Calcium homeostasis in bovine somatotrophs: calcium oscillations and calcium regulation by growth hormone-releasing hormone and somatostatin.
The free intracellular calcium ion concentration ([Ca2+]i) was measured in single cells of a population containing 65-80% somatotrophs, using the fluorescent Ca(2+)-indicator Fura-2 and digital imaging microscopy. Spontaneous oscillations in [Ca2+]i ranging in frequency up to 1.5 oscillations per minute were observed in 30% of somatotrophs. These Ca2+ oscillations were blocked by the Ca2+ channel blocker CoCl2 and were thus proposed to be the result of influx of Ca2+ into the cell, possibly as the result of spontaneous electrical activity. GHRH (10-100 nM) increased [Ca2+]i in 61% of the cells studied, although the amplitude and dynamics of the response varied from cell to cell. Typically [Ca2+]i rose from 170 +/- 26 nM to 321 +/- 44 nM (n = 13) in response to a challenge with 66 nM GHRH. GHRH also increased the frequency of Ca2+ oscillations in a number of cells, and some previously quiescent cells showed Ca2+ oscillations following addition of GHRH. Forskolin, which raises cAMP levels in bovine anterior pituitary cells, also stimulated a sustained rise in [Ca2+]i in 10 out of 14 cells tested. Somatostatin (SS) (10-80 nM) rapidly reduced basal [Ca2+]i, blocked Ca2+ oscillations, and blocked the [Ca2+]i response to GHRH. The Ca2+ channel blocker CoCl2 (4 mM) had similar actions on [Ca2+]i to those of SS. These results suggest that GHRH and SS may regulate GH release by modulating Ca2+ entry into the cell through the cell membrane. The [Ca2+]i oscillations seen in a proportion of the somatotrophs were modulated in frequency by GHRH and SS, and are probably generated by influx of Ca2+ through channels in the cell membrane. Thus GH secretion may be regulated by changes in the mean level of [Ca2+]i, which in turn, may be influenced by the frequency of [Ca2+]i oscillations in bovine somatotrophs.
Rawlings
S R
SR
Department of Neuroendocrinology, AFRC Institute of Animal Physiology and Genetics Research, Babraham, Cambridgeshire, UK.
Hoyland
J
J
Mason
W T
WT
D000818:Animals; D002118:Calcium / Q000032:analysis / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects*; D002417:Cattle; D003035:Cobalt / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D004347:Drug Interactions; D016257:Fura-2; D013007:Growth Hormone-Releasing Hormone / Q000378:metabolism / Q000494:pharmacology*; D006706:Homeostasis / Q000187:drug effects* / Q000502:physiology; D008297:Male; D009991:Oscillometry; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000378:metabolism*; D013004:Somatostatin / Q000494:pharmacology*; D013050:Spectrometry, Fluorescence
D015220:Calcium Channels; D005576:Colforsin; D003035:Cobalt; D013004:Somatostatin; D013007:Growth Hormone-Releasing Hormone; C018021:cobaltous chloride; D002118:Calcium; D016257:Fura-2