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Endogenous Hypothalamic Somatostatins Differentially Regulate Growth Hormone Secretion from Goldfish Pituitary Somatotropes in Vitro

Department of Biological Sciences (W.K.Y., S.S., C.G., P.J.D., S.U., R.E.P., J.P.C.), Faculty of Science, University of Alberta, Edmonton, Alberta, Canada T6G 2E9; and Faculty of Medicine and Dentistry (W.K.Y.), University of Alberta, Edmonton, Alberta, Canada T6G 2R7; and The Salk Institute (J.E.R.), La Jolla, California 92037

Address all correspondence and requests for reprints to: Dr. J. P. Chang, Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9. E-mail: john.chang{at}ualberta.ca.

Using Southern blot analysis of RT-PCR products, mRNA for three different somatostatin (SS) precursors (PSS-I, -II, and -III), which encode for SS₁₄, goldfish brain (gb)SS₂₈, and [Pro²]SS₁₄, respectively, were detected in goldfish hypothalamus. PSS-I and -II mRNA, but not PSS-III mRNA, were also detected in cultured pituitary cells. We subsequently examined the effects of the mature peptides, SS₁₄, gbSS₂₈, and [Pro²]SS₁₄, on somatotrope signaling and GH secretion. The gbSS₂₈ was more potent than either SS₁₄ or [Pro²]SS₁₄ in reducing basal GH release but was the least effective in reducing basal cellular cAMP. The ability of SS₁₄, [Pro²]SS₁₄, and gbSS₂₈ to attenuate GH responses to GnRH were comparable. However, gbSS₂₈ was less effective than SS₁₄ and [Pro²]SS₁₄ in diminishing dopamine- and pituitary adenylate cyclase-activating polypeptide-stimulated GH release, as well as GH release resulting from the activation of their underlying signaling cascades. In contrast, the actions of a different 28-amino-acid SS, mammalian SS₂₈, were more similar to those of SS₁₄ and [Pro²]SS₁₄. We conclude that, in goldfish, SSs differentially couple to the intracellular cascades regulating GH secretion from pituitary somatotropes. This raises the possibility that such differences may allow for the selective regulation of various aspects of somatotrope function by different SS peptides.

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