Translocation mechanism of cardiac Na-Ca exchange.

In single cardiac ventricular cells of guinea pig, we have studied the ionic translocation mechanism of the electrogenic Na-Ca exchange, that is, whether Na and Ca ions countercross the membrane simultaneously or consecutively. The dose-response relations between the external Ca ([Ca]o) and the outward Na-Ca exchange current were measured at three different internal Na concentrations ([Na]i) in the absence of external Na. Hyperbolic regression curves and Hanes-Woolf linear plots of the dose-response relation revealed that apparent Km values for external Ca (K'mCao) decrease progressively as [Na]i decreases. The ratio of K'mCao to apparent Imax value (I'max) showed a slight increasing tendency as [Na]i decreased. We previously interpreted the data as consistent with the simultaneous mechanism but without statistical analysis. Here we performed careful statistical analysis, which indicated that the K'max/I'max values were not significantly different among the different [Na]i at most of the potentials. This result suggests that Na-Ca exchange is likely to be a consecutive mechanism.