Intracellular calcium concentration and growth hormone secretion in individual somatotropes: effects of growth hormone-releasing factor and somatostatin.
The cytosolic free calcium concentration and cumulative GH release were measured simultaneously in normal pituitary cells. This was made possible by a novel combination of fluorescence microscopy using the calcium indicator fura-2 and a reverse hemolytic plaque assay. GRF (10 nM) rapidly increased the intracellular free calcium concentration ([ Ca2+]i) from a basal level of 234 +/- 17 nM (mean +/- SE) to a peak value of 480 +/- 61 nM 1 min after stimulation. This GRF-induced calcium rise was totally abolished in calcium-free medium or in the presence of calcium channel blockers cobalt chloride (2 mM) and verapamil (100 microM). When somatostatin (SRIF; 1 nM) was added after basal recordings, cytosolic calcium decreased to 96 +/- 23 nM in identified somatotropes. [Ca2+]i returned to baseline upon the removal of SRIF inhibition. This rebound was higher when a sequential treatment of SRIF followed by GRF was applied. Exposing cells to a combination of GRF (10 nM) plus SRIF (1 nM) resulted in a decrease in [Ca2+]i identical to that caused by SRIF treatment alone. Despite the 10-fold excess of GRF, SRIF not only inhibited hormone secretion, but also totally overcame the GRF-induced rise of [Ca2+]i. In summary, stimulation by GRF increases cytosolic calcium in normal somatotropes. This increase is proposed to be due to the influx of calcium through membrane ion channels. In contrast, SRIF decreases [Ca2+]i. This might explain the cAMP-independent effects of this peptide. The effect of SRIF dominates over that of GRF with respect to both changes in [Ca2+]i and hormone release. Changes in the GH secretory rate are, therefore, accompanied by parallel changes in [Ca2+]i, both of which are primarily regulated by SRIF.
D000818:Animals; D001572:Benzofurans; D002118:Calcium / Q000378:metabolism*; D003035:Cobalt / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism; D005456:Fluorescent Dyes; D016257:Fura-2; D013006:Growth Hormone / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D006462:Hemolytic Plaque Technique; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism; D008297:Male; D008856:Microscopy, Fluorescence; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D013004:Somatostatin / Q000494:pharmacology*; D014700:Verapamil / Q000494:pharmacology
D001572:Benzofurans; D005456:Fluorescent Dyes; D007473:Ion Channels; D003035:Cobalt; D013004:Somatostatin; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D014700:Verapamil; C018021:cobaltous chloride; D002118:Calcium; D016257:Fura-2