Somatostatin actions on a protein kinase C-dependent growth hormone secretagogue cascade |
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Authors: | Yunker W K Chang J P |
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Institution: | Department of Biological Sciences, CW 405 Biological Sciences Building, Faculty of Science, University of Alberta, Edmonton, T6G 2E9, Alberta, Canada. |
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Abstract: | In mammals, the ability of somatostatin (SS) to block growth hormone (GH) secretion is due, in part, to the inhibition of two key intracellular mediators, cAMP and Ca2+. We examined whether or not inhibition of Ca2+ signaling was mediating SS-induced inhibition basal, as well as gonadotropin-releasing hormone (GnRH; a protein kinase C (PKC)-dependent growth hormone secretagogue)-stimulated growth hormone (GH) release. Although SS reduced basal GH release from populations of pituitary cells, parallel reductions in Ca2+]i were not observed within single, identified somatotropes. Similarly, application of GnRH and the PKC activator DiC8 elicited increases in Ca2+]i and GH release, but abolition of the Ca2+ responses did not accompany SS inhibition of the GH responses. Surprisingly, while DiC8 potentiated SS inhibition of GH release, SS paradoxically increased DiC8-stimulated increases in Ca2+]i. These data establish that abolition of Ca2+ signals is not a primary mechanism through which SS lowers basal, or inhibits GnRH-stimulated hormone release. |
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