GROWTH-HORMONE REGULATES CYTOSOLIC-FREE CALCIUM IN RAT FAT-CELLS BY MAINTAINING L-TYPE CALCIUM CHANNELS

In freshly isolated individual rat adipocytes, cytosolic free Ca2+ con centration ([Ca2+](i)) as measured with fura 2 slowly declined during incubation but was sustained, or even somewhat increased, by brief tre atment with growth hormone (GH) at the beginning of a 3-h incubation p eriod. GH-treated adipocytes were more permeable to Ca2+ than GH-depri ved cells as indicated, using Mn2+ as a surrogate and monitoring in fl ux by the rate of quenching of fura 2 fluorescence. Blockade of Ca2+ c hannels with 100 nM nimodipine lowered [Ca2+](i) in GH-treated cells t o the level seen in GH-deprived cells. Increases in [Ca2+](i) or the r ate of Mn2+ entry were twofold greater in GH-treated than in GH-depriv ed cells when extracellular K+ was increased to 30 mM. Similarly, the Ca2+ channel agonist BAY K 5552 or the diacylglycerol analogue 1,2-dio ctanoyl-sn-glycerol increased [Ca2+]i more in GH-treated than in GH-de prived adipocytes. Ca2+-ATPase activity was two times higher in plasma membranes isolated from GH-treated than from GH-deprived cells. Conti nued synthesis of Ca2+-ATPase may depend on [Ca2+](i), since the effec ts of GH on [Ca2+](i) and Ca2+-ATPase were blocked by cycloheximide or verapamil. We suggest that voltage-sensitive L-type Ca2+ channels reg ulate steady-state [Ca2+](i) in rat adipocytes and that GH maintains t he number or functional integrity of these channels.