Role of Renal Prostaglandins in Sympathetically Mediated Renin Release in the Rat

Renal prostaglandins (PG) appear to mediate renin release due to stimulation of the intrarenal baroreceptor, but not that due to activation of the macula densa. However, as the role of PG in sympathetically mediated renin release remains unclear, a possible interrelationship between these factors was examined in conscious rats. Hydralazine increased the serum renin levels from 3.1+/-0.8 to 16.7+/-3.0 ng/ml per h at a dose of 1 mg/kg. Indomethacin (5 mg/kg) suppressed urinary PGE(2) and PGF(2alpha) excretion by 89 and 74%, respectively, arachidonate hypotension by 82%, and inhibited the elevated renin levels from hydralazine by 100% without altering the hypotensive effect of the drug. Another PG synthetase inhibitor, meclofenamate, was also effective in attenuating hydralazine-induced renin release, urinary PGE(2) and PGF(2alpha) excretion, and arachidonate hypotension. Isoproterenol, a nonselective beta-adrenergic agonist, increased heart rate, lowered blood pressure, and also stimulated the release of renin when administered intraperitoneally. However, intrarenal infusion of the drug only resulted in increased renin release. Indomethacin inhibited isoproterenol-induced renin release by 66 and 67%, respectively, without altering the hemodynamic effects associated with the intraperitoneal administration of the drug. The selective beta(1) agonist, H133/22, increased the release of renin and heart rate in a dose-related manner without altering blood pressure. H133/22-induced renin release was inhibited by 80% by indomethacin pretreatment. Finally, intrarenal infusions of dibutyryl cyclic AMP (3 mg/kg per min) increased the serum activity from 4.1+/-0.2 to 20.4+/-3.9 ng/ml per h without altering mean arterial pressure. Indomethacin inhibited this renin response to dibutyryl cyclic AMP by 96%. Thus, renal PG appear to be important mediators of sympathetically stimulated renin release acting as a site distal to the beta-adrenergic receptor.     

Abnormal Adrenal Responsiveness and Angiotensin II Dependency in High Renin Essential Hypertension

Adrenal responsiveness to angiotensin II (AII) and the diastolic blood pressure responses to saralasin were studied in 19 patients with high renin essential hypertension (HREH) on a 10-meq Na(+)/100 meq K(+) diet. The increment in plasma renin activity (PRA) between supine and upright positions was used as an estimate of the acute stimulation of the adrenal gland by endogenous AII; the normal increment in plasma aldosterone divided by the increment in PRA was >3.8. 7 of 19 had abnormal upright posture responses with significantly greater mean PRA increments (24+/-6 ng/ml per h) and significantly smaller plasma aldosterone increments 47 +/- 16 ng/dl) (P < 0.036) compared to the increments observed in HREH patients with normal adrenal responsiveness (PRA = 15 +/- 1 ng/ml per h; plasma aldosterone = 87 +/- 17 ng/dl). When AII was infused at doses of 0.1-3 ng/kg per min, only patients with normal posture responses had normal plasma aldosterone increments; plasma aldosterone levels failed to significantly increase even at the highest infusion rate in the patients with the abnormal upright posture responses. The AII competitive inhibitor, saralasin (0.3-30 mug/kg per min) was then infused to study the occurrence of angiotensinogenic hypertension in both HREH subgroups. The mean decline in diastolic blood pressure to saralasin in the subnormal adrenal responsive patients (-15 +/- 3 mm Hg) was significantly greater than in the normal adrenal responsive group (-3 +/- 2 mm Hg) (P < 0.02).It is concluded that patients with HREH are not a homogeneous population; approximately one-third have AII-dependent hypertension. In these patients, the mechanism responsible for the elevated renin and blood pressure could be a compensatory increase secondary to decreased adrenal responsiveness to AII. In the remainder, the high PRA levels have little, if any, causal role in the pathogenesis of the hypertension but could reflect a marker of other pathophysiologic processes.     

The effect of indomethacin and other anti-inflammatory drugs on the renin-angiotensin system.

The administration of two different doses of indomethacin, 9 and 18 mg/kg, to two different groups of rabbits was followed 6 h later by a significant decrease in plasma renin activity, and these levels were not increased by hemorrhage. The administration of 2 mg/kg of indomethacin did not alter the basal levels of plasma renin activity, but it was effective in diminishing the peripheral increase of renin produced by hemorrhage. Similar effects were obtained in other groups of rabbits treated with 9 mg/kg of meclofenamate or 18 mg or aspirin. The lowering effect of indomethacin on plasma renin activity is not specifically related to hemorrhage because it also prevented the increase in plasma renin activity elicited by 5 mg/kg of furosemide. Further studies showed that indomethacin did not exert any significant effect in vivo on the plasma level of renin substrate or on the generation of angiotensin from normal plasma by exogenous renin. And indomethacin did not interfere with the binding capacity of anti-angiotensin I for angiotensin I in the radioimmunoassay reaction or with the in vitro formation of angiotensin from hog renin-nephrectomized rabbit plasma reaction. The results thus indicate that the lowering effect of indomethacin on plasma renin activity is due to the interference with renal renin release. That this effect may be related to the blockade of prostaglandin synthesis is suggested by the similar effect exhibited by other blockers of prostaglandin synthesis.     

Atrial natriuretic peptide inhibits the effect of endogenous angiotensin II on plasma aldosterone in conscious sodium-depleted rats

Previous studies have shown that atrial natriuretic peptide (ANP) inhibits the secretion of aldosterone by isolated adrenal glomerulosa cells stimulated by angiotensin II, adrenocorticotropic hormone and potassium in vitro. We have also demonstrated that this inhibitory effect of ANP on plasma aldosterone induced by angiotensin II and adrenocorticotropic hormone can be reproduced in vivo in conscious unrestrained rats. In this study, we have investigated the effect of an intravenous infusion of ANP on plasma aldosterone in conscious unrestrained sodium-depleted rats. During sodium depletion, the rise in plasma renin activity which determines an increment in the circulating concentration of angiotensin II was accompanied by a rise in aldosterone secretion as expected. ANP infused intravenously at a dose which increased the plasma concentration of the peptide three- to five-fold, produced a significant decrement in the concentration of aldosterone in plasma after an infusion period of 120 min. There was no significant effect of ANP on plasma renin activity and plasma corticosterone concentration. Since the increase in plasma aldosterone levels in sodium-depleted rats is mainly dependent on the activation of the renin-angiotensin system, we conclude that ANP may modulate the effect of endogenous as well as exogenous angiotensin II on plasma aldosterone secretion.     

Specific action of the lipoxygenase pathway in mediating angiotensin II-induced aldosterone synthesis in isolated adrenal glomerulosa cells.

Angiotensin II (AII) in adrenal glomerulosa cells activates phospholipase C resulting in the formation of inositol phosphates and diacylglycerol rich in arachidonic acid (AA). Although glomerulosa cells can metabolize AA via cyclooxygenase (CO), this pathway plays little role in aldosterone synthesis. Recent evidence suggests that the lipoxygenase (LO) pathway may be important for hormonal secretion in endocrine tissues such as the islet of Langerhans. However, the capacity of the glomerulosa cell to synthesize LO products and their role in aldosterone secretion is not known. To study this, the effect of nonselective and selective LO inhibitors on AII, ACTH, and potassium-induced aldosterone secretion and LO product formation was evaluated in isolated rat glomerulosa cells. BW755c, a nonselective LO inhibitor dose dependently reduced the AII-stimulated level of aldosterone without altering AII binding (91 +/- 6 to 36 +/- 4 ng/10(6) cells/h 10(-4) M, P less than 0.001). The same effect was observed with another nonselective LO blocker, phenidone, and a more selective 12-LO inhibitor, Baicalein. In contrast U-60257, a selective 5-LO inhibitor did not change the AII-stimulated levels of aldosterone (208 +/- 11% control, AII 10(-9) M vs. 222 +/- 38%, AII + U-60257). The LO blockers action was specific for AII since neither BW755c nor phenidone altered ACTH or K+-induced aldosterone secretion. AII stimulated the formation of the 12-LO product 12-hydroxyeicosatetraenoic acid (12-HETE) as measured by ultraviolet detection and HPLC in AA loaded cells and by a specific RIA in unlabeled cells (501 +/- 50 to 990 +/- 10 pg/10(5) cells, P less than 0.02). BW755c prevented the AII-mediated rise in 12-HETE formation. In contrast, neither ACTH nor K+ increased 12-HETE levels. The addition of 12-HETE or its unstable precursor 12-HPETE (10(-9) or 10(-8) M) completely restored AII action during LO blockade. AII also produced an increase in 15-HETE formation, but the 15-LO products had no effect on aldosterone secretion. These studies suggest that the 12-LO pathway plays a key role as a new specific mediator of AII-induced aldosterone secretion.     

Functional studies of nonpeptide angiotensin II receptor subtype-specific ligands: DuP 753 (AII-1) and PD123177 (AII-2)

DuP 753 and PD123177 are two nonpeptide angiotensin II (AII)-specific ligands, which show high affinities for two respective and distinct subtypes of AII binding sites, i.e., AII-1 and AII-2 sites, respectively, in the rat adrenal gland, brain and uterus. The objective of this study is to identify the functions of these subtype binding sites in the adrenal, sympathetic ganglia, brain and vascular smooth muscle. In conscious rats, DuP 753 at 1, 3 and 10 mg/kg i.v. but not PD123177 at 30 and 100 mg/kg i.v. inhibited the AII-induced aldosterone increase. In the isolated perfused rat adrenal gland, DuP 753 at 10(-6) and 10(-4) M but not PD123177 at 10(-3) M blocked the AII-induced epinephrine secretion. In control and chemically sympathectomized pithed rats, the pressor and tachycardiac responses to AII were blocked by DuP 753 at 10 mg/kg i.v. but not by PD123177 at 100 mg/kg i.v. In conscious rats, DuP 753 at 10 mg/kg s.c. but not PD123177 at 100 mg/kg s.c. inhibited the AII-induced water drinking. In the rabbit aorta, DuP 753 at 10(-6) M but not PD123177 at 10(-4) M inhibited the contractile effect of AII. In conscious renal artery-ligated hypertensive rats, DuP 753 but not PD123177 at 0.1 to 10 mg/kg i.v. lowered blood pressure. In summary, a function of the PD123177-sensitive AII binding site (AII-2) has not yet been identified. However, the DuP 753-sensitive site (AII-1) appears to mediate the AII-induced responses such as adrenal aldosterone and catecholamine secretion, release of catecholamine from sympathetic ganglia, drinking and vasoconstriction.     

In vitro pharmacology of a nonpeptidic angiotensin II receptor antagonist, SC-51316.

The properties of a novel nonpeptidic angiotensin II (AII) receptor antagonist, 2,5-dibutyl-2,4-dihydro-4-([2-(1H-tetrazol-5-yl)(1,1'-biphenyl) -4'-yl]methyl)-3H-1,2,4-triazol-3-one (SC-51316), are described. SC-51316 inhibited [125I]AII binding selectively to the AT1 receptor with IC50 values of 3.6 and 5.1 nM in rat adrenal cortical and rat uterine membrane preparations, respectively. The compound was a competitive and reversible antagonist of AII-mediated contraction of rabbit aortic rings with a pA2 of 8.86. In addition, SC-51316 inhibited AII-induced aldosterone release from rat adrenal zona glomerulosa cells and blocked inhibition of renin release by AII from rat kidney slices with pA2 values of 8.62 and 8.9, respectively. The agent (0.1 mM) did not inhibit angiotensin-converting enzyme or plasma renin activity. These data demonstrate that SC-51316 is a potent AII receptor antagonist which may prove to be useful as a pharmacologic tool for studying the role of the renin-angiotensin system in cardiovascular diseases.     

Pharmacology of DuP 532, a selective and noncompetitive AT1 receptor antagonist

DuP 532 (2-propyl-4-pentafluoroethyl-1-[(2'-(1H-tetrazol-5-yl)bip hen yl- 4-yl)methyl]imidazole-5-carboxylic acid) inhibited the specific binding of [125I]angiotensin II (AII) for the subtype receptor AT1 in rat adrenal cortical membranes with an IC50 of 3.1 X 10(-9) M, but not the [125I]AII binding for the subtype AT2 sites in rat adrenal medulla tissues. It inhibited the contractile response to AII selectively and noncompetitively in the isolated rabbit aorta with a KB value of 1.1 X 10(-10) M. The selective AII antagonism was confirmed in the guinea pig ileum and the pithed rat. In conscious rats, DuP 532 inhibited the AII-induced pressor effect, aldosterone secretion, and water drinking induced by AII. In conscious renal hypertensive rats, DuP 532 decreased blood pressure with i.v. and p.o. ED30 of 0.02 and 0.21 mg/kg, respectively. The antihypertensive effect of DuP 532 at 0.3 to 3 mg/kg p.o. lasted for at least 24 hr. In conscious spontaneously hypertensive rats, DuP 532 given i.v. or p.o. at 0.3 to 3 mg/kg reduced blood pressure dose-dependently. DuP 532, at doses up to 100 mg/kg i.v., did not cause a pressor response in conscious normotensive rats, suggesting lack of agonism. DuP 532 exerted selective AII antagonism in conscious dogs. In conscious furosemide-treated dogs, DuP 532 given either at 0.3 and 1 mg/kg i.v. or at 1 to 10 mg/kg p.o. decreased blood pressure. As the AT1 receptors are responsible for AII-induced vasoconstriction, aldosterone secretion, and water drinking, our study indicates that DuP 532 is a potent, orally active, selective, and noncompetitive AT1 receptor antagonist and antihypertensive agent.     

Comparative effects of angiotensin II and angiotensin III in rabbit adrenal and aortic tissues

The addition of angiotensin II (AII) and angiotensin III (AIII) to isolated tissue baths produced the same maximal contractile response of rabbit aortic strips. AIII was about 10 times less potent, the slope of its concentration-response curve was less steep and its rate of onset slower than that of AII. The responses of both AII and AIII were inhibited with equal potency by the surmountable AII antagonist Phe4, Tyr8-AII and its unsurmountable analog Sar1, Leu8-AII but the kinetic patterns of inhibition by both were less well defined with the agonist AIII than with AII. The addition of AIII to tissues which had exhibited a maximal response to AII did not increase the level of contraction, in contrast to the case when norepinephrine was added to tissues contracted by AII. Both AII and AIII displaced [125I]AII binding from rabbit adrenal membranes; AIII was 6 times less potent than AII but displayed competitive kinetics as an inhibitor of [125I]AII binding. In further studies two binding sites for [125I]AII were identified in adrenal membranes, having KD values of 2.0 +/- 0.2 and 19.6 +/- 2.3 nM, respectively. Each site was inhibited by both AII and AIII and the ratio of the apparent Ki values for the two hormones was not significantly different. The Hill coefficient for the high affinity site was, however, lower for AIII than AII. We interpret our data to suggest that AII and AIII act on the same receptors. AIII apparently binds less efficiently than does AII in both rabbit adrenal membranes and rabbit aortic strips.     

The role of adrenal medulla in endogenous dopaminergic inhibition of aldosterone secretion

Evidence has accumulated that aldosterone secretion is under endogenous dopaminergic inhibition. To examine potential sources of the dopamine thus inhibitorily acting in the adrenal zona glomerulosa, the responsiveness of aldosterone, plasma renin activity, prolactin, and plasma catecholamines to haloperidol, a dopaminergic antagonist, was studied in rats 6 weeks after unilateral adrenalectomy (Group B), 6 weeks after unilateral adrenal demedullation followed by contralateral adrenalectomy 5 days later (Group C), and in controls without any pretreatment (Group A). In Group C, there were increases in basal levels of norepinephrine (P less than 0.01), prolactin (P less than 0.02), and aldosterone (P less than 0.01). Basal plasma renin activity was also increased (P less than 0.05), epinephrine concentrations were decreased. Two hours after haloperidol 1 mg/kg b.wt. i.p., aldosterone levels were increased in Groups A + B (P less than 0.01) but unresponsive in Group C. Haloperidol-induced stimulation of prolactin and norepinephrine was not impaired by the surgical procedures. Epinephrine levels were increased by haloperidol only in groups A + B (P less than 0.002). In none of the groups were plasma renin activity or dopamine levels influenced by haloperidol. It is concluded that dopaminergic inhibition of aldosterone production is brought about neither by circulating dopamine nor by potential dopaminergic nerves accompanying arterial blood supply of the adrenal cortex but by dopamine originating directly in adrenal medulla.     

The effect of metoclopramide and haloperidol on plasma renin activity and aldosterone levels in rats

To gain further information on dopaminergic inhibition of renin release and aldosterone secretion, we studied the effect of 0.1, 1.0, or 5.0 mg metoclopramide or haloperidol/kg body weight i.p. on plasma renin activity (PRA) and aldosterone concentration in serum, and of furosemide pretreatment or dietary sodium restriction (14 days) on PRA and aldosterone responses to 1.0 and 5.0 mg haloperidol/kg b.wt. i.p. in groups of eight male Sprague-Dawley rats. Aldosterone levels in serum were increased very similarly by 0.1 and 1.0 mg metoclopramide and haloperidol/kg. Whereas PRA and aldosterone were unaffected by 5.0 mg metoclopramide/kg, both were maximally stimulated by 5.0 mg haloperidol/kg. Furosemide pretreatment increased PRA and aldosterone concentration and blunted the aldosterone response to haloperidol. PRA response to 5.0 mg haloperidol/kg was not changed. After sodium restriction, aldosterone concentration as inappropriately high and did not respond to 1.0 mg haloperidol/kg. However, PRA and aldosterone response to 5.0 mg haloperidol/kg was magnified. Our study confirms both dopaminergic inhibition of PRA and stimulation of aldosterone secretion by dopamine antagonists in rats. A feedback regulatory mechanism becomes conceivable which comprises aldosterone secretion, sodium turnover, volume homeostasis, and dopaminergic activity.     

Angiotensin II induces reduced oxytocin but normal corticotropin release in rats with lesions of the subfornical organ

Summary— The subfornical organ (SFO) was suggested to be the site of the central nervous system which mediates the stimulatory effect of angiotensin II (AII) on corticotropin (ACTH) release. To verify this hypothesis, ACTH response to peripherally administered All was measured in rats with electrolytic lesion of the SFO. Increase in ACTH levels in response to AII (0.5 μg/kg or 2.0 μg/kg iv within 2 min) in conscious cannulated rats was dose-related and it was not affected by SFO lesion. The short infusion of AII (2.0 μg/kg) was enough to induce an elevation in plasma oxytocin. Oxytocin response to AII was reduced while that of aldosterone and blood pressure was not modified by SFO lesion. Our data show that an intact SFO is needed for a full response of oxytocin but not of ACTH release to peripherally injected AII.     

Hyperreninemic hypoaldosteronism after chronic stress in the rat.

The effects of chronic stress on the renin-angiotensin-aldosterone system were studied by analysis of plasma hormone levels, kidney renin mRNA levels, adrenal angiotensin II receptors, and steroidogenesis in rats subjected to repeated immobilization (2 h daily) or intraperitoneal injections of 1.5 M NaCI for 14 d. 24 after the last stress in both stress models, plasma aldosterone levels were reduced in spite of significant increases in plasma renin activity. Repeatedly intraperitoneal hypertonic saline-injected rats showed plasma renin activity responses to acute immobilization similar to controls, but markedly reduced plasma aldosterone responses. Concomitant with the increases in plasma renin activity, renin mRNA levels in the kidney were significantly increased in intraperitoneal hypertonic saline-injected rats, and these increases were prevented by beta-adrenergic receptor blockade with propranolol. In isolated adrenal glomerulosa cells from chronically stressed rats, maximum aldosterone responses to angiotensin II, ACTH, and 8-Br-cAMP were significantly decreased, whereas pregnenolone responses were increased. P450-aldosterone synthetase mRNA levels and binding of 125I-[Sar1,Ile8] angiotensin II were significantly reduced in the adrenal zona glomerulosa of stressed rats. These studies show that chronic repeated stress leads to renin stimulation due to sympathetic activation, and inhibition of aldosterone secretion due to inhibition of the late steroidogenic pathway. The data provide evidence for a role of chronic stress in the development of hyperreninemic hypoaldosteronism.     

Vasodilating antihypertensive drug-induced aldosterone release--a study of endogenous angiotensin-mediated aldosterone release in the rat.

The vasodilatory drugs, minoxidil and hydralazine, induce renin release in the rat, man and the dog. Previous reports suggest that the rat adrenal cortex was insensitive to angiotensin stimulation. As a result these studies were designed to obtain evidence for or against the hypothesis that the control of aldosterone release in the rat is unique among mammalian species. Minoxidil and hydralazine induced a time-related increase in both serum renin activity and serum aldosterone. Minoxidil caused a dose-related, proportional increase in serum renin and aldosterone. This response was blocked by prior bilateral nephrectomy but was not affected by hypophysectomy. A competitive angiotensin antagonist, saralasin (1-Sar-8-Ala angiotensin II), impaired minoxidil-induced aldosterone release in a dose-related manner while potentiating minoxidil-induced renin release. Pretreatment with propranolol, a beta adrenergic blocking drug, impaired minoxidil-induced renin and aldosterone release. Only small changes in serum corticosterone occurred after minoxidil or hydralazine administration. These results indicate that minoxidil-induced aldosterone release was mediated by the endogenous angiotensin II formed from renin release. They also support the unanesthetized rat as an appropriate animal model for study of the renin-angiotensin-aldosterone axis and its modification by drugs.     

Effect of atrial peptides on aldosterone production.

This study examines the effects of the synthetic atrial peptides (atriopeptin I, II, and III) on aldosterone and corticosterone production by rat adrenal cell suspensions. Furthermore, we studied the effect of atriopeptin II infusion on the plasma aldosterone response to angiotensin II in the rat in vivo. Atriopeptin I, II, and III decreased aldosterone release from zona glomerulosa cells in a dose-dependent fashion. 10 pM atriopeptin II inhibited basal aldosterone release significantly (P less than 0.01), and 10 nM atriopeptin II or III lowered it by 79%. Atriopeptin II decreased the sensitivity of the glomerulosa cells to adrenocorticotropic hormone (ACTH) and angiotensin II. Atriopeptin II had no effect on basal or ACTH-stimulated corticosterone release by fasciculata-medullary cells. In vivo infusions of angiotensin II with or without simultaneous infusions of atriopeptin II showed that atriopeptin II significantly inhibited the aldosterone response to angiotensin II. This inhibition by atriopeptin II was independent of any effect on plasma renin activity, serum potassium, or ACTH. These data raise the possibility that the atrial natriuretic peptides may affect sodium excretion by the kidney, not only directly, but also indirectly through the inhibition of aldosterone production.     

Defect in the sodium-modulated tissue responsiveness to angiotensin II in essential hypertension.

In normal subjects, dietary sodium intake modulates renovascular, adrenal, and pressor responses to infused angiotensin II (AII). To examine the hypothesis that this modulation is abnormal in some patients with essential hypertension, we studied 18 hypertensives and 9 normal subjects twice--during dietary sodium restriction and during loading. Paraaminohippurate (PAH) clearance was used to assess renal plasma flow. AII was infused in graded doses (0.3-3.0 ng/kg per min). Plasma aldosterone, cortisol, renin activity, AII, sodium, potassium, and PAH clearance were measured at the onset and end of each AII dose. During dietary sodium repletion, eight of the subjects with essential hypertension showed a normal renovascular response (greater than 125 ml/min per 1.73 m2) to AII infusion (3 ng/kg per min). The decrement in renal blood flow in these normal responders (NR) was 168 +/- 10, which was comparable to the range in normotensive subjects (206 +/- 25 ml/min per 1.73 m2). All of the remaining hypertensive patients, designated abnormal responders (AbR), had lower (less than 125) renal blood flow responses to the same dose of infused AII (mean decrement: 84 +/- 11 ml/min per 1.73 m2) compared with the NR and normotensive subjects. Renal blood flow responses to all AII doses were statistically greater on a high-vs.-low salt diet in the NR (P less than 0.001, chi-square) and normotensives (P = 0.004, chi-square) but sodium intake had no effect on this response in the AbR. Basal renal blood flow in NR increased significantly (P less than 0.001, paired t test) with dietary sodium repletion, from 491 +/- 36 (low salt) to 602 +/- 40 ml/min per 1.73 m2 (high salt), but was almost identical in the AbR on differing dietary sodium intakes (429 +/- 24 vs. 425 +/- 26 ml/min per 1.73 m2). The adrenal responses to sodium intake and infused AII also differed in the two subgroups. In the NR, the adrenal response to AII was significantly greater (P = 0.011, Wilcoxon signed rank test) after sodium restriction. In contrast, there was no significant difference in the aldosterone response to AII infusion between the low and high sodium diets in the AbR. Thus, a substantial subgroup of essential hypertensives has an abnormality in responsiveness to AII in two systems central to volume homeostasis: the kidney and adrenal. They fail to modulate their renal blood flow and aldosterone responses to AII with changes in dietary sodium intake. Moreover, basal renal blood flow does not increase appropriately with increased sodium intake. These abnormalities, which may be due to an increased local production of AII or a defect in the AII receptors in these three target tissues, could contribute to the elevated blood pressure.     

Nonpeptide angiotensin II receptor antagonists. I. Pharmacological characterization of 2-n-butyl-4-chloro-1-(2-chlorobenzyl)imidazole-5-acetic acid, sodium salt (S-8307)

2-n-Butyl-4-chloro-1-(2-chlorobenzyl)imidazole-5-acetic acid, sodium salt (S-8307) displaced [3H]angiotensin II (All) from its specific binding sites in rat adrenal cortical membranes with an IC50 of 4 x 10(-5) M. In rabbit aorta, S-8307 competitively inhibited the contractile response to All with a pA2 value of 5.49 but at 10(-4) M it did not alter the response to norepinephrine or KCI. Similarly, a specific AII antagonism was shown in vivo in the spinal pithed rat model. In anesthetized rats, S-8307 did not potentiate the bradykinin vasodepressor response. In renal artery-ligated rats, a high renin model, S-8307 decreased mean blood pressure at 10 and 30 mg/kg i.v. as well as at 100 mg/kg p.o. In anesthetized rats, furosemide enhanced the hypotensive effect of S-8307. Blockade of the renin-angiotensin system by captopril, saralasin or bilateral nephrectomy inhibited significantly but did not abolish completely the hypotensive effect of S-8307 in furosemide-treated rats. Inhibition of prostaglandin synthesis by indomethacin did not significantly reduce the hypotensive effect of S-8307. Our results identify S-8307 as a selective antagonist of AII receptors. However, at higher doses, mechanisms other than AII receptor blockade may partly account for its acute hypotensive effect.     

Stimulation of Aldosterone Biosynthesis in Adrenal Mitochondria by Sodium Depletion

The effect of various factors on the conversion of corticosterone to aldosterone was studied in an isolated mitochondrial system from rat adrenal glands. The adrenal mitochondrial fraction from rats on a low sodium diet has a greater capacity for converting corticosterone to aldosterone than mitochondria from rats fed a normal diet. After 1 day on a low sodium diet the amount converted was 162% and after the 2nd and 4th day the amounts converted were 239 and 242%, respectively, compared to a value of 100% for the control rats. Sodium and(or) potassium added in vitro did not affect the conversion of corticosterone to aldosterone.The specificity of the sodium depletion stimulus on the conversion of corticosterone to aldosterone was established by comparing two other mitochondrial enzymes from glomerulosa cell mitochondria. Succinic dehydrogenase and 11 beta-hydroxylase were measured in normal and sodium-depleted rats and no difference in activity of either enzyme was found.The data are consistent with the view that sodium depletion stimulates the last step in aldosterone biosynthesis by causing a specific enzymatic change in adrenal mitochondria.     

Unresponsiveness of plasma mineralocorticoids to angiotensin II in diabetic patients with asymptomatic normoreninemic hypoaldosteronism

offlated hypoaldosteronism with or without hyperkalemia in patients with diabetes mellitus has been shown to exist occasionally without hyporeninemia. To assess in detail the adrenal function in this disorder, the responses of plasma aldosterone (PA) and its precursor steroids to angiotensin II (AII) infusion and adrenocorticotropic hormone (ACTH) injection were studied in seven patients with asymptomatic normoreninemic hypoaldosteronism (ANH) and 11 age-matched normal subjects. The ANH diabetic patients had, by definition, a low PA level after furosemide (80 mg orally) plus upright posture (4 hours) stimulation, low PA and high plasma renin activity (PRA) increases after the stimulation (a low delta PA/delta PRA ratio), and normokalemia. Plasma inactive renin and the inactive renin/total renin ration were similar in the ANH diabetic patients and in the normal subjects. Under the pre-AII condition, plasma DOC and corticosterone levels tended to be low, and the plasma 18-OHB and PA levels were low in the ANH diabetic patients compared with the normal subjects. The ratio of plasma 18-OHB to PA was similar in the two groups. All infusion produced no increases in plasma 18-OHB and PA in the ANH diabetic patients, whereas the infusion caused dose-dependent increases in these steroids in the normal subjects. Plasma DOC and corticosterone levels remained unchanged during AII infusion in the two groups. ACTH injection produced appropriate PA increases relative to the basal PA in the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulation of angiotensin II receptors in the rat adrenal cortex by dietary electrolytes.

The binding affinity and concentration of specific angiotensin II receptor sites of rat adrenal cortical cells and homogenates were determined after 1 and 6 wk of altered sodium and potassium intake. Sodium deprivation caused marked increases in plasma renin, blood angiotensin II, and plasma aldosterone, and was accompanied by a significant increase (+74%) in the number of specific angiotensin II receptor sites per adrenal cortical cell. High potassium intake was followed by increased serum potassium and markedly elevated plasma aldosterone, with subnormal levels of renin and angiotensin II and a 170% increase in the number of angiotensin II receptors per cell after 1 wk. Sodium loading and potassium deprivation were followed by the opposite effect upon adrenal receptors, with reduction of the angiotensin II-binding capacity. None of the dietary electrolyte changes were accompanied by an ancrease in receptor affinity above the control value of 2 nM-1. A decrease in receptor affinity was noted after 6 wk of either low sodium or low potassium intake, when the renin and angiotensin II levels were increased by 104-129%. The adrenals of normal rats infused acutely with synthetic angiotensin II, or anesthetized with ether or sodium pentobarbital, which markedly increased plasma renin activity, contained fewer angiotensin receptors. These reductions in binding site concentration were not accompanied by changes in affinity and were attributed to occupancy by angiotensin II. These studies have demonstrated that chronic changes in sodium or potassium balance and acute changes in blood angiotensin II levels can exert modulating effects upon the adrenal content and/or affinity of specific receptor sites for angiotensin II.