Effects of enhancement and antagonism of 5-hydroxytryptamine activity on metoclopramide-induced aldosterone secretion in man

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.

Effect of metoclopramide, ondansetron and granisetron on aldosterone secretion in man

The plasma aldosterone response following the administration of drugs with antagonist and agonist activity at Serotonin 3 and 4 (5-HT3&4) receptors has been examined in 9 healthy male volunteers receiving the following four treatments i.v. in a randomised, cross-over sequence: ondansetron 8 mg, granisetron 3 mg, metoclopramide 20 mg, and saline 20 ml. Metoclopramide significantly increased the mean plasma aldosterone level to 196% of basal level at 5 min. It rose to 234% at 15 min and remained at more than 185% of basal level for the duration of the experiment. The response to ondansetron and granisetron did not differ significantly from placebo. If dopamine antagonism is discounted, the results suggest that metoclopramide-induced aldosterone secretion results from its agonist activity at 5-HT4 receptors, although slow neuronal depolarization via an unidentified receptor remains a possibility. Antagonism at the 5-HT3 receptor plays no role, as the selective antagonist, granisetron, did not elicit a significant response. It seems unlikely that the 5-HT4 receptor is the second, low affinity binding site of ondansetron, unless it had no agonist activity at this receptor.

Adrenal mitochondrial benzodiazepine receptors are sensitive to agents active at the dopamine receptor

Male rats were treated for 21 days with drugs known to affect prolactin secretion, in order to assess the effects of these drugs on mitochondrial benzodiazepine receptors (MBRs). Sulpiride, a selective dopamine D2 receptor antagonist and hyperprolactinemic agent, decreased MBR density in the adrenal gland (49%; P < 0.005), whereas metoclopramide, another dopamine antagonist with a preference for dopamine D2 receptors, increased adrenal gland MBR density (31%; P < 0.05). Bromocriptine, a specific dopamine agonist, increased MBR density in this organ (87%; P < 0.001). None of the three agents influenced kidney or testicular MBRs. These data indicate that the mechanism of organ-specific alterations in MBRs seems to be prolactin independent.