Free intracellular Ca2+ concentration and growth hormone (GH) release from purified rat somatotrophs. III. Mechanism of action of GH-releasing factor and somatostatin.
GH-releasing factor (GRF)-stimulated GH release is dependent on a biphasic increase in free intracellular Ca2+ concentration [( Ca2+]i), resulting from an influx of Ca2+ into somatotrophs, while the inhibitory action of somatostatin (SRIF) on basal and GRF-induced GH release results from its ability to lower [Ca2+]i by inhibiting Ca2+ influx. This study was carried out to investigate the mechanism by which GRF and SRIF regulate [Ca2+]i to control GH release. The roles of ion channels, cAMP-dependent processes, and protein kinase-C (PKC) were investigated by measuring changes in [Ca2+]i, 45Ca influx, and GH release when purified rat somatotrophs were exposed to high K+, cAMP analogs, prostaglandin E2, as well as the PKC activators 1,2-dioctanoyl-glycerol and phorbol 12-myristate 13-acetate. High K+ depolarization produced a rapid and transient increase in [Ca2+]i, while cAMP and prostaglandin E2 led to a sustained elevated [Ca2+]i. PKC activators produced a transient increase in [Ca2+]i, followed by a decrease to below baseline. All secretagogues tested raised [Ca2+]i by stimulating Ca2+ influx through L-type voltage-sensitive Ca2+ channels (VSCC), since the increases in [Ca2+]i were blocked by incubation in Ca2(+)-free medium and by the dihydropyridine Ca2+ antagonist nifedipine. SRIF lowered [Ca2+]i by blocking the Ca2+ influx stimulated by all of these GH secretagogues except high K+. These results are consistent with the model in which GRF initiates its action by increasing Na+ conductance to depolarize the somatotroph via cAMP. This depolarization would stimulate Ca2+ influx through VSCC, which would result in the first phase of the GRF-dependent increase in [Ca2+]i. This increase in [Ca2+]i would stimulate Ca2+ removal from the cytosol by activating Ca-ATPase via Ca-calmodulin and/or PKC. This would result in the lowering of [Ca2+]i to the plateau level of the second phase of the GRF response. SRIF prevents the GRF-induced increase in [Ca2+]i by increasing K+ conductance and, thus, hyperpolarizing the cell. Hyperpolarization would close VSCC, leading to a decrease in Ca2+ influx, with a subsequent drop in [Ca2+]i.
Lussier
B T
BT
Department of Physiology, University of Western Ontario, London, Canada.
French
M B
MB
Moor
B C
BC
Kraicer
J
J
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D003994:Bucladesine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002469:Cell Separation / Q000379:methods; D002499:Centrifugation, Density Gradient / Q000379:methods; D000242:Cyclic AMP / Q000378:metabolism; D005456:Fluorescent Dyes; D013006:Growth Hormone / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D007211:Indoles; D007700:Kinetics; D008297:Male; D008954:Models, Biological; D009543:Nifedipine / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013004:Somatostatin / Q000494:pharmacology*; D013050:Spectrometry, Fluorescence
D005456:Fluorescent Dyes; D007211:Indoles; D013004:Somatostatin; D003994:Bucladesine; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D000242:Cyclic AMP; D009543:Nifedipine; C048960:indo-1; D011188:Potassium; D002118:Calcium