| Abstract: | Continuous stimulation of cultured chicken pituitary cells with a native chicken hypothalamic GnRH, Gln8-GnRH, caused LH release, followed by rapid desensitization. Continuous exposure to calcium ionophore A23187 also produced a short-lived LH response, followed by desensitization. In contrast, continuous stimulation with a phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), caused prolonged release of LH, reaching maximum amplitude after 1 h and slowly decreasing thereafter. Cells desensitized to GnRH remained fully responsive to A23187 and TPA, indicating that GnRH-mediated desensitization occurs at a level before protein kinase C activation and calcium mobilization. Simultaneous addition of A23187 and TPA resulted in a synergistic response which was independent of the order of addition of the two compounds. Synergism was also demonstrated between depolarizing concentrations of potassium and TPA and between veratridine and TPA, indicating that calcium entering via voltage-dependent channels also synergizes with phorbol ester. Despite the prolonged action of TPA, rapid pulsatile changes in LH release could be induced in cells treated with TPA and veratridine by rapidly changing the extracellular calcium concentration. This suggest that protein kinase C could function as an amplifier of the calcium signal generated by GnRH. Synergism between calcium and TPA was blocked by trifluoperazine, chlorpromazine, and W-7 N-(6-aminohexyl)5-chloro-1-napthalenesulfonamide] at concentrations that had no effect on LH release mediated by TPA alone. This suggests that synergism is mediated via calmodulin and not a direct effect of calcium on protein kinase C. |
