Effect of cisapride, a serotonin-4 receptor agonist, on aldosterone secretion: absence of age-related change

5-Hydroxytryptamine receptors in the human cardiovascular system

The human cardiovascular system is exposed to plasma 5-hydroxytryptamine (5-HT, serotonin), usually released from platelets. 5-HT can produce harmful acute and chronic effects. The acute cardiac effects of 5-HT consist of tachycardia (preceded on occasion by a brief reflex bradycardia), increased atrial contractility and production of atrial arrhythmias. Acute inotropic, lusitropic and arrhythmic effects of 5-HT on human ventricle become conspicuous after inhibition of phosphodiesterase (PDE) activity. Human cardiostimulation is mediated through 5-HT4 receptors. Atrial and ventricular PDE3 activity exerts a protective role against potentially harmful cardiostimulation. Chronic exposure to high levels of 5-HT (from metastatic carcinoid tumours), the anorectic drug fenfluramine and its metabolites, as well as the ecstasy drug 3,4-methylenedioxymethamphetamine (MDMA) and its metabolite 3,4-methylenedioxyamphetamine (MDA) are associated with proliferative disease and thickening of cardiac valves, mediated through 5-HT2B receptors. 5-HT2B receptors have an obligatory physiological role in murine cardiac embryology but whether this happens in humans requires research. Congenital heart block (CHB) is, on occasion, associated with autoantibodies against 5-HT4 receptors. Acute vascular constriction by 5-HT is usually shared by 5-HT1B and 5-HT2A receptors, except in intracranial arteries which constrict only through 5-HT1B receptors. Both 5-HT1B and 5-HT2A receptors can mediate coronary artery spasm but only 5-HT1B receptors appear involved in coronary spasm of patients treated with triptans or with Prinzmetal angina. 5-HT2A receptors constrict the portal venous system including oesophageal collaterals in cirrhosis. Chronic exposure to 5-HT can contribute to pulmonary hypertension through activation of constrictor 5-HT1B receptors and proliferative 5-HT2B receptors, and possibly through direct intracellular effects.

Pharmacological profile of serotonergic receptors in the adrenal gland

The secretory activity of the adrenal gland is mainly regulated by peptidergic hormones (ACTH, angiotensin II) and ions. However, there is now increasing evidence that local factors, including neuropeptides and neurotransmitters, can also participate in the control of adrenocortical cells. In particular, serotonin (5-HT), produced by adrenochromaffin cells in frog and rat as well as by mast cells in the adrenal gland of rat and human, stimulates corticosteroid secretion. In both frog and human adrenal gland, the benzamide derivative (R,S)-zacopride induces a robust increase in corticosteroid release suggesting that the effect of 5-HT on steroidogenesis is mediated through activation of 5-HT4 receptors. In contrast, in rat, the stimulatory effect of 5-HT on aldosterone secretion is clearly not mediated by 5-HT4 receptors. In all three species, incubation of adrenocortical fragments with 5-HT induces a significant increase in cAMP formation. Our data suggest that 5-HT, released within the adrenal cortex, may act as a paracrine factor to stimulate steroid secretion. Although the corticotropic effect of 5-HT has been conserved from amphibians to primates, the type of receptors involved in the action of 5-HT markedly differs across species.