Sarcolemmal glucose transport in Ca2+-tolerant myocytes from adult rat heart. Calcium dependence of insulin action

Cardiac myocytes were isolated from adult rat ventricles by a method which preserves their functional integrity, including long survival in physiological concentrations of Ca²⁺. Sarcolemmal glucose transport was assessed by measuring linear initial uptake rates of the nonmetabolized glucose analog3-O-methyl-d-glucose. Transport was saturable and showed competition byd-glucose and other features of chemical and stereo-selectivity. Transport was stimulated by insulin in a dose-dependent manner, resulting in an almost 5-fold increase inV_max, with little change inK_m. Stimulation of 3-methylglucose transport by insulin was largely Ca²⁺ -dependent. Omission of Ca²⁺ from the incubation medium caused a minor rise in basal 3-methylglucose uptake but the insulin-stimulated rise inV_max was only 30%. The Ca²⁺ antagonist D600 also antagonized stimulation of hexose transport by insulin. In all the above respects, 3-methylglucose transport in myocytes is identical to that in intact heart muscle. In addition, the decrease in insulin response by Ca²⁺ emission was partially reversed by subsequent return to a Ca²⁺ -containing medium. ATP levels remained stable in the absence of Ca²⁺, showing that the Ca²⁺ dependence did not reflect nonspecific cell damage.