In vitro control of prolactin (PRL) and growth hormone secretion of neonatal rat pituitary glands: effects of ovine PRL, salmon calcitonin, endothelin-3, angiotensin II, bromocryptine and somatostatin.

Bromocryptine potently decreased prolactin (PRL) secretion of pituitary glands of 2-day-old rats in vitro (up to 85% inhibition; ED50 between 0.1 and 1.0 nM) without altering the bioactivity to immunoreactivity (B/I) ratio. Bromocryptine tended to suppress growth hormone (GH) secretion although the effect did not reach statistical significance. Angiotensin-II (A-II; 1-1000 nM) stimulated PRL secretion in a dose-dependent manner without affecting secretion of GH. The B/I ratio of PRL secreted in response to A-II was increased. Somatostatin (SRIF) had no effect on PRL secretion but inhibited GH secretion in a dose-dependent manner; significant inhibition (50%) was observed at 100 nM. A 6-h exposure to ovine PRL (oPRL) in concentrations equipotent with 1.2-120 ng/ml rat PRL (rPRL) in the Nb2 bioassay had no effect on immunoreactive rPRL secretion. Salmon calcitonin (sCT) and endothelin-3 (ET-3; 0.1-100 nM) failed to inhibit secretion of PRL or GH. PRL secretion was slightly stimulated by sCT with no apparent dose-response relationship. The present findings suggest that neonatal pituitary glands do not display autoregulation of PRL secretion, and sCT and ET-3 (either endogenous or milk-derived) may not function as PRL inhibiting factors in 2-day-old pups. Thus, the receptors of PRL, sCT and ET-3 on lactotropes, or their functional coupling with inhibition of basal PRL secretion, occur at a later stage of development. The specificity of the PRL releasing factor (PRF) activity of A-II at this age is unique for established PRFs and might reflect a physiological function of PRL in osmoregulation. The increased B/I ratio of PRL secreted in response to A-II may be due to the release of specific PRL variants, and might be a sign of functional heterogeneity among lactotropes. The differential sensitivity of PRL and GH to the applied secretagogues suggests that the intracellular regulation of PRL and GH are compartmentalized in the mammosomatotrope cell.