MODULATION OF ACTIVIN-A ACTION AND SPECIFICITY IN THE RAT GONADOTROPE BY PROTEIN-KINASE-C

This study was undertaken to assess the role of protein kinase C (PKC) in activin action in the rat pituitary. Pretreatment with 500 nm phorbol 12-myristate 13-acetate (PMA) for 22-24 h reduced subsequent FSH and LH release (percentage of total cellular FSH and LH released) in response to 100 nm PMA. This action persisted for 2 days after the pretreatment. Pretreatment with 500 nM 4alpha-phorbol 12,13-didecanoate (4alphaPDD, a phorbol ester which does not activate PKC) did not affect cell responsiveness to 100 nm PMA. Both PKC-down-regulated cells and cells with a full complement of PKC responded similarly to 100 nm GnRH and 100 mum A23187 during this period. Incubation with 50 ng/ml activin A for 48 h significantly increased both FSH release and total FSH (extracellular plus intracellular) compared to corresponding basal values in PMA-pretreated cells, as well as in vehicle- or 4alphaPDD-pretreated cells. Activin stimulation of basal FSH release and total FSH was significantly more potent in PMA-pretreated cells than in cells not pretreated with PMA. Activin did not alter basal LH release or total LH in vehicle- or 4alphaPDD-pretreated cells but significantly increased both in PMA-pretreated cells. When PMA was present only during the initial 2 h of the 22- to 24-h pretreatment period at 50 nm, PKC was not down-regulated. In these cells, the potency of activin stimulation of basal FSH release was not affected, but stimulation of basal LH release by activin was still observed. These results suggest that PKC is not required for activin to stimulate FSH release but is involved as a modulator of potency and specificity of the activin action.