Free intracellular Ca2+ concentration ([Ca2+]i) and growth hormone release from purified rat somatotrophs. I. GH-releasing factor-induced Ca2+ influx raises [Ca2+]i.
This study was carried out to investigate the role of free intracellular Ca2+ ([Ca2+]i) in the action of GH-releasing factor (GRF) by determining whether GRF causes and increase in [Ca2+]i and whether this increase results from changes in Ca2+ influx/efflux and/or mobilization of intracellular Ca2+ stores. We used a purified preparation of normal rat somatotrophs and examined the changes in 45Ca uptake, [Ca2+]i measured with indo-1, intracellular cAMP, and GH release induced by GRF. GRF stimulated a concentration-related biphasic increase in [Ca2+]i. Both the GRF-dependent increase in [Ca2+]i and GH release were blocked by incubation in low Ca2+ medium and by the organic Ca2+ antagonists nifedipine and diltiazem. The measurement of 45Ca uptake, in both steady state and nonsteady state conditions, demonstrated directly that GRF stimulates Ca2+ influx into somatotrophs. These data demonstrate that the GRF-stimulated increase in [Ca2+]i is dependent on Ca2+ influx. Redistribution of intracellularly stored Ca2+ could not be detected, even though intracellular Ca2+ stores were present. Therefore, the increase is due to Ca2+ influx, and the biphasic nature of the increase in [Ca2+]i induced by GRF is due to a difference in the rate of activation of Ca2+ influx and Ca2+ removal from the cytosol.
Lussier
B T
BT
Department of Physiology, University of Western Ontario, London, Canada.
French
M B
MB
Moore
B C
BC
Kraicer
J
J
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002469:Cell Separation / Q000379:methods; D002478:Cells, Cultured; D002499:Centrifugation, Density Gradient; D000242:Cyclic AMP / Q000378:metabolism; D004110:Diltiazem / Q000494:pharmacology; D005456:Fluorescent Dyes; D013006:Growth Hormone / Q000378:metabolism; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D007211:Indoles; D007700:Kinetics; D008297:Male; D009543:Nifedipine / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013050:Spectrometry, Fluorescence
D005456:Fluorescent Dyes; D007211:Indoles; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D000242:Cyclic AMP; D004110:Diltiazem; D009543:Nifedipine; C048960:indo-1; D002118:Calcium