EVIDENCE THAT TRH CONTROLS PROLACTIN-RELEASE FROM RAT LACTOTROPHS BY STIMULATING A CALCIUM INFLUX

Prolactin (PRL) release and intracellular free calcium concentration [ Ca2+](i) were measured in two populations of normal rat lactotrophs (l ight and heavy fractions) in culture. Spontaneous PRL release of heavy fraction cells was more sensitive to dihydropyridines (DHPs; Bay K 86 44 and nifedipine) when compared to the light fraction lactotrophs. Th e stimulatory effect of thyrotropin-releasing hormone (TRH) on PRL rel ease from heavy fraction cells was inhibited by Cd2+ and mimicked by B ay K 8644. Indo-1 experiments revealed that TRH-increased [Ca2+](i) wa s reversibly inhibited by Cd2+. In a Ca2+-free EGTA-containing medium, TRH did not modify [Ca2+](i).