Interactions of cyclic adenosine 3',5'-monophosphate, protein kinase-C, and calcium in dopamine- and gonadotropin-releasing hormone-stimulated growth hormone release in the goldfish

Abstract

In the goldfish, GH release is under the stimulatory control of multiple neuroendocrine factors, including dopamine (DA) and GnRH. We have previously shown that DA-stimulated GH release in the goldfish is mediated via D1 receptors and is dependent on cAMP, whereas the GH response to GnRH involves protein kinase-C (PKC) and extracellular Ca2+ entry through voltage- sensitive Ca2+ channels (VSCC). In this study, the interactions of PKC, cAMP, and Ca2+ in the regulation of GH release responses to DA and GnRH were examined using dispersed goldfish pituitary cells. The GnRH-induced GH secretion was unaffected by a protein kinase-A (PKA) inhibitor, but was additive to the GH responses to 8-bromo-cAMP (8-Br-cAMP), the phosphodiesterase inhibitor isobutylmethylxanthine, or the D1 agonist SKF38393. Similarly, the GH responses to PKC activators were additive to those to 8-Br-cAMP, forskolin, SKF38393, or the general DA agonist apomorphine. In contrast, depletion of cellular PKC content inhibited the GH responses to GnRH and PKC activators, but not those to DA, SKF38393, 8-Br- cAMP, or forskolin. Furthermore, the GH response to 8-Br-cAMP or SKF38393 was unaffected by the selective PKC inhibitor H7. Taken together, these results suggest that GnRH and DA D1 mechanisms stimulate GH release in the goldfish independently via PKC- and cAMP-dependent intracellular signaling pathways, respectively. However, both the cAMP (D1) and PKC (GnRH) mechanisms also interacted with VSCC. The VSCC agonist Bay K8644 potentiated, whereas the VSCC blocker verapamil reduced, the GH responses to GnRH, PKC activators, SKF38393, and activators of the cAMP pathway. A hypothesis is proposed based on these findings. GnRH and DA initiate GH release independently by activation of PKC- and cAMP-dependent mechanisms. The secondary activation of VSCC by either PKC or cAMP/PKA further enhances the GH response.