No difference between micro-and macroprolactinomas in the prolactin responsiveness to metoclopramide and dopamine administration

Differences between micro- and macroprolactinomas, as regards the prolactin secretory pattern in response to pharmacological challenges, have been reported in in vivo and in vitro models, and interpreted as being due to different dopaminergic regulation of prolactin release. In 32 patients with prolactin-secreting tumors, 19 with microprolactinomas and 13 with macroprolactinomas, and ten healthy volunteers, we evaluated the prolactin secretion in response to pharmacological manipulations of central dopaminergic tone. To this end, three tests were performed, in random order: (1) 4-h saline infusion; (2) 10 mg metoclopramide as i.v. bolus; (3) 4-h dopamine infusion (0.01 microgram/kg/min) with a 10-mg metoclopramide bolus given after the second hour of infusion. Dopamine infusion, compared to saline, caused a significant prolactin decrease in all the three groups of subjects, without significant difference between micro- and macroprolactinoma patients. In prolactinoma patients, administration of metoclopramide induced a significant rise in plasma prolactin which, however, was significantly lower than the one displayed by controls. Again, no difference was observed between the two groups of hyperprolactinemic patients. Dopamine infusion induced a significant and comparable increase in the prolactin response to metoclopramide in micro- and macroprolactinoma patients, while it was ineffective in control subjects. In conclusion, no differences appear to exist between micro- and macroprolactinoma patients as regards the prolactin secretory pattern during pharmacological modifications of the dopaminergic tone. A central dopaminergic defect and an increased prolactin turnover with attendant reduction of the intracellular hormone pool may both be involved in the reduced prolactin release following provocative stimuli in patients with prolactinoma.