Stimulation of glucose transport in rat adipocytes by calcium

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Abstract
Glucose transport in rat adipocytes was studied by monitoring the conversion of [1-14C]-glucose to 14CO2 in a system where glucose transport was made rate-limiting by increasing the flux through the pentose phosphate pathway with phenazine methosulphate, an agent which rapidly reoxidizes NADPH. Ca increased both basal and insulin-stimulated apparent rates of glucose transport by .apprx. 40%. The maximum velocity of the apparent rate rates of glucose transport was increased by extracellular Ca in the presence or absence of insulin. There was no change in the glucose concentration required for half-maximal rates of 14CO2 production. Ca enhanced the stimulation of apparent rates of glucose transport by insulin over a ragne of hormone concentrations. Adipocyte c[cyclic]AMP concentrations were significantly lowered by Ca under conditions which led to increased apparent rates of glucose transport. Co and NI, antagonists of Ca action, elevated adipocyte cAMP levels and inhibited apparent rates of glucose transport. Agents which inhibit transmembrane Ca (verapamil, tetracaine and procaine) inhibited apparent rates of glucose transport despite a reduction in adipocyte cAMP concentration. Apparently Ca may increase apparent rates of glucose transport in rat adipocytes by lowering intracellular cAMP concentration and by a further mechanism independent of changes in the level of cAMP. Glucose transport in rat adipocytes may be controlled, in part, by a cAMP-induced phosphorylation mechanism.