Dopaminergic blockade of the renin-angiotensin-aldosterone system: effect of high and low sodium intakes

Aldosterone biosynthesis, regulation, and classical mechanism of action

Circulating aldosterone is principally made in the glomerulosa zone of the adrenal cortex by a series of enzyme steps leading to the conversion of cholesterol to aldosterone. Uniquely, aldosterone's production is regulated at two critical enzyme steps: (1) early in its biosynthetic pathway (the conversion of cholesterol to pregnenolone cholesterol side chain cleavage enzyme) and (2) late (the conversion of corticosterone to aldosterone by aldosterone synthase). A variety of factors modify aldosterone secretion--the most important are angiotensin II (AngII), the end-product of the renin-angiotensin system (RAS), and potassium. However ACTH, neural mediators and natriuretic factors also contribute at least over the short run. Aldosterone's classical epithelial effect is to increase the transport of sodium across the cell in exchange for potassium and hydrogen ions. Although still controversial, there is an increasing body of data that supports the hypothesis that aldosterone can be synthesized in tissues outside of the adrenal cortex, specifically in the heart and the vasculature. Aldosterone's biosynthesis appears to be regulated in these tissues similar to what occurs in the adrenal cortex. The role of this extra adrenal aldosterone production in health and disease is as of yet undetermined.

Renal and neurohumoral effects of ibopamine and metoclopramide in normal man

The effects of 100 mg ibopamine, an orally active nonselective dopamine agonist on renal haemodynamics, sodium excretion, blood pressure (BP), heart rate (HR) and neurohumoral parameters were investigated in 10 healthy volunteers, with and without metoclopramide pretreatment. A small and temporary rise of glomerular filtration rate (GFR) was found after ibopamine without but not with metoclopramide pretreatment. No differences in effective renal plasma flow (ERPF) but a small rise in sodium excretion were observed comparing ibopamine with control. Metoclopramide induced a fall in sodium excretion which was not reversed by ibopamine. Ibopamine failed to affect BP and HR and no changes of PRA or plasma aldosterone concentration (PAC) were found. Metoclopramide induced a pronounced increase of PAC which was blunted by ibopamine. Plasma and urinary catecholamines were unchanged for all study days. We conclude that ibopamine induces natriuresis probably not by the observed small and temporary renal haemodynamic effects but by direct stimulation of DA1 dopamine receptors in the proximal tubule.

The role of dopamine in the control of corticosteroid secretion and metabolism

The relation between aldosterone and its trophins is altered by electrolyte status and in some hypertensive conditions in man by a mechanism or mechanisms not understood. Dopamine has been suggested as the agent for the altered sensitivity of plasma aldosterone to angiotensin II based on the results of studies with dopamine itself, both in vivo and in vitro, and with pharmacological agonists and antagonists. The evidence derived from these studies is presented and discussed. Questionable specificity of the agents used makes interpretation difficult. Similarly, dopamine infusion rates used in man and animals have resulted in plasma concentrations far in excess of those found normally and these pharmacological concentrations have been shown to alter both the clearance rate of exogenous angiotensin II, and the pattern of steroid response to ACTH. Direct study of adrenal tissue has provided more promising results. The adrenal cortex possesses specific dopamine receptors and dopamine has been shown to modify aldosterone biosynthesis in vitro. Moreover, dopamine is present in adrenocortical tissue in concentrations in the range calculated to operate the receptors. However, there is, as yet, no evidence that dopamine concentrations change in a physiological meaningful way, for example, during changes in sodium status.

Aldosterone response to metoclopramide is mediated through the autonomic nervous system in man

This study primarily examines the role of the autonomic nervous system in the aldosterone response to metoclopramide since there is conflicting evidence as to the involvement of a dopaminergic mechanism in this response. Six normal male volunteers in metabolic balance at 100 mmol sodium/day and 60 mmol potassium/day constant intake received metoclopramide, 10 mg i.v., on five different occasions. The dosing was either metoclopramide alone or combined with ganglionic, muscarinic, beta-adrenergic or calcium-channel blockade. Metoclopramide increased serum aldosterone significantly to 163.3% of basal level at 10 min. Atropine blunted this response and the 10 min level was significantly reduced to 116.03% of the basal value. The highest aldosterone levels were attained when metoclopramide was administered during a trimethaphan infusion and a peak of 292.8% of basal level occurred at 90 min. In the presence of atenolol, with or without nifedipine, the metoclopramide-induced aldosterone response was significantly greater at 15 min than with metoclopramide alone. The results of this investigation suggest that the aldosterone response to metoclopramide is mediated by acetylcholine released from post-ganglionic cholinergic nerve terminals, and that an adrenergic mechanism exerts a tonic inhibitory influence on aldosterone secretion in man.

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.

Changes in adrenal responsiveness and potassium balance with shifts in sodium intake

Dietary sodium modulates the aldosterone response to angiotensin, but available evidence does not indicate whether there is a gradual change in adrenal responsiveness with intermediate sodium intakes or a sharp shift from a low to a high responsive level at some threshold sodium intake. Nine normal subjects received angiotensin II infusions while in balance on five levels of sodium intake over two orders of magnitude, 3 to 300 mEq sodium per day. Basal plasma renin activity, plasma aldosterone and plasma angiotensin II concentrations gradually fell as dietary sodium intake increased. The adrenal was quite sensitive to the state of sodium balance since a shift in sodium intake of as little as 20 mEq (e.g. 10 to 30 mEq/day) induced a significant change in basal plasma aldosterone. Adrenal responsiveness to infused angiotensin varied inversely with the log of urinary sodium excretion over the entire range of sodium intake, indicating that there is a gradual modulation of adrenal responsiveness with changes in dietary sodium. In addition, despite a constant dietary potassium intake, serum potassium gradually fell from 4.55 +/- .06 to 3.98 +/- .07 mEq/l, (P less than 0.02), as sodium intake increased, partly due to increased urinary potassium excretion (mean cumulative potassium loss of 129 mEq). Thus, changes in potassium balance may be important in modulating the sodium-associated variation in aldosterone secretion.

Effects of dietary sodium on dopamine content of rat adrenal cortex

Dopamine occurs in the adrenal cortex and appears to provide maximum tonic inhibition of aldosterone secretion. In the present experiments, the effects of high or low sodium (Na+) intake on concentrations of dopamine in the adrenal cortex of adult male Sprague-Dawley rats were examined. In the first experiment, sham-operated and adrenal demedullated rats were provided with 1.5% NaCl or tap water for drinking for 5 days. In the second experiment, sham-operated and adrenal demedullated rats were fed regular laboratory chow (approximately 0.45% NaCl) or low Na+ chow (0.001-0.005% NaCl) for 5 days. High Na+ intake was attended by dramatic increases in fluid consumption, urine production and Na+ excretion. However, high Na+ intake did not affect levels of dopamine, norepinephrine or epinephrine in the adrenal glands. Approximately 45% of adrenal dopamine but less than 4% of either norepinephrine or epinephrine was localized in the adrenal cortex. In the second experiment, animals fed a diet low in Na+ had a significant reduction in Na+ excretion but levels of adrenal catecholamines were unaffected. Approximately 36% of adrenal dopamine but less than 5% of either norepinephrine or epinephrine was localized in the adrenal cortex. These findings suggest that adrenal cortical dopamine concentrations remain relatively constant in spite of presumed differences in turnover rates of this neurohormone.

Stimulation of aldosterone secretion by metoclopramide is not affected by chronic converting enzyme inhibition

To assess if dopaminergic control of aldosterone secretion is mediated by the renin-angiotensin system, the effect of chronic angiotensin converting enzyme inhibition by enalapril on the aldosterone response to metoclopramide has been studied in 10 patients with mild to moderate essential hypertension. Enalapril reduced supine blood pressure and increased the heart rate significantly. Plasma renin activity and urinary sodium excretion rose significantly. PRA was not changed by metoclopramide, neither during placebo nor during enalapril treatment. Metoclopramide induced a two-fold increase in plasma aldosterone, the peak response being reached within 15 min. Enalapril treatment did not alter the aldosterone response to metoclopramide. Dopaminergic control of aldosterone secretion appears to be independent of the renin-angiotensin system.