Renal nerves and renal responses to head-up tilt in dogs

Transcription

1 Renal nerves and renal responses to head-up tilt in dogs THOMAS V. PETERSON, NANCY L. HURST, AND JAME A. RCHARDSON Department of Medical Physiology, Texas A&M University College of Medicine, College Station, Texas PETERSON, THOMAS, V., NANCY L. HURST, AND JAME A. RCHARDSON. Renal nerves and renal responses to head-up tilt in dogs. Am. J. Physiol. 252 (Regulatory ntegrative Comp. Physiol. 21): R979-R986,1987.-Experiments were performed in anesthetized dogs to compare the effects of acute and chronic unilateral renal denervation on the renal responses to head-up tilt and to assess denervation hypersensitivity to infused norepinephrine (NE). Responses of the denervated kidney were compared with those of the contralateral innervated kidney in each animal. With acute denervation, 40 min of 45 head-up tilt decreased urine fiow (V) 37%, absolute sodium excretion (UN& 53%, and fractional sodium excretion (FEN,+) 44% in the innervated kidneys, but no decreases occurred in the denervated kidneys. NE infusion (125 ng= kg- l rni~ l) increased arterial pressure by 11 mmhg and increased V, U,,V, and FEN,+ in both kidneys. n the chronically denervated animals (2-4 wk prior to experiment) tilt decreased V by 32%, U,V by 44%, and FEN,+ by 21% in the innervated kidneys, but again no changes occurred in the denervated kidneys. NE infusion in this group also increased arterial pressure ~11 mmhg and caused V, U& and FEN*+ to increase in the innervated kidneys but decrease in the denervated kidneys. These results demonstrate that the renal responses to tilt are abolished by both acute and chronic renal denervation even though the chronically denervated kidney is hypersensitive to NE-stimulated fluid reabsorption. Therefore endogenous plasma NE levels must not increase enough during tilt such that this hypersensitivity phenomenon can compensate for chronic ablation of the renal nerves. orthostasis; renal denervation; antidiuresis; antinatriuresis EXPERMENTS designed to investigate the significance of the renal sympathetic nerves in controlling renal sodium and water excretion have, for the most part, concluded that these neural elements may play an important role in maintaining body fluid homeostasis, pthculdy in species such as the dog and rat (reviewed in Refs. 8 and 14). Results from these studies have shown that stimulation of the renal nerves either electrically or reflexly via baroreceptor mechanisms increases renal sodium reabsorption, oftentimes without concurrent changes in renal blood flow or glomerular filtration rate. Conversely reflex withdrawal of renal nerve activity or acute denervation of the kidney decreases renal sodium reabsorption, this effect again not being dependent on a change in renal hemodynaics. This functional importance of the renal nerves in mediating renal excretion is consistent with the anatomical evidence for a direct neural innervation of the renal tubules in many species (1,21). Studies that have addressed the issue of the necessity of the renal nerves for eliciting appropriate renal excretory responses to blood volume changes have traditionally employed a renal denervation approach (8,14), That is, during the volume alteration, the response of the denervated kidney is compared with that of the innervated kidney in the same or a different animal. However, the results obtained in many renal denervation studies may be influenced by a number of factors. First, since excessive handling and manipulation of the kidney may depress renal hemodynamic function as much as 50% (15, 20), studies employing acute renal denervation, as the experimental model, may be difficult to interpret. That is, the failure of the denervated kidney to respond to the experimental intervention may be due to trauma of the organ rather than renal denervation per se. A nondepressed control function of the denervated kidney compared with that of the contralateral innervated kidney or the former kidney before denervation should be demonstrated. Second, although the technique of chronic as opposed to acute renal denervation allows~experiments to be performed in conscious animals and ordinarily circumvents the potential problem of acute postdenervation depressed renal function, the potential development of denervation hypersensitivity to circulating norepinephrine may pose another problem. That is, since it has been demonstrated that the chronically denervated kidney is supersensitive to vascular (3, 18) and tubular (24, 26) effects of this catecholamine, this phenomenon may partially or completely compensate for the absence of the renal nerves. The present experiments were designed to take some of these issues into account. Our objective was to determine whether acute and chronic renal denervation have similar effects on the renal responses of the dog to headup tilt, a maneuver that decreases central blood volume and reflexly elicits a decrease in renal excretion. Although renal perfusion pressure was held constant, we aimed to evaluate the relative role of the renal nerves within the framework of some of the other factors that could be involved in this response. Thus the tilt was prolonged enough (40 min) to potentially allow humoral factors to exert their effects. The design of the experiments utilized unilateral renal denervation so that the responses of the denervated kidney could be compared with that of the contralateral innervated kidney in each animal. n addition we also assessed the effect of an infusion of norepinephrine on the function of the denervated and innervated kidneys to determine whether the /87 $1.50 Copyright the American Physiological Society 3979

2 kg- min- R980 RENAL NERVES AND HEAD-UP TLT N DOGS results obtained during tilt may have been influenced a norepinephrine hypersensitivity effect. METHODS The experiments were performed on mongrel dogs (n = 17) of both sexes with an average weight of 13.0 t 0.6 kg. All animals were maintained on a normal diet. Prior to each experiment or chronic surgery, each animal was fasted overnight but allowed water ad libitum. Surgical preparation. Chronic unilateral renal denervation was performed in eight of the dogs 2-4 wk before the experiment. For this surgical procedure each animal was anesthetized with pentobarbital sodium (30 mg/kg) adininistered through an intracath inserted into a cephalic vein. A cuffed endotracheal tube was then inserted. Under aseptic conditions the left kidney was exposed through a flank incision. The renal artery and vein were carefully isolated, stripped of all visible nerves, and painted with 10% phenol in 95% ethanol. The ureter was stripped of its fibrous coat near the renal pelvis and also painted with phenol. Any other nervous or connective tissue near the renal hilus was also transected. The incision was then sutured closed and each animal given an intramuscular injection of penicillin (Longicil, 300,000 U). Another penicillin injection was given on the second postoperative day. Exp;imen~al procedure. For the tilt and norepinephrine infusion experiment each animal was anesthetized with pentobarbital sodium as described above and intubated with a cuffed endotracheal tube. A brachial artery was cannulated with a polyethylene catheter for withdrawing arterial blood samples. A femoral cutdown was performed and another polyethylene catheter inserted into a femoral vein for infusing solutions and supplemental anesthetic. A Millar transducer-tipped catheter was inserted into the femoral artery and advanced into the abdominal aorta such that its tip was below the origin of the renal arteries. This renal perfusion pressure was monitored continuously throughout the experiment with a Grass 7D polygraph and heart rate calculated from a fast trace of the pressure recording. Acute left renal denervation was then performed in the dogs (n = 9), which had not been subjected to the previous chronic denervation. Through the flank incision used for exposing the left kidney, a snare of umbilical tape was also placed around the suprarenal aorta to control renal perfusion pressure. The chronically denervated dogs also had a left-flank incision made at this time and aortic snare placed. n both groups of dogs the right kidney was then exposed through a right-flank incision. n most of the animals the ureters were cannulated through the flank incisions, but in some dogs they were cannulated close to the bladder via a separate suprapubic incision. All incisions made during this preparative procedure were closed with surgical clips. After this preparation was complete each animal was secured to a wood platform in the recumbent position. A test tilt (45, head-up) of -5min duration was performed to determine the effect of tilt on renal perfusion pressure. The animal was then returned to the recumbent position and the aortic snare tightened slightly such that renal by perfusion pressure was less than or equal to that during tilt. A priming dose of creatinine (33 mg/kg) and p- aminohippurate (PAH) (8 mg/kg) was then given, followed by a sustaining infusion (1.5 ml/min) containing 1.6 g/l each of creatinine and PAH in phosphate-buffered 0.6% NaCl. An hour later timed urine collections consisting of consecutive 5-min samples were started, the urine being collected separately from each kidney in graduated centrifuge tubes. Arterial blood samples were taken every 20 min, and after centrifugation and decanting of the plasma the blood cells were resuspended in 6% dextran in isotonic saline (6% Gentran 75, Travenol) and returned to the animal at the time of the next blood sample. After two to three stable urine flow values were obtained, the head end of the platform and attached animal were elevated to an angle of 45. Five minutes were allowed for the animal to stabilize <and for adjustment of the aortic snare. Samples were then collected for 40 min of tilt. The animal was then returned to the recumbent position, and after another 5-min stabilization period, two 5-min posttilt collections obtained. The norepinephrine infusion protocol was started 30 min after the tilt experiment was over. For this protocol, urine and blood samples were taken the same as described earlier, but renal perfusion pressure was not controlled. Two 5-min control collections were obtained, and then each animal received an intravenous infusion of norepinephrine bitartrate (Levophed, Breon Laboratories). The norepinephrine was diluted in normal saline and infused at 125 ng l l at a pump rate of 0.38 ml/min. This infusion was continued for 30 min after which two additional postinfusion samples were taken. This norepinephrine infusion procedure was performed in all of the chronic denervation dogs (n = 8) and seven of the nine acute denervation dogs. At the end of this entire procedure, renal denervation was functionally verified in most of the animals using the criteria suggested by DiBona (8). This was done by first exposing the innervated right kidney, placing an electromagnetic flow probe (Carolina) on the renal ar- tery, and assessing the renal vasoconstrictor response to electrical stimulation of the suprarenal lumbar sympathetic chain or to bilateral carotid arterial occlusion. The same procedure was then repeated on the left side, and the absence of vasoconstriction by the left kidney was considered evidence of the effectiveness of the denervation procedure. The animal was then euthanized with an anesthetic overdose and the kidneys were removed and weighed. Analysis. Plasma and urine concentrations of creatinine and PAH were determined using the methods described by Smith (25) and creatinine and PAW clearances calculated to estimate glomerular filtration rate and effective renal plasma flow, respectively. These were converted to clearances per gram kidney weight. Sodium. concentrations were determined by flame photometry (nstrumentation Laboratories 643) and both absolute and fractional sodium excretion were calculated. Since these experiments consisted of repeated measurements of each variable, changes within each group of kidneys (innervated or denervated) were analyzed

3 RENAL NERVES AND HEAD-UP TLT N DOGS R981 statistically with an analysis of variance with repeated measures design and the Newman-Keuls multiple range test. Differences between the innervated and denervated kidneys in each animal group (acute or chronic denervation) were assessed with the paired t test. P < 0.05 was considered statistically significant. RESULTS Acute renal deneruation: tilt. The urine flow and sodium excretion responses of the innervated and acutely denervated kidneys to 40 min of head-up tilt are depicted in Fig. 1. A denervation diuresis and natriuresis were present in the control condition in that urine flow and fractional sodium excretion were significantly higher in the denervated compared with innervated kidneys, although differences in absolute sodium excretion between the two kidneys did not reach statistical significance. None of these parameters decreased in the denervated kidneys during tilt whereas, in the innervated kidneys, urine flow decreased by 37%, urinary sodium excretion by 53%, and fractional sodium excretion by 44%. With return to recumbency, these values increased back to control levels. Although not shown in Fig. 1, tilt caused creatinine clearance to decrease from 0.75 * 0.06 to a nadir of (mumin) UNav (peq/min) FENa (96) t 0.06 ml. min- l g-l and PAH clearance from 1.94 t 0.21 to 1.40 t 0.22 ml. min- l g-l in the innervated kid- neys. Neither of these parameters changed in the denervated kidneys during tilt. Mean renal perfusion pressure was adequately controlled in that it was 122 t 7 mmhg during the last control period (C2) and did not change significantly during tilt. Acute renal denervation: norepinephrine infusion. The effects of the norepinephrine infusion in the acute renal denervation dogs are shown in Fig. 2. This infusion significantly increased renal perfusion pressure from mmhg to a peak value of 127 t 10 mmhg. The renal effects were similar in both the innervated and denervated kidneys in that urine flow and absolute and fractional sodium excretion increased slightly but signifi- cantly, whereas PAH clearance did not change. Creatinine clearance (not shown) also was unchanged in both kidneys. Chronic renal deneruation: tilt. The effects of tilt in the innervated and denervated kidneys of the dogs subjected to chronic unilateral renal denervation are shown in Fig. 3. Similar to the effects observed with acute denervation, a denervation diuresis and natriuresis were again present and, in fact, the differences in excretion between the two kidneys were greater than in the acute TLT ru1 ru1 r-*1 ru1 r*1 r*1 f-u1 ru1 FUl ru1 r -U1 T 0.0 Cl c2 11 T2 T3 T4 T5 T6 T7 T8 Rl R2 0 NNERVATED KDNEY 1 m DENERVATEO KDNEY FG. 1. Renal excretory responses to head-up tilt after acute unilateral renal denervation. V, urine flow; UN& absolute sodium excretion; and FENa+, fractional sodium excretion. Values are means t SE for successive 5-min periods during control (C), tilt (T), and recovery (R). Five-minute stabilization periods with no measurements occurred before Tl and after T8. * Significantly different from average control value or differences between kidneys as indicated at P < PER100

4 g RENAL NERVES AND HEAD-UP TLT N DOGS 9 bntlmin) - NNERWiTED KDNEY - - DENERWiTED KDNEY NOREPNEPHRNE chronic denervation dogs (Fig. 4) were similar to those in the acute denervation group in that renal perfusion pressure increased by 11 mmhg (138 t 3 mmhg control to 149 $- 4 mmhg peak effect during infusion). However, the renal excretory effects of this infusion differed markedly in the two kidneys. The innervated kidneys responded similarly to both the innervated and denervated kidneys in the acute denervation group in that urine flow and absolute and fractional sodium excretion increased significantly. n contrast, in the denervated kidneys, these excretory parameters not only failed to increase but significantly decreased. Both PAH and creatinine clearances were unaffected in both kidneys. hmhq) -r *21 T i T T T -r * * * * * * / T T Od, i MNUTES FG. 2. Renal effects of norepinephrine infusion after acute unilateral renal denervation. 9, urine flow; UN& absolute sodium excretion; FEN.+, fractional sodium excretion; CPU, p-aminohippurate clearance; and RPP, renal perfusion pressure. Values are means t SE for successive fi-min periods. * Signifkantly different from average preinfusion control value or differences between kidneys as indicated at P < 0;05. denervation group. The effects of chronic denervation on the excretory responses to tilt were the same as those obtained with acute denervation. That is, antidiuretic or antinatriuretic responses did not occur in the denervated kidneys. n contrast, in the innervated kidneys, urine flow decreased by 32%, absolute sodium excretion by 44%, and fractional sodium excretion by 21%. The hemodynamic effects of tilt were also similar to those in the acute denervation study in that creatinine and PAH clearances decreased only in the innervated kidneys, these changes being from 0.81 t 0.02 to 0.59 t 0.08 ml. min- l and 2.36 t 0.20 to 1.79 t 0.24 ml. min-. g-l, respectively. Mean renal perfusion pressure was 137 & 3 mmhg during the last control period and again was adequately controlled such that it did not decrease during tilt. This pressure was slightly but significantly higher in this group compared with the acute denervation group. Chronic renal denervation: norepinephrine infudon. The pressor effects of the norepinephrine infusion in the DSCUSSON The results from the present experiments have demonstrated that renal denervation abolishes the antidiuretic and antinatriuretic responses to 40 min of headup tilt in the dog. Furthermore, this effect of renal denervation was present whether the kidneys were acutely denervated the same day as the experiment or chronically denervated 2-4 wk earlier. Our results with acute denervation confirm and extend those of DiBona and Johns (9), who also reported that this denervation abolishes the antinatriuresis of tilt. However, there are some differences between their experiment and ours. DiBona and Johns (9) collected urine unilaterally and thus used two groups of dogs, one for innervated kidney data and the other for denervated kidney data, whereas we used a paired design to assess the responses of the innervated and denervated kidneys in the same animal. Therefore, in our dogs, both the innervated and denervated kidneys in each animal would be exposed to the same arterial pressure and the same humoral environment, and this approach also allowed us to ensure that our acute denervation, in particular, did not depress renal function. ndeed, not only were excretory and hemodynamic functions of the denervated kid-- ney not depressed compared with the contralateral innervated kidney, but a denervation diuresis and natriuresis were apparent. The disadvantage of this paired-. kidney design is that, because of the existence of renorenal reflexes, unilateral renal denervation has been shown to cause an increase in efferent renal nerve activity to the contralateral kidney and subsequent antidiuresis and antinatriuresis (6,11,28). Therefore, its function as a reference, innervated control, may be altered. With regard to our experiments, this could mean that the magnitude of the decrease in renal excretion occurring in this kidney during tilt was possibly different from what would have occurred had the contralateral kidney also been innervated. Another difference between our study and that of DiBona and Johns (9) was that the latter authors were interested primarily in the initial or early responses to tilt such that the animals in their study were only tilted for 15 min, with urine being collected between 5 and 15 min of tilt. Also their animals received a constant infusion of vasopressin at maximal antidiuretic concentrations so that this variable was clamped and thus they did not report their urine flow data. n contrast, in our experiments, we did not clamp

5 l RENAL NERVES AND HEAD-UP TLT N DOGS R v (ml/min) UNaV (peq/min) FENa (96) Cl c2 Tl T2 T3 T4 T5 T6 Tf T8 Rl R2 PERloo q NNERVATED KDNEY if8 DENERVATED KONEY FG. 3. Renal excretory responses to head-up tilt after chronic unilateral renal denervation. Same definitions and format as in Fig. 1. or block the actions of any hormones and subjected our animals to a more prolonged (40 min) tilt. Since increases in plasma renin activity and plasma levels of circulating catecholamines, vasopressin and aldosterone have been reported to occur within 5-30 min of tilt (5, 7, 10, 27), we felt that our longer-tilt protocol would be more appropriate to evaluate the importance or necessity of the renal nerves in this response. The only variable that was controlled was renal perfusion pressure. We obtained the same abolition of the excretory responses when the kidneys were denervated 2-4 wk before the experiment. Although the effect of chronic renal denervation on the responses to tilt has not been reported previously, our results with this denervation model are somewhat similar to those of Fewell and Bond s study (12) on continuous positive-pressure ventilation (CPPV), an intervention that also decreases central blood volume and leads to a decrease in renal excretion. These investigators found that the antidiuretic and antinatriuretic responses to 60 min of CPPV were abolished by chronic renal denervation performed 4-5 days earlier. They did not report values for fractional sodium excretion and concluded that the normal CPPV decrease in renal excretion that they obtained in their dogs innervated kidneys was due to a decrease in renal hemodynamics secondary to an increase in renal nerve activity. n our experiments the decreases in renal plasma flow and glomerular filtration rate with tilt were quite small such that fractional sodium excretion also decreased. This is consistent with the numerous studies that have provided evidence supporting a direct effect of the renal nerves on renal tubular sodium reabsorption (8,14). Another aspect of our study, to compare the effects of chronic and acute renal denervation on the responses to tilt, was based on the phenomenon of denervation hypersensitivity and its potential as a complicating factm in chronic rend denervation experiments. With, regard to this issue, it has been demonstrated that the chronically denervated kidney is supersensitive to the vasoconstrictor (3, 18) and tubular sodium reabsorptive actions (24, 26) of infused norepinephrine. Thus it has been suggested (24, 26) that the chronic renal denervation model may not be appropriate for studying the role of the renal nerves in conditions where sympathetic tone and thus plasma norepinephrine levels are elevated. That is, the supersensitivity of the denervated kidney to norepinephrine-stimulated sodium reabsorption and/or vasoconstriction may mask any differences that the experiment is designed to determine between the responses of the innervated and denervated kidneys. n the present experiments we did indeed provide evidence for norepinephrine denervation hypersensitivity. n our chronic denervation animals, norepinephrine infusion increased arterial pressure and increased renal excretion by the innervated kidney. Although other studies have reported that norepinephrine increases renal

6 kg-. R984 RENAL NERVES AND HEAD-UP TLT N DOGS v (ml/mid UNaV (peq/min) fena+ (% - NNERVATED KDNEY - - DENERVATED KDNEY *2 GMH 1.7 (mllminlg) pf++ +- g&+&+ 1.2 o RPP 130 (mmhg) 110 1, 0 h1...l-;, = L -J,,,,, 1,, MNUTES FG. 4. Renal effects of norepinephrine infusion after chronic unilateral renal denervation. Same definitions and format as in Fig. 2. sodium reabsorption (2-4, 13, 16, 22), we undoubtedly used a high enough infusion rate such that the pressure increase and subsequent diuresis and natriuresis effects offset any stimulatory action of norepinephrine on renal tubular fluid reabsorption. However, the effects of norepinephrine on the chronically denervated kidneys were opposite to this in that renal excretion decreased. Thus, since creatinine and PAH clearances did not change, these kidneys were hypersensitive to norepinephrine s effect on the renal tubule, and this effect predominated over any tendency for renal excretion to increase secondary to the rise in arterial pressure. This hypersensitivity phenomenon does not appear to be a rapidly developing effect (hours) since, in our acute denervation dogs, norepinephrine infusion increased renal excretion similar amounts in both the innervated and denervated kidneys. However, despite this demonstrated denervation hypersensitivity, no antidiuretic or antinatriuretic responses occurred in the chronically denervated kidneys during head-up tilt, the same as the results obtained with acute renal denervation. Although we did not measure plasma norepinephrine levels, it seems a safe assumption that they did not increase enough with tilt to exert any effects on the norepinephrine-hypersensitive denervated kidneys. There are three possible reasons for this. One is that this hypersensitivity may be a pharmacological rather than physiological phenomenon in that endogenous norepinephrine levels may never attain high enough concentrations for these effects to be detectable. t is obvious that the norepinephrine infusion rates used in our experiments and those of the chronic denervation study of Sadowski and Portalska (24) were quite high, since they both increased arterial pressure and did not decrease renal excretion in the innervated kidneys in contrast to the decreased renal excretion obtained with lower infusion rates or when arterial pressure is not allowed to increase (3, 4, 13). Our infusion rate of 125 ng l min- has been shown by Johnson et al. (16) to increase plasma norepinephrine to over 1,600 pg/ml. Since hypotensive hemorrhage in conscious dogs only raises plasma norepinephrine up to approximately half this value (17, 29), our infusion rate was very likely elevating plasma norepinephrine above a physiological level. A second possible reason that chronic denervation abolished the responses to tilt despite denervation hypersensitivity could be that the latter phenomenon is present with physiological levels of norepinephrine but these must be above a certain threshold level to be effective. This threshold may not be attained during head-up tilt but may be with other maneuvers such as hemorrhage. This could explain the results of Lifschitz (19) who reported similar hemorrhage-induced decreases in sodium excretion in innervated and chronically denervated kidneys in conscious dogs, a finding that they partially ascribed to denervation hypersensitivity. A third reason for our results could be that anesthesia may have been a factor, since Zimpfer et al. (29) have demonstrated blunted increases in circulating catecholamine levels during hemorrhage in pentobarbital-anesthetized dogs compared with conscious dogs. f there is a threshold plasma level of norepinephrine needed to elicit this hypersensitivity response, this level may not have been reached in our animals because the anesthetic attenuated the increases in catecholamines occurring during tilt. A recent paper by Szenasi et al. (26) has also focused on this renal denervation hypersensitivity issue. Similar to our experiments these authors found that the chronically denervated kidney shows tubular supersensitivity to infused norepinephrine, these studies also being performed in pentobarbital-anesthetized dogs. More importantly, they also concluded that this effect may be important at physiological levels of norepinephrine. This was based on their findings that intrarenal infusion of the a-adrenergic blocker phenoxybenzamine increased urine flow and sodium excretion in chronically denervated kidneys (l-3 wk) but had no effect in acutely denervated kidneys. n fact the increased renal excretion obtained with cu-blockade in the former kidneys was of similar magnitude to the increased excretion occurring when they infused the blocker into innervated kidneys, thus suggesting that norepinephrine hypersensitivity essentially completely compensated for the absence of the renal nerves. This could explain why their chronically denervated dogs did not show a denervation diuresis or

7 RENAL NERVES AND HEAD-UP TLT N DOGS R985 natriuresis compared with their intact animals, although, curiously, this was also not apparent in their acutely denervated dogs. n contrast we detected denervation diuresis and natriuresis in both our acute and chronic denervation groups, although we used unilateral renal denervation as opposed to the bilateral denervation approach used by Szenasi et al. (26). As mentioned earlier, unilateral denervation may have led to an increased sympathetic tone to the contralateral innervated kidney and exaggerated the differences in excretion between the two kidneys. However, these differences did not become less with chronic denervation and, in fact, appeared to be greater than in our acute denervation group, suggesting that there was no developing norepinephrine hypersensitivity effect to decrease excretion of the chronically denervated kidney and attenuate the differences between it and the contralateral innervated kidney. Furthermore, the study by Szenasi et al. (26), although indicating that norepinephrine hypersensitivity compensates for the absence of the renal nerves under basal conditions, did not address the question whether this effect can compensate enough to elicit decreases in renal excretion under conditions of increased sympathetic outflow such as headup tilt. Our data would suggest that it cannot. n any event the issue of denervation hypersensitivity after chronic renal denervation needs to be further investigated. t should also be noted that there may be species differences with regard to many of the above issues, since we previously reported that chronic renal denervation in the monkey does not attenuate the magnitude of the antidiuretic and antinatriuretic responses to head-up tilt (23). This result could have been due to complete compensation by norepinephrine denervation hypersensitivity, compensation by other factors or simply no importance of the renal nerves in mediating these responses in this species. n conclusion our results have demonstrated that both acute and chronic renal denervation abolish the renal responses of the dog to 40 min of head-up tilt even though the chronically denervated kidney is hypersensitive to norepinephrine-stimulated fluid reabsorption. Therefore, in this anesthetized model, endogenous plasma norepinephrine levels must not increase enough during tilt to enable this hypersensitivity effect to compensate for the absence of the renal nerves. Whether this phenomenon is of physiological importance under other conditions or in other species remains to be determined. The authors gratefully acknowledge the technical assistance of Andrew Richardson and Gregory Savage and secretarial assistance of Helen Higginbotham and Martha Larson. This research was supported by National Heart, Lung and Blood nstitute Grant HL T. V. Peterson is a recipient of National Heart, Lung and Blood nstitute Research Career Development Award HL Received 24 March 1986; accepted in final form 8 January REFERENCES 1. BARAJAS, L., K. POWERS, AND P. WANG. nnervation of the renal cortical tubules: a quantitative study. Am. J. Physiol. 247 (Renal Fluid Electrolyte Physiol. 16): F50-F60, BELLO-REUSS, E. Effect of catecholamines on fluid reabsorption by the isolated proximal convoluted tubule. Am. J. Physiol. 238 (Renal Fluid Electrolyte Physiol. 7): F347-F352, BERNE, R. M., W. K. HOFFMAN, JR., A. KAGAN, AND M. N. LEVY. Response of the normal and denervated kidney to 1-epinephrine and 1-norepinephrine.,Am. J. Physiol. 171: 564~571, BESARAB, A., P. SLVA, L. LANDSBERG, AND F. H. EPSTEN. Effect of catecholamines on tubular function in the isolated perfused rat kidney. Am. J. Physiol. 233 (Renal Fluid Electrolyte Physiol. 2): F39-F45, CAMPESE, V. M., M. ROMOFF, V. DEQUATTRO, AND S. G. MASSRY. Relationship between plasma catecholamines, plasma renin activity, aldosterone, and arterial pressure during postural stress in normal subjects. J. Lab. Clin. Med. 95: , COLNDRES, R. E., W. S. SPELMAN, N. G. Moss, W. W. HAR- RNGTON, AND C. W. GOTTSCHALK. Functional evidence for renorenal reflexes in the rat. Am. J. Physiol. 239 (Renal Fluid Electrolyte Physiol. 8): F265-F270, DAVES, R., J. D. H. SLATER, M. L. FORSLNG, AND N. PAYNE. The response of arginine vasopressin and plasma renin to postural change in normal man, with observations on syncope. Clin. Sci. Mol. Med. 51: , DBONA, G. F. The functions of the renal nerves. Reo. PhysioZ. Biochem. Pharmacol. 94: , DBONA, G. F., AND E. J. JOHNS. A study of the role of renal nerves in the renal responses to 60 head-up tilt in the anesthetized dog. J. Physiol. LAM&. 299: ,198O. 10. DBONA, G. F., E. J. JOHNS, AND J. L. OSBORN. The effect of vagotomy on sodium reabsorption and renin release in anaesthetized dogs subjected to 60 head-up tilt. J. Physiol. Land. 320: ,198l. 11. DBONA, G. F., AND L. L. ROS. Renal nerves in compensatory renal response to contralateral renal denervation. Am. J. Physiol. 238 (Renal Fluid Electrolyte Physiol. 7): F26-F30, FEWELL, J. E., AND G. C. BOND. Renal denervation eliminates the renal response to continuous positive-pressure ventilation. Proc. Sot. Exp. Biol. Med. 161: , GLL, J. R., JR., AND A. G. T. CASPER. Effect of alpha-adrenergic stimulation on proximal tubular sodium reabsorption. Am. J. Physiol. 223: 1201-liO5,1972. GOTTSCHALK, C. W., N. G. Moss, AND R. E. COLNDRES. Neural controlof renal function in health and disease. n: The Kidney: Physiology and Pathophysiology, edited by D. W. Seldin and G. Giebisch. New York: Raven, 1985, p HANDLEY, C. A., AND J. H. MOYER. Unilateral renal adrenergic blockade and the renal response to vasopressor agents and to hemorrhage: J. Pharmacol. Exp. Ther. 112: l-7,1954. JOHNSON, a. D., AND A. C. BARGER. Circulating catecholamines in control of renal electrolyte and water excretion. Am. J. Physiol. 240 (Renal Fluid Electrolyte Physiol. 9): Fl92-Fl99,1981. JOHNSON, M. D., D. N. SHER, AND A. C. BARGER. Circulating catecholamines and control of plasma renin activity in conscious dogs. Am. J. Physiol. 236 (Heart Circ. Physiol. 5): H463-H470, KLNE, R. L., AND P. F. MERCER. Functional reinnervation and development of supersensitivity to NE after renal denervation in rats. Am. J. Physiol. 238 (Regulatory ntegrative Comp. Physiol. 7): R353-R358,1980. LFSCHTZ, M. D. Lack of a role for the renal nerves in renal sodium reabsorption in conscious dogs. Clin. Sci. Mol. Med. 54: ,1978. MERCER, P. F. Effects of bilateral vagotomy on renal function in the rat. Can. J. Physiol. Pharmacol. 49: ,197l. MULLER, J., AND L. BARAJAS. Electron microscopic and histochemical evidence for a tubular innervation in the renal cortex of the monkey. J. Ultra&u&. Res. 41: , PEARSON, J. E., AND R. L. WLLAMS. Analysis of direct renal actions of alpha and beta adrenergic stimulation upon sodium excretion compared to acetylcholine. Br. J. Pharmacol. Chemother. 33: ,1968. PETERSON, T. V., N. L. CHASE, AND D. K. GRAY. Head-up tilt in the nonhuman primate. Effects of renal denervation. Renal Physiol. 7: ,1984. SADOWSK, J., AND E. PORTALSKA. Denervated and intact kidney responses to norepinephrine infusion in conscious dogs. J. Auton. Nerv. Syst. 6: , SMTH, H. W. Principles of Renal Physiology. New York: Oxford

8 R986 RENAL NERVES AND HEAD-UP TLT N DOGS Univ. Press, 1960, p SZENAS, G., P. BENCSATH, AND L. TAKACS. Supersensitivity of the renal tubule to catecholamines in the chronically denervated canine kidney. pfluegers Arch. 406: 57-59, VANDONGEN, R., L. DAVDSON, L. 3. BEXLN, AND A. E. BARDEN. Effect of /J-adrenergic receptor blockade with propranolol on the response of plasma catecholamines and renin activity to upright tilting in normal subjects. Br. J. Clin. Pharmacol. 12: , ZANCH~, A., A. STELLA, R. GOLN, AND S. GENOVES. Neural control of the kidney-are there reno-renal reflexes? Clin. Exp. Hypertens. Part A Theory Pmt. 6: ,1984. ZMPPER, M., W. T. MANDERS, A. C. BARGER, AND S. F. VATNER. Pentobarbital alters compensatory neural and humoral mechanisms in response to hemorrhage. Am. J. Physid Physiol. 12): H713-H721, (Heart Circ.