Insulin action on cardiac glucose transport. Studies on the role of the sodium pump.

Isolated muscle cells from adult rat heart have been used to study the relationship between myocardial glucose transport and the activity of the Na+, K+ pump. 86Rb+ uptake by cardiac cells was found to be linear up to 2 min, with a steady state reached by 40-60 min, and was used to monitor the activity of the sodium pump. Both the ouabain-sensitive and ouabain-insensitive 86Rb+ uptake by cardiac cells were found to be unaffected by insulin treatment under conditions in which a significant stimulation of 3-O-methylglucose transport occurred. 86Rb+ uptake was markedly reduced by the presence of calcium or magnesium or both, but remained unresponsive toward insulin treatment. Inhibition of the sodium-pump activity by ouabain and a concomitant shift in the intracellular Na+/K+ ratio did not affect basal or insulin-stimulated rates of 3-O-methylglucose transport in cardiac myocytes. The data argue against a functional relationship between the myocardial Na+, K+ pump and the glucose-transport system.