In intact sheep gonadotropes, the protein kinase inhibitor, staurosporine, inhibited the stimulatory effect of phorbol 12-myristate 13-acetate (PMA) on luteinizing hormone (LH) secretion. Under the same conditions staurosporine enhanced gonadotrophin-releasing hormone (GnRH)-stimulated LH exocytosis without altering the EC50 of GnRH and without affecting basal LH exocytosis. These results suggest that PKC does not play a major role in mediating acute GnRH-stimulated LH exocytosis. Furthermore, they demonstrate that staurosporine enhances GnRH stimulus-secretion coupling. Both extracellular Ca2+ -dependent and Ca2+ -independent components of GnRH-stimulated LH secretion were enhanced by the drug. Staurosporine had no effect on GnRH stimulation of cAMP and inositol phosphate synthesis. In permeabilized cells staurosporine did not enhance Ca2+ and cAMP-stimulated LH exocytosis. Based on these results we hypothesize that staurosporine inhibits a protein kinase which is activated by GnRH and which negatively modulates GnRH stimulus-secretion coupling.
