The effects of α- and β-adrenergic agents, Ca 2+ and insulin on 2-deoxyglucose uptake and phosphorylation in perfused rat heart

Abstract

Insulin (0.1 μM) and 1 μM epinephrine each increased the uptake and phosphorylation of 2-deoxyglucose by the perfused rat heart by increasing the apparent V max without altering the K m. Isoproterenol (10 μM), 50 μM methoxamine and 10 mM CaCl 2 also increased uptake. Lowering of the perfusate Ca 2+ concentration from 1.27 to 0.1 mM Ca 2+, addition of the Ca 2+ channel blocker nifedipine (1 μM) or addition of 1.7 mM EGTA decreased the basal rate of uptake of 2-deoxyglucose and prevented the stimulation due to 1 μM epinephrine. Stimulation of 2-deoxyglucose uptake by 0.1 μM insulin was only partly inhibited by Ca 2+ omission, nifedipine or 1 mM EGTA. Half-maximal stimulation of 2-deoxyglucose uptake by insulin occurred at 2 nM and 0.4 nM for medium containing 1.27 and 0.1 mM Ca 2+, respectively. Maximal concentrations of insulin (0.1 μM) and epinephrine (1 μM) were additive for glucose uptake and lactate output but were not additive for uptake of 2-deoxyglucose. Half-maximal stimulation of 2-deoxyglucose uptake by epinephrine occurred at 0.2 μM but maximal concentrations of epinephrine (e.g., 1 μM) gave lower rates of 2-deoxyglucose uptake than that attained by maximal concentrations of insulin. The addition of insulin increased uptake of 2-deoxyglucose at all concentrations of epinephrine but epinephrine only increased uptake at sub-maximal concentrations of insulin. The role of Ca 2+ in signal reversal was also studied. Removal of 1 μM epinephrine after a 10 min exposure period resulted in a rapid return of contractility to basal values but the rate of 2-deoxyglucose uptake increased further and remained elevated at 20 min unless the Ca 2+ concentration was lowered to 0.1 mM or nifedipine (1 μM) was added. Similarly, removal of 0.1 μM insulin after a 10 min exposure period did not affect the rate of 2-deoxyglucose uptake, which did not return to basal values within 20 min unless the concentration of Ca 2+ was decreased to 0.1 mM. Insulin-mediated increase in 2-deoxyglucose uptake at 0.1 mM Ca 2+ reversed upon hormone removal. It is concluded that catecholamines mediate a Ca 2+-dependent increase in 2-deoxyglucose transport from either α or β receptors. Insulin has both a Ca 2+-dependent and a Ca 2+-independent component. Reversal studies suggest an additional role for Ca 2+ in maintaining the activated transport state when activated by either epinephrine or insulin.