Sodium-calcium exchange current. Dependence on internal Ca and Na and competitive binding of external Na and Ca.

Na-Ca exchange current was measured at various concentrations of internal Na ( Nai) and Ca ( Cai) using intracellular perfusion technique and whole-cell voltage clamp in single cardiac ventricular cells of guinea pig. Internal Ca has an activating effect on Nai-Cao exchange beginning at approximately 10 nM and saturating at approximately 50 nM with a half maximum Cai (KmCai) of 22 nM (Hill coefficient, 3.7). Measurement of Nai-Cao exchange current at various concentration of Nai revealed an apparent KmNai of 20.7 +/- 6.9 mM (n = 14) with imax of 3.5 +/- 1.2 microA/microF. For Cai transported by the exchange, a KmCai of 0.60 +/- 0.24 microM (n = 8) with an imax of 3.0 +/- 0.54 microA/microF was obtained by measuring Nao-Cai exchange current. These values are apparently different from the values for the external binding site which have been reported previously. Whether Na and Ca compete for the external binding site, and if so, how it affects the binding constants was then investigated. Outward Nai-Cao exchange current became larger by reducing Nao. The double reciprocal plot of the current magnitude and Cao at different Nao revealed a competitive interaction between Na and Ca. In the absence of competitor Nao, an apparent KmCao of 0.14 mM was obtained. When comparing internal and external Km values, the external value is markedly larger than the internal one and thus we conclude that binding sites of the Na-Ca exchange molecule are at least apparently asymmetrical between the inside and outside of the membrane.