Mechanisms of action of pituitary adenylate cyclase-activating polypeptide (PACAP) on growth hormone release from dispersed goldfish pituitary cells |
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Authors: | Noel R Wirachowsky Patrick Kwong Warren K Yunker James D Johnson John P Chang |
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Institution: | (1) Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada |
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Abstract: | The mechanisms of pituitary adenylate cyclase activating polypeptide (PACAP) action on goldfish growth hormone (GH) release were investigated by examining GH release responses from dispersed goldfish pituitary cells to a synthetic mammalian (m)PACAP38 peptide. It was established that GH release stimulated by 2-h exposure to mPACAP38 was concentration-dependent, attenuated by the PACAP receptor antagonist mPACAP6–38, and subject to neuroendocrine modulation by somatostatin. Maximal mPACAP38-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 mPACAP38, forskolin and 8-bromo-cAMP, either alone or in combination, were abolished by H89, a protein kinase A (PKA) inhibitor. SQ22536, an adenylate cyclase inhibitor, attenuated forskolin- and mPACAP38-stimulated GH release. In contrast, mPACAP38-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 mPACAP38 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 mPACAP38 in static incubation; conversely, mPACAP38 increased intracellular Ca2+] in identified, single goldfish somatotropes. |
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Keywords: | signal transduction cyclic AMP protein kinase A protein kinase C [Ca2+]i voltage-sensitive Ca2+ channels |
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