Somatostatin-14 actions on dopamine- and pituitary adenylate cyclase-activating polypeptide-evoked Ca2+ signals and growth hormone secretion.
Using single-cell Ca(2+) imaging and a growth hormone (GH) radioimmunassay, we investigated somatostatin-14 (SS(14)) inhibition of cAMP-dependent, stimulated GH secretion from primary cultures of dispersed goldfish pituitary cells. The dopamine-D1 receptor agonist SKF-38393, and the hypothalamic neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) both elevated intracellular Ca(2+) concentration ([Ca(2+)](i)) and stimulated GH release. When increases in [Ca(2+)](i) were prevented by intracellular loading of BAPTA, a Ca(2+) chelator, SKF-38393- and PACAP-stimulated GH release were inhibited, suggesting that these Ca(2+) signals are required for stimulated GH release. SS(14) inhibited SKF-38393- and PACAP-stimulated GH release, but did not prevent these Ca(2+) signals. Kinetic analysis revealed that SS(14) lowered the maximum amplitude of the SKF-38393- and PACAP-evoked Ca(2+) responses, but had no effect on other aspects of the Ca(2+) signal. We then examined the ability of SS(14) to act subsequent to dopamine-D1 or PACAP receptor activation using the adenylate cyclase activator forskolin, or the membrane permeant cAMP analogue 8Br-cAMP. Forskolin and 8Br-cAMP both increased [Ca(2+)](i) and GH secretion and, as expected, SS(14) inhibited the resultant GH release. Although SS(14) significantly increased the time to maximum amplitude of the forskolin-evoked Ca(2+) signals, it had no detectable effect on any of the kinetic parameters used to describe the Ca(2+) signals evoked by 8Br-cAMP. Taken together, these results establish that SS(14) has the ability to suppress Ca(2+)-dependent exocytosis by acting distal to elevations in [Ca(2+)](i). Furthermore, it appears likely that the cellular mechanisms underlying the observed effects of SS(14) on Ca(2+) signalling are upstream of cAMP and may be unrelated to those responsible for inhibiting GH release.
Department of Biological Sciences, Faculty of Science, University of Alberta, Edmonton, Alberta, Canada.
W K
WK
Yunker
J P
JP
Chang
D000818:Animals; D020013:Calcium Signaling / Q000502:physiology*; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004298:Dopamine / Q000378:metabolism*; D015536:Down-Regulation; D005260:Female; D005456:Fluorescent Dyes / Q000378:metabolism; D016257:Fura-2 / Q000378:metabolism; D006054:Goldfish; D013006:Growth Hormone / Q000378:metabolism*; D007424:Intracellular Fluid / Q000378:metabolism; D008297:Male; D009479:Neuropeptides / Q000378:metabolism*; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D010902:Pituitary Gland / Q000166:cytology / Q000201:enzymology / Q000378:metabolism*; D017447:Receptors, Dopamine D1 / Q000378:metabolism; D013004:Somatostatin / Q000502:physiology*
D005456:Fluorescent Dyes; D009479:Neuropeptides; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D017447:Receptors, Dopamine D1; D013004:Somatostatin; D013006:Growth Hormone; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D016257:Fura-2; D004298:Dopamine