Regulation of pituitary somatotropes by endogenous somatostatins.

摘要

The regulation of pituitary somatotrope function in goldfish ( Carassius auratus) by the somatostatin (SS) neuropeptide family was examined in the present study. Using reverse transcription-polymerase chain reaction and Southern blot analysis, mRNA for three different SS precursors (PSS-I, -II, and -III), which encode for the putative peptides SS14, goldfish brain (gb)SS28 and [Pro2]SS14, respectively, were detected in goldfish hypothalamus. Interestingly, PSS-I and -II mRNA, but not PSS-III mRNA, were also detected in cultured pituitary cells. The effects of SS14, gbSS28 and [Pro2 ]SS14 on somatotrope signalling and growth hormone (GH) secretion were subsequently examined. SS14 and [Pro2]SS 14 were similar in their effects on somatotrope 3',5 '-cyclic adenosine 5'-monophosphate levels and GH secretion but differed markedly from those of gbSS28. This suggests that, in goldfish, different SS peptides may be responsible for selectively regulating different aspects of somatotrope function.;The ability of SS14 to inhibit basal and stimulated GH secretion was further investigated using single-cell Ca2+ imaging and time-matched column perifusion GH release experiments. As expected, application of various natural GH-releasing ligands, as well as pharmacological activators of their respective intracellular signalling cascades, stimulated GH release and increased intracellular Ca2+ concentration ([Ca 2+]i) within single identified somatotropes. Furthermore, buffering these increases in [Ca2+]i with a Ca 2+ chelator impaired the corresponding GH responses. Surprisingly, SS14 reduced basal GH release without altering [Ca2+] i in single identified somatotropes. Furthermore, stimulated increases in [Ca2+]i persisted despite SS14 inhibition of stimulated GH secretion. These results establish that, in goldfish, SS 14 does not abolish stimulated Ca2+ signals as a means of inhibiting stimulated GH secretion. This type of regulatory mechanism would allow for the differential regulation of hormone release and other Ca 2+-dependent cellular processes by SS14. The data also suggest that the cellular mechanisms underlying the observed effects of SS 14 on Ca2+ signalling may be unrelated to those responsible for inhibiting GH release.;Overall, by comparing the effects of three closely related endogenous SS isoforms and subsequently examining, more closely, the intracellular mechanisms of one these peptides, this thesis provides evidence for the differential regulation of cellular functions at both the extracellular and intracellular levels.