HUMAN GROWTH HORMONE INCREASES THE INTRACELLULAR FREE CALCIUM IN CULTURED HUMAN IM-9 LYMPHOCYTES

Abstract

We studied the effects of human GH (hGH) on intracellular free calcium concentration ([Ca2+]i) in cultured IM-9 lymphocytes an established human cell line possessing typical somatogenic receptors. [Ca2+]i was measured on cell suspensions by fluorescence spectrometry using the fluorescent probe fura-2. Resting [Ca2+]i levels were 93 ± 15 nM. After the addition of 5 nM hGH, [Ca2+]i levels remained unchanged for about 20 min, then increased steadily and continuously, up to 186 ± 20 nM 40 min after GH stimulation. No change in [Ca2+]i was observed for up to 1 h in unstimulated cells. The effect of hGH was both specific (not observed after either insulin or IGF-1) and dose-dependent {maximal at 5 nM), and depended on the presence of calcium (1 nM) in the extracellular medium, suggesting a calcium influx. Somatostatin-14 (SMS) in the range 10−10 to 10−8 M had no effect on [Ca2+]i but, when added together with hGH, completely prevented the GH-induced rise in [Ca2+]i. Preincubation of the cells with pertussis toxin (PTX) did not have any significant effects on the GH-stimulated[Ca2+]i increase, but it completely suppressed the inhibition of the GH effect by SMS. In conclusion: 1) GH promoted a slow, progressive and sustained increase in [Ca2+]i in cultured human IM-9 lymphocytes. 2) This effect probably involves a cation channel negatively regulated by a PTX-sensitive G-protein. 3) Measurements of [Ca2-]i provide an additional tool for investigating the mechanisms involved in the transduction of the GH signal.