Changes of the subsarcolemmal Na+ concentration in internally perfused cardiac cells.

Transients of Na+/K+ pump and of Na+/Ca2+ exchange current occur during whole-cell recording from cardiac cells upon quick changes of active Na+ efflux. The transients reflect a temporary loss of control of the subsarcolemmal Na+ concentration. Even in the steady state the control is not complete is certain cells. Quantitative studies on ion transport by whole-cell recording are meaningful only if an adequate control of the submembranal ionic composition is demonstrated.