Translocation mechanism of Na-Ca exchange in single cardiac cells of guinea pig

Abstract

We have studied in single cardiac ventricular cells of guinea pig the ionic translocation mechanism of the electrogenic Na-Ca exchange, i.e., whether Na and Ca ions countercross the membrane simultaneously or consecutively with "ping pong" kinetics. The dose-response relation between the external Ca concentrations ([Ca]o) and the current density of the outward Na-Ca exchange current were measured at three different intracellular Na concentrations ([Na]i) in the absence of external Na. Nonlinear regression curves of the dose-response relation obtained by computer revealed Michaelis-Menten type hyperbola from which the [Ca]o giving a half-maximal response (apparent KmCao or Km′Cao) and the apparent maximum current magnitude (Imax′) were estimated at each [Na]i. As [Na]i increased, the Km′JCao increased progressively and the value of Km′Cao/Imax′ tended to decrease. These results are consistent with the simultaneous mechanism. The Km′Cao/Imax′ values, however, were small and close to each other, so it was not possible to completely preclude a consecutive mechanism.