 1, 2, and 4 fig/kg per min. Large symbls are resting values. Lwer relatinship between the change in resting MAP and the shift in BP K derived frm individual measurements in each rabbit given phenylephrine at 1 and 2 iig/kg per min. dse f phenylephrine, the MAP-RSNA relatin had returned t the cntrl lcatin (Fig. 11). Discussin The renal nerve electrde permitted recrding f neural activity ver a lnger perid and in a greater prprtin f animals than with previus implanted TABLE 2 Parameters and Resting Values Obtained with Different Dses f Nitrprusside in Eight Cnscius Rabbits Hyptensive value (NU) Upper plateau (NU) Resting RSNA (NU) Lwer plateau (NU) Hypertensive value (NU) RSNA range (NU) Average gain (NU/mm Hg) BP M (mm Hg) Resting MAP (mm Hg) Hyptensive reversal (NU) Hypertensive reversal Dse SED ± ±2.6 ±2.2 ±5.4 ±6.8 ±1.4 ±1.4 ±1.1 ±7.6 ±5.0 A 7.4* * * 106.9* 11.3* 2.1 F-value B 7.6* * 6.3* 5.9* * 80.2* 13.5* 2.3 Dses are infusin rates f nitrprusside (jigag per min). F-values given are ft linear cmparisns between dses (clumn A) and fr cmparisns f averages f first tw and last tw dses (clumn B). P<0.05.

12 Dnvard et al./renal Sympathetic Barreflex 629 TABLE 3 Parameters and Resting Values Obtained with Different Dses f Phenylephrine in Five Cnscius Rabbits Hyptensive value (NU) Upper plateau (NU) Resting RSNA (NU) Lwer plateau (NU) Hypertensive value (NU) RSNA range (NU) Average gain (NU/mm Hg) BP M (mm Hg) Resting MAP (mm Hg) Hyptensive reversal (NU) Hypertensive reversal Dse SED ±14.1 ±10.1 ± 2.4 ± 2.2 ± 2.4 ±10.1 ± 2.1 ± 2.4 ± 1.7 ±12.0 ± 2.8 F-value A C * 7.4* 26.8* * * 10.0* 13.6* * 13.0* * Dses are infusin rates f phenylephrine (MgAg per min). F-values given are fr linear cmparisns between dses (clumn A), and fr cmparisns f average f middle tw dse with that f first and final dses (clumn Q. P<0.05. electrdes (Schad and Seller, 1975; Lewis and Haeusler, 1975; Ninmiya et al., 1976; Riedel et al., 1982; Iriki and Kzawa, 1983). Thus, the main experiments culd be perfrmed after full recvery frm the peratin fr implanting the electrde. Renal nerve recrding in the cnscius animal is free f the inertia assciated with measurement f vascular resistance and f the many lcal factrs (e.g., the renin-angitensin system) that can affect the latter variable. We used perivascular ballns t prduce slw changes in MAP frm the resting value, s that ur findings cncerning the renal barreflex wuld be relevant t steady state cnditins. Our findings suggest that in nrmal rabbits the arterial and cardiac barreceptrs exerted inhibitry effects n virtually every cmpnent f the renal sympathetic barreflex. In ur analysis, the rle f the cardiac barreceptrs was assessed frm the respnses btained ver a perid f 45 minutes after prcaine, while that f the arterial barreceptrs was assessed frm the difference in pattern befre and 24 hurs after SAD. Ludbrk and Graham (1984) have shwn that the rabbit's cardivascular respnse t exercise is altered within minutes after SAD, and that this altered respnse subsequently remains cnstant ver the next 7 days. This makes it unlikely that the difference in the times f measurements will alter ur cnclusins abut the cntributin f the tw sets f barreceptrs n the RSNA respnses f nrmal rabbits. Intrapericardial prcaine prduced rapid effects n the reflex (Fig. 8), mst f which had reversed 2 hurs after stpping the drug. This methd f blcking the cardiac nerves was nt assciated with the large, transient cnstrictr effects that have been reprted in anesthetized rabbits after cervical vagtmy (Gu et al., 1982). Intrapericardial lcal anesthesia has the advantage that it eliminates nt nly vagal but als cardiac sympathetic afferents, and that it avids remval f afferents frm lung inflatin receptrs which will affect cnstrictr tne (Daly and Rbinsn, 1968). The inhibitry effects f the arterial barreceptrs n the sigmid cmpnent f the barreflex, including resting RSNA, lwer plateau, and gain, were greater than thse f the cardiac receptrs. During venus balln-induced falls in bld pressure belw resting, the inhibitin f RSNA mediated thrugh the cardiac receptrs increased prgressively, whereas that mediated thrugh the arterial barreceptrs decreased. The magnitude f the inhibitin mediated thrugh the cardiac receptrs was independent f the presence r absence f the arterial barreceptrs (Fig. 7). In the nrmal rabbit, the inhibitry actin f the cardiac receptrs limits the magnitude f the upper RSNA plateau during unlading f the arterial barreceptrs. During artic balln-induced rises in bld pressure, cardiac receptr influences culd nt be demnstrated when the arterial barreceptrs were intact. Hwever, an inhibitry effect became bvius after SAD, where it was apparent at relatively high pressure levels (Fig. 7, curves c and ). This suggests that the cardiac receptrs als prvide inhibitry drive t the lwer plateau in nrmal rabbits. Their inhibitry effect and that mediated thrugh the arterial barreceptrs is nt ne f simple summatin, but is a nnadditive interactin affecting cmmn sympathetic neurns, similar t the cclusin respnses described by Sherringtn (1906) in his analysis f spinal reflexes. In cnscius rabbits, the lwer plateau always remained abve nise level even when resting MAP was substantially raised with phenylephrine (Table 3), althugh, under these cnditins, it was reduced frm the level bserved in the nrmal resting state. We have nt analyzed the surces f afferent drive

13 630 invlved in the greater reductin f the lwer plateau with phenylephrine, but the prnunced inhibitry drive frm cardiac receptrs at these higher bld pressures culd accunt fr the effect (cf. Fig. 7, difference between curves c and ). Only during anesthesia was it easy t reduce the lwer plateau t nise level by raising the bld pressure abve resting. In the cnscius animal, a small cmpnent f the renal sympathetic mtneurn pl appears resistant t inhibitin by pressure rises, particularly t rises in arterial pressure. Alfathesin anesthesia eliminates the surces f excitatin t this part f the pl, s that rises in bld pressure ablish RSNA. The renal hyptensive reversal respnse has nt, t ur knwledge, been described previusly in animals with intact afferents (Irisawa et al., 1973; Ninmiya et al., 1973; Iriki et al., 1977; Drward and Krner, 1978; Iriki and Kzawa, 1983). Prbably this is due t the susceptibility t anesthesia f this cmpnent f the renal barreflex. Our findings indicate that the large hyptensive reversal respnse, characteristic f cnscius rabbits, depends n afferents traveling in bth sinartic and cardiac nerves, and that remval f bth sets f nerves ablished the respnse. When the cardiac nerves alne are intact, the small residual reversal respnse can be explained by the prgressive increase in inhibitin f RSNA mediated thrugh the cardiac receptrs, which eventually prduces depressin f the upper plateau. When the sinartic nerves alne are intact, a residual hyptensive reversal respnse is mre difficult t explain. Incmplete blckade with prcaine appears unlikely, since cmbined SAD + prcaine cmpletely ablished this cmpnent f the reflex. A paradxical increase in firing f the arterial barreceptrs prduced by distrtin f the cllapsing cartid sinus has been described at abut the same MAP values as bserved in the reversal respnse (Landgren, 1952; Green, 1967). An alternative explanatin f the residual reversal respnse after blckade f the cardiac nerves culd be thrugh interactins between arterial chemreceptrs and barreceptrs, r between arterial and pulmnary barreceptrs (Paintal, 1973), resulting in attenuatin f the excitatin f RSNA prduced by full unlading f the arterial barreceptrs. In the intact rabbit, the hyptensive reversal respnse thus results frm nn-additive facilitatry interactins resulting frm changes in activity f between tw t fur sets f afferents. We may speculate that the turning-ff f sympathetic cnstrictr tne during the hyptensive reversal respnse can be cnsidered a last resrt mechanism t ablish reflex vascnstrictin. This will help limit the extent f the fall in cardiac utput when this becmes marked, which ccurs with venus balln inflatin (Krner, unpublished data). It is nt surprising that the mst cmplex cmpnents f the barreflex are almst cmpletely Circulatin Research/Vl. 57, N. 4, Octber 1985 eliminated by anesthesia. The level f alfathesin used in ur experiments was adequate fr surgical anesthesia and deeper than in ur previus study (Blake and Krner, 1981). This explains the marked depressin f the vagal and sympathetic cmpnents f the barreceptr-heart rate reflex. Indeed, the marked depressin f the sigmid cmpnent f the barreceptr-heart rate reflex cntrasts with the relatively minr effects n this cmpnent f the renal sympathetic barreflex, emphasizing the selective nature,f the central depressin f particular anesthetics. In nrmal rabbits, the hypertensive RSNA reversal respnse was smaller than the hyptensive reversal and was assciated with reductin in reflex bradycardia. Bth the arterial and cardiac receptrs exerted a substantial inhibitry effect n the hypertensive reversal respnse, with that thrugh the arterial barreceptrs larger than that mediated thrugh the cardiac receptrs. Hwever, the inhibitin thrugh the latter afferents was substantial (Fig. 7, curves c and ); its enhancement during phenylephrine infusins culd accunt fr the ablitin f the respnse under these circumstances. The huge reversal respnse evked after remving bth arterial and cardiac barreceptr influences was twice the nrmal upper plateau level. This was similar t the RSNA respnse evked by naspharyngeal stimulatin, which wuld have reduced renal bld flw t nearly zer. Gu et al. (1982) have described an analgus reflex respnse in hindlimb vascular resistance f anesthetized SAD rabbits, prduced by large elevatins f systemic bld pressure. Hwever, this was nt bserved in animals with afferents intact. In cntrast t ur findings in cnscius rabbits, the "hindlimb hypertensive resistance respnse* was assciated with enhancement f bradycardia. The hypertensive reversal respnse represents failure f the cmbined effects f the arterial and cardiac barreceptrs t inhibit the excitatry effects n RSNA f large rises in bld pressure and/r ischemia (see belw). Previus investigatrs have demnstrated inhibitry influences arising frm cardiac receptrs during falls in bld pressure (Oberg and White, 1970a; Oberg and Thren, 1971; Chen et al., 1978; Thren, 1979; Thames et al., 1982; Abbud and Thames, 1984). Of particular relevance t the present study is the pssible enhanced activity f left ventricular receptrs in the empty heart (Oberg and Thren, 1971) and the demnstratin f inhibitry effects evked frm left ventricular receptrs by mycardial ischemia (Thames and Abbud, 1979). Linden et al. (1981) shwed that sme f the effects arising frm left atrial receptrs can influence renal sympathetic nerve activity, independent f effects arising frm the arterial barreceptrs. These receptrs culd cntribute drive t resting RSNA thrugh simple summatin f afferent influences.

14 Drward et a/./renal Sympathetic Barreflex Enhanced inhibitry effects thrugh left ventricular receptrs have been evked by increasing ventricular afterlad (Mark et al., 1973; Thren, 1979; Malliani, 1982). These prbably cntributed t the inhibitin f the lwer plateau and hypertensive respnse. Our experiments prvide n infrmatin n whether the inhibitry influences frm the cardiac receptrs evked during rises in bld pressure invlved the same receptrs as prduce enhancement f inhibitin during pressure falls. After cmbined SAD + cardiac receptr blckade, RSNA was still altered by balln inflatin. Pssible afferent surces include pulmnary barreceptrs (Paintal, 1973) and a range f C-fiber afferents mediating spinal and supraspinal reflexes (Malliani, 1982). Lwer bdy ischemia culd have cntributed t the large hypertensive reversal respnse, althugh it was transient, and nt sufficient t cause the animals apparent distress. Mrever, the findings f Gu et al. (1982) with phenylephrine suggest that it is the increase in MAP, rather than ischemia, that evked the hypertensive reversal respnse, since the ischemia in their experiments was prbably cnsiderably smaller with phenylephrine than with artic balln inflatin. Resetting We have previusly fund that nitrprusside-induced falls and phenylephrine-induced rises in MAP alter the threshld f the arterial barreceptrs by abut 0.4 mm Hg/mm Hg A resting MAP. This degree f resetting remained stable frm between 15 minutes and 2 hurs after the attainment f the new resting MAP (Drward et al., 1982). Resetting f the whle artic nerve activity during similar drug infusins is smewhat larger [abut 0.55 mm Hg/ mm Hg A MAP (Drward, unpublished bservatins)]. In the present experiments, the changes in threshld f the renal sympathetic barreflex (assessed frm the changes in BP ) with nitrprusside were significantly greater (0.74 mm Hg/mm Hg MAP), suggesting that additinal factrs cntributed t its resetting. Additinal factrs als were present in relatin t reflex resetting with phenylephrine in view f the highly nnlinear relatinship between dse f phenylephrine and change in threshld, as well as in the changes in the upper RSNA plateau. Prbably the ther determinants f reflex resetting were central interactins between the arterial barreceptrs and ther afferents, particularly thse arising frm the cardiac receptrs. Cardiac receptr invlvement is suggested by the reductin in the hyptensive reversal respnse bserved with the tw higher dses f nitrprusside and by the prgressive reductin in lwer curve plateau and in the hypertensive reversal respnse with phenylephrine (see abve). The findings with nitrprusside help t explain the reductin in renal barreflex threshld bserved during spntaneus falls in resting MAP (e.g., be- 631 tween cntrl and recvery curves in Figure 5) and during the reductin in resting MAP which accmpanied anesthesia. With bth vasactive drugs, there prbably are changes in resting cardiac vlumes which may alter the cntributin made by the cardiac receptrs t the afferent input prfile t the central nervus system. Fr example, with high dses f phenylephrine, the acute rise in resting MAP prbably increases left ventricular resting vlume, enhancing wall stress and prducing mre prnunced alteratins in cardiac receptr activity during subsequent pressure changes (cf. Mark et al., 1973). With nitrprusside, the reflex resetting was related t dse (and changes in resting MAP) ver the entire range. Hwever, with phenylephrine, changes in reflex threshld and in ther curve parameters ccurred at nly the tw lwer dses; with the higher dse, the MAP-RSNA curve was restred back t cntrl. In cntrast, ur previus studies f arterial barreceptr resetting shwed linear increases in receptr threshlds with all dses f phenylephrine (Drward et al., 1982). We have nt investigated the reasn fr the nnlinearity in reflex resetting with increasing dses in phenylephrine and increasing rises in resting MAP; a disprprtinate increase in inhibitry influences arising frm the cardiac receptrs, due t a large increase in resting chamber vlume, culd accunt fr the bserved effect. The average gain f the sigmid cmpnent f the MAP-RSNA relatinship was nt altered with either nitrprusside r phenylephrine (Figs. 10 and 11). With nitrprusside, resting RSNA remained n the steep prtin f the curve and increased in prprtin t the fall in resting MAP. This allws the animal t respnd relatively nrmally t transient rises and falls in bld pressure f mderate magnitude (cf. Drward et al., 1982). In cntrast, rises in resting MAP during phenylephrine infusins, shifted resting RSNA nt the lwer plateau f the curve at the tw highest dses. This has imprtant cnsequences fr circulatry hmestasis: with resting RSNA n the lwer plateau, nly a fairly large fall in MAP will elicit reflex elevatin in RSNA. Thus the apparent gain f the reflex ver a mderate range f pressures abut resting is greatly reduced, even thugh the gain f the sigmid cmpnent f the functin curve is relatively unaffected. This becmes apparent nly frm analysis f the full reflex functin curve. Incrrect cnclusins abut gain wuld arise frm analysis f RSNA respnses evked by smaller pressure changes abut the resting value. In this type f acute elevatin f resting MAP, the mre prnunced sympathetic inhibitin f resting RSNA helps t limit the hemdynamic lad n the heart, but at the cst f diminished effectiveness f neural regulatin during mderate pressure changes abut resting. During alfathesin anesthesia, the resetting f the

15 632 threshld f the sigmid cmpnent f the reflex and the small rise in resting RSNA were very similar t the effects f nitrprusside in the cnscius animal. Alf athesin has peripheral vasdilatr prperties (Blake and Krner, 1981) which culd be the main determinant f the abve changes. In cntrast, the effects f the drug n the lwer RSNA plateau and hyptensive reversal respnse are clearly mediated thrugh its effects n the central nervus system. Cnclusin Our analysis suggests that changes in intravascular pressures assciated with a circulatry disturbance prduce simultaneus changes in the afferent signals arising frm several grups f pressure-sensitive receptrs. In the nrmal rabbit, the cardiac and arterial barreceptrs each cntributes significant inhibitry drive t the renal sympathetic barreflex ver the entire range f bld pressure. These effects are apprximately additive ver the highgain part f the sigmid cmpnent f the functin curve, but becme increasingly nnlinear at the extremes f MAP. The mst cmplex interactins, including the hyptensive reversal respnse and the lwer plateau, are mst susceptible t the effects f anesthesia, emphasizing the advantage f unanesthetized animals fr the analysis f cmplex neural events. Changes in resting MAP prduce resetting f the renal sympathetic barreflex, which accunts fr the shifts in reflex threshld during spntaneus alteratins in bld pressure. Reflex resetting is explained in part by resetting f the threshld f the arterial barreceptrs and in part by central interactins assciated with alteratins in afferent input, including that arising frm the cardiac receptrs. We are grateful t Marjrie Nichlsn wh prepared the manuscript. This study was supprted by grants frm the Natinal Health and Medical Research Cuncil and the Life Insurance Medical Research Fund f Australia and New Zealand. Dr. Riedel's present address is: Max Planck Institute, D-63 Bad Nauheim, Parkstrasse 1, West Germany. Address fr reprints: Prfessr P.I. 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Aust J Exp Bil Med Sci 51:17-31 Zimpfer M, Manders WT, Barger AC, Vatner SF (1982) Pentbarbital alters cmpensatry neural and humral mechanisms in respnse t hemrrhage. Am J Physil 243: H713-H721 INDEX TERMS: Anesthesia Arterial barreceptrs Cardiac receptrs Cnscius rabbit Implanted electrde Naspharyngeal reflex Renal sympathetic barreflex Barreflex resetting