Role of nitric oxide in responses to renin-angiotensin system inhibition in sodium-depleted guinea pig and rat

Previous reports have suggested that NO is an important mediator of the antihypertensive effects of renin-angiotensin system (RAS) inhibition. We examined the effects of the NO synthase inhibitor L-NNA on the hypotensive effects of captopril, the Ang II antagonist EXP 3174, or the renin inhibitor terlakiren. In sodium-depleted guinea pigs (GPs), L-NNA (3 mg/kg) increased MAP by 15-21% for at least 5 hours. L-NNA partially blocked the hypotensive effects of captopril (1 mg/kg, iv), but not those of EXP 3174 (1 mg/kg, iv) or terlakiren (3 mg/kg). In sodium-depleted rats, 10 mg/kg L-NNA (iv) increased MAP by 16-22%, and partially or fully blocked the hypotensive effect of EXP 3174 (1 mg/kg, iv) or captopril (3 mg/kg, iv), respectively. Thus, in contrast to the rat, NO in GPs appears to participate only in the hypotensive action of ACE inhibition and does not appear to be strongly involved in the hypotensive action of AII antagonism or renin inhibition. The involvement of NO in the hypotensive effects of RAS antagonists other than ACE inhibitors may be species-dependent.

Discovery of inhibitors of human renin with high oral bioavailability

Knowledge of the sequence of a bioactive protein (angiotensinogen) and the availability of a natural product inhibitor lead (pepstatin) were the starting point for discovery of potent penta- and hexapeptide renin inhibitors. Study of the metabolism and disposition of these substances forced the discovery of simpler inhibitors leading to the discovery of oral activity in Terlakiren (22). Modification of physical properties led to the synthesis of aminopiperidine 30, which was identified by oral efficacy profiling. Structural modification to give enzymatic stability produced the bioavailable benzylsuccinate inhibitor 34. Its bioactive monomethylamine metabolite (35, CP-108,671) was subsequently found to have uniformly high oral bioavailability and activity in various species including primates.

CP-71,362: a pentapeptide renin inhibitor selective for the canine enzyme

Most renin inhibitors are primate-specific. In the present paper, we describe the effects of CP-71,362, a pentapeptide which preferentially inhibits canine (and to a lesser extent, rat) plasma renin. Vs. the canine enzyme, its affinity (IC50 = 3.3 x 10(-12) M) is 1000x greater than for rat renin (IC50 = 3.3 x 10(-9) M), and 1000x greater than for human (IC50 = 2.3 x 10(-8) M), cynomolgus monkey (IC50 = 1.6 x 10(-8) M), or guinea pig (IC50 = 5.2 x 10(-8) M) enzyme. In anesthetized, sodium-depleted dogs, intravenous infusion of CP-71,362 (ED50 = 1.1 micrograms/kg/min) resulted in dose-dependent decreases (up to -35 mm Hg) in mean arterial pressure (MAP). The maximum fall in MAP was equivalent to that produced by i.v. captopril (5 mg/kg). Similar falls in MAP were observed in conscious sodium-depleted SHR (ED50 = 5 micrograms/kg/min). Via bolus injection, the action of CP-71,362 was relatively brief in dog, guinea pig, and SHR. We conclude that CP-71,362 is a potent canine/rat renin inhibitor and causes profound MAP lowering in these species.

Vasoconstrictor action of angiotensin I-convertase and the synthetic substrate (Pro11,D-Ala12)-angiotensin I

A chymase (also referred to as angiotensin I-convertase) specific for the conversion of angiotensin (Ang) I to Ang II has been identified in human heart. This serine protease is also present in dog and marmoset vasculature. We examined the vasoconstrictor effects of Ang II putatively generated from an angiotensin-converting enzyme (ACE)-resistant convertase synthetic substrate (SUB) in vivo and in vitro. In marmosets, SUB (7 to 700 micrograms/kg i.v.) or Ang I (0.1 to 30 micrograms/kg) caused similar dose-dependent increases in mean arterial pressure (10 to 100 mm Hg) and decreases in heart rate. Pressor effects of SUB were slightly attenuated at low (but not high) doses by captopril (CAP, 1 mg/kg i.v.) and blocked by losartan (5 mg/kg i.v.); in contrast Ang I pressor effects were substantially blocked by both. In isolated canine superior mesenteric artery, Ang I-induced contraction was eliminated by losartan and reduced but not eliminated by 10 mumol/L CAP. When combined with the serine protease inhibitor chymostatin, CAP eliminated Ang I-induced contraction, but chymostatin alone had no effect. SUB-induced contraction was not blocked by CAP but was equally blocked by chymostatin (25 mumol/L) alone or by the combination of CAP (10 mumol/L) and chymostatin (25 mumol/L); losartan (10 mumol/L) eliminated SUB-induced responses. Previous studies have suggested that Ang I-convertase is important for production of Ang II in the heart. Our results are consistent with a potential role for Ang I-convertase in the production of Ang II in the vasculature, resulting in Ang II-mediated vasoconstriction.

Central DuP 753 does not lower blood pressure in spontaneously hypertensive rats

Oral administration of the angiotensin II receptor subtype 1 (AT1) antagonist DuP 753 causes long-lasting lowering of mean arterial pressure in spontaneously hypertensive rats. We examined whether the antihypertensive action of DuP 753 is a result of inhibition of brain angiotensin II. In normal spontaneously hypertensive rats, we found that intracerebroventricular DuP 753 (10 micrograms) blocked the pressor action of intracerebroventricular angiotensin II (100 ng); however, intracerebroventricular DuP 753 (10 micrograms) had no effect on the pressor response to 300 ng/kg angiotensin II administered intravenously (48 +/- 3 mm Hg in the presence of intracerebroventricular DuP 753 versus 49 +/- 4 mm Hg in its absence). In both normal and furosemide-treated spontaneously hypertensive rats (low Na+ diet plus furosemide), intracerebroventricular DuP 753 alone at 10 or 100 micrograms caused transient but significant pressor responses; however, no significant reduction in pressure (versus controls) was observed over the next 48 hours. In contrast to its central effects, we found that oral DuP 753 (10 or 30 mg/kg) in normal spontaneously hypertensive rats resulted in sustained mean arterial pressure decreases of up to -74 mm Hg. These data suggest that, although the pressor effect of brain angiotensin II is mediated by the AT1 receptor, blockade of these receptors does not lower blood pressure in spontaneously hypertensive rats. In the spontaneously hypertensive rat, DuP 753 depresses blood pressure by blockade of peripheral, not central, AT1 receptors.

Area postrema: essential for support of arterial pressure after hemorrhage in rats

Previous studies have indicated that the area postrema (AP) of the rat is necessary for the development of chronic angiotensin-dependent hypertension. The present study assesses the role of the AP in the maintenance of arterial pressure during hemorrhage. Sprague-Dawley rats were given sham or AP lesions 1 wk before the experiment. They were instrumented with femoral arterial and venous catheters 2 days before the experiment. On the day of the experiment, base-line mean arterial pressure (MAP) was measured for 1 h before hemorrhage. During the following 45 min, each rat was subjected to one 7-ml/kg hemorrhage every 15 min for a total of three hemorrhages. MAP was monitored by computerized data acquisition. As shown previously, MAP was slightly but significantly lower in AP-lesion rats compared with sham-lesion rats before the hemorrhage procedure. In AP-lesion rats, hemorrhage resulted in a significantly greater fall in arterial pressure than in sham-lesion rats. In spite of larger drops in pressure in AP-lesion rats, hemorrhage caused equivalent increases in plasma renin and vasopressin in both groups. In AP-lesion rats compared with sham-lesion rats, significant bradycardia was present before hemorrhage. Hemorrhage caused bradycardia in both sham- and AP-lesion rats relative to the prehemorrhage heart rates, but AP-lesion rats showed greater bradycardia than did sham-lesion rats during every time period. We conclude that the AP may play an important role in the defense of arterial pressure against hemorrhage.

Effect of circulating vasopressin on arterial pressure regulation in rats

It has been hypothesized that moderately increased blood levels of arginine vasopressin (AVP) contribute to the development and/or maintenance of hypertension. In this study, male Sprague-Dawley rats on a fixed 1 meq daily sodium intake received 10-day intravenous infusions of 0.2 and 2.0 ng.kg-1.min-1 AVP. The higher infusion rate was above the acute vasoconstrictor threshold for AVP administration and also produced a maximal antidiuretic effect. During chronic AVP administration, however, daily mean arterial pressure, heart rate, and body fluid composition were not changed, despite a maintained antidiuresis. To test the hypothesis that circulating AVP failed to cause hypertension as a result of sensitization of the baroreflex or a direct sympathoinhibitory effect of the peptide, additional experiments were performed in rats subjected to sinoaortic denervation (SAD) or ablation of the area postrema (APX). Infusion of AVP for 10 days into SAD or APX rats caused a sustained antidiuresis but did not change arterial pressure, heart rate, or body fluid composition. In all groups of rats, the depressor response to ganglionic blockade (20 mg/kg hexamethonium) was used to estimate the autonomic component of resting arterial pressure; no change in autonomic cardiovascular control was found using this method in any of the groups during AVP infusion. Long-term elevation of plasma AVP in rats, therefore, does not cause hypertension or significantly affect autonomic regulation of arterial pressure.

Lesion of the area postrema region attenuates hypertension in spontaneously hypertensive rats

To determine whether the area postrema contributes to the development of hypertension in spontaneously hypertensive rats (SHR), sham or electrolytic lesions of the area postrema (AP) were made in 4-week-old SHR and Wistar-Kyoto (WKY) controls. From weeks 5 through 16, systolic pressure was measured via tail plethysmography. While blood pressure rose markedly in sham-operated SHR, increases in pressure were small in AP-ablated SHR and similar to those seen in all WKY. Subsequent direct measurements of mean arterial pressure in the same rats showed a significant correlation (r = 0.87, p less than 0.01) with the pressure data acquired via weekly tail-cuff measurement, thereby confirming that hypertension in AP-ablated SHR had indeed been attenuated. Analysis of several hundred computer-acquired measurements of mean arterial pressure from each rat showed that AP ablation shifted the distribution of mean arterial pressure to a lower range in SHR but not WKY. Ablation of the AP also decreased resting heart rate in SHR but not WKY. Suppression of heart rate in response to intravenous phenylephrine was equivalent in sham-operated and AP-ablated rats, suggesting that baroreflex-mediated slowing of heart rate was not impaired. In response to intravenous angiotensin II, suppression of heart rate was similar in sham and AP-ablated SHR, and actually was enhanced in AP-ablated WKY. Histological evaluation of the lesions indicated that visible damage to the adjacent nuclei of the solitary tracts was confined to a small portion of the commissural nucleus.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation of spontaneous hypertension in rats by a vasopressin antagonist

Although abnormalities in the vasopressin system have been reported in spontaneously hypertensive rats (SHR), neither short-term nor long-term administration of the vasopressin antagonist d(CH2)5-Tyr(Me)arginine vasopressin (AVP), which selectively blocks the action of vasopressin on vascular (V1) receptors, altered the course of hypertension in SHR. In the current study, long-term administration of a different vasopressin antagonist, d(CH2)5-D-Tyr(Me)VAVP, to SHR and Wistar-Kyoto rats (WKY) from 4 to 12 weeks of age significantly attenuated the development of systolic hypertension in SHR (p less than 0.05) without altering blood pressure in normotensive WKY. The antagonist was delivered subcutaneously by osmopump at 0.1 microgram/hr. Systolic blood pressure was monitored twice weekly by tail plethysmography beginning at 5 weeks of age. In a second group of SHR, the drug infusion was continued until 18 weeks of age. In this group, the attenuation of systolic hypertension by the drug was extended and became more prominent (p less than 0.007). Resting mean arterial pressure measured by indwelling catheters in the conscious state at 18 weeks of age was significantly reduced in the antagonist-treated SHR (144 +/- 4 vs 157 +/- 4 mm Hg; p less than 0.05). Heart rate also was significantly reduced by the drug (351 +/- 6 vs 392 +/- 7 beats/min; p less than 0.001). Following measurement of mean arterial pressure in the rats at 18 weeks of age, the osmopumps were removed and systolic blood pressure, mean arterial pressure, and heart rate were observed until 22 weeks of age.(ABSTRACT TRUNCATED AT 250 WORDS)

Area postrema ablation and vascular reactivity in deoxycorticosterone-salt-treated rats

In rats, central administration of the neurotoxin 6-hydroxydopamine prevents hypertension and certain functional vascular changes after deoxycorticosterone (DOC)-salt treatment. In this study, the effect of electrolytic ablation of the area postrema on blood pressure and vascular reactivity in DOC-salt-treated rats was examined. Four treatment groups of rats were studied (n = 5 in each): area postrema lesion, DOC-salt (DOC pivalate, 5 mg/wk s.c. for 5 weeks); sham lesion, DOC-salt; area postrema lesion, control; and sham lesion, control. Helically cut strips of carotid artery, aorta, and mesenteric artery were prepared for isometric force recording. Area postrema lesion attenuated hypertension in DOC-salt rats (mean arterial pressure, 107 vs 123 mm Hg in area postrema lesion and sham lesion rats, respectively; chronic aortic catheter). Vascular strips from sham lesion-control rats. These changes in vascular reactivity also were observed in area postrema lesion-DOC-salt rats. DOC treatment in rats on a normal sodium intake did not result in hypertension or increased vascular reactivity. In summary, integrity of the area postrema is necessary for hypertension but not for changes in vascular reactivity, in DOC-salt rats. It appears that 1) changes in vascular reactivity may be necessary, but they are not sufficient to produce DOC-salt hypertension, and 2) if these vascular changes are secondary to a central nervous system effect, they are mediated by a pathway distinct from the area postrema.

Area postrema and cardiovascular regulation in rats

The area postrema (AP) of the dog mediates pressor responses to angiotensin II and plays a role in the maintenance of normal arterial pressure. Ablation of the AP (APX) in the rat has been reported to have little or no effect on cardiovascular regulation. In the present study, computer data acquisition techniques were used to investigate this question. APX in the rat significantly lowered resting mean arterial pressure within 1 h (P less than 0.005) and lowered heart rate within 1 day (P less than 0.05) following the lesion. Increases in heart rate following atropine injections were significantly greater (P less than 0.05) in AP relative to sham lesion rats, suggesting higher vagal tone in the AP lesion rats. In addition, APX significantly enhanced the baroreflex control of heart rate in response to intravenous phenylephrine (P less than 0.05). Lability of pressure was not affected by the lesion. The hypotension and bradycardia produced by APX were still present 1 wk after APX. These AP lesions apparently did not produce significant damage to the function of the nearby NTS, since 1) histological analysis revealed minimal NTS damage, 2) arterial pressure lability was not increased, and 3) APX enhanced rather than impaired baroreflex control of heart rate. We conclude that the AP may have a role in the maintenance of resting arterial pressure and heart rate in the rat.

Vasoconstrictor and vasodilator sites within anteroventral third ventricle region

Previous studies have shown that electrical stimulation of the rat anteroventral third ventricle (AV3V) region produces a characteristic pattern of hemodynamic effects, i.e., renal and mesenteric vasoconstriction, and hindquarters vasodilation. In the present study, we localized the vasoconstrictor and vasodilator effects to specific subregions of the AV3V. In urethan-anesthetized rats prepared with arterial catheters and pulsed Doppler flow probes, we assessed the effects of electrical stimulation of four nuclei within AV3V on mean arterial pressure and renal, mesenteric, and hindquarters resistance. These nuclei were the organum vasculosum lamina terminalis (OVLT), ventral nucleus medianus (median preoptic nucleus), anterior (precommissural) nucleus medianus (median preoptic nucleus), and periventricular preoptic nuclei. Stimulation was carried out by use of a tungsten microelectrode. Stimulation of the OVLT consistently provoked stimulus-locked increases in arterial pressure coupled with increases in mesenteric and renal vascular resistance. Ganglionic blockade with chlorisondamine prevented these responses, demonstrating that they were mediated neurogenically. Stimulation of the three remaining nuclei produced decreases in arterial pressure, hindquarters vasodilation, and little change in mesenteric and renal vascular resistance. No changes in heart rate were observed with stimulation of any of the four nuclei. These results suggest that the vasoconstrictor and pressor functions of the AV3V region are localized in or near the OVLT region, whereas the remaining nuclei of the AV3V region mediate vasodilator and depressor responses.

The area postrema in deoxycorticosterone-salt hypertension in rats

Ablation of the area postrema in rats prevents sustained hypertension during angiotensin II infusion and after unilateral renal artery constriction (two-kidney, one clip hypertension). The current experiment was performed to determine whether an intact area postrema is required for hypertension development in a low renin model of experimental hypertension in rats. In 11 rats, the area postrema was destroyed using electrical current; the extent and specificity of each lesion was confirmed later by blind histological analysis. In 12 rats, sham operations were performed. All rats were uninephrectomized and drank saline. During once-weekly injections of deoxycorticosterone pivalate (5 mg/wk) for 4 weeks, sham-operated rats (n = 10) showed a significant increase in mean arterial pressure (Days 6-28) and saline intake (Days 12-28), but no significant increase in sodium or water retention. Deoxycorticosterone-treated rats with area postrema ablation (n = 9) exhibited no change in arterial pressure, sodium retention, or water retention, but a significant increase in saline intake (Days 17-28). Plasma renin activity was equally suppressed in both groups of rats. The depressor response to ganglion blockade with hexamethonium (20 mg/kg i.v.) was significantly increased during the 2nd, 3rd, and 4th weeks of steroid treatment in sham-operated, but not area postrema-ablated, rats. Four rats (2 sham-operated; 2 ablated) showed no change in any variable over 28 days in the absence of steroid treatment. It is concluded that the area postrema may be important in some non-angiotensin-dependent forms of experimental hypertension, possibly by affecting neurogenic control mechanisms.

Area postrema is critical for angiotensin-induced hypertension in rats

The effect of surgical ablation of the area postrema on acute (5-10 minutes) and chronic (5-10 days) increases in mean arterial pressure produced by intravenous infusion of angiotensin II in conscious, instrumented rats was studied. In agreement with previous studies, pressor responses of area postrema-ablated rats (n = 11) to acute angiotensin II infusion were identical to those of control sham-lesioned rats (n = 13). In these same rats, however, a 5-day infusion of angiotensin II produced a sustained hypertension in the sham-lesioned group whereas mean arterial pressure was increased only transiently (1-3 days) in the area postrema-ablated rats. No differences before infusion of arterial pressure, heart rate, water intake, urinary sodium excretion, and urinary potassium excretion were observed between sham-lesioned and area postrema-ablated rats; only arterial pressure was changed significantly during angiotensin II infusion in either group. Twenty-four hours after terminating angiotensin II infusion, mean arterial pressure was within the normotensive range in both sham-lesioned and area postrema-ablated rats. In a separate group of sham-lesioned (n = 13) and area postrema-ablated (n = 12) rats, angiotensin II was infused intravenously for a 10-day period; mean arterial pressure was increased significantly over the entire 10-day infusion in sham-lesioned rats, but for only 1 day in area postrema-ablated rats. An intact area postrema appears necessary for the development of chronic, but not acute, hypertension during intravenous infusion of angiotensin II in the rat.

Median preoptic nucleus ablation does not affect angiotensin II-induced hypertension

Previous studies implicated the ventral median preoptic nucleus (MNPOv) in cardiovascular responses to circulating and intracerebroventricular angiotensin II (ANG II) and in normal cardiovascular and fluid homoeostasis. In the present experiments, chronically catheterized rats received continuous (24 h/day) intravenous infusions of ANG II (10 ng/min) for 5 days, and changes in mean arterial pressure, heart rate, water intake and urinary electrolyte and water excretion were determined daily. Three groups of rats were compared as follows: 1) sham-operated control rats (n = 12), 2) rats with 20-70% of the MNPOv ablated electrolytically (n = 6), and 3) rats with over 90% of the MNPOv ablated (n = 5). The organum vasculosum of the lamina terminalis was intact in all three groups. Base-line values of all measured variables were identical in the three groups on two control days preceding ANG II infusion and on two recovery days after infusion. During the administration of ANG II for 5 days, mean arterial pressure rose significantly (and similarly) in all three groups of rats; no other variable was significantly affected by ANG II infusion. These results suggest that neural pathways originating in, or passing through, the MNPOv region are not critical in the pathogenesis of ANG II-induced hypertension in the rat.

Mechanisms of hemodynamic responses to electrical stimulation of subfornical organ

The rat subfornical organ (SFO) is involved in the pressor response to circulating angiotensin II, and recent evidence indicates that SFO electrical stimulation also produces a pressor response. In the present experiments we examined the hemodynamic, neural, and humoral mechanisms that underlie the pressor response to electrical stimulation of the SFO. Rats were anesthetized with urethan and instrumented with femoral arterial catheters and with pulsed Doppler flow probes on the superior mesenteric and renal arteries and on the abdominal aorta. Constant-current stimulation, delivered to the SFO via tungsten microelectrodes, resulted in stimulus-locked frequency-dependent pressor responses and vasoconstriction in all vascular beds tested. The stimulation-evoked increases in vascular resistance were greatest in the mesenteric circulation and least in the renal. Movement of the electrode away from the SFO produced significantly smaller responses. Ganglionic blockade abolished the responses to electrical stimulation, whereas vasopressin blockade significantly attenuated the responses. The responses of baroreceptor-denervated rats were qualitatively similar to but approximately double in magnitude of those of normal rats. We conclude that electrical stimulation of the SFO elicits widespread regional vasoconstriction that is most pronounced in the mesenteric circulation. The sympathetic nervous system appears responsible for these effects, but there may be facilitation of the responses by vasopressin.

Subfornical organ. Does it protect against angiotensin II-induced hypertension in the rat?

The purpose of this study was to examine the contribution of the subfornical organ to the chronic hypertension produced by intravenous angiotensin II infusion in rats. Male rats were instrumented with permanent arterial and venous catheters and housed in metabolism cages for daily measurement of arterial pressure, heart rate, water intake, water balance, and urinary electrolyte excretion. Angiotensin II was infused intravenously at a rate of 10 ng/minute for 5 consecutive days, preceded by 2 control days, and followed by 2 recovery days. Normal rats with an intact subfornical organ (n = 7), and rats with an electrolytic lesion placed such that greater than 80% of the subfornical organ was destroyed (n = 9), were studied using this infusion protocol. An additional group of eight rats did not receive angiotensin II, and, thus, served as a time control. Both groups of rats receiving angiotensin II exhibited significant elevations in arterial pressure during the 5-day hormone infusion period, but pressure in rats with subfornical organ lesions (delta 23-29 mm Hg) was increased significantly more than that of intact rats (delta 14-20 mm Hg). Water intake was significantly increased on the 3rd, 4th, and 5th days of angiotensin II infusion only in rats with lesions in the subfornical organ. In contrast to previous studies showing that an intact subfornical organ is required for normal pressor and drinking responses to acute elevations in circulating angiotensin II in the rat, the current experiments indicate that the presence of the subfornical organ actually inhibits these same responses during more chronic increases in plasma angiotensin II levels.

The cardiovascular effects of centrally and peripherally administered indoramin in conscious rats

Indoramin has centrally mediated hypotensive effects in anesthetized animals. In the present study, the cardiovascular effect of central and peripheral indoramin was determined in conscious, freely moving rats. Animals were instrumented with femoral arterial and venous catheters and miniaturized pulsed-Doppler flow probes were placed on the superior mesenteric and renal arteries and lower abdominal aorta. Injection of indoramin (25-100 micrograms) in the lateral cerebroventricle produced an immediate (1.5 min) increase in arterial pressure which was accompanied by vasoconstriction in all three vascular beds. By 10 min all values had returned to control except for heart rate which was decreased. Vehicle alone or intravenous indoramin (100 micrograms) had no effect. In baroreceptor-denervated rats smaller effects were seen. Intravenous indoramin (3.0-13.5 mg/kg) produced dose-related decreases in arterial pressure, heart rate, and hindquarter vascular resistance. The 13.5 mg/kg dose blocked to a similar degree the cardiovascular effect produced by intravenous norepinephrine or stimulation of the paraventricular nucleus. These data suggest that indoramin is an effective peripheral alpha-adrenergic receptor antagonist but does not appear to be centrally active as a hypotensive agent.

Comparative central and peripheral antihypertensive mechanisms of urapidil and prazosin

The central effects of urapidil were investigated in conscious rats with sinoaortic denervation. Intraventricular urapidil administration (40 to 100 micrograms) produced a transient depressor response followed by a pressor response coupled with tachycardia. In comparison, intraventricular prazosin administration (2.5 to 5.0 micrograms) produced only a prolonged depressor effect. The effect of intravenously administered urapidil (3 mg/kg) on arterial pressure, heart rate, and mesenteric, renal, and hindquarter resistances was then compared with that of prazosin (0.5 mg/kg) in conscious rats with sinoaortic denervation, instrumented with pulsed Doppler flow probes. Both agents caused similar significant decreases in arterial pressure and vascular resistances, but urapidil decreased renal resistance significantly more than did prazosin. Prazosin increased heart rate, whereas no change was found with urapidil. Prazosin blocked the pressor and regional constrictor effects of intravenously administered norepinephrine more effectively than urapidil.

Role for the subfornical organ in vasopressin release

The effect of subfornical organ (SFO) lesions on plasma vasopressin and drinking was tested in rats given either of two stimuli: subcutaneous injection of hypertonic saline or injection of a single dose of angiotensin II (AII) into the dorsal third cerebral ventricle. Drinking in response to hypertonicity was significantly attenuated, whereas AII-induced drinking was unaffected by the lesions. In contrast, SFO lesions were associated with significantly reduced vasopressin responses following either of these potent stimuli to vasopressin secretion. Partial lesions that damaged structures adjoining the SFO (fornix or septum) had no significant effects. These results demonstrate that, in rats, the SFO is in some way necessary for vasopressin responses to both AII and hypertonicity.

[Sar,Ala]angiotensin II in cerebrospinal fluid blocks the binding of blood-borne [125I]angiotensin II to the circumventricular organs

There is a specific, high affinity uptake of angiotensin II in the circumventricular organs when the peptide is injected systemically. The question of whether angiotensin II in cerebrospinal fluid can reach angiotensin receptors in the circumventricular organs was investigated in rats by determining the effect of intraventricular administration of the angiotensin II receptor blocking peptide [Sar,Ala]angiotensin II (saralasin) on the binding of blood-borne [125I]angiotensin II. Other rats received intraventricular saline, intraventricular ACTH as a peptide control, or intravenous saralasin. The brains of the rats were then sectioned and subjected to radioautography. ACTH had no effect on angiotensin II uptake. Intraventricular saralasin reduced the uptake of blood-borne angiotensin II in the median eminence and organum vasculosum of the lamina terminalis to the same degree as intravenous saralasin, and reduced uptake in the subfornical organ and area postrema to a lesser extent. Uptake was reduced 40% in the anterior lobe of the pituitary by intraventricular saralasin and 73% by intravenous saralasin, indicating that some saralasin entered the portal vessels. Uptake in the posterior lobe was unaffected by intraventricular saralasin, but reduced by intravenous saralasin. The data indicate that saralasin, and so presumably angiotensin II, in the cerebrospinal fluid can reach angiotensin II receptors in the circumventricular organs which bind blood-borne angiotensin II. Consequently, the effects of intraventricular angiotensin II that are also produced by intravenous angiotensin II can probably be explained by the peptide acting on the circumventricular organs.

Deficits in drinking and vasopressin secretion after lesions of the nucleus medianus

The effects of ablation of the nucleus medianus on drinking and vasopressin secretion were studied in male Long-Evans rats. The amount of water drunk in 1 h was assessed after subcutaneous injection of 5.8% NaCl (13.34 mosm/kg) or of angiotensin II (1.5 mg/kg). In a separate test with no water available, plasma vasopressin was measured 15 min after the above dose hypertonic saline. Ablation of the nucleus medianus, or the dorsal and anterior portions of the nucleus medianus, blocked drinking to hypertonic saline or angiotensin II and attenuated the vasopressin response to hyperosmolality. Animals with septal or diagonal band lesions showed responses comparable to sham-operated rats. These results indicate that a neural pathway important for fluid balance passes through, or terminates in, the nucleus medianus.

Drinking and pressor responses after acetylcholine injection into subfornical organ

Mean arterial pressure and drinking were measured in rats before and after intracranial injection of acetylcholine (ACh) into the subfornical organ (SFO), the circumventricular organ of the dorsal third ventricle. Arterial pressure was recorded in unanesthetized unrestrained animals by means of an abdominal aortic catheter. Maximal increases in arterial pressure and drinking followed the application of ACh to SFO, while significantly smaller effects were obtained after ACh injections into near-SFO control sites. Inasmuch as the effects also occurred with the shortest latencies after SFO injections, the possibility of ACh diffusing to act at other than SFO sites seems unlikely. The pressor and drinking responses to ACh were blocked by SFO pretreatment with atropine, indicating muscarinic receptor involvement. The SFO-mediated pressor response to ACh was reduced after systemic injection of phentolamine, suggesting at least a partial role for the sympathetic nervous system in the response. The present experiments suggest that ACh acts at the SFO to generate not only drinking but also increases in arterial pressure.

Subfornical organ lesions reduce the pressor effect of systemic angiotensin II

The pressor effect of intravenous angiotensin II (AII) was compared in the unrestrained rat before and after electrolytic lesions of the subfornical organ (SFO), the circumventricular organ of the dorsal third cerebral ventricle. Abdominal aortic and venal caval catheters were used to measure arterial pressure and to infuse solutions, respectively. The pressor effect of AII was significantly reduced following complete SFO lesions but was unaffected by partial SFO lesions or by control lesions in adjacent tissue. The pressor effect of intravenous infusion of the alpha-adrenergic agonist, phenylephrine, was unaffected by any of these lesions. In agreement with experiments demonstrating that SFO injection of AII increases arterial pressure, the present results suggest that a significant portion of the pressor action of circulating AII is centrally mediated, and that the SFO participates in this mediation.

Subfornical organ: forebrain site of pressor and dipsogenic action of angiotensin II

Intracranial injection of angiotensin II (AII) at three brain sites elicited near simultaneous dipsogenic and pressor effects in rats. Both effects were maximal, occurred with the shortest latencies, and at the lowest doses of AII when the cannula terminated precisely within the parenchyma of the subfornical organ (SFO). Pressor effects were produced by SFO injection of a dose of AII (0.1 pg) which approximates plasma AII concentrations at the high end of the physiological range. Both the drinking and pressor effects were blocked by saralasin. Injections of AII at sites immediately adjacent to SFO produced smaller effects with longer latencies. These results ruled out the possibility that SFO injections were effective via leakage to alternative sites. The pressor effect of AII at the SFO remained in animals under chloralose anesthesia, demonstrating that it is not an artifact of drinking behavior. These results indicate that the SFO is a site of AII pressor action, and confirm previous demonstrations that the structure is a site of AII drinking action.

Pharmacologic independence of subfornical organ receptors mediating drinking

In rats with chronically implanted cannulae in the subfornical organ (SFO), the relationship between cholinergic- and angiotensin (AII)-induced drinking was investigated pharmacologically. All substances were injected via SFO cannulae which did not rupture ventricular ependyma. Pretreatment with low doses of the muscarinic antagonist atropine abolished carbachol-induced drinking, while nicotinic antagonists had no effect. Nonetheless, pretreatment with much larger doses of atropine had no effect on AII-induced drinking. Similarly, relatively small doses of the AII antagonist, saralasin, blocked AII-induced drinking, yet a much larger dose of saralasin had no effect on carbachol-induced drinking. The receptors mediating cholinergic- and AII-induced drinking therefore cannot be in series and must be in parallel. A hypothesis is proposed to account for this independence and for the significance of the SFO cholinergic innervation.

Central receptor sites for angiotensin-induced drinking: a critical review

A review of proposed sites of the dipsogenic action of angiotensin II is presented. Techniques used for such localization are critically discussed, and it is suggested that convergence of evidence from several different experimental techniques is required for localization of dipsogenic receptors. Loci suggested as such sites of action include the preoptic regions, the subfornical organ, and the tissue proximal to the optic recess of the third ventricle, including the organum vasculosum laminae terminalis. Current evidence suggests that there are at least two loci within the forebrain that possess dipsogenic receptors for angiotensin II.