Dopamine infusion enhances the adrenocorticotropic hormone and cortisol response to metoclopramide in hyperprolactinemic patients but not in normal subjects

Based on the facts that prolactin and adrenocorticotropic hormone (ACTH) each seem to influence the secretion of the other, that dopamine is the established inhibitory factor for prolactin secretion and negatively modulates ACTH release, and finally that alterations of the central dopaminergic tone have been postulated in tumorous hyperprolactinemia, we studied the effects of pharmacological manipulations of the dopaminergic system on ACTH and cortisol secretion in patients bearing a prolactinoma and in normal subjects. Twenty-seven patients with a prolactin-secreting pituitary tumor and 12 healthy controls were submitted to three tests: (a) 4-h saline infusion; (b) 10 mg metoclopramide (MTC) as an intravenous bolus after a 2-h saline infusion; and (c) 4-h dopamine ingusion atthe dose of 0.01 μg/kg/min with a 10-mg intravenous bolus of MTC given at the second hour of dopamine infusion. Administration of MTC, compared to saline, caused a moderate (not significant) plasma ACTH increase, and a significant cortisol increase (p <0.05), both in hyperprolactinemic and normal subjects, without statistically significant differences between the two groups. When MTC was administered during dopamine infusion, the ACTH and cortisol elevation was significantly potentiated in prolactinoma patients while it was similar in magnitude to that recorded after MTC alone in control subjects. These findings support the concept of an inhibitory role exerted by dopamine and are compatible with a stimulatory influence exerted by prolactin on corticotropin-releasing hormone and ACTH secretion, and also favor the view of a reduced central dopaminergic tone in patients with tumorous hyperprolactinemia.