Mechanisms of action of pituitary adenylate cyclase-activating polypeptide (PACAP) on growth hormone release from dispersed goldfish pituitary cells

The mechanisms of pituitary adenylate cyclase activating polypeptide (PACAP) action on goldfish growth hormone (GH) release were investigated by examining CH release responses from dispersed goldfish pituitary cells to a synthetic mammalian (m)PACAP(38) peptide. It was established that GH release stimulated by 2-h exposure to mPACAP(38) was concentration-dependent, attenuated by the PACAP receptor antagonist mPACAP(6-38), and subject to neuroendocrine modulation by somatostatin. Maximal mPACAP(38)-stimulated GH release was not additive to the responses elicited by either the adenylate cyclase activator forskolin or the cyclic (c)AMP analog 8-bromo-cAMP. The GH responses to mPACAP(38), forskolin and 8-bromo-cAMP. either alone or in combination, were abolished by H89, a protein kinase A (PKA) inhibitor. 3Q22536. an adenylate cyclase inhibitor, attenuated forskolin- and mPACAP(38)-stimulated GH release. in contrast, mPACAP(38)-stimulated GH release were additive to the responses to two protein kinase C (PKC) activators and unaffected by two PKC inhibitors. These results suggest that the stimulatory action of PACAP on GH secretion is mediated through a cAMP-/PKA-dependent mechanism, whereas the involvement of PKC appears unlikely. The ability of mPACAP(38) to further enhance maximal GnRH (PKC)-dependent GH release, but not dopamine D1 agonist (PKA)-dependent GH secretion, is consistent with this hypothesis. A possible involvement of Ca2+ in PACAP action is also suggested. Two inhibitors of voltage-sensitive Ca2+ channel reduced the GH responses to mPACAP(38) in static incubation; conversely, mPACAP(38) increased intracellular [Ca2+] in identified, single goldfish somatotropes.