The separate and combined influences of common carotid occlusion and nonhypotensive hemorrhage on kidney blood flow

Baroreceptor and chemoreceptor contributions to the hypertensive response to bilateral carotid occlusion in conscious mice

This study aimed to characterize the role played by baroreceptors and chemoreceptors in the hypertensive response to bilateral carotid occlusion (BCO) in conscious C57BL mice. On the day before the experiments the animals were implanted with pneumatic cuffs around their common carotid arteries and a femoral catheter for measurement of arterial pressure. Under the same surgical approach, groups of mice were submitted to aortic or carotid sinus denervation or sham surgery. BCO was performed for 30 or 60 s, promoting prompt and sustained increase in mean arterial pressure and fall in heart rate. Compared with intact mice, the hypertensive response to 30 s of BCO was enhanced in aortic-denervated mice (52 ± 4 vs. 41 ± 4 mmHg; P < 0.05) but attenuated in carotid sinus-denervated mice (15 ± 3 vs. 41 ± 4 mmHg; P < 0.05). Suppression of peripheral chemoreceptor activity by hyperoxia [arterial partial pressure of oxygen (Pa(O(2))) > 500 mmHg] attenuated the hypertensive response to BCO in intact mice (30 ± 6 vs. 51 ± 5 mmHg in normoxia; P < 0.05) and abolished the bradycardia. It did not affect the hypertensive response in carotid sinus-denervated mice (20 ± 4 vs. 18 ± 3 mmHg in normoxia; P < 0.05). The attenuation of the hypertensive response to BCO by carotid sinus denervation or hyperoxia indicates that the hypertensive response in conscious mice is mediated by both baro- and chemoreceptors. In addition, aortic denervation potentiates the hypertensive response elicited by BCO in conscious mice.

Otolithic activation on visceral circulation in humans: effect of aging

Engagement of the otolith organs elicits differential activation of sympathetic nerve activity and vascular responses to muscle and skin in humans. Additionally, aging attenuates the otolith organ-mediated increases in muscle sympathetic nerve activity in older adults. In this study, we hypothesized that 1) the vestibulosympathetic reflex (VSR) would elicit visceral vascular vasoconstriction and 2) visceral vascular response to the VSR would be attenuated in older subjects compared with young. To test these hypotheses, heart rate, mean arterial blood pressure, and renal, celiac trunk, and superior mesenteric arterial blood velocity (Doppler ultrasound) were measured in 22 young (25+/-1 yr) and 18 older (65+/-2 yr) healthy subjects during head-down rotation (HDR), which selectively activates the otolith organs. Mean arterial pressure and heart rate did not change from baseline during HDR in young or older subjects. Renal blood velocity (Delta -2+/-1 cm/s) and vascular conductance (Delta -0.03+/-0.01 cm.s(-1).mmHg(-1)) significantly decreased from baseline during HDR (P<0.05) in young subjects. In contrast, renal blood velocity and conductance did not change in older subjects (Delta -0.2+/-1 cm/s and Delta0.02+/-0.08 mmHg.cm(-1).s(-1), respectively) during HDR. Superior mesenteric and celiac blood velocity and vascular conductance did not change in response to HDR in either the young or older subjects. These data suggest that renal vasoconstriction occurs during otolith organ activation in young but not older humans. Together with our previous studies, we conclude that the VSR elicits a diverse patterning of sympathetic outflow that results in heterogeneous vascular responses in humans and that these responses are significantly attenuated in older humans.

Sympathetic stimulation of renin is independent of direct regulation by renal nitric oxide

Nitric oxide (NO) regulates renin secretion through various pathways. The possibility that renal neuronal nitric synthase (nNOS) may mediate beta-adrenergic control of renin was tested. In six Inactin-anesthetized rats, renin secretion rate (RSR) was measured in response to the beta-agonist isoproterenol with and without selective inhibition of nNOS using 7-nitroindazole (7-NI, 50 mg/kg body weight [BW]). 7-NI had no effect on blood pressure (BP) or renal hemodynamics, while isoproterenol increased RSR by 9 ng AngI/h/min (P<.05) similarly with or without 7-NI. Isoproterenol decreased BP by 20 mm Hg (P<.001), but this depressor response was completely blocked by 7-NI. When acute endogenous stimulation of renal sympathetic nerve activity (RSNA) was induced by bilateral carotid occlusion, BP in 12 rats (105+/-5 mm Hg) rose transiently to peak at 121+/-6 mm Hg (P<.005) within 5 min, returning to baseline within 10 min. RSR rose threefold (2.1+/-0.5 to 7.6+/-3.3 ng AngI/ml/min/g kidney weight [KW]; P<.05). Next, 7-NI had no effect on BP (108+/-5 mm Hg), but subsequent carotid occlusion increased and sustained BP by 27+/-5 mm Hg (P<.001), but RSR did not change (2.46+/-0.94 ng AngI/ml/min/g KW). However, if after 7-NI treatment followed by carotid occlusion, the renal perfusion pressure was not allowed to rise, but held constant at 111+/-3 mm Hg, RSR increased from 3.03+/-0.79 to 12.97+/-3.41 ng AngI/ml/min/g KW (P<.025). Thus, neither beta-adrenergic stimulation of RSR with isoproterenol nor direct stimulation of RSR by activation of RSNA with carotid occlusion was modified by selective nNOS inhibition. These data suggest an important nNOS component in the regulation of BP in response to carotid occlusion, but do not support a direct role of renal nNOS mediating sympathetic regulation of RSR.

Vestibular stimulation leads to distinct hemodynamic patterning

Previous studies demonstrated that responses of a particular sympathetic nerve to vestibular stimulation depend on the type of tissue the nerve innervates as well as its anatomic location. In the present study, we sought to determine whether such precise patterning of vestibulosympathetic reflexes could lead to specific hemodynamic alterations in response to vestibular afferent activation. We simultaneously measured changes in systemic blood pressure and blood flow (with the use of Doppler flowmetry) to the hindlimb (femoral artery), forelimb (brachial artery), and kidney (renal artery) in chloralose-urethane-anesthetized, baroreceptor-denervated cats. Electrical vestibular stimulation led to depressor responses, 8 +/- 2 mmHg (mean +/- SE) in magnitude, that were accompanied by decreases in femoral vasoconstriction (23 +/- 4% decrease in vascular resistance or 36 +/- 7% increase in vascular conductance) and increases in brachial vascular tone (resistance increase of 10 +/- 6% and conductance decrease of 11 +/- 4%). Relatively small changes (<5%) in renal vascular tone were observed. In contrast, electrical stimulation of muscle and cutaneous afferents produced pressor responses (20 +/- 6 mmHg) that were accompanied by vasoconstriction in all three beds. These data suggest that vestibular inputs lead to a complex pattern of cardiovascular changes that is distinct from that which occurs in response to activation of other types of somatic afferents.

Modulation of erythropoietin formation by changes in blood volume in conscious dogs

1. A possible influence of the filling of the circulatory system on the plasma concentration of erythropoietin, which is the major regulator of erythrocyte formation, was investigated in conscious dogs. 2. Over an experimental period of 5 h, the animals were subjected to either haemorrhage (hypovolaemia), blood volume expansion (hypervolaemia), or exchange transfusion of blood with dextran (isovolaemic anaemia). 3. A reduction of blood volume by 20% induced by haemorrhage increased plasma erythropoietin levels approximately 1.5-fold in the absence of significant changes in haematocrit. 4. An expansion of blood volume by 12% induced by an intravenous infusion of dextran did not change plasma erythropoietin levels, although the haematocrit decreased by 0.04. 5. A reduction of the haematocrit by 0.12 in the absence of changes in blood volume induced by an isovolaemic exchange transfusion (dextran vs. blood) increased plasma erythropoietin levels approximately 3-fold. 6. Total renal oxygen supply did not change in any of the three experimental protocols. 7. These data indicate that in dogs the erythropoietin production rate is modulated by changes in blood volume, and suggest a possible role of erythropoietin in the regulation of blood volume.

Sympathetic modulation of renal hemodynamics, renin release and sodium excretion

In anesthetized animals it has been shown previously, that the influence of electrical stimulation of efferent renal nerves on renal function with increasing stimulation frequencies can be graded; renin release is affected at low, sodium excretion at intermediate and vascular resistance at high stimulation frequencies. Experiments in conscious dogs are reviewed, which present evidence for a similar functional dissociation under physiological conditions. Moderate activations of the renal sympathetic nerves, which do not change renal blood flow 1) decrease sodium excretion independent of changes in angiotensin II, 2) interact with the pressure-dependent mechanism of renin release by resetting its threshold pressure and 3) modulate autoregulation by increasing the lower limits of glomerular filtration rate and renal blood flow-autoregulation. These findings may contribute to our understanding of the role of the renal nerves in the pathophysiology of congestive heart failure and hypertension.

Vagal afferents inhibit the antidiuresis and antinatriuresis secondary to bilateral carotid occlusion in the chloralose-anesthetized dog

It has been reported that bilateral carotid occlusion (BCO) does not alter renal excretory function in conscious dogs on a high-salt diet with intact vagi provided renal perfusion pressure (RPP) is held constant. In contrast, low carotid sinus pressures in chloralose-anesthetized dogs with severed vagi elicit significant reductions in renal excretory function which were mediated by renal sympathetic nerves. The purpose of the present study was to investigate the influence of BCO on renal function in chloralose-anesthetized, volume expanded dogs with and without intact cervical vagi and with RPP held constant. A total of 10 dogs, volume expanded with hypotonic saline, were prepared to measure systemic arterial pressure (SAP), carotid sinus pressure (CSP), RPP, and urine flow. With the cervical vagi intact, BCO elicited an increase in SAP from 132 +/- 4 mm Hg to 172 +/- 5 mm Hg (p less than 0.01). This was associated with significant and paradoxical increases in urine flow (+35%), sodium excretion (+63%), osmolar clearance (+32%), and free water clearance (+33%). These changes were also accompanied by small but significant increases in ERBF (+11%) and GFR (+12%). Bilateral cervical vagotomy alone (Vx) increased SAP (129 +/- 6 mm Hg to 146 +/- 5 mm Hg, p less than 0.05). Urine flow and free water clearance were decreased but sodium excretion and osmolar clearance were increased. No change in ERBF or GFR was measured. BCO after Vx elicited a greater increase in SAP as compared to BCO alone (194.5 +/- 6.2 mm Hg) which was accompanied by significant decreases in urine flow (-60%), sodium excretion (-55%), osmolar clearance (-43%), ERBF (-13%), and GFR (-19%). Therefore, the results of this study demonstrate that the contrasting influences of BCO on renal function reported previously in conscious and anesthetized animal models may be due to the presence or absence of the inhibitory influences of afferent fibers contained in the cervical vagi, probably cardiac in origin.

Reflex responses to baroreceptor, chemoreceptor and nociceptor inputs in single renal sympathetic neurones in the rabbit and the effects of anaesthesia on them

Reflex responses of renal postganglionic neurones to stimulation of arterial baroreceptors, arterial and central chemoreceptors and cutaneous nociceptors, and the rhythmicity of their resting activity were studied in paralyzed, artificially ventilated rabbits, anaesthetized with either alfathesin or chloralose-urethane. A 'vasoconstrictor' response pattern was seen in all units. Perivascular balloon-induced falls in blood pressure increased firing while pressure rises silenced 90% of units and reduced firing in the rest. Resting activity was linked to pressure changes within the cardiac cycle and to the artificial respiratory cycle. The largest excitation occurred during hypoxia and injections of CO2 saturated solutions into the carotid artery while hypercapnia and stimulation of cutaneous nociceptors only slightly increased firing. Parameters characterizing rhythmicities and reflex responses were unimodally distributed with no apparent subgrouping of units on quantitative grounds. Unit response patterns were similar to those recorded in the whole renal nerve. With one exception, no silent units were found which responded to the afferent inputs studied. Nor was there a small-spike fibre group which was excited by angiotensin. However, reflex responses were significantly influenced by the anaesthetic regime selected for use. Under alfathesin, baroreceptor and chemoreceptor reflexes were double those found with chloralose-urethane. Under chloralose-urethane, hypoxia increased both rhythmicities, while under alfathesin, cardiac rhythmicity was decreased and respiratory rhythmicity was variably affected. We concluded that renal sympathetic neurones are a functionally uniform population which behave like vasoconstrictors.

Effects of hemorrhage on renal nerve activity in conscious dogs

We studied the effects of slow continuous hemorrhage (0.5 ml/kg per min) on measurements of arterial and left atrial pressures, and renal nerve activity in conscious dogs with all reflexes intact, or after sinoaortic baroreceptor denervation, cardiac denervation, or sinoaortic baroreceptor denervation plus vagal denervation. In intact dogs, mean arterial pressure remained relatively constant at 101 +/- 4 mm Hg until 20 +/- 4 ml/kg of hemorrhage, when renal nerve activity increased by 211 +/- 53%. At 39 +/- 2 ml/kg hemorrhage, mean arterial pressure fell by 48 +/- 3 mm Hg, and renal nerve activity returned to the prehemorrhage control level. Cardiac denervation did not affect the response of mean arterial pressure to hemorrhage, whereas, after sinoaortic baroreceptor denervation and sinoaortic baroreceptor plus vagal denervation, mean arterial pressure remained at its control level only through 8 +/- 1 and 4 +/- 1 ml/kg hemorrhage, respectively. The increases in renal nerve activity during nonhypotensive hemorrhage were significantly attenuated by either sinoaortic baroreceptor or cardiac denervation, and were completely blocked by sinoaortic baroreceptor plus vagal denervation. However, the decline in renal nerve activity with hypotensive hemorrhage was not blocked by either cardiac or sinoaortic baroreceptor denervation, and was enhanced after sinoaortic baroreceptor plus vagal denervation. Our data indicate that nonhypotensive hemorrhage in the conscious dog elicits a striking increase in renal nerve activity, which then returns to control levels during hypotensive hemorrhage. Both sinoaortic and cardiopulmonary baroreceptors are involved in mediating the increase in renal nerve activity, whereas the decline in renal nerve activity is not due to either of these baroreflexes.

Baroreflex sympathetic activation increases threshold pressure for the pressure-dependent renin release in conscious dogs

Stimulus-response curves relating renal-venous-arterial plasma renin activity difference (P.R.A.-difference) to mean renal artery pressure (R.A.P.) were studied in seven chronically instrumented conscious foxhounds with a daily sodium intake of 6.1 mmol/kg. R.A.P. was reduced in steps and maintained constant for 5 min using an inflatable renal artery cuff and a pressure control system. The stimulus-response curve obtained during control conditions (C) or during common carotid artery occlusion (C.C.O.) could be approximated by two linear sections: a rather flat section or plateau-level of P.R.A.-difference at normal blood pressure or above, and a very steep section between a distinct threshold pressure and 65-70 mm Hg. While the parameters of the curves varied from dog to dog, the curves kept their unique shape in the individual dog for at least 1 week. C.C.O. had no effect on the plateau-level of the P.R.A.-difference (C:0.98 +/- 0.14, C.C.O:0.99 +/- 0.14 ng AI . ml-1 . h-1) and on the slope of the curve below threshold pressure (C:-0.379 +/- 0.041, C.C.O:-0.416 +/- 0.082 ng AI . ml-1 . h-1 . mm Hg-1) but shifted the stimulus-response curve to the right and increased threshold pressure (C:92.7 +/- 2.8, C.C.O:109.7 +/- 4.1 mm Hg; P less than 0.05). Renal blood flow, which was measured simultaneously in three of the dogs, showed good autoregulation down to 70 mm Hg under resting conditions and was not affected by C.C.O. except for a 30% reduction of renal blood flow at the lowest pressure step (70 mm Hg).(ABSTRACT TRUNCATED AT 250 WORDS)

The reflex effects of changes in carotid sinus pressure upon renal function in dogs

In chloralose-anaesthetized and artificially ventilated dogs, the carotid sinuses were vascularly isolated and perfused with arterial blood. Mean aortic pressure was held constant at 100 +/- 2 mmHg (mean +/- S.E. of mean, n = 19) by means of a pressure bottle connected to the aorta. Both vagus nerves were sectioned in the neck and propranolol hydrochloride (0.5 mg kg-1) was administered every 30 min. The left renal blood flow was measured by an electromagnetic flowmeter (wrap-round probe), glomerular filtration rate by creatinine clearance and urinary sodium by flame photometry. Decreasing pressure in the isolated carotid sinuses from 186 +/- 10 to 63 +/- 5 mmHg resulted in significant decreases in renal blood flow from 281 +/- 35 to 177 +/- 30 ml min-1 100 g-1 renal mass; glomerular filtration rate from 40.0 +/- 7.8 to 12.3 +/- 4.4 ml min-1 100 g-1; urine flow from 0.31 +/- 0.05 to 0.12 +/- 0.03 ml min-1 100 g-1 and sodium excretion from 21.7 +/- 7.2 to 8.2 +/- 3.0 mumol min-1 100 g-1. Increasing carotid sinus pressure back to 188 +/- 11 mmHg resulted in increases in all the variables to values not significantly different from their initial values. Tying renal sympathetic nerves at low carotid sinus pressure (73 +/- 11 mmHg) caused an increase in all of the variables. After denervation there was no response to changes in carotid sinus pressure. These results show that changes in carotid sinus pressure can result in significant reflex effects on renal function and that these effects are mediated by renal sympathetic nerves.

Threshold pressure for the pressure-dependent renin release in the autoregulating kidney of conscious dogs

The effect of varying renal artery pressure between 160 and 40 mm Hg on renal blood flow and renin release was studied in seven conscious foxhounds under beta-adrenergic blockade receiving a normal sodium diet (4.1 mmol/kg/day). Pressure was either increased by bilateral common carotid occlusion or reduced in steps and maintained constant by a control-system using an inflatable renal artery cuff. Carotid occlusion itself had no influence on renal blood flow and renin release when renal artery pressure was kept constant and the beta-receptors in the kidney were blocked. Between 160 mm Hg and resting pressure there was no change in renal blood flow; between resting blood pressure and the lower limit of autoregulation (average 63.9 mm Hg) renal blood flow increased slightly (average 7%) indicating a high efficiency of renal blood flow autoregulation. The relationship between renal artery pressure and renin release could be approximated by two linear sections: a low sensitivity to a pressure change (average slope: -0.69 +/- 0.26 ng AI/min/mm Hg) was found above a threshold pressure (average: 89.8 +/- 3.3 mm Hg) and a high sensitivity to a pressure change (average slope: -64.4 +/- 20.8 ng AI/min/mm Hg) was observed between threshold pressure and 60 mm Hg. There was no further increase of renin release between 60 and 40 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of carotid occlusion and of perfusion pressure on renal function in conscious dogs

We studied the effect of bilateral common carotid occlusion (implanted pneumatic cuffs) on renal blood flow (electromagnetic flowmeter) and renal function (implanted ureteral catheter) in nine chronically instrumented, conscious dogs on a high sodium diet (14 mmol/kg body weight per day). By means of suprarenal aortic constriction (pneumatic cuff) the influence of renal perfusion pressure was investigated. There was no change in renal blood flow or glomerular filtration rate (inulin clearance) with either reflexly increasing (+49.6%) or constant renal perfusion pressure. Carotid occlusion caused an increase of urine output by 80.5% and of sodium excretion by 85.3% due to a fall in fractional sodium reabsorption (-0.9%) when renal perfusion pressure was allowed to rise. Neither an increase of diuresis or sodium excretion nor an antinatriuresis was observed when renal perfusion pressure was kept constant during carotid occlusion. We conclude that, in conscious dogs at rest, the moderate sympathetic activation associated with carotid occlusion is too small to induce renal sympathetic vasoconstriction or antinatriuresis. The "carotid sinus polyuria" is a pressure-diuresis.

Renal chemoreceptors

A study of the renal receptors and types of stimuli which give origin to supraspinal and spinal-mediated autonomic reflexes is presented. Multiunit and single unit recordings from the afferent renal nerves of male Sprague-Dawley rats have revealed two groups of renal chemosensitive receptors (chemoreceptors). These we have called renal R1 and R2 "chemoceptive" receptors. R1 receptors do not have a resting discharge but are activated after 38.7 +/- 3.3 (S.E) sec (n = 40) of complete renal ischemia (occlusion of the renal artery). Other activating stimuli are associated with a marked impairment in renal blood flow (prolonged occlusion of the renal vein and the hypotension of systemic asphyxia or hemorrhage). Their discharge is characterized by trains of impulses which cease abruptly upon re-entry of blood into the kidney. They are not responsive to increases or decreases in renal perfusion pressure or to increases in renal venous or ureteral pressure. In contrast, R2 receptors have a resting discharge and respond vigorously to backflow of normal urine (nondiuretic) into the renal pelvis. The results of the backflow into the pelvis of different test solutions (diuretic and nondiuretic urine, 1 M urea, 1 M mannitol and solutions of NaCl and KCl) indicate that this response is dependent upon the composition of the fluid bathing the renal pelvis rather than the increase in pelvic pressure or pelvic distension. The resting discharge rate is highest in nondiuretic conditions and declines substantially after diuresis is induced by extracellular volume expansion. R2 receptors are also activated by renal ischemia produced by clamping the renal artery. It is concluded that these two groups of afferent sensory units are renal chemosensitive receptors, (chemoreceptors) which respond to the chemical environment of renal interstitium.

Interaction between perfusion pressure and sympathetic nerves in renin release by carotid baroreflex in conscious dogs

1. The effect of bilateral carotid occlusion on carotid sinus pressure, systemic blood pressure, heart rate, renal blood flow, renal-venous and arterial plasma renin activity was studied in twelve trained conscious foxhounds on a normal sodium diet (4.7 mmol/kg per day). 2. When renal perfusion pressure was allowed to rise with systemic pressure by 51.0 +/- 6.3 mmHg during carotid occlusion, renin release decreased by 72.3 +/- 22.2 ng/min (78% of control; P less than 0.05) while renal blood flow remained at its resting level of 232.7 +/- 20.1 ml/min (n = 8 dogs). 3. When renal perfusion pressure was maintained constant at 93.0 +/- 3.6 mmHg during carotid occlusion (suprarenal aortic cuff), renin release increased by 154.6 +/- 60.4 ng/min (73% of control; P less than 0.05), again there was no significant change of renal blood flow (n = 7 dogs). 4. After beta-adrenergic blockade carotid occlusion increased systemic blood pressure by 47.7 +/- 7.8 mmHg, decreased renin release by 34.6 +/- 9.9 ng/min (67% of control; P less than 0.05) and had no effect on renal blood flow (n = 4 dogs). 5. When renal perfusion pressure was controlled at its resting level, no significant change of renin release and renal blood flow was observed during carotid occlusion in the surgically denervated kidney (n = 3 dogs) or in the intact kidney after beta-adrenergic blockade (n = 4 dogs). 6. It is concluded that a reduction of carotid sinus pressure in the conscious dog increases renin release by a direct beta-adrenergic stimulation without exerting vasomotor effects provided renal perfusion pressure is maintained at control level. The vascular receptor mechanism can effectively counteract the stimulating influence of the renal sympathetic nerves when perfusion pressure is allowed to rise.