Ca2+ channel agonists enhance thyrotropin-releasing hormone-induced inositol phosphates and prolactin secretion

The dihydropyridine Ca2+ channel activator BAY K 8644 (1 microM) stimulated basal prolactin secretion from perifused primary cultures of anterior pituitary cells and potentiated the stimulation of prolactin secretion by 1 microM thyrotropin-releasing hormone (TRH) 5-fold over 30 min. This potentiation was mimicked by other dihydropyridine agonists CGP 28392 and (+)-SDZ 202-791 and by (-)-BAY K 8644 (1 microM), but not by (+)-BAY K 8644. The Ca2+ channel antagonist nimodipine, at a concentration sufficient to block BAY K 8644-stimulated 45Ca2+ uptake in GH4C1 anterior pituitary tumor cells, decreased basal prolactin secretion and blocked the enhancement of basal and TRH-stimulated secretion by BAY K 8644. These results suggest that dihydropyridine agonists potentiate TRH-induced secretion through interaction with known stereospecific sites on Ca2+ channels. In GH4C1 cells, BAY K 8644 alone did not affect inositol polyphosphate accumulation, but potentiated TRH-stimulated accumulation of inositol 1,3,4-trisphosphate and inositol 1,3,4,5-tetrakisphosphate. Accumulation of the Ca(2+)-mobilizing isomer inositol 1,4,5-trisphosphate was not potentiated, suggesting that potentiation of TRH-stimulated hormone secretion by BAY K 8644 does not result from synergistic stimulation of phospholipase C, but may correlate with enhanced inositol trisphosphate-3-kinase activity.