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.