Characteristics of active Na transport in intact cardiac cells.

An active Na transport maintains the Na and K concentration gradients across the cell membrane of many cells and restores them following excitation. Heart muscle cells display frequent electrical discharges and thus the cardiac Na pump is of fundamental functional significance. Some methods for studying active Na transport are described. The active Na efflux from heart muscle cells is activated by an increase in the intracellular Na and the extracellular K concentration. The linkage between active Na efflux and active K influx varies widely according to the experimental conditions. The cardiac Na pump is electrogenic and can contribute directly to the membrane potential of the cells. The effects of active Na transport on contraction and intercellular coupling in myocardium are discussed.