Water and sodium intake of wild and New Zealand Rabbits following angiotensin

Two rabbit strains, New Zealand (laboratory) rabbits and Australian wild rabbits, both members of the Oryctolagus cuniculus genus were studied. New Zealand rabbits under control conditions consumed 2-5 times more water and 8-30 times more 0.5 M NaCl/kg body weight than wild rabbits. Single injections of angiotensin II or III administered ICV did not induce water drinking in either strain. Acute ICV infusion of angiotensin II also did not influence water intake, but after several days of administration, induced increased sodium intake. Intravenous infusion of graded doses of angiotensin II induced diuresis only at the higher doses in both strains. In New Zealand rabbits, this was accompanied by a commensurate and concurrent increase in water intake. Intravenous infusion of angiotensin II also induced urinary sodium loss that was either accompanied or followed by increased sodium intake. The development of salt appetite in both strains was preceded by sodium loss.     

Interaction between the natriuretic effects of renal perfusion pressure and the antinatriuretic effects of angiotensin and aldosterone in control of sodium excretion

Aldosterone has been recognized as an important sodium retaining hormone for many years. Recently we have demonstrated that angiotensin II has a much more powerful antinatriuretic effect than that of aldosterone. The importance of angiotensin II in regulation of sodium excretion has been observed in experiments in which angiotensin II has been infused intravenously or into the renal artery in acute and chronic situations, and in studies involving blockade of angiotensin II formation. In other experiments we have studied the effects of changes in renal perfusion pressure on sodium excretion. While earlier work by others indicated that an acute 10 mm Hg increase in perfusion pressure would increase sodium excretion 60%-70% we observed that a chronic 10 mm Hg change in perfusion pressure would result in a 300% change in sodium excretion. In view of evidence suggesting that changes in the ability of the kidney to excrete sodium normally at normal arterial pressure is an important element in hypertension we studied the effects of aldosterone and angiotensin II on arterial pressure regulation in normal dogs. High physiological levels of each hormone were infused intravenously for several weeks. Both produced sustained hypertension. Aldosterone hypertension was a typical volume loading type with sodium retention, increased blood volume and extracellular fluid volume and a slow rise in arterial pressure. Angiotensin hypertension was a typical vasoconstrictor type with high peripheral resistance, normal or decreased blood volume, decreased cardiac output, a rapid rise in arterial pressure and only initial sodium retention.(ABSTRACT TRUNCATED AT 250 WORDS)     

The adrenal and vascular effects of angiotensin II and III in sodium depleted rats

The effects of angiotensin II and angiotensin III on mean arterial pressure, serum aldosterone, and serum corticosterone were studied in normal and sodium depleted, conscious rats. In normal rats, angiotensin III was 76% (p > 0.10) as potent as angiotensin II on aldosterone release but only 31% (p < 0.001) as potent on blood pressure. Following sodium depletion, the pressor responses to both angiotensin II and III were reduced (p < 0.001) (65% and 86% respectively). In addition, the release of aldosterone by both peptides was potentiated by sodium depletion as indicated by an increase in the slope of the dose-response curves. However, in the sodium depleted rats, angiotensin III was only 20% (p < 0.001) as potent as angiotensin II in stimulating aldosterone release. Small changes in serum corticosterone were noted following infusions of both peptides, but unlike the case with aldosterone, sodium depletion did not alter the serum corticosterone responses to the peptides. These $\text{in}$$\text{vivo}$ experiments taken with $\text{in}$$\text{vitro}$ studies support the interpretation that angiotensin III could function to control aldosterone release in altered sodium states either as a circulating hormone if present in concentrations far in excess of those of angiotensin II or as a local hormone formed in the adrenal from angiotensin II.     

Increased sodium intake is somehow induced in rats by intravenous angiotensin II

Adult male rats maintained on dry food, water, and 3% NaCl solution received continuous infusion of angiotensin II (A II) via right atrial catheters. A II was delivered in 0.315 ml/hr at doses of 15, 30, and 60 ng/min/rat for 3 to 5 days. At the higher doses mean daily salt solution intake rose from very low preinfusion levels to 18.7 and 19.6 ml, respectively. Daily water intakes increased in some animals and decreased in others on the first day of infusion but were double preinfusion levels by the second day. Persistence of the sodium appetite after the end of A II infusion was seen in most of the rats which received the highest dose. The mechanisms which might underlie the effect of blood-borne A II on sodium intake are discussed and three possibilities considered: (1) that A II may act on the brain to stimulate sodium appetite, (2) that A II may act via the adrenal cortex, and that the sodium appetite may arise as a result of increased plasma levels of adrenal steroids, and (3) that A II may act via the kidney, producing a natriuresis. According to this view, sodium appetite would arise as a result of loss of body sodium and/or blood volume.     

Effect of metoclopramide on adrenal secretion in sheep: influence of dexamethasone and sodium intake

Metoclopramide, a competitive dopamine antagonist, stimulates aldosterone in man and monkey without affecting cortisol secretion. In sheep, metoclopramide also stimulates aldosterone but ist action on adrenocortical secretion is more controversial. To clarify the action of metoclopramide in conscious sheep, the response of plasma aldosterone, cortisol, angiotensin II and potassium were studied after 0.16 and 0.64 mg/kg metoclopramide, with and without pretreatment with dexamethasone. The effect of sodium status on the response was also studied by repeating the experiments after 7 days of dietary sodium restriction. In the absence of dexamethasone, plasma aldosterone was significantly increased by metoclopramide in both sodium-replete and restricted sheep. In sodium-replete sheep, plasma cortisol was also increased by 0.64 mg/kg, and by both doses when salt-restricted. However all cortisol responses were completely suppressed by dexamethasone pretreatment. Dexamethasone also suppressed the aldosterone response to metoclopramide in sodium-replete but not in sodium-restricted sheep where significant responses of aldosterone to both doses of metoclopramide still occurred without changes in plasma angiotensin II or potassium. While a nonspecific stress effect of metoclopramide can contribute to the aldosterone response, these results show that the sheep's adrenal glomerulosa is capable of responding to metoclopramide without change in ACTH, angiotensin or potassium.     

19-hydroxyandrostenedione and 6 beta-hydroxyandrostenedione: new steroids regulated by the renin-angiotensin system in man

The changes of plasma 19-hydroxyandrostenedione (19-OH-A-dione) and 6 beta-hydroxyandrostenedione (6 beta-OH-A-dione) during the infusion of angiotensin II were evaluated and were compared with those of plasma aldosterone in man. Angiotensin II was infused into 5 normal subjects with an infusion pump at rates of 0.5, 1.0, 2.0 and 4.0 ng/kg per min. Each dose was infused for 20 min. Plasma 19-OH-A-dione rose significantly following the infusion of angiotensin II at a rate of 0.5 ng/kg per min and plasma 6 beta-OH-A-dione rose significantly following the infusion of angiotensin II at a rate of 1.0 ng/kg per min. In contrast, plasma aldosterone did not change significantly until the infusion rate reached 4.0 ng/kg per min. These results indicate that the secretion of 19-OH-A-dione and 6 beta-OH-A-dione is under the control of angiotensin II and the release of 19-OH-A-dione and 6 beta-OH-A-dione is induced earlier by the smaller doses of angiotensin II prior to the secretion of aldosterone. As 19-OH-A-dione and 6 beta-OH-A-dione amplify the action of aldosterone in bioassays using adrenalectomized rats and work as sodium-retaining and hypertensinogenic agents in intact rats, they are newly recognized biologically active steroids which are regulated by the renin-angiotensin system in man.     

Neuroendocrine factors in salt appetite.

We dedicate this paper to Curt P. Richter, father of the study of salt appetite, who died recently at the age of 94. Richter first demonstrated that the adrenalectomized rat's voracious appetite for salt kept it alive (1936) and showed the same in humans (1940). Our first paper in 1955 demonstrated that salt appetite was an innate response to salt depletion. Since then, we have pursued the notion that the neuroendocrine consequences of sodium depletion create a brain state that raises salt appetite. In Epstein's laboratory, it was shown that angiotensin and aldosterone, the hormones of salt retention in the periphery, act synergistically in the brain to produce salt appetite in the rat. Block either hormone and the appetite is reduced by half; block both and the appetite is eliminated despite severe bodily need. With repeated depletions or treatments of the brain with angiotensin and aldosterone, salt ingestion increases, reaching an asymptote by the third depletion. Need-free intake of NaCl also increases, especially in female rats which ingest more NaCl than male rats. In Stellar's laboratory, running speed to salt solutions in a runway is used as a measure of salt appetite. When the appetite is raised with large doses of DOCA, a mimic of aldosterone, rats run rapidly for a taste of strong salt solutions as high as 24% (almost 4 molar). Using ingestion as a measure, the role of the atrial natriuretic peptide (ANP), an antagonist of angiotensin's physiological effect, was investigated as a modulator of salt appetite. When angiotensin is involved is producing salt appetite, following sodium depletion by a diuretic combined with a low-salt diet, ANP reduced salt intake by 40%. When salt appetite was raised by DOCA, however, ANP either had no effect or reduced salt ingestion by only 10%. The subfornical organ, the lateral preoptic area, and the central and medial nuclei of the amygdala are being investigated as major components of the limbic circuit underlying salt appetite produced by the actions of angiotensin, aldosterone and ANP in the brain.     

Dopaminergic modulation of aldosterone secretion: effect of sodium balance and postural changes

The influence of an increased endogenous production of angiotensin II and of sodium homeostasis upon the response of plasma aldosterone to metoclopramide administration has been investigated in 5 normal volunteers. Our results show that the increase of plasma aldosterone after metoclopramide administration is independent of angiotensin II, ACTH and potassium, and that it increases even further due to the endogenous production of angiotensin II induced by postural changes. The state of sodium balance seems to influence the response of plasma aldosterone to metoclopramide administration as it occurs with other stimuli of aldosterone secretion.     

What makes wild rabbits drink?

Wild rabbits Oryctolagus cuniculus (L) introduced to Australia over a century ago successfully colonized diverse environments in a large part of the continent varying from arid desert, alps, to lush grasslands and coastline where water and salt may be either abundant or very scarce. Wild rabbits caught in Northern Victoria were studied under laboratory conditions, where they adapted to dry pelleted food and drank regularly water and a cafeteria of electrolyte solutions offered. Intracerebroventricular (IVT) infusion of angiotensin II (AII) in doses 10, 50 and 500 ng/h did not increase their water drinking, but increased salt appetite, although it was delayed one or more days after the beginning of AII infusion. IVT infusion of AII 500 ng/h for one day caused a halving in water intake and a tenfold increase in sodium excretion. These were followed by compensatory changes in water and 0.5 M NaCl intake on the consecutive days. IVT infusion of AII 50 ng/h for one day induced an increased urinary sodium excretion, a negative sodium balance which was not followed by an increased salt appetite. IVT infusion of AII 10 ng/h for five days caused a progressive increase in sodium excretion and salt appetite which were significant on the fourth day of infusion and both remained eight-ten times greater than control levels for three days after the cessation of infusion. Water intake was unchanged. IVT infusion of 0.3 M Na-CSF for two days reduced water and food intake, and caused a negative sodium balance on the second day of infusion which was not followed by increase in salt appetite.(ABSTRACT TRUNCATED AT 250 WORDS)     

Atrial natriuretic peptide inhibits water and sodium intake in rabbits

The effect of atrial natriuretic peptide (ANP) on water and sodium intake was investigated in wild rabbits, a species which does not drink water following i.c.v. or i.v. administration of angiotensin II but develops sodium appetite following i.c.v. infusion of angiotensin II. ANP was given during or after depletion of extracellular fluid volume: hemorrhage, fluid deprivation and administration of furosemide. Systemically administered ANP reduced the water, but not the sodium intake of wild rabbits. I.c.v. administration of ANP inhibited both water and sodium intake. The suppression of thirst following both i.v. and i.c.v. administration of ANP indicates that inhibition of the effect of angiotensin II is not the exclusive mechanism and the circumventricular organs are probably not the exclusive sites of action for ANP. The inhibition of sodium appetite in wild rabbits was consistent with earlier proposals that ANP acts through the inhibition of the effects of angiotensin II. Reduction of food intake coincident with administration of ANP was also noted, but dose-dependent decrease was not observed.     

The central role of the brain aldosterone–“ouabain” pathway in salt-sensitive hypertension

Na⁺-transport regulating mechanisms classically considered to reflect renal control of sodium homeostasis and BP, i.e. aldosterone–mineralocorticoid receptors (MR)—epithelial sodium channels (ENaC)—Na⁺/K⁺-ATPase have now been demonstrated to also be present in the central nervous system. This pathway is being regulated independently of the peripheral/renal pathway and contributes to regulation of cerebrospinal fluid [Na⁺] by the choroid plexus, of brain tissue [Na⁺] by the ependyma and to neuronal responses to e.g. Na⁺ or angiotensin II. Increases in CSF [Na⁺] by central infusion of Na⁺-rich aCSF or by high salt intake in Dahl S or SHR cause sympatho-excitation and hypertension. These responses appear to depend on activation of a CNS cascade starting with aldosterone–MR–ENaC–“ouabain,” the latter lowering neuronal membrane potential leading to enhanced angiotensin II release in e.g. the PVN. Specific CNS blockade of any of the steps in this cascade from aldosterone synthase blockade to AT₁-receptor blockade prevents the sympathetic hyperactivity and hypertension on high salt intake, irrespective of the presence of a “salt-sensitive kidney.” We propose that in salt-sensitive hypertension an increase in CSF [Na⁺] causes a local increase in aldosterone biosynthesis which activates an aldosterone dependent neuromodulatory pathway which enhances activity of angiotensinergic sympatho-excitatory pathways leading to hypertension.     

Propranolol induces acute natriuresis by beta blockade and dopaminergic stimulation.

dl-Propranolol (0.8-1.6 mg/kg - h for 1 h) produced a transient two- to three-fold increase in sodium excretion in nondiuretic rats infused with Pitressin and aldosterone and in water diuretic rats. Sodium excretion increased more in rats depleted of renin by chronic Doca and salt administration than in rats maintained on a low salt diet. An angiotensin inhibitor (1,sarcosine-8,valine angiotensin II) decreased sodium excretion. Therefore the natriuresis was not mediated by antidiuretic hormone, aldosterone, or renin-angiotensin. d-Propranolol did not produce a natriuresis. Prior treatment with phenoxybenzamine did not prevent the natriuretic response but chlorisondamine pretreatment did. The natriuresis is produced by beta blockade and requires post ganglionic nerve function but is independent of alpha receptors. dl-Propranolol decreased heart rate and cardiac output but systemic pressure did not fall and renal blood flow increased. This suggests a dopamine-mediated renal vasodilation and natriuresis. Haloperidol and pimozide, both dopamine blocking agents with minimal beta blocking effects, prevented the natriuretic response. We conclude that propranolol may increase sodium excretion directly by blocking beta receptors in the distal nephron and indirectly by dopamine-mediated renal vasodilation.     

Enhanced water and salt intake in transgenic mice with brain-restricted overexpression of angiotensin (AT1) receptors

To address the relative contribution of central and peripheral angiotensin II (ANG II) type 1A receptors (AT(1A)) to blood pressure and volume homeostasis, we generated a transgenic mouse model [neuron-specific enolase (NSE)-AT(1A)] with brain-restricted overexpression of AT(1A) receptors. These mice are normotensive at baseline but have dramatically enhanced pressor and bradycardic responses to intracerebroventricular ANG II or activation of endogenous ANG II production. Here our goal was to examine the water and sodium intake in this model under basal conditions and in response to increased ANG II levels. Baseline water and NaCl (0.3 M) intakes were significantly elevated in NSE-AT(1A) compared with nontransgenic littermates, and bolus intracerebroventricular injections of ANG II (200 ng in 200 nl) caused further enhanced water intake in NSE-AT(1A). Activation of endogenous ANG II production by sodium depletion (10 days low-sodium diet followed by furosemide, 1 mg sc) enhanced NaCl intake in NSE-AT(1A) mice compared with wild types. Fos immunohistochemistry, used to assess neuronal activation, demonstrated sodium depletion-enhanced activity in the anteroventral third ventricle region of the brain in NSE-AT(1A) mice compared with control animals. The results show that brain-selective overexpression of AT(1A) receptors results in enhanced salt appetite and altered water intake. This model provides a new tool for studying the mechanisms of brain AT(1A)-dependent water and salt consumption.     

Effect of dietary sodium intake on the pressor reactivity to angiotensin II in rats with experimental cirrhosis of the liver

The present experiments were designed to evaluate vascular reactivity to angiotensin II in rats with experimental cirrhosis of the liver (induced with CCl4 and phenobarbital) before ascites appearance. The systemic pressor response to angiotensin II in conscious animals and the contractile effect of angiotensin II in isolated femoral arteries were studied. In addition, the effect of high sodium intake on these parameters was also analyzed. Both renin and aldosterone plasma concentrations were similar in control and cirrhotic rats on the normal or on the high sodium diet. Basal mean arterial pressure was higher in control rats than in cirrhotic rats on the normal sodium (116 +/- 4 vs. 101 +/- 4 mmHg (1 mmHg = 133.3 Pa), p less than 0.05) or on the high sodium diet (118 +/- 7 vs. 98 +/- 6 mmHg). No differences in plasma renin activity or plasma aldosterone were found between control and cirrhotic rats. Upon injection of angiotensin II, control rats show a dose-dependent increase in mean arterial pressure which is higher in high sodium than in normal sodium rats. Cirrhotic rats showed a lower hypertensive response to angiotensin II than their corresponding control rats. In addition, no difference between pressor responses to angiotensin II was observed when normal sodium and high sodium cirrhotic rats were compared. On application of angiotensin II, femoral arteries of control and cirrhotic rats exhibited a dose-dependent contraction. However, maximal contraction was higher in high sodium control rats (145 +/- 12 mg) than in normal sodium control rats (99 +/- 6 mg, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Sodium pumps,ouabain and aldosterone in the brain: A neuromodulatory pathway underlying salt-sensitive hypertension and heart failure

Accumulating evidence obtained over the last three decades has revealed a neuroendocrine system in the brain that mediates long term increases in blood pressure. The system involves distinct ion transport pathways including the alpha-2 isoform of the Na,K pump and epithelial sodium channels, as well as critical hormone elements such as angiotensin II, aldosterone, mineralocorticoid receptors and endogenous ouabain. Activation of this system either by circulating or central sodium ions and/or angiotensin II leads to a cascading sequence of events that begins in the hypothalamus and involves the participation of several brain nuclei including the subfornical organ, supraoptic and paraventricular nuclei and the rostral ventral medulla. Key events include heightened aldosterone synthesis and mineralocorticoid receptor activation, upregulation of epithelial sodium channels, augmented synthesis and secretion of endogenous ouabain from hypothalamic magnocellular neurons, and sustained increases in sympathetic outflow. The latter step depends upon increased production of angiotensin II and the primary amplification of angiotensin II type I receptor signaling from the paraventricular nucleus to the rostral ventral lateral medulla. The transmission of sympathetic traffic is secondarily amplified in the periphery by increased short- and long-term potentiation in sympathetic ganglia and by sustained actions of endogenous ouabain in the vascular wall that augment expression of sodium calcium exchange, increase cytosolic Ca²⁺ and heighten myogenic tone and contractility. Upregulation of this multi-amplifier system participates in forms of hypertension where salt, angiotensin and/or aldosterone are elevated and contributes to adverse outcomes in heart failure.     

Captopril in renovascular hypertension: long-term use in predicting surgical outcome.

The angiotensin converting-enzyme inhibitor captopril was used as long-term preoperative treatment in a series of hypertensive patients with unilateral renal arterial disease. There were immediate and sustained falls in plasma angiotensin II and aldosterone concentrations, with converse increases in circulating renin and angiotensin I. In patients with sodium and potassium deficiency and secondary aldosterone excess before treatment captopril corrected the sodium and potassium deficits; in these cases the initial hypotensive response was profound but the later effect was less pronounced. When sodium and potassium state was initially normal it remained unchanged during captopril treatment, while the full hypotensive effect took up to three weeks to be attained. The immediate, but not long-term, falls in arterial pressure with captopril were proportional to the immediate decrements of plasma angiotensin II. Nevertheless, while the immediate blood-pressure reduction with captopril variously overestimated and underestimated the eventual surgical response, the absolute blood-pressure values during long-term captopril related well with those after operation. Pretreatment plasma renin and angiotensin II concentrations, while closely predicting the immediate captopril response, are fallible guides to surgical prognosis. In contrast, long-term treatment with converting-enzyme inhibitors may provide an accurate indication of surgical outcome.     

Monophasic and biphasic effects of angiotensin II and III on norepinephrine uptake and release in rat adrenal medulla.

Angiotensin II and III have hypertensive effects. They induce vascular smooth muscle constriction, increase sodium reabsorption by renal tubules, stimulate the anteroventral third ventricle area, increase vasopressin and aldosterone secretions, and modify catecholamine metabolism. In this work, angiotensin II and III effects on norepinephrine uptake and release in rat adrenal medulla were investigated. Both angiotensins decreased total and neuronal norepinephrine uptake. Angiotensin II showed a biphasic effect only on evoked neuronal norepinephrine release (an earlier decrease followed by a later increase), while increasing the spontaneous norepinephrine release only after 12 min. On the other hand, angiotensin III showed a biphasic effect on evoked and spontaneous neuronal norepinephrine release. Both angiotensins altered norepinephrine distribution into intracellular stores, concentrating the amine into the granular pool and decreasing the cytosolic store. The results suggest a physiological biphasic effect of angiotensin II as well as angiotensin III that may be involved in the modulation of sympathetic activity in the rat adrenal medulla.     

Human alpha atrial natriuretic peptide inhibits angiotensin stimulated aldosterone biosynthesis in bovine adrenal glands

The behaviour of aldosterone output was evaluated in isolated and superfused bovine adrenal glands during superfusion with human alpha atrial natriuretic peptide on its own or with angiotensin II or a antagonist dopaminergic drug: metoclopramide. H alpha-ANP even in high concentrations did not reduce the basal amount of aldosterone released from bovine adrenal glands, nor did it modify aldosterone response to metoclopramide, but it partially inhibited aldosterone stimulation by angiotensin II. These data suggest that atrial natriuretic factor may affect sodium secretion through the modulation of aldosterone secretion.     

RALES, EPHESUS and redox

In RALES, low doses of the mineralocorticoid receptor (MR) antagonist spironolactone, added to standard of care for severe heart failure, improved survival by 30% and lowered hospitalization by 35%. Animal studies with the selective MR antagonist eplerenone have similarly shown MR blockade to prevent the cerebral, renal and coronary vascular inflammatory response to elevated aldosterone levels. There is now general acceptance that aldosterone concentrations inappropriate for salt status have major deleterious effects on the cardiovascular system.

In many instances, however (e.g. Randomized Aldactone Evaluation Study (RALES), EPHESUS) aldosterone levels are normal and salt status unremarkable and yet MR blockade has unquestioned benefits. In these instances, there is increasing evidence that coronary and cardiac MR are activated by normal circulating cortisol levels, in the cellular context of generation of reactive oxygen species (ROS) and/or alteration in intracellular redox status.

MR in VSMC and cardiomyocytes are normally predominantly occupied by cortisol in tonic inhibitory mode. Blockade of 11β hydroxysteroid dehydrogenase type II (11βHSD2) or ROS generation both serve to activate cortisol–MR complexes, thus mimicking the effects of mineralocorticoid/salt imbalance on blood vessels and the heart. In RALES and EPHESUS, it is likely that the antagonists are blocking normal levels of cortisol, not aldosterone, from activating MR in the context of tissue damage and ROS generation. If this is the case, MR antagonists may be of wide therapeutic potential in cardiovascular disease and not confined to those characterized by aldosterone/salt excess. Finally, the pathophysiologic roles of always-occupied MR in ‘unprotected’ tissues such as cardiomyocytes or neurons in response to altered intracellular redox status remain to be explored.     

Mechanisms of enhanced angiotensin II–stimulated signal transduction in vascular smooth muscle by aldosterone

We tested the hypothesis that mineralocorticoids potentiate angiotensin II–stimulated phospholipase C activation through an increased number of angiotensin II receptors in cultured rat aortic vascular smooth muscle cells. Exposure of cells to aldosterone for 24 h resulted in concentration-dependent increases in angiotensin II receptor binding. Via studies of angiotensin II displacement by non-peptide receptor antagonists, both basal and upregulated angiotensin II receptors were found to be of the AT₁, subtype. Incubation with 1 μM aldosterone resulted in 50%–100% enhancement of angiotensin II (100 nM)–stimulated diacylglycerol formation and intracellular calcium mobilization. Exposure to 100 nM 1,25-(OH)₂ VitD₃, which did not upregulate angiotensin II receptors, did not potentiate stimulated inositol phosphate formation. Incubation with aldosterone resulted in potentiation of inositol phosphate formation upon receptor occupation (100 nM angiotensin II) but not upon post-receptor stimulation (25 mM NaF/10 μM AlCl₃). Aldosterone did not increase basal phospholipase C activity or content of the inositol trisphosphate precursor phosphatidylinositol-4,5-bisphosphate. These data are consistent with the hypothesis that aldosterone potentiates angiotensin II–stimulated, phospholipase C-dependent intracellular signals solely by coupling to an increased number of angiotensin II receptors. This mechanism may contribute to the sensitized vascular responses to angiotensin II observed in states of mineralocorticoid excess. © 1994 Wiley-Liss, Inc.