Effect of serotonin on plasma aldosterone in man

Primary Aldosteronism: Changing Definitions and New Concepts of Physiology and Pathophysiology Both Inside and Outside the Kidney

In the 60 years that have passed since the discovery of the mineralocorticoid hormone aldosterone, much has been learned about its synthesis (both adrenal and extra-adrenal), regulation (by renin-angiotensin II, potassium, adrenocorticotrophin, and other factors), and effects (on both epithelial and nonepithelial tissues). Once thought to be rare, primary aldosteronism (PA, in which aldosterone secretion by the adrenal is excessive and autonomous of its principal regulator, angiotensin II) is now known to be the most common specifically treatable and potentially curable form of hypertension, with most patients lacking the clinical feature of hypokalemia, the presence of which was previously considered to be necessary to warrant further efforts towards confirming a diagnosis of PA. This, and the appreciation that aldosterone excess leads to adverse cardiovascular, renal, central nervous, and psychological effects, that are at least partly independent of its effects on blood pressure, have had a profound influence on raising clinical and research interest in PA. Such research on patients with PA has, in turn, furthered knowledge regarding aldosterone synthesis, regulation, and effects. This review summarizes current progress in our understanding of the physiology of aldosterone, and towards defining the causes (including genetic bases), epidemiology, outcomes, and clinical approaches to diagnostic workup (including screening, diagnostic confirmation, and subtype differentiation) and treatment of PA.

Paracrine control of steroidogenesis by serotonin in adrenocortical neoplasms

Serotonin (5-hydroxytryptamine; 5-HT) is able to activate the hypothalamo-pituitary-adrenal axis via multiple actions at different levels. In the human adrenal gland, 5-HT, released by subcapsular mast cells, stimulates corticosteroid production through a paracrine mode of communication which involves 5-HT receptor type 4 (5-HT4) primarily located in zona glomerulosa. As a result, 5-HT is much more efficient to stimulate aldosterone secretion than cortisol release in vitro and administration of 5-HT4 receptor agonists to healthy individuals is followed by an increase in plasma aldosterone levels without any change in plasma cortisol concentrations. Interestingly, adrenocortical hyperplasias and tumors responsible for corticosteroid hypersecretion exhibit various cellular and molecular defects which tend to reinforce the intraadrenal serotonergic tone. These pathophysiological mechanisms, which are summarized in the present review, include an increase in adrenal 5-HT production and overexpression of 5-HT receptors in adrenal neoplastic tissues. Altogether, these data support the concept of adrenal serotonergic paracrinopathy and suggest that 5-HT and its receptors may constitute valuable targets for pharmacological treatments of primary adrenal diseases.

Role of 5-HT in the regulation of the brain-pituitary-adrenal axis: effects of 5-HT on adrenocortical cells

Serotonin (5-HT) plays a pivotal role in the regulation of the brain-pituitary-adrenal axis. In particular, 5-HT has been shown to control the activity of hypothalamic CRF neurons and pituitary corticotrope cells through activation of 5-HT1A and (or) 5-HT2A/2C receptor subtypes. 5-HT, acting through 5-HT2 receptors, can also trigger the renin-angiotensin system by stimulating renin secretion and consequently can enhance aldosterone production. At the adrenal level, 5-HT produced locally stimulates the secretory activity of adrenocortical cells through a paracrine mode of communication. The presence of 5-HT in the adrenal gland has been demonstrated immunohistochemically and biochemically in various species. In the frog, rat, and pig adrenal gland, 5-HT is synthesized by chromaffin cells, while in the mouse adrenal cortex, 5-HT is contained in nerve fibers. In man, 5-HT is present in perivascular mast cells. In vivo and in vitro studies have shown that 5-HT stimulates corticosteroid secretion in various species (including human). The type of receptor involved in the mechanism of action of 5-HT differs between the various species. In frogs and humans, the stimulatory effect of 5-HT on adrenocortical cells is mediated through a 5-HT4 receptor subtype positively coupled to adenylyl cyclase and calcium influx. In the rat, the effect of 5-HT on aldosterone secretion is mediated via activation of 5-HT7 receptors. Clinical studies indicate that 5-HT4 receptor agonists stimulate aldosterone secretion in healthy volunteers and in patients with corticotropic insufficiency and primary hyperaldosteronism. Local serotonergic control of corticosteroid production may be involved in the physiological control of the activity of the adrenal cortex as well as in the pathophysiology of cortisol and aldosterone disorders.Key words: HPA axis, renin-angiotensin system, adrenal gland, corticosteroid secretion, serotonergic receptors.

Regulation of plasma aldosterone levels by metoclopramide: a reappraisal of its mechanism from dopaminergic antagonism to serotonergic agonism

It has been thought, since the late 1970s, that dopamine exerts a tonic suppression of plasma aldosterone levels in human subjects. This action, however, had not been established directly using dopamine and dopamine mimetic drugs, which do not, in fact, affect the aldosterone levels. Rather, the conclusion was arrived at indirectly, based on the increase in aldosterone levels seen with dopamine receptor blockers; metoclopramide in particular, considered at the time of its discovery in the 1960s to be a new generation dopamine antagonist. However, metoclopramide is not a pure drug and in fact, shows intermediate affinity at certain serotonin receptor subtypes. Studies have been recently carried out in human subjects on the role of serotonergic transmission in mediating the metoclopramide as an aldosterone secretagogue effect. Here we briefly review this work and attempt to reassess the action of metoclopramide as an aldosterone secretagogue, from dopamine D2 antagonism to serotonin 5-HT4 partial agonism.

Serotoninergic stimulation of aldosterone secretion in vivo: role of the hypothalamo-pituitary adrenal axis

The control of aldosterone secretion in vivo by serotonin was studied in conscious rats. Serial blood samples were taken from indwelling arterial cannulae before and after i.p. administration of 1 ml (4 g/l) 5-hydroxytryptophan (5-HTP), the precursor of serotonin (5-HT), or saline, and analysed for 5-HTP, serotonin, 5-hydroxyindoleacetic acid, plasma renin activity (PRA), corticosterone, aldosterone, sodium and potassium concentration. The relative contribution of the hypothalamo-pituitary adrenal axis was investigated in animals pretreated with the synthetic glucocorticoid dexamethasone. 5-HTP caused a significant increase in all parameters within 45 min except for plasma sodium and potassium. Saline administration showed no significant effect. Dexamethasone pretreatment significantly impaired the corticosterone and aldosterone response to 5-HTP, although the aldosterone response was merely attenuated. No other parameter was affected by dexamethasone pretreatment. The results show that administration of 5-HTP, which increases serum serotonin levels, stimulates PRA, corticosterone and aldosterone secretion. Dexamethasone pretreatment inhibits the aldosterone response, though not completely, suggesting that the stimulatory action of 5-HTP involves the release of ACTH, which stimulates corticosterone and aldosterone secretion by the adrenal cortex. The failure of dexamethasone to block the aldosterone response completely, suggests the involvement of other mechanisms such as the renin-angiotensin system or a direct action of serotonin on the adrenal zona glomerulosa.

Benzamide derivatives provide evidence for the involvement of a 5-HT4 receptor type in the mechanism of action of serotonin in frog adrenocortical cells

We have previously shown that serotonin (5-HT) is a potent stimulator of corticosterone and aldosterone secretion by frog adrenocortical cells and we have demonstrated that the action of 5-HT is not mediated by the classical 5-HT receptor subtypes i.e. 5-HT1, 5-HT2 and 5-HT3. Recently, a non-classical 5-HT receptor (termed 5-HT4) has been characterized using 4-amino-5-chloro-2-methoxy-benzamide derivatives as serotonergic agonists. In the present report, we have investigated the possible involvement of the 5-HT4 receptor subtype in the mechanism of action of 5-HT on steroid secretion. Increasing concentrations of benzamide derivatives (zacopride, cisapride and BRL 24924) gave rise to a dose-related stimulation of corticosteroid production, zacopride being the most potent compound of this series to enhance steroidogenesis. Prolonged administration (230 min) of zacopride induced a rapid increase in corticosterone and aldosterone output followed by a gradual decline of corticosteroid secretion. During prolonged exposure of adrenal tissue to zacopride (10(-5) M), the corticotropic activity of 5-HT (10(-6) M) was totally abolished. The stimulatory effects of 5-HT and zacopride were abolished by the non-selective 5-HT3 antagonist ICS 205 930. In contrast methysergide, a 5-HT1 receptor antagonist, and MDL 72222, a selective 5-HT3 antagonist did not block zacopride-induced corticosteroid secretion. Both 5-HT and zacopride induced a dose-related increase in cAMP production by frog adrenal slices. Taken together, these results indicate that the stimulatory effect of 5-HT on frog adrenocortical tissue is mediated by activation of a 5-HT4 receptor subtype positively coupled to adenylate cyclase.

Effect of repeated doses of L-5-hydroxytryptophan and carbidopa on prolactin and aldosterone secretion in man

To evaluate changes in serum prolactin and plasma and urine aldosterone after a serotonergic challenge, 8 healthy men (19 to 42 yr), taking dexamethasone (0.75 mg qid), received the serotonin precursor L-5-hydroxytryptophan (L5HTP; 100 mg qid) with the peripheral decarboxylase inhibitor carbidopa (C; 50 mq qid) or matching placebos in a randomized, crossover manner. Serum prolactin concentration increased in all subjects after L5HTP/C in comparison to placebo, mean (SD) prolactin (ng/ml) at 8 h after dosing was 19.8 +/- 6.3 after L5HTP/C and 12.0 +/- 3.1 after placebo (p less than 0.05). In contrast, in comparison to values on placebo, L5HTP/C had no apparent effect on mean plasma concentration at all observation times; mean (SD) aldosterone (ng/dl) at 8 h after dosing was 12.0 +/- 5.1 and 12.0 +/- 3.8 after placebo (NS). Mean (SD) urinary aldosterone (micrograms/24 h), Na+(mEq/24 h) and K+(mEq/24 h) excretion were 7.0 +/- 4.4, 49.3 +/- 30.6, 30.1 +/- 11.2, after L5HTP/C and 7.4 +/- 5.8, 59.7 +/- 23.9, 33.3 +/- 7.4 after placebo (NS). Under these study conditions, subacute serotonergic stimulation with oral L5HTP/C resulted in prolactin but not aldosterone release.

Mechanism of action of serotonin on frog adrenal cortex

The mechanism of action of serotonin (5-HT) on frog adrenal cortex has been investigated in vitro using the perifusion system technique. The direct effect of 5-HT on corticosteroid secreting cells was demonstrated, using enzymatically dispersed adrenocortical cells. Melatonin and 5-HTP appeared to be less potent than 5-HT to enhance corticosteroid secretion. In contrast Trp and 5-HIAA were totally devoid of effect on steroid secretion. To investigate the type of receptor involved in the stimulatory effect of 5-HT on adrenocortical cells, adrenal slices were stimulated with 5-HT in absence or presence of various antagonists. We observed that classical antagonists of 5-HT1, 5-HT2 and 5-HT3 type receptors failed to block 5-HT-induced corticosteroid secretion in our model. These results show that 5-HT exerts a direct effect on corticosteroid-secreting cells. Our data also indicates that the type of receptor involved in the action of 5-HT in frog adrenal cortex differs from mammalian 5-HT receptors.

Effect of serotonin on cytosolic free calcium in adrenal glomerulosa and vascular smooth muscle cells

In view of its vasoconstricting action and of its stimulating effect on aldosterone biosynthesis, serotonin (5-hydroxytryptamine, 5-HT) could play a role in the genesis and/or maintenance of hypertension. The effects are mediated by different specific receptors whose transmembrane signaling system is not elucidated. We have used the fluorescent probe quin 2 to study the effect of 5-HT on cytosolic free calcium in enzymatically dispersed bovine adrenal glomerulosa cells and in cultured rat aortic smooth muscle cells. We also examined the effect of 5-HT on prostacyclin production by rat aortic smooth muscle cells. Serotonin did not modify the level of cytosolic free calcium in adrenal glomerulosa cells. In contrast, serotonin induced rapid, concentration-dependent (10(-8) -10(-5) M) rises of cytosolic free calcium in monolayers of cultured rat aortic smooth muscle cells, from a basal level of 153 +/- 27 nM to peak levels of about 400 nM. Ketanserin (10(-6) M), a specific 5-HT2-receptor antagonist completely blocked the free calcium rise induced by 5-HT. 5-HT induced a concentration-dependent increase in 6-keto-PGF1 alpha production in smooth muscle cells, which was suppressed by ketanserin, indomethacin or removal of calcium from the incubation medium. In contrast nifedipine (10(-6) M) did not modify the response to 5-HT while it abolished the response to vasopressin and did not modify the response to angiotensin II. We conclude that the 5-HT receptors of adrenal glomerulosa cells and vascular smooth muscle cells are linked to two distinct signalling systems which mediate the different biological responses.

Effects of ketanserin on cardiovascular, sympatho-adrenal and endocrine systems during physical exercise in man

Blood pressure, heart rate, oxygen consumption, plasma concentrations of catecholamines, renin, aldosterone and lactate were measured in 6 normotensive volunteers during a randomized cross-over study of oral ketanserin (20 mg X 7) and placebo; measurements were made at rest and during maximal dynamic exercise on a bicycle ergometer. At rest ketanserin reduced blood pressure without modifying heart rate or plasma noradrenaline and adrenaline. Duration of exercise and blood lactate levels did not differ between the ketanserin and the control group. During exercise only systolic blood pressure was significantly decreased on ketanserin at maximal work rate whereas heart rate did not change. Plasma noradrenaline was significantly increased and plasma aldosterone significantly decreased during exercise in ketanserin-treated subjects whereas plasma renin activity and plasma adrenaline remained unchanged. Finally, under ketanserin oxygen consumption during exercise was reduced. The results suggest that ketanserin might interfere with the sympathetic nervous system and aldosterone secretion in man.

Central serotonergic stimulation of aldosterone secretion

Serotonin stimulates aldosterone secretion both in vitro and in vivo, and serotonin antagonism decreases plasma aldosterone levels in patients with idiopathic aldosteronism. This study was designed to assess the effects of the serotonin precursor, 5-hydroxytryptophan (5HTP), upon aldosterone secretion in man, and to determine whether stimulatory effects of 5HTP are mediated through the central nervous system. Oral 5HTP, administered as a single 200-mg dose, increased plasma aldosterone levels from 4.7 +/- 0.6 to 13.3 +/- 2.8 ng/dl in dexamethasone-pretreated, normal volunteers. Peripheral inhibition of decarboxylation of 5HTP, achieved by pretreatment with carboxydopa, 25 mg three times daily for 3 d, significantly increased the stimulatory effects of 5HTP on aldosterone levels (P less than 0.001). No change in aldosterone levels occurred in subjects who received placebo after pretreatment with dexamethasone and carboxydopa. Increased aldosterone was not accompanied by increases in plasma levels of renin activity, potassium, or ACTH. Plasma levels of 5HTP were markedly increased by carboxydopa pretreatment, but peak plasma levels of serotonin were not significantly altered. Four patients with idiopathic aldosteronism all had an increase in plasma aldosterone levels after 5HTP administration, whereas the response in four patients with aldosterone-producing adenoma was variable. Incubation of isolated human and rat adrenal glomerulosa cells with serotonin resulted in increased aldosterone secretion by both sets of cells, whereas 5HTP was ineffective in stimulating aldosterone secretion in vitro. We conclude that central serotonergic pathways are involved in the stimulation of aldosterone induced by administration of 5HTP. This mechanism may be an important etiologic factor in the hypersecretion of aldosterone that occurs in patients with idiopathic aldosteronism.

Effect of metergoline on the aldosterone-stimulating properties of metoclopramide

The mechanism by which metoclopramide stimulates aldosterone secretion is still unclear, since it cannot be explained by known regulatory systems. On the other hand we have recently shown that serotonin is also a potent aldosterone-stimulatory substance in man and its action can be blunted by the antiserotoninergic drug, metergoline. To test if metoclopramide could act through a serotoninergic pathway we have studied the effect of metergoline on the aldosterone response to metoclopramide in 8 normal subjects. The effect of metoclopramide (10 mg i.v.) on plasma aldosterone, PRA, cortisol, prolactin and K was studied before and after acute metergoline administration. Metergoline pretreatment significantly reduced the aldosterone response to metoclopramide, together with a slight reduction of prolactin, whereas no changes were seen on PRA, cortisol and K. In vitro, metoclopramide induced a significant aldosterone output from human adrenal adenoma cells, which was partially reduced by metergoline and not by bromocriptine. Despite a lack of specificity of metergoline these data may suggest the hypothesis of a serotoninergic component in the aldosterone-stimulating properties of metoclopramide.