Effects of protein kinase C activation on cyclic AMP and testosterone production of rat Leydig cells in vitro.

The role of protein kinase C in modulation of the endocrine function of rat Leydig cells was studied. Percoll-purified rat Leydig cells were stimulated with hCG, forskolin, cholera toxin, pertussis toxin and 8-bromo-cAMP in the presence and absence of two activators of protein kinase C, 12-0-tetradecanoylphorbol 13-acetate (TPA) or 1-oleoyl-2-acetyl-sn-glycerol (OAG). The two activators had no effect on basal cAMP, but decreased hCG-stimulated, and increased cholera toxin- and forskolin-stimulated cAMP production. Cells pre-incubated with pertussis toxin showed enhanced rate of cAMP production in response to forskolin, but were no more responsive to TPA and OAG stimulation. These findings suggest that protein kinase C activation may on one hand inhibit the LH-receptor and Gs-protein coupling and on the other hand inhibit the Gi-protein mediated suppression of adenylyl cyclase activity. TPA and OAG effects on testosterone production were measured in the absence and presence of 8-bromo-cAMP stimulation. TPA enhanced basal testosterone production, but this effect was shifted to inhibition when steroidogenesis was stimulated by 8-bromo-cAMP. The OAG effect on testosterone production was inhibitory throughout the dose-response curve of 8-bromo-cAMP. The basal stimulation of testosterone production by TPA was probably due to a marginal increase of cAMP caused by inhibition of the Gi-protein, since a similar effect was observed by pertussis toxin, and thereafter TPA was without effect on testosterone. The inhibition of stimulated testosterone production by TPA and OAG indicates that protein kinase C activity also affects steroidogenesis at a step(s) beyound cAMP formation.