CURRENT-VOLTAGE RELATIONS AND STEADY-STATE CHARACTERISTICS OF NA-CA2+EXCHANGE - CHARACTERIZATION OF THE 8-STATE CONSECUTIVE TRANSPORT MODEL()

An analytical expression for Na+-Ca2+ exchange currents in cardiac cel ls has been obtained for an eight-state model, The equation obtained h as been used to derive theoretical expressions for current-voltage rel ationships, maximum N+-Ca2+ exchange currents, and half-saturating con centrations for Na+ and Ca2+. These equations were analyzed over a wid e 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 expe riments are presented. Rate constants from published reports were used to evaluate turnover rates for Na+-Ca2+ exchange in the forward and r everse directions, A factor, epsilon, is introduced that permits predi ction of the extent to which the Na+-Ca2+ exchange cycle is under volt age or diffusion control. This factor can be conveniently used for dat a interpretation and comparison. The derived equations also provide a foundation for continuing experimental evaluation of the fidelity of t his model.