Current-voltage relations and steady-state characteristics of Na+-Ca2+ exchange: characterization of the eight-state consecutive transport model.

An analytical expression for Na+-Ca2+ exchange currents in cardiac cells has been obtained for an eight-state model. The equation obtained has been used to derive theoretical expressions for current-voltage relationships, maximum Na+-Ca2+ exchange currents, and half-saturating concentrations for Na+ and Ca2+. These equations were analyzed over a wide range of cytoplasmic and extracellular Na+ and Ca2+ concentrations, under forward and reverse "zero-trans" conditions. Correspondence of theoretical results with those obtained from giant excised patch experiments are presented. Rate constants from published reports were used to evaluate turnover rates for Na+-Ca2+ exchange in the forward and reverse directions. A factor, epsilon, is introduced that permits prediction of the extent to which the Na+-Ca2+ exchange cycle is under voltage or diffusion control. This factor can be conveniently used for data interpretation and comparison. The derived equations also provide a foundation for continuing experimental evaluation of the fidelity of this model.