Regulation of Growth Hormone Release in Common Carp Pituitary Cells by Pituitary Adenylate Cyclase-Activating Polypeptide: Signal Transduction Involves cAMP- and Calcium-Dependent Mechanisms

Abstract

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of the glucagon/secretin peptide family and its molecular structure is highly conserved among vertebrates. In this study, the role of PACAP in regulating growth hormone (GH) secretion in fish was examined in vitro using common carp pituitary cells under column perifusion. A dose-dependent increase in GH release was observed after exposing pituitary cells to increasing doses of ovine PACAP38 (oPACAP38) and PACAP27 (oPACAP27), but not vasoactive intestinal polypeptide (VIP). A lack of GH response to VIP stimulation is consistent with the pharmacological properties of PAC-1 receptors, suggesting that this receptor subtype may be involved in PACAP-induced GH secretion in carp species. Although the maximal GH responses induced by oPACAP38 and oPACAP27 were similar, the minimal effective dose and ED50 value for oPACAP38 were significantly lower than that for oPACAP27. These results may indicate that common carp PAC-1 receptors are more sensitive to stimulation by oPACAP38 than by oPACAP27. In parallel studies, oPACAP38 and oPACAP27 were also effective in increasing cAMP release, cellular cAMP content, total cAMP production, and intracellular Ca²⁺ ([Ca²⁺]_i) levels in common carp pituitary cells. Besides, the rise in [Ca²⁺]_i induced by oPACAP38 was blocked by removing extracellular Ca²⁺ ([Ca²⁺]_e) or by treatment with nifedipine, an inhibitor of voltage-sensitive Ca²⁺ channels (VSCC). The dose dependence of PACAP-stimulated GH release in common carp pituitary cells was mimicked by activating adenylate cyclase using forskolin, inhibiting cAMP degradation using IBMX, increasing functional levels of intracellular cAMP using CPT-cAMP, or inducing [Ca²⁺]_e entry using the Ca²⁺ ionophore A23187. In contrast, the GH-releasing effect of oPACAP38 was suppressed by treatment with the adenylate cyclase inhibitor MDL12330A, protein kinase A inhibitor H89, and VSCC blocker nifedipine, or by perifusion with a Ca²⁺-free culture medium. These results, as a whole, suggest that PACAP functions as a GH-releasing factor in common carp by activating pituitary receptors resembling mammalian PAC-1 receptors. Apparently, the GH-releasing action of PACAP is mediated through the adenylate cyclase/cAMP/protein kinase A pathway and [Ca²⁺]_e influx through VSCC.