Calmodulin and protein kinase C activation duplicates the biphasic secretion of luteinizing hormone

Ionomycin, which activates the Ca2+-calmodulin system, and phorbol 12-myristate 13-acetate (PMA), which activates protein kinase C (PKC), were used to investigate potential roles of these systems as mediators of the biphasic secretion of luteinizing hormone. Quartered pituitaries from diestrous II female rats were perifused at 37 degrees C, and sequential effluent fractions collected every 10 min. Gonadotropin-releasing hormone administration resulted in a biphasic response: an initial, protein synthesis-independent secretion, followed 60 min later by a secondary, augmented, protein synthesis-dependent component. Ionomycin-stimulated gonadotropin secretion was immediate and partially independent of protein synthesis, whereas the PMA-induced secretion was delayed (approximately 70 min), and was completely dependent on protein synthesis. Simultaneous infusions of ionomycin and PMA resulted in an initial, protein synthesis-independent response followed by the secondary, augmented, protein synthesis-dependent component, which exhibited synergistic interactions between calmodulin and PKC. These results suggest that calmodulin mediates the initial, protein synthesis-independent secretion, PKC mediates part of the secondary, augmented response, while calmodulin and PKC synergize to mediate the remaining component of the secondary response.