Differential Effects of a Low Dose Dopamine Infusion on Prolactin Secretion in Normal and Hyperprolactinemic Subjects*

Publisher Website
Google Scholar
Abstract
The PRL inhibitory effect of dopamine (DA) in human in vivo studies has been previously demonstrated with DA infusions at rates generally exceeding 2 μg/kg-min. We report here the effects of a DA infusion administered at a rate of 0.02 μg/kg-min for 180 min to 10 normal subjects and 25 hyperprolactinemic patients with pituitary tumors (13 microprolactinomas, 8 macroprolactinomas, and 4 expanding nonsecreting pituitary adenomas). Serum free DA concentrations during the 3-h DA infusion reached an average of 0.8 ± 0.1 ng/ml (about an 8- to 10-fold rise from basal levels). DA produced a significant (P μ 0.001) decline in plasma PRL levels in both normal subjects and hyperprolactinemic patients. There was a negative linear correlation between the serum DA concentrations and the percent PRL variation from basal levels (r = −0.58; Pμ 0.001). The comparison of PRL responses between the different groups revealed that the mean percent overall PRL inhibition was significantly lower in patients with microprolactinomas than in normal subjects (P μ 0.02). On the other hand, PRL inhibition was greater in patients with nonsecreting adenomas than in either patients with microprolactinomas or those with macroprolactinomas (P μ 0.001). From 90–180 min, PRL suppression was also greater in patients with nonsecreting adenomas than in normal controls (P μ 0.05). The present study shows that 1) slight elevations of plasma DA are sufficient to inhibit PRL secretion, suggesting that DA acts as a major physiological PRL-inhibiting factor; 2) there is a relative PRL resistance to DA inhibition in microprolactinoma patients; and 3) PRL is hyperresponsive to DA in expanding nonsecreting pituitary tumors.