MODULATION OF CA2+ OSCILLATION AND APAMIN-SENSITIVE, CA2+-ACTIVATED K+ CURRENT IN RAT GONADOTROPES

In rat pituitary gonadotropes, gonadotropin-releasing hormone (GnRH) stimulates rhythmic release of Ca2+ from stores sensitive to inositol 1,4,5-trisphosphate [Ins(1,4,5)P-3], which in turn induces an oscillatory activation of apamin-sensitive Ca2+-activated K+ current, I-K(Ca). Since GnRH also activates protein kinase C (PKC), we investigate the action of PKC while simultaneously measuring intracellular Ca2+ concentration ([Ca2+](i)) and I-K(Ca). Stimulation of PKC by application of phorbol 12-myristate 13-acetate (PMA) did not affect basal [Ca-2+](i) However, PMA or phorbol 12,13-dibutyrate (PdBu), but not the inactive 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD reduced the frequency of GnRH-induced [Ca2+](i) oscillation and augmented the I-K(Ca) induced by any given level of [Ca2+](i). The slowing of oscillations and the enhancement of I-k(Ca) were mimicked by synthetic diacylglycerol (1,2-dioctanoyl-sn-glycerol) and could be induced during ongoing oscillations that had been initiated irreversibly in cells loaded with guanosine 5'-O-(3-thiotriphosphate) (GTP-[gamma S]). In contrast, when oscillations were initiated by loading cells with Ins(1,4,5)P-3, phorbol esters enhanced I-K(Ca) without affecting the frequency of oscillation. The protein kinase inhibitor, staurosporine, reduced I-K(Ca) without affecting [Ca2+](i) and partially reversed the phorbol-ester-induced slowing of oscillation. Therefore, activation of PKC has two rapid effects on gonadotropes. It slows [Ca2+](i) oscillations probably by actions on phospholipase C, and it enhances I-K(Ca) probably by a direct action on the channels.