Interaction of Vasopressin and the Baroreceptor Reflex System in the Regulation of Arterial Blood Pressure in the Dog

Transcription

1 Interactin f Vaspressin and the Barreceptr Reflex System in the Regulatin f Arterial Bld Pressure in the Dg By Allen W. Cwley, Jr., Emil Mns, and Arthur C. Guytn ABSTRACT The hemdynamic effects f 1-hur intravenus infusins f vaspressin were evaluated in trained, unanesthetized dgs in the nrmal state and fllwing sinartic barreceptr denervatin. Pressr sensitivity t vaspressin was greatly enhanced fllwing barreceptr denervatin; threshld sensitivity was increased 11-fld and sensitivity at higher dse levels was increased fld. Infusin f physilgical levels f vaspressin caused an average increase in arterial bld pressure f 33 mm Hg in cnscius, barreceptr-denervated dgs cmpared with an increase f 5 mm Hg in nrmal dgs. In cntrast, similar intravenus infusins f nrepinephrine at physilgical levels resulted in a 3-fld increase in pressr sensitivity with n change in threshld dse. Hy- - pphysectmy f barreceptr-denervated dgs did nt significantly alter their pressr sensitivity t vaspressin in the cnscius state. The arterial bld pressure respnse t intravenus vaspressin infusins was greatly depressed when a high backgrund level f circulating vaspressin was present. Decapitated, spinal, anesthetized dgs maintained with a small cntinuus infusin f nrepinephrine exhibited the greatest sensitivity t vaspressin; the threshld dse fr a pressr respnse was similar t that in cnscius barreceptr-denervated dgs, but pressr sensitivity at physilgical dse levels was increased nearly 8,000-fld. The elevatins in arterial bld pressure resulting frm vaspressin infusins f less than 1.0 munits/kg min" 1 were large enugh t implicate the direct pressr effect f vaspressin in the nrmal cntrl f arterial bld pressure. KEY WORDS dse-respnse curve cardiac utput nrepinephrine diabetes insipidus areflexic dg cntinuus data cllectin barreceptr denervatin arginine vaspressin hypphysectmy pressure-frequency distributin curves' Cntrary t generally held views, reprts frm several different labratries have indicated that vaspressin participates in the nrmal daily regulatin f arterial bld pressure. Szczepanska- Sadwska (1, 2) has recently reprted that, fllwing a mild nnhyptensive hemrrhage in nrmal cnscius dgs, the elevatin in plasma vaspressin is sufficient t be significant in the maintenance f arterial bld pressure and bld vlume. Als, the increased plasma vaspressin cncentratins bserved during hypvlemia and mild water deprivatin exceed thse prducing antidiuresis (3, 4). If these plasma levels f vaspressin have significant vasactive prperties, then the hypthalamic-hypphyseal antidiuretic system culd Frm the Department f Physilgy and Biphysics, University f Mississippi Schl f Medicine, Jacksn, Mississippi This wrk was supprted by U. S. Public Health Service Grants HL and HL frm the Natinal Heart and Lung Institute. This wrk was dne during Dr. Cwley's tenure as an Established Investigatr f the American Heart Assciatin. Dr. Mns' present address is Semmelweis University Schl f Medicine, Budapest, Hungary. Received Nvember 30, Accepted fr publicatin January 30, Circulatin Raeanh, Vl. XXXIV, April 1974 functin as a mderately rapid-acting mechanism fr maintaining a nrmal level f arterial bld pressure. Supprt fr this hypthesis is fund in the results f Rcha E Silva and Rsenberg (5), wh have shwn that the amunt f vaspressin secreted in respnse t mild hemrrhage in anesthetized dgs is sufficient t cause a pressr respnse when the reflex feedback cntrl lps f the barreceptr system are eliminated. Mns et al. (6) have bserved that nrmal subpressr infusins f vaspressin result in an increase in arterial bld pressure fllwing hypphysectmy; this bservatin suggests that endgenus vaspressin nrmally influences vascular tne. Further supprt fr this idea cmes frm the reprt that dgs with diabetes insipidus are mre susceptive than nrmal dgs t lw arterial bld pressure fllwing hemrrhage (7). It has als been shwn that the pressr sensitivity t vaspressin can be greatly enhanced under certain cnditins. Fr example, the pressr respnses t catechlamines are ptentiated by infusins f physilgical cncentratins f vaspressin (7), and persns with primary autnmic insufficiency exhibit marked pressr respnses t vaspressin infusins within the nrmal physilgical range (8). 505

2 506 COWLEY, MONOS, GUYTON The purpse f the present experiment was t examine the ptential rle f vaspressin in the regulatin f arterial bld pressure, perhaps even under nrmal cnditins. The experiments differed frm thse previusly cnducted in several imprtant aspects. First, trained, unanesthetized dgs with chrnically implanted arterial and venus catheters were used. In all previus studies except ne (1), the pressr effects f vaspressin were evaluated in anesthetized animals, but anesthesia alters bth the reflex respnses t pressr agents (9) and the plasma cncentratins f vaspressin (10). Secnd, new techniques were used t precisely quantify the variable arterial bld pressures which ccur in unanesthetized, barreceptrdenervated dgs. Thus, the pressr effects f small dses f vaspressin were bserved in the absence f the barreceptr reflexes and withut the depressant effects f anesthesia. Third, vaspressin respnses were quantified fr the first time in resting, unanesthetized, hypphysectmized, barreceptr-denervated dgs. Furth, vaspressin was administered t decapitated, areflexic dgs s that direct pressr effects culd be studied withut any interactins with the central nervus system r any endgenus release f vaspressin. Finally, a wide dse range f vaspressin was tested t generate a cmplete dse-respnse curve characterizing each type f dg preparatin. These experiments indicated that vaspressin, infused s as t apprximate physilgical secretin rates, culd significantly affect systemic resistance vessels and, under certain cnditins, cntribute t bth shrt- and lng-term regulatin f arterial bld pressure. CHRONIC EXPERIMENTS Methds The hemdynamic effects f intravenusly infused vaspressin ( munits/kg min" 1 ) were evaluated in 11 trained, unanesthetized dgs. Over a perid f 1 year these dgs were evaluated in several different physilgical states. Nrmal, Unanesthetized Dgs. At least 2 weeks prir t the experiment, catheters were chrnically implanted in the femral artery and vein f 11 dgs, tunneled subcutaneusly t the back fr prtectin, and kept patent by filling with 1,000 USP units f heparin slutin. Seven f the dgs were studied withut anesthesia t cmpare the hemdynamic effects f vaspressin infusins with thse f nrepinephrine infusins. Three f the dgs were used t cmpare the hemdynamic effects f Parke-Davis Pitressin (a mixture f lysine and arginine vaspressin) with thse f pure synthetic arginine vaspressin, 1 the naturally ccurring neurhypphyseal vaspressin prduced by dgs. Unanesthetized, Sinartic Barreceptr-Denervated Dgs. In six f the dgs, the sinartic barreceptrs were successfully denervated; the details f the surgery have been previusly described (11). Several weeks after denervatin, similar vaspressin and nrepinephrine infusin studies were repeated in the unanesthetized dgs. Electrmagnetic flw transducers (Bitrnex 5000 series) had been placed n the ascending artas f tw f these dgs befre the time f barreceptr denervatin fr determinatin f cardiac utput. Unanesthetized, Hypphysectmized, Sinartic Barreceptr-Denervated Dgs. Three f the riginal 11 nrmal dgs were successfully subjecte'd t barreceptr denervatin and hypphysectmy. The pituitary gland was apprached transpharyngeally, and the dura was pened at the midline fr the remval f the entire hypphysis as has been previusly described (6). These dgs were then chrnically maintained with intramuscular injectins f adrencrtitrphic hrmne three times weekly. Within 48 hurs after the surgery, the effects f vaspressin (Pitressin) infusins were studied in these dgs in the unanesthetized state. At this time all 3 dgs shwed the classic signs f diabetes insipidus. EXPERIMENTAL PROTOCOL AND ANALYSIS OF DATA The experimental prtcl cnsisted f a 1-hur cntrl perid f cntinuus cllectin f hemdynamic data fllwed by a 1-hur perid f drug infusin. When the pressure had returned t cntrl levels, cntrl data were cllected fr anther 1-hur perid and then anther randmly selected dse f drug was infused. At least five different dses f vaspressin and nrepinephrine were studied in each dg. The dgs were assumed t be nrmally hydrated, since drinking water was freely available befre and thrughut the experimental prcedures. The dgs were fitted with a backpack husing a Statham P23DC pressure transducer at heart level. The infusin tubes and wires frm the pressure and flw transducers were prtected, brught ut f the tp f the pen husing the dg, and cnnected t a Grass mdel 7 recrder. Since the arterial bld pressure f the barreceptr-denervated dgs is extremely labile and greatly influenced by surrundings, as is the endgenus secretin f vaspressin, the experiments were perfrmed in a quiet, islated rm with n ne present except t start and stp the infusin pump. The dgs were put in a large pen in which they had a high degree f freedm f mvement. Heart rates were mnitred using a Grass mdel 7P44A tachgraph triggered frm the first derivative f the arterial pulse wave. Cardiac utputs were btained using a Bitrnex mdel BL-610 bld flwmeter. The data cllectin and analysis system has been described recently in detail (11). This system permitted arterial bld pressures, heart rates, and cardiac utputs recrded n plygraph recrds t be cnverted t Supplied by Dr. Rderich Walter, Munt Sinai Schl f Medicine, New Yrk. Circulatin Research, VL XXXIV. April 1974

3 'AS0PRE8SIN PRE88OR ACTIVITY 507 jlectrical signals fr cmputer analysis. A fur-channel malg curve-reading system with fiber ptic scanning >ens generated analg vltages frm the recrded ink races. The analg vltages in turn were fed int an inalg-t-digital cnverter that changed them t digital ignals fr analysis by a PDP-9 cmputer. The infrmain was used by the cmputer t calculate average lurly arterial bld pressure, heart rate, cardiac ut- )ut, and standard statistical infrmatin. Such pr- :edures enabled the frequency f ccurrence f lifferent mean arterial bld pressures t be tabulated >y the cmputer (Fig. 1). Nearly 1,800 sample pints fr :ach variable were stred fr each hur f recrded ime. This prcedure permitted very accurate measurenent f all recrded hemdynamic data. Mrever, the nethd allwed the variable pressures f barreceptrlenervated dgs t be presented by graphing frequency listributin curves thrughut the desired perid f ex- )erimentatin. ICUTE EXPERIMENTS Dgs used fr acute studies were anesthetized with a hrt-acting barbiturate (Suratal, 30 mg/kg); a rachetmy was perfrmed and catheters were placed n the femral artery and vein. A 12-ml spinal injectin if 80% ethanl was then administered fllwed by a apid decapitatin prcedure requiring less than 12 secinds. These dgs were then maintained with psitiveiressure ventilatin, and arterial bld pressure was tabilized at 100 mm Hg by infusing nrepinephrine at a»nstant rate between 0.01 and 0.1 Mg/kg min" 1. The letails f this preparatin have been described previusly (12) LU I 12.8 rr 8 «* Li. O 6,4 O rr j 3.2 Cntrl Nrmal Vaspressin UJ O One hur after decapitatin, when arterial bld pressure had been stabilized at 100 ± 5 mm Hg fr 15 minutes, a vaspressin-pressure dse-respnse curve was btained by infusing vaspressin intravenusly at randmly selected rates between 0.05 and 100 munits/ kg min" 1. In these acute studies, each rate f infusin was maintained fr 10 minutes, which was sufficient t establish the plateau f the pressure respnse. Fllwing each infusin, pressure was permitted t return t cntrl values; within 10 minutes after the infusin, pressure was generally within ±5 mm Hg f cntrl, indicating a stable preparatin. Results Typical Respnse t Lw Rates f Vaspressin Infusin befre and after Barreceptr Denervatin. The results f 1 hur f vaspressin infusin at 1.6 munits/kg min" 1 in an unanesthetized dg befre and after sinartic barreceptr denervatin are illustrated in Figure 1; the frequency distributin curves fr mean arterial bld pressure befre and during infusins are shwn. In the nrmal state, the infusin f vaspressin did nt have a significant effect n arterial bld pressure (P>0.5). Fllwing denervatin f the sinartic barreceptrs, hwever, the same dse f vaspressin resulted in an average pressure rise f 32 mm Hg during the 1-hur perid. This rise is clearly demnstrated by the shift in the pressurefrequency distributin curve. 7.0 rr 5.6 rr O 42 U. O UJ 2.8 UJ 1.4 rr UJ Barreceptr Denervated Cntrl Vaspressin MEAN ARTERIAL PRESSURE (mmhg) FIGURE 1 Frequency f ccurrence f different mean arterial bld pressures fr 1 hur befre and 1 hw during intravenus vaspressin infusins (1.6 munitsfkg min' 1 ) in a single unanesihetized nrmal dg befre (left) and several weeks after (right) sinartic barreceptr denervatin. (rcuiattn Rtteanh, VL XXXIV. April 1974

4 508 COWLEY, MONOS, QUYTOI All infusins in barreceptr-denervated dgs and in mst f the nrmal dgs were analyzed in this manner, and a statistical cmparisn was made between each cntrl perid and its crrespnding infusin perid (Student's t-test). Since each distributin perid cntained 1,800 sample data pints, the technique permitted fine discriminatin between the results during cntrl and infusin perids, even with the randmly fluctuating pressure f barreceptr-denervated dgs. Average Arterial Bld Pressure Respnses t Vaspressin in Nrmal, Barreceptr-Denervated, and Decapitated Dgs. Figure 2 illustrates three dse-respnse curves cmparing the increases in arterial bld pressure during 1-hur vaspressin infusins in nrmal dgs, barreceptr-denervated dgs, and decapitated, areflexic dgs. Respnses were btained frm 34 vaspressin infusins in seven nrmal dgs, 27 infusins in six barreceptrdenervated dgs, and 32 infusins in five decapitated dgs. Cmparisn f the respective regressin equatins shws that the unanesthetized, barreceptr-denervated dgs were cnsiderably mre sensitive t vaspressin than were the nrmal dgs. The regressin equatin fr nrmal dgs was Y lg X with a cefficient f crrelatin ( r) 0.75; fr barreceptr-denervated dgs it was Y = 23.0 lg X , r = There was an Nrmal O Barrs - + Decapitated QOI 0JO VASOPRESSIM ( mu/kg/min) FIGURE 2 Dse-respnse curves btained during 1-hur intravenus vaspressin infusins in seven nrmal dgs (34 infusins), six barreceptr-denervated dgs (27 infusins), and five decapitated dgs (32 infusins). 11-fld difference in threshld sensitivity betwee the tw grups. Pressr sensitivity was fl higher in the barreceptr-denervated dgs at th higher dse levels, which is apparent frm the sig nificant difference between the slpes f the re gressin equatins (P<0.01). Fr example, the re gressin equatins shw that vaspressin infused i 0.42 munits/kg min~' in a barreceptr-denervate dg will result in a 25-mm Hg rise in arti pressure, althugh 30.0 munits/kg min" 1 is require t btain the same respnse in nrmal dgs; thus, 70-fld sensitivity difference exists. The average arterial bld pressure elevatin i barreceptr-denervated dgs at infusin rates be tween 0.2 and 2.0 munits/kg min" 1 was +33 mm Hj This dse range was examined specifically, since represents secretin rates which are bserved in number f different physilgical cnditins. I cmparisn, Figure 2 shws that nrmal d generally respnded with less than a 5.0-mm H rise in mean bld pressure at this dse range. Decapitated, spinal, anesthetized dgs exhibite the greatest sensitivity t vaspressin infusins, i illustrated by the steep regressin line in Figure (Y g X , r ). Althugh th threshld dse was similar t that fr barrecer. tr-denervated dgs, the dse f vaspressin re quired t achieve a 25-mm Hg pressure rise in th nrmal dgs was 188-fld greater than in th decapitated preparatin. T achieve a 50-mm H arterial bld pressure rise, 8,000 times mr vaspressin was required in a nrmal dg than w needed in an areflexic dg. The average mean arterial bld pressure duj ing the cntrl perids in the nrmal dgs infuse with vaspressin was ± 5.7 (SE) mm Hj in the barreceptr-denervated dgs it w; ± 6.0 mm Hg, and in the decapitated dgs was ± 0.2 mm Hg. Each pint pltted abv zer n the Y axis in these regressin equatins w; statistically different (P < 0.05) frm its wn ci trl value accrding t the ttal sampled pin used in the calculatin f the frequency distribi tin curves fr the cntrl and infusin peric (Student's t-test). The transient respnses f arterial blc pressure at the beginning f a vaspressin infusic were nearly the same in all three preparatins; tli plateau f the pressure respnse was reached witl in 4-8 minutes. The transient pressure respnses ; the end f the infusins were quite variable in bt the nrmal and the barreceptr-denervated dg return f arterial bld pressure t cntrl leve Circulatin Ratardi, VL XXXIV, April 197

5 VASOPRESSIN PRESSOR ACTIVITY 509 NORMAL BARORECEPTOR DENERVATED < e u <t VASOPRESSIN ( mu/kg/min) FIGURE 3 Heart rate changes during varius rates f intravenus vaspressin infusin in seven nrmal dgs (left) and six barreceptr-denervated dgs (right). tk frm 10 minutes t 2 hurs. Hwever, in decapitated dgs the pressures cnsistently returned t cntrl levels in an average f abut 10 minutes. Heart Rate Respnses t Vaspressin. In nrmal dgs heart rate generally decreased at prgressively higher levels f vaspressin (Y = lg X -8.7, r = 0.45), but in barreceptr-denervated dgs heart rate was nt affected in any cnsistent manner by the increasing infusin rates (r = 0.00l), as shwn in Figure 3. A similar lack f crrelatin between the dse f infused vaspressin and heart rate ccurred in decapitated dgs. In nrmal intact dgs the prgressive decrease in heart rate was assciated with a rise in mean arterial bld pressure and yielded a crrelatin cefficient f r= 0.7. When barreceptrs were remved, the crrelatin between arterial bld pressure and heart rate was very lw ( r= +0.15). The average cntrl heart rate in nrmal dgs was 90.4 ± 5.9 (SE) beats/min, and it was ± 3.1 beats/min in barreceptr-denervated dgs. Cardiac Output Respnses t Vaspressin. Cardiac utput respnses t vaspressin were determined in tw dgs befre and after denervatin. Fur dse levels f vaspressin between 1.0 and 20 munits/kg min" 1 were tested in each dg. There was a cnsistent decrease in cardiac utput in bth the nrmal and the barreceptr-denervated dgs ranging between 5.0% at lw infusin rates (less than 2.0 munits/kg min" 1 ) and 30.0% at higher infusin rates (greater than 10 munits/kg min" 1 )- The respnses were t variable t detect any statistically significant difference between the emulatin Research. VL XXXIV. April 1974 nrmal and the barreceptr-denervated dgs. Arterial Bld Pressure Respnses t Vaspressin Infusins in Barreceptr-Denervated Dgs Cmpared with Thse in Hypphysectmized, Barreceptr-Denervated Dgs. The changes in mean arterial bld pressure during 15 infusins in three hypphysectmized, barreceptr-denervated dgs are indicated by the circled crsses n the dse-respnse diagram in Figure 4. The pen circles are the same respnses btained in the six barreceptr-denervated dgs cnsidered in Figure 2. There was little difference between the results btained in the tw grups f loon VASOPRESSIN (mu/kg/min.) FIGURE 4 Changes in mean arterial bld pressure during 15 infusins in three hypphysectmized, barreceptr-denervated dgs are shwn by circled crsses. These changes are superimpsed n the same dse-respnse curves shwn in Figure 2 fr nrmal (triangles) and sinartic barreceptr-denervated (circles) dgs.

6 510 COWLEY, MONOS, GUYTON UJ OH V) ft cc a. ui t * < E 5 5 ui X I00 9a SO- SO M Nrmal (N<=6) EZ2 Bar-denervated (N«5) CD Decapitated (N«5) iil Bar-denervated-hypphysectmized (N-3) E23 Bar-denervated-hypphysectmized -elevated plasma ADH (N-3) h0.05-q09h ho.io-0.49h I-0OO-I.59-I I I I-M-M H H4.0-I6.0H VASOPRESSIN (mu/kg/min.) FIGURE 5 Changes in mean arterial bld pressure btained at the indicated dse ranges f vaspressin fr all grups f dgs. Particular attentin shuld be given t the last tw clumns in each dse range which are the respnses btained frm the unanesthetized, hypphysectmized, barreceptr-denervated dgs (N 3) and the vaspressin-preinjected dgs (N 3). ADH antidiuretic hrmne. dgs, althugh the regressin equatin fr the barreceptr-denervated dgs was slightly lwer when it was recalculated t include the hypphysectmized dgs (upper slid line, Fig. 4 [Y = 26.8lg X , r = 0.73]). Thus, hypphysectmy resulted in little change in the respnse f barreceptr-denervated dgs t vaspressin. Depressin f Vaspressin Sensitivity by Prir Intramuscular Vaspressin Injectins. Twenty days after hypphysectmy and barreceptr denervatin, three dgs were given an intramuscular injectin f 10 units f vaspressin (Pitressin); 30 minutes was allwed fr absrptin int the circulatry system. Then a 30-minute cntrl recrd was btained fllwed by a 1-hur infusin f vaspressin in dses ranging frm 0.5 t 15.0 munits/kg min" 1. The effects f vaspressin preinjectin are summarized in Figure 5. Fr rapid visual cmparisn, the results frm the nrmal, barreceptr-denervated, and decapitated dgs are als summarized in this figure by gruping them int the different indicated dse ranges. The last tw clumns in each dse range are the results frm the unanesthetized, hypphysectmized, barreceptr-denervated dgs (N = 3) and the vaspressin-preinjected dgs, respectively. The arterial bld pressure respnse was greatly depressed as a result f the preinjectin f vaspressin, which presumably caused a high backgrund level f circulating vaspressin. Sensitivity t Pitressin Cmpared with That t Pure Synthetic Arginine Vaspressin. The arterial bld pressure respnse t intravenus infusin f Pitressin was cmpared with that t pure synthetic arginine vaspressin in three nrmal dgs and in tw hypphysectmized, barreceptr-denervated dgs fr 2 days fllwing surgery. The results frm 12 infusins f arginine vaspressin were cmpared with thse frm 12 identical infusins f Pitressin ranging between 0.4 and 5.0 munits/kg min" 1 )- A paired variance statistical analysis fr the same infusin rate in each individual dg established n statistical difference between the pressure changes resulting frm the tw drugs (P > 0.2). Hemdynamic Respnses t Nrepinephrine in Nrmal and Barreceptr-Denervated Dgs. Nrepinephrine was infused t determine whether the increased sensitivity t vaspressin bserved in barreceptr-denervated dgs was a specific r a nnspecific respnse. Figure 6 cmpares the mean arterial bld pressure changes resulting frm 19 nrepinephrine infusins in five unanesthetized, nrmal dgs with thse resulting frm 20 infusins in five unanesthetized, barreceptr-denervated dgs. This regressin analysis shws that the sensitivity difference t nrepinephrine between the nrmal and the barreceptr-denervated dgs was cnsiderably less than that t vaspressin. In nr- Circutatin Research, Vl. XXXIV, April 1974

7 VASOPRESSIN PRESSOR ACTIVITY 511 U I < Nrmal O Barreceptr Denervated QOI QIO I.00 NOREPINEPHRINE (pg/kg/min) FIGURE 6 Dse-respnse curves btained during 1-hur intravenus nrepinephrine infusins in five nrmal dgs (19 infusins) and five barreceptr-denervated dgs (20 infusins). mal dgs the regressin equatin was Y = 34.6 lg X + 43, r = 0.8, and in barreceptr-denervated dgs it was Y = 55.8 lg X + 79, r=0.8. These equatins indicate that an infusin rate f 0.3 /ig/ kg min" 1 was required t btain a 25-mm Hg rise in mean arterial bld pressure in nrmal dgs and that a rate f nly 0.1 /u.g/kg min" 1 was required t achieve the same pressure rise in barreceptrdenervated dgs. The average cntrl mean arterial bld pressure was ± 3.3 (SE) mm Hg in the nrmal dgs and 89.4 ± 5.4 mm Hg in the barreceptr-denervated dgs. A decrease in heart rate was generally bserved during nrepinephrine infusins in nrmal dgs; the decrease was similar t that bserved with vaspressin infusins, but it had a higher negative cefficient f crrelatin ( r= 0.74). N crrelatin was btained in the barreceptr-denervated dgs ( r = 0.08) between heart rate and amunt f nrepinephrine administered. The mean cntrl heart rate befre nrepinephrine infusin was ±10.1 (SE) beats/min in the nrmal dgs and 92.4 ± 2.6 beats/min in the barreceptr-denervated dgs. Cardiac utput during infusins f nrepinephrine in the same tw dgs used t test cardiac utput during vaspressin infusins was elevated. The nrmal dgs respnded with a maximum increase f nly 10%. The elevatins were greater fllwing denervatin; the dgs then respnded with a maximum increase f 50% at dses exceeding 0.17 /xg/kg min" 1. Circulatin Reiearch, VL XXXIV. April 1974 Discussin The physilgical imprtance f the pressr activity f vaspressin has remained an unanswered questin in the mind f many investigatrs. It has been generally cncluded that the cncentratins f circulating vaspressin required t prduce a significant pressr respnse are rarely achieved under physilgical cnditins (13). Reexaminatin f the earlier studies n which this cnclusin is based shws that the pressr effects were studied primarily in acute experiments using animals anesthetized with sdium pentbarbital a situatin in which bth central nervus system reflexes and vascular reactivity are altered (9). Furthermre, it is well knwn nw that the stress f surgery, as well as the effects f anesthesia, can increase the cncentratin f endgenus vaspressin (10). Since the vaspressin-pressure dserespnse relatinship is an expnential functin (Fig. 2), many f the previus investigatrs prbably determined pressr activity when the animals were already clse t the plateau level f the dserespnse curve because f endgenus vaspressin. Fr this reasn, very large dses f vaspressin appeared t have n pressr effect (14, 15), but cardiac depressant effects were cmmnly bserved, pssibly due t cnstrictin f the crnary vasculature. Hwever, a few recent reprts by ther investigatrs, as well as the present study, indicate that the pressr activity f nrmally released vaspressin is greater than was previusly suspected. In particular, Szczepanska-Sadwska (1, 2), using a femral artery puncture technique fr measurements in cnscius dgs, cncluded that vaspressin culd be imprtant in nrmal pressure regulatin, and even in his experiments the dgs were subjected t abnrmal stress which perhaps elevated endgenus vaspressin release. Sme f the majr prblems f previus studies were vercme in the present experiment in which the arterial bld pressure respnses were quantified in ttally undisturbed, cnscius, nrmally hydrated dgs. The results in these dgs shwed that vaspressin infusins at very lw dses culd result in mild pressr respnses. But, even mre imprtant, greatly enhanced pressr respnses were bserved when the sinartic barreceptrs were remved and the sensitivity t vaspressin was tested in the same undisturbed cnscius state. And, finally, when all central nervus system influences were remved by decapitating the dgs, which als cmpletely eliminated the endgenus surce f vaspressin secretin, the pressr

8 512 COWLEY, MONOS, GUYTON respnses t physilgical dses f vaspressin were dramatic. These findings are f particular interest, since they imply that the direct pressr effects f vaspressin culd be f cnsiderable physilgical significance fr three reasns. First, the plasma levels f vaspressin in the present experiments were still within the physilgical range when the infusin rates were less than 2.0 munits/kg min" 1. As seen in Figures 2,4, and 5, infusin rates in this range induced significant pressr repnses in bth barreceptr-denervated and decapitated dgs. Tagawa et al. (16) have shwn that under nrmal steady-state cnditins the plasma cncentratin f vaspressin can be estimated frm the rate f infusin. They infused vaspressin int dgs at rates f 0.5, 1.0, and 2.0 munits/min and reprted increases in vaspressin cncentratins f 1.0,1.2, and 4.0 /aunits/ml plasma, respectively, frm an average cntrl level f 1.2 /u.units/ml, as measured by biassay prcedures. These results are very clse t thse that can be calculated using the reprted 4-5-minute half-life f vaspressin and a vlume distributin equal t 10% f bdy weight (17). Fr example, an infusin f 2.0 munits/min int a 20-kg dg wuld result in a calculated vaspressin plasma elevatin f 4.0 piunits/ml, the same rise that was fund by Tagawa et al. (16) by their biassay prcedures. Similarly, infusin f munits/kg min" 1 ( munits/min in a 20-kg dg), as was dne in ur unanesthetized dgs (Fig. 2), shuld have resulted in calculated plasma elevatins f piunits/ ml plasma. These infusin levels are the same as thse that resulted in arterial bld pressure elevatins f mm Hg (Fig. 2) in barreceptrdenervated dgs and mm Hg in decapitated dgs, even thugh nrmal dgs shwed less than a 5-mm Hg pressure elevatin at this dse range. This dse range f vaspressin shuld yield the same plasma vaspressin cncentratins that have been demnstrated t ccur during mild surgical stress, 24-hur water deprivatin, and nnhypertensive hemrrhage. Fr example, Bnjur and Malvin (10) have reprted that nrmally hydrated, cnscius dgs have a vaspressin cncentratin f 0.9 /xunits/ml plasma which is elevated t 1.7 /nunits/ml after 24 hurs f water deprivatin, 3.9 ptunits/ml during minr surgery, and 34.5 /xunits/ml during majr surgery. In anther study, mild hyptensin induced by sdium pentbarbital anesthesia (decrease in diastlic pressure ranging frm 21 t 30 mm Hg) prduced an average furfld increase in the cncentratin f vaspressin in bld (5). Als, a nnhyptensive 15% decrease in ttal bld vlume elevated plasma vaspressin levels frm an average f abut 1.5 /^units/ml t nearly 20 /xunits/ml within 50 minutes after hemrrhage (1). Maximum renal cncentrating pwer has been reprted t be btained at intravenus vaspressin infusin rates f munits/kg min" 1 (18). It. therefre, appears that many f the lwer dses infused in the present study were within physilgical secretin ranges; yet, these dses caused large elevatins f arterial bld pressure in bth barreceptr-denervated dgs and decapitated dgs. Secnd, vaspressin culd serve as anther efferent limb f bth the lw-pressure atrial stretch receptrs and the high-pressure barreceptrs fr shrt term cntrl f arterial bld pressure. Distentin f the left atrium has been shwn t inhibit vaspressin release (19), althugh enhanced release in respnse t decreasing atrial pressures remains t be clearly demnstrated. Similarly, bth chemreceptrs and cartid sinus barreceptrs can significantly influence vaspressin secretin (20, 21). In the event f decreased circulating bld vlume, the afferent limb f these reflex arcs wuld stimulate bth increased peripheral sympathetic activity and increased release f vaspressin. Since the present experiment demnstrated that vaspressin has cnsiderable pressr activity in the absence f the barreceptrs, its simultaneus release culd cntribute t an enhanced pressr respnse. Furthermre, it has been reprted that the pressr effect f circulating nrepinephrine can be cnsiderably enhanced by vaspressin (7). Third, adaptatin f the barreceptrs has been shwn t ccur when the vascular system is subjected t prlnged stress such as Gldblatt hypertensin (22, 23). The acute pressure respnses btained in the unanesthetized, barreceptr-denervated dgs shuld thus represent the pressr respnse btainable with prlnged elevatins f plasma vaspressin, prvided that ther mechanisms such as alteratin f thirst by the central nervus system and fluid vlume regulatin by the kidneys d nt verride the pressr respnses. Pssible Reasns fr Enhanced Sensitivity. The reasns fr the enhanced sensitivity t vaspressin are nt entirely clear frm the results f this experiment. Certainly sme f the increased pressr sensitivity resulted frm lss f reflex cmpensatin by the barreceptrs. Hwever, merely pening the reflex feedback lp shuld prduce an in- Circuhtim Research, Vl. XXXIV, April 1974

9 VASOPRESSIN PRESSOR ACTIVITY 513 crease in sensitivity nly as great as that caused by the nrepinephrine infusins in barreceptrdenervated dgs (Fig. 6) r that caused by angitensin (24). Since the change in pressr respnsiveness at similar dses f nrepinephrine fllwing barreceptr denervatin was less than a seventh f that btained with vaspressin fllwing barreceptr denervatin (at physilgical dses), it appears that less than 15% f the enhanced sensitivity t vaspressin can be accunted fr by the specific lack f reflex cmpensatin. Circulating plasma levels f vaspressin appear t determine, t sme extent, the pressr sensitivity t infused vaspressin (Figs. 2 and 5). Chrnically barreceptr-denervated dgs may have depressed levels f plasma vaspressin; indeed, this pssibility is supprted by the bservatin that nly slight differences in pressr sensitivity ccur between barreceptr-denervated dgs and hypphysectmized, barreceptr-denervated dgs. Anther mechanism pssibly cntributing t enhanced sensitivity culd be the interactin f catechlamines with infused vaspressin. This mechanism wuld have been f special significance in the decapitated, areflexic dgs, since their arterial bld pressure was maintained by cnstant intravenus infusin f nrepinephrine, which prbably resulted in smewhat elevated levels f circulating catechlamines. Therefre, the tremendus additinal sensitivity t vaspressin in decapitated dgs cmpared with that in bth nrmal and barreceptr-denervated, unanesthetized dgs culd have resulted frm the reprted ptentiatin f the pressr effect f nrepinephrine by vaspressin (7). Significance f the Study. The elevatins f arterial bld pressure which resulted frm vaspressin infusins f less than 1.0 munits/kg mhv 1 were f sufficient magnitude t suggest that the direct pressr effect f vaspressin must be cnsidered t be yet anther mechanism which can significantly cntribute t the cntrl f arterial bld pressure. The release f vaspressin in respnse t afferent signals frm varius receptr sites such as atrial stretch receptrs and barreceptrs culd serve as anther efferent limb in the reflex cntrl f arterial bld pressure. The rapidly acting pressr effects f vaspressin seen in the present study alng with the pssible ptentiatin f the pressr activity f the catechlamines indicate that this system deserves further study and additinal quantificatin t determine the relative imprtance f these mechanisms in the verall Circulatin Research, VL XXXIV, April 1974 daily regulatin f arterial bld pressure. References 1. SZCZEPANSKA-SADOWSKA, E.: Activity f the hypthalamhypphyseal antidiuretic system in cnscius dgs. Pfluegers Arch 335: , SZCZEPANSKA-SADOWSKA, E.: Hemdynamic effects f a mderate increase f the plasma vaspressin level in cnscius dgs. Pfluegers Arch 338: , GAUEB, O.H., AND TATA, P.S.: Vaspressin studies in the rat: II. Amunt f water reabsrbed by the rat kidney after a single i.v. injectin f vaspressin: Vaspressin water equivalent. Pfluegers Arch 290: , RAISZ, L.G., MCNEELY, W.F., AND SAXON, L.: Studies n the renal cncentrating mechanism: I. Rle f vaspressin. J Lab Clin Med 52: , ROCHA E SILVA, M., JR., AND ROSENBERG, M.: Release f vaspressin in respnse t hemrrhage and its rle in the mechanism f bld pressure regulatin. J Physil (Lnd) 202: , MONOS, E., KOLTAY, E., AND KOVACH, A.G.B.: Adrenal bld flw and crticsterid secretin: III. Effect f vaspressin n bld circulatin and crticsterid secretin in the dg befre and after acute hypphysectmy. Acta Physil Acad Sci Hung 31: , BARTELSTONE, H.J., AND NASMYTH, P.A.: Vaspressin ptentiatin f catechlamine actins in dg, rat, cat, and rat artic strip. Am J Physil 208: , WAGNER, H.N., AND BRAUNWALD, E.: Pressr effect f the antidiuretic principle f the psterir pituitary n rthstatic hyptensin. J Clin Invest 35: , PRICE, H.L.: General anesthesiz and circulatry hmestasis. Physil Rev 40: , BONJOUR, J.P., AND MALVIN, R.L.: Plasma cncentratins f ADH in cnscius and anesthetized dgs. Am J Physil 218: , COWLEY, A.W., JR., LIARD, J.F., AND GUYTON, A.C.: Rle f the barreceptr reflex in daily cntrl f arterial bld pressure and ther variables in dgs. Circ Res 32: , COWLEY, A.W., JR., AND GUYTON, A.C.: Quantificatin f intermediate steps in the renin-angitensin-vascnstrictr feedback lp in the dg. Circ Res 30: , SAWYER, W.H.: Neurhypphyseal hrmnes. Pharmacl Rev 13: , EMERSON, T.E., JR.: Effects f angitensin, epinephrine, nrepinephrine, and vaspressin n venus return. Am J Physil 210: , FRIEDMAN, S.M., AND PAULS, H.: Effect f pitressin infusin n bld pressure f the rabbit, cat, and rat. Am Heart J 44: , TAGAWA,H., VANDER, A.J., BONJOUR, J.P., AND MALVIN, R.L.: Inhibitin f renin secretin by vaspressin in unanesthetized sdium-deprived dgs. Am J Physil 220: , LAUSON, H.D.: Metablism f antidiuretic hrmnes. Am J Med 42: , LAUSON. H.D.: Prblem f estimating the rate f secretin f antidiuretic hrmne in man. Am J Med 11: , JOHNSON, J.A., MOORE, W.W., AND SEGAR, W.E.: Small

10 514 COWLEY, MONOS, GUYTON changes in left atrial pressure and plasma antidiuretic 22. hrmne titers in dgs. Am J Physil 217: , SHARE, L., AND LEVY, M.N.: Cartid sinus pulse pressure: A determinant f plasma antidiuretic hrmne cncentra- 23. tin. Am J Physil 211: , SHARE, L., AND LEVY, M.N.: Effect f cartid chemreceptr 24. stimulatin n plasma antidiuretic hrmne titer. Am J Physil 210: ,1966. KEZDI, P., AND SPRICKLER, W.: Evidence fr resetting f the barreceptrs in hypertensin. In Barreceptrs and Hypertensin, edited by P. Kezdi. Lndn, Pergamn Press, KRIECER, E.M.: Time curse f barreceptr resetting in acute hypertensin. Am J Physil 218: , COWLEY, A.W., JR., AND GUYTON. A.C.: Sensitivity t angitensin II in sin-artic denervated dgs (abstr.). Fed Prc 31:367,1972. Circulatin Research, Vl. XXXJV, April 1974