Role of vasopressin in salt-induced hypertension in baroreceptor-denervated uninephrectomized rabbits

Animal models for the study of arterial hypertension

Hypertension is one of the leading causes of disability or death due to stroke, heart attack and kidney failure. Because the etiology of essential hypertension is not known and may be multifactorial, the use of experimental animal models has provided valuable information regarding many aspects of the disease, which include etiology, pathophysiology, complications and treatment. The models of hypertension are various, and in this review, we provide a brief overview of the most widely used animal models, their features and their importance.

Interaction between AVP and sympathetic system in subtotal nephrectomy-saline hypertension: role of alpha and V1 receptors

The development process of subtotal nephrectomy-salt hypertension is still unclear. The aim of the present study was to determine the role of the interaction between vasopressin and sympathetic system in the development of this hypertension by using AVP V1 antagonist and alpha blocker phentolamine under anesthesia condition. For this purpose, we carried out about 73% subtotal nephrectomy on male Wistar rats. One group of these rats (normotensive group) was given a low-salt diet and the other group (hypertensive group) was given a high-salt diet for 4 weeks. Finally, eight groups of rats were formed according to the kind(s) of the injected drug(s): (1) normotensive and hypertensive groups injected only V1 antagonist, (2) normotensive and hypertensive groups injected only phentolamine, (3) normotensive and hypertensive groups injected first V1 antagonist and then phentolamine, (4) normotensive and hypertensive groups injected first phentolamine and then V1 antagonist. Either V1 or alpha blockage separately led to a higher reduction in the mean blood pressure (MAP) of the hypertensives than, of the normotensives (p < 0.05). The combined blockage of V1 and alpha receptors, also caused a higher decrease in the MAP of hypertensive group than, of normotensive group, not depending on the order of the injections (p < 0.01). The heart rate increase recorded as a response to the phentolamine injection in normotensive group, did not develop in hypertensive group (p < 0.05). There was no significant difference between the two groups with regard to plasma electrolytes and osmolality. A positive correlation was found between systolic blood pressure and plasma osmolality in hypertensive group (r = 0.40, p < 0.05), but not in normotensive group. We conclude that the increase in V1 and alpha pressor activities contributes to the subtotal nephrectomy-saline hypertension and the augmentation of alpha pressor activity by vasopressin may participate in this contribution.

Possible mechanisms of salt-induced hypertension in Dahl salt-sensitive rats

Genetic factors, diet, and salt sensitivity have all been implicated in hypertension. To further understand the mechanisms involved in salt-induced hypertension, cardiovascular, hemodynamics, and biochemical parameters in Dahl salt-sensitive rats were evaluated in animals on high- and low-sodium diets. During a 4-week treatment period, blood pressure was significantly elevated in the high (8.0%) salt group compared to the low (0.3%) salt group (p< or =0.05 for weeks 2 and 4, respectively). No significant changes were observed in heart rate. The increase in blood pressure was associated with significant increases in lower abdominal aortic and renal vascular resistance, along with a reduction in blood flow. A fourfold increase in arginine vasopressin was observed in animals on the high-salt diet. In contrast, there was no effect on plasma sodium, potassium, or aldosterone levels during the treatment period. As measured in isolated aortic rings, the high-salt diet also caused a significant elevation in stimulated norepinephrine release and a reduction in cyclic GMP levels. These data suggest that salt-induced elevation in blood pressure is due to activation of both the sympathetic and arginine vasopressin systems via mechanisms involving decreased cyclic GMP generation in vascular smooth muscle.

Arterial baroreceptor denervation impairs long-term regulation of arterial pressure during dietary salt loading

Experiments were performed to examine the contribution of arterial baroreceptors to long-term regulation of mean arterial pressure (MAP) during changes in dietary salt intake. Normotensive Sprague-Dawley rats were subjected to either sinoaortic denervation (SAD; n = 8) or Sham surgery (n = 6) and instrumented 1 wk later with radiotelemetry transmitters for continuous minute-to-minute monitoring of MAP and heart rate (HR) over the 8-wk protocol. Rats consumed three levels of dietary NaCl: 0.4% NaCl (week 1), 4.0% NaCl (weeks 2-4), and 8.0% NaCl (weeks 5-7). Rats returned to a 0.4% NaCl diet during the eighth week of the experiment. During week 1 (0.4% NaCl), there were no differences between Sham and SAD groups for 24-h averages of MAP or HR. However, by the third week of 4.0% NaCl, 24-h MAP was elevated significantly from baseline in SAD (10 +/- 2 mmHg) but not Sham (1 +/- 1 mmHg) rats. By the end of the third week of 8.0% NaCl diet, 24-h MAP was elevated 15 +/- 2 mmHg above control in SAD rats compared with a 4 +/- 1 mmHg increase in Sham rats (P < 0.05). Hourly analysis of the final 72 h of each level of dietary salt revealed a marked effect of dietary NaCl on MAP in SAD rats, particularly during the dark cycle. MAP increased approximately 20 and 30 mmHg in SAD rats over the 12-h dark cycle for 4.0 and 8.0% NaCl diets, respectively. In contrast, increased dietary NaCl had no effect on MAP during any phase of the light or dark period in Sham rats. These data support the hypothesis that arterial baroreceptors play a critical role in long-term regulation of MAP under conditions of altered dietary salt intake. Finally, hourly analysis of MAP revealed that the majority of the hypertensive response to increased NaCl occurs during the dark cycle in SAD rats. Hence, previous investigations may have underestimated the magnitude of the hypertensive response to increased dietary NaCl in animals with baroreceptor dysfunction.

Cardiovascular, endocrine, and body fluid-electrolyte responses to salt loading in mRen-2 transgenic rats

We previously demonstrated that mRen-2 transgenic [Tg(+)] rats are sensitive to chronic high NaCl intake, showing increased arterial pressure and vasopressin (VP) secretion. In this study, we determined the effect of a chronic osmotic challenge, 4 days of drinking 2% NaCl, on direct arterial blood pressure, heart rate, fluid-electrolyte balance, circadian rhythm of mean arterial pressure (MAP), and changes in plasma VP and catecholamines. Under baseline conditions, male Tg(+) rats showed a significant shift in the peak in circadian MAP into the light portion of the day-night cycle. Substitution of 2% NaCl for drinking water caused a rapid increase in MAP, 20 +/- 5 mmHg in Tg(+) rats within 6 h. Whereas the amplitude of circadian MAP fluctuations increased in salt-loaded Tg(+) rats, there was no significant change in the circadian timing of peak MAP with salt loading. Tg(+) rats showed exaggerated osmotic-induced increases in plasma VP, norepinephrine (NE), and epinephrine (Epi) compared with Tg(-) rats. Plasma NE and Epi were increased two- and fourfold, respectively, in the hypertensive rats with no significant change in the Tg(-) rats. Intravenous administration of a VP antagonist did not alter arterial pressure in either Tg(+) or Tg(-) rats. Tg(+) and Tg(-) rats showed a positive sodium balance with no significant difference observed between the groups. Tg(+) rats showed a significant increase in salt consumption, plasma sodium, osmolality, and hematocrit, accompanied by a negative water balance. We conclude that Tg(+) rats are sensitive to acute and chronic osmotic stimuli in terms of blood pressure, fluid-electrolyte balance, and plasma VP and catecholamines. Whereas elevated plasma VP does not contribute to the hypertensive response, increased sympathetic drive may mediate the salt-induced blood pressure changes in this model.

Sinoaortic denervation produces sodium retention in Dahl salt-sensitive rats

The role of the arterial baroafferent signals in sodium (Na) homeostasis was examined in salt-sensitive rats. Sodium balances, water balances, and systolic arterial pressure (SAP) were measured for 3 weeks in sinoaortic denervated (SAD) or sham denervated (Sham) Dahl salt-sensitive (S) and salt-resistant (R) rats fed a standard-salt diet (0.4% NaCl). In R rats, there was no significant difference in Na balance, water balance, or SAP between the SAD and Sham animals. In the S rats, urinary Na excretion was suppressed in the SAD but not in the Sham animals, resulting in Na retention. SAD significantly increased SAP and mean arterial pressure. High salt challenge (8% NaCl diet) markedly increased SAP and Na and water balances in both S-SAD and S-Sham rats. However, no significant difference was found in SAP or cumulative Na and water balances between S-SAD and S-Sham rats. These results suggest that the baroafferent signals may have some role on the regulation of Na balance in salt-sensitive animals under a standard-salt condition, although a high-salt load masked the baroafferent-dependent sodium excretion.

Role of renal nerves in hemodynamic and natriuretic responses to saline in rabbits with impaired baroreflex sensitivity

The hemodynamic and urinary Na+ excretory response to a 2.5-fold increase in NaCl by i.v. infusion were assessed in conscious male rabbits with either high (BShi, salt-insensitive) or low (BSlo, salt-sensitive) cardiac baroreflex sensitivity, before, and 11-14 days after bilateral renal denervation. Effective renal plasma flow (ERPF) and proximal tubular Na+ reabsorption were measured by para-amino-hippurate (PAH) and Li+ clearances, respectively, before and after NaCl infused for 2 hr at a rate of 0.11 mL/kg/min. Intact BShi rabbits, showed a significant natriuresis within 30 min which was associated with an increase in ERPF and inhibition of proximal tubular reabsorption. The Na+ excretion rate was much slower in BSlo rabbits, while ERPF and proximal tubular reabsorption remained unchanged. Renal denervation reduced MAP, increased basal ERPF, Na+ and Li+ excretion in both groups, and abolished the difference in the renal hemodynamic re-sponse and Li+ excretion to increased NaCl, but not that in the rate of Na+ excretion. The data suggest that BSlo rabbits do not increase their ERPF and Li+ in response to saline because of an inability to bring about an inhibition of renal sympathetic nerve activity. This could be due to an impairment in the sensitivity of their cardiopulmonary baroreceptors. The difference in the rate of natriuresis in the two groups of rabbits which remained after renal denervation could involve an additional hormonal or a local renal mechanism.

Sympathoexcitatory response to cyclosporin A and baroreflex resetting

We postulate that the sympathoexcitatory response associated with the immunosuppressive agent cyclosporin A is due to an upward resetting of the arterial baroreflex. We performed studies in conscious intact and sinoaortic-denervated rabbits instrumented with catheters and renal nerve electrodes. In intact rabbits, cyclosporin A (20 mg/kg i.v., 30 minutes) produced significant increases in renal sympathetic nerve activity (100% to 269 +/- 74%, P < .05) but did not increase mean arterial pressure. In intact rabbits, we determined arterial baroreflex curves relating renal sympathetic nerve activity and heart rate to mean arterial pressure by producing ramp increases (intravenous phenylephrine) and decreases (intravenous nitroprusside) in mean arterial pressure. Cyclosporin A treatment produced a shift of the midrange of the baroreflex control of heart rate (78.0 +/- 4.1 to 84.6 +/- 4.7 mm Hg, P < .05) and renal sympathetic nerve activity (74.6 +/- 3.9 to 87.0 +/- 4.8 mm Hg, P < .05). Vehicle administration produced no effects on arterial baroreflex curves relating renal sympathetic nerve activity and heart rate to mean arterial pressure. Compared with vehicle treatment, cyclosporin A reduced the maximum gain of heart rate (-5.6 +/- 0.6 versus -3.1 +/- 0.8 beats per minute per millimeter of mercury, P < .05) but had no effect on the maximum gain of renal sympathetic nerve activity. In conscious sinoaortic-denervated rabbits, cyclosporin A had no effect on mean arterial pressure (95.7 +/- 7.3 to 91.8 +/- 10.8 mm Hg), renal sympathetic nerve activity (100% to 110 +/- 6%). and heart rate (287 +/- 10 to 279 +/- 8 beats per minute). However, when the same sinoaortic-denervated rabbits were anesthetized with sodium pentobarbital, cyclosporin A (20 mg/kg i.v.) produced increases in renal sympathetic nerve activity (100% to 189 +/- 27%). These data indicate (1) that the sympathoexcitatory response to cyclosporin A depends on baroreceptor afferent input in the conscious state and (2) that this response involves an upward resetting of the arterial baroreflex.

Baroreceptor function is restored by antihypertensive therapy through lowering of blood pressure in adult SHR

1. We investigated the effects of antihypertensive treatment (8 weeks) with four different agents (trichlormethiazide, atenolol, nicardipine and enalapril) on baroreceptor function in 28 week old spontaneously hypertensive rats (SHR) to measure aortic depressor nerve (ADN) activity. 2. Threshold pressure (Pth) of ADN activity was elevated and the gain sensitivity of the pressure-activity curve, as determined by the maximum gain (Gmax) of a logistic function curve, was depressed in untreated SHR compared to those in untreated Wistar-Kyoto (WKY) rats. 3. Treatment with the four agents similarly reduced blood pressure in SHR. Each of the four agents induced a decrease in Pth and an increase in Gmax to a similar extent in SHR. 4. These findings suggest that antihypertensive therapy in chronic hypertension augments baroreceptor function through the lowering of blood pressure but not through specific pharmacological actions.

Effect of nonpeptide vasopressin receptor antagonists on developing, and established DOCA-salt hypertension in rats

Efficacy of orally available, selective vasopressin V1 and V2 receptor antagonists on the developing and established stage of DOCA-salt hypertension was investigated. Twenty-nine Wistar rats were heminephrectomized, and administered DOCA (50 mg/kg; intraperitoneally twice a week) and salt (5% NaCl diet) from week 0 to the end of study. Group 1 rats were served as control. Group 2 and 5 rats were given a V1 antagonist, and groups 3 and 6 rats were given a V2 antagonist, while groups 4 and 7 rats received both V1 and V2 antagonists. Each drug was started to groups 2, 3 and 4 at week 0, and to groups 5, 6 and 7 at week 4. Significant amelioration of the increase in blood pressure was observed in groups 3 and 4 at week 10, and a reduction in blood pressure occurred in groups 6 and 7 at week 10. Otherwise, a V1 antagonist alone slightly attenuated blood pressure rise in the group 2 without significance, and failed to lower blood pressure of the group 5 during the study. These results suggest that both V1 and V2 agonisms are involved in an increase in blood pressure at the developing stage of DOCA-salt hypertension, and that V2 agonism, but not V1 plays a major role in the maintenance of high blood pressure at the established stage.

Effects of antihypertensive agents on baroreceptor function in early hypertensive rats

To investigate the effects of antihypertensive treatment with four currently used agents (trichlormethiazide, atenolol, nicardipine, and enalapril) on the arterial baroreceptor function at the early phase of hypertension, we administered the agents to spontaneously hypertensive rats and Wistar-Kyoto rats from 8 to 10 weeks of age and examined the aortic nerve activity function. In untreated spontaneously hypertensive rats, the relation between the arterial pressure and aortic nerve activity was shifted to the right, that is, to a higher pressure level (threshold pressure, 90 +/- 3 versus 76 +/- 1 mm Hg, P < .05), and the maximum gain which was obtained by logistic function analysis was depressed (1.55 +/- 0.08% versus 2.18 +/- 0.13% maximum/mm Hg, P < .01) as compared with untreated Wistar-Kyoto rats. An equivalent decrease in arterial pressure with each of the four agents (-20 +/- 1 mm Hg, P < .01) produced a leftward shift of the arterial pressure-aortic nerve activity relation to a similar extent (threshold pressure, 77 +/- 1 mm Hg, P < .05) in spontaneously hypertensive rats. In addition, treatment with the four agents equally augmented the maximum gain in spontaneously hypertensive rats (2.13 +/- 0.09% maximum/mm Hg, P < .05). The antihypertensive agents affected neither the blood pressure nor the aortic nerve activity in Wistar-Kyoto rats. These findings suggest that antihypertensive treatment with the four classes of agents equally enhances the arterial baroreceptor function through blood pressure reduction but not through specific depressor mechanisms at the early stage of hypertension.

Losartan inhibits sympathetic and cardiovascular responses to carotid occlusion

We have reported that inhibition of angiotensin-converting enzyme with captopril attenuates the cardiovascular responses to bilateral carotid occlusion in conscious rabbits and proposed that the attenuation results from removal of a facilitatory action of angiotensin II on the sympathetic nervous system. The aim of the present study was to assess the effect of carotid occlusion on renal sympathetic nerve activity in conscious rabbits and to investigate the effect of the angiotensin II subtype 1 (AT1) receptor antagonist losartan on the cardiovascular and renal sympathetic nerve activity responses to carotid occlusion. In seven conscious, aortic depressor nerve-sectioned rabbits, carotid occlusion elicited prompt and reproducible increases in mean arterial pressure from 75 +/- 2 to 124 +/- 5 mm Hg (P < .001), heart rate from 285 +/- 8 to 317 +/- 9 beats per minute (P < .01), and renal sympathetic nerve activity to 165 +/- 11% of control (P < .01). In the same rabbits, losartan (5 mg/kg i.v.) decreased mean arterial pressure by 9 +/- 2 mm Hg (P < .01), increased renal sympathetic nerve activity to 143 +/- 13% of control (P < .05), but did not alter heart rate. Losartan significantly attenuated (P < .01) the mean arterial pressure (66 +/- 2 to 81 +/- 2 mm Hg), heart rate (282 +/- 9 to 289 +/- 7 beats per minute), and renal sympathetic nerve activity (143 +/- 13% to 159 +/- 15% of control) responses to carotid occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Central and peripheral vasopressin interact differently with sympathetic nervous system and renin-angiotensin system in renal hypertensive rabbits

This study was designed to elucidate how central and peripheral arginine vasopressin (AVP) interacts with the sympathetic nervous system and the renin-angiotensin system to maintain blood pressure in two-kidney, one-clip hypertensive rabbits. We recorded renal sympathetic nerve activity (RSNA) in the conscious state as an index of sympathetic nervous system function. The changes in mean arterial pressure, heart rate, and RSNA were recorded continuously for 60 minutes after intravenous administrations of captopril (2.5 mg/kg) and nicardipine (3.2 micrograms.kg-1.min-1) in eight identical rabbits. Despite equivalent reductions in mean arterial pressure (10 +/- 1 mm Hg), the increase in RSNA was significantly larger with captopril than that with nicardipine, and the plasma concentration of AVP was elevated (from 100% to 255 +/- 24%) with captopril. Mean arterial pressure was reduced, and RSNA was increased by intravenous infusion of AVP antagonist d(CH2)5Tyr(Me)AVP (n = 8), whereas vertebral artery infusion of the antagonist (n = 6) did not change RSNA. During central and peripheral infusions of AVP antagonist, RSNA was exaggerated by blood pressure reduction with nicardipine as well as with captopril. Increases in RSNA induced by captopril and nicardipine were larger by central infusion of AVP antagonist than by intravenous infusion. The decrease in mean arterial pressure by captopril (30 +/- 4 mm Hg) in eight sinoaortic-denervated hypertensive rabbits was larger than that in hypertensive rabbits with intact baroreflex. These data suggest that compensatory activation of RSNA was revealed by central and peripheral attenuation of AVP and that the sympathetic nervous system became the most important mechanism for blood pressure maintenance in the absence of AVP. The interaction of AVP with the sympathetic nervous system may be independent of the state of the renin-angiotensin system, since the exaggeration of RSNA by AVP antagonist was qualitatively the same with nicardipine as with captopril. In conscious renal-hypertensive rabbits, AVP in the central nervous system played a substantial role when blood pressure was reduced, although it did not contribute to blood pressure maintenance in the basal condition.

Salt-dependent hypertension in the sinoaortic-denervated rat

To determine the extent to which baroreceptor function is a determinant of salt-dependent hypertension, we studied the cardiovascular and renal responses to increasing dietary sodium chloride in sinoaortic-denervated (n = 9) and sham-denervated (n = 9) Sprague-Dawley rats. Rats were instrumented with an arterial catheter for measurement of arterial pressure and were individually housed for daily measurements of water intake, sodium intake, urinary output, and urinary sodium excretion. Arterial pressure was monitored daily over a 30-minute period by computer. After 3 days of control measurements (0.4% sodium chloride diet), dietary sodium chloride was increased to 8.0% for 21 days, followed by a 3-day recovery period (0.4% sodium chloride). Ingestion of an 8.0% sodium chloride diet resulted in a 20- to 25-fold increase in sodium intake and a fivefold increase in water intake in both groups. In sinoaortic-denervated rats, arterial pressure increased approximately 10 mm Hg on days 5-10, 20 mm Hg on days 11-18, and 30 mm Hg on days 19-21 of 8.0% sodium chloride. Arterial pressure returned to control levels within the first 24 hours of the recovery period. Elevated sodium intake had no significant effect on arterial pressure in the sham-denervated group. Finally, there were no significant differences between groups in urine output or urinary sodium excretion at any time during the study. We conclude that a primary impairment in the afferent limb of the arterial baroreceptor reflex results in salt-dependent hypertension in the Sprague-Dawley rat.