LONG-TERM EXPOSURE TO DOPAMINE REVERSES THE INHIBITORY EFFECT OF ENDOTHELIN-1 ON PROLACTIN SECRETION

This study was undertaken to assess the possibility that endothelin-1 (ET-1) and dopamine (DA) can act in concert to modulate PRL secretion. Enzymatically dispersed anterior pituitary cells obtained from random cycling female rats were perifused with Dulbecco's Modified Eagle's Medium supplemented with 0.2% BSA and 100 mu M ascorbic acid. In the absence of dopamine, ET-1 (applied at 20 nM for 60 min) rapidly evoked a small transient elevation of PRL release, followed by a sustained inhibitory phase. Overnight perfusion with 500 nM DA-supplemented medium did not change the basic character of ET-1's effects on PRL secretion. Continuation of DA exposure for 48 h dramatically shifted the responsiveness of the lactotrophs to ET-1; the fast stimulatory response was robustly enhanced, whereas the inhibitory phase was replaced by a modest secondary elevation of basal PRL secretion. The stimulatory effect of ET-1 on PRL secretion after DA pretreatment was blocked by an ET(A) receptor antagonist, BQ-123. The effect of DA can be mimicked completely by a specific D-2 receptor agonist (+)-2-(N-phenyl-N-propyl)amino-5-hydroxytetraline hydrochloride, whereas pretreatment with a D-1 agonist, SKF-39393, failed to change the responsiveness of lactotrophs to ET-1. Our data indicate that persistent activation of D-2 receptors, a condition most closely resembling the in vivo environment of the lactotrophs, uncouples the inhibitory signaling pathway from the ET(A) receptor while synergistically affecting signal transduction, which mediates the ET-induced stimulation of PRL secretion.