Growth hormone increases intracellular free calcium in rat adipocytes: correlation with actions on carbohydrate metabolism.

Adipocytes that have been preincubated for 3 h or more in hormone-free medium respond to GH with a transient insulin-like increase in glucose metabolism, followed by a period of refractoriness to further insulin-like stimulation. Adipocytes freshly isolated from normal rats and adipocytes that were exposed to GH in the first hour of a 4-h incubation period are refractory to this insulin-like effect. Because earlier studies revealed a relationship between refractoriness and cellular calcium, we examined the effects of GH on the intracellular calcium concentration ([Ca2+]i) under a variety of conditions in which sensitivity to insulin-like stimulation or refractoriness is known to be affected. A dual nitrogen laser imaging microscope with computer-assisted image processing to measure fluorescence changes in individual adipocytes loaded with fura-2 AM was used to measure [Ca2+]i. After prolonged incubation in vitro, resting [Ca+2]i was 120 +/- 6 nM and remained unchanged for more than 1 h after the addition of 100 ng/ml human GH (hGH), which doubled the rate of incorporation of [3-3H] glucose into triglycerides in this interval. Lipogenesis declined in the second hour, and [Ca+2]i slowly increased, reaching 324 +/- 49 nM by the end of the third hour (P less than 0.05). When added with GH, actinomycin-D, an inhibitor of RNA synthesis, caused the accelerated rate of lipogenesis to persist for at least 5 h and blocked the delayed increase in resting [Ca+2]i. Resting [Ca2+]i in freshly isolated, and hence refractory, adipocytes was 342 +/- 34 nM and declined to 112 +/- 11 nM concomitant with acquisition of insulin-like sensitivity to GH. The addition of 100 ng/ml hGH to these cells at the beginning of incubation, under conditions known to sustain refractoriness prevented the decline in resting [Ca2+]i and enabled them to exhibit a further acute increase in [Ca2+]i in response to a second exposure to hGH in the fourth hour. When added 60 min after GH, actinomycin-D blocked the ability to raise [Ca2+]i acutely in response to GH, but did not interfere with GH's action to sustain resting [Ca2+]i, which remained at about 300 nM. The concentration of GH needed to increase [Ca2+]i acutely in refractory cells is about 6-fold higher than that needed to maintain resting [Ca+2]i. Differences in the time of sensitivity to actinomycin-D and dose dependency suggest that sustaining resting [Ca2+]i and production of the acute increase in [Ca2+]i are separate phenomena.(ABSTRACT TRUNCATED AT 400 WORDS)