Gonadotropin-releasing hormone-stimulated luteinizing hormone (LH) release from ovine gonadotrophs in culture is separate from phorbol ester-stimulated LH release.

The proposed involvement of protein kinase-C (PKC) as a mediator of GnRH action in pituitary cells was investigated. Treatment of dispersed ovine pituitary cells with the PKC activator phorbol 12-myristate-13-acetate (PMA) for 6 h at a dose of 10 nM stimulated the release of 5- to 10-fold more LH than did GnRH at the same dose. Analogs of PMA also stimulated LH release, but at levels parallel to their reported abilities to activate PKC. Treatment with GnRH plus PMA stimulated more LH release than either one alone in a strictly additive manner at all dose combinations from 10 pM to 100 nM. The PKC-activating diacylglycerol sn-1,2-dioctanoylglycerol also stimulated LH release that was additive with GnRH-stimulated LH release. Furthermore, retinal, an inhibitor of PKC, inhibited only PMA-stimulated LH release and not GnRH-stimulated LH release. These results do not support the theory that PKC activation is a necessary part of the mechanism of GnRH-stimulated LH release. The theory that calcium mobilization is also part of the mechanism of GnRH-stimulated LH release was, however, supported by the finding that GnRH did not stimulate LH release from ovine pituitary cells incubated in calcium-free medium. This demonstrates the absolute requirement for calcium in GnRH action, as proposed by others. In addition, the calcium ionophores A23187 and ionomycin were able to trigger LH release by themselves as well as augment GnRH- and PMA-stimulated LH release. This further strengthens the hypothesis that calcium mobilization is important in LH secretion.