Sodium-calcium exchange current: Dependence on internal ca and na and competitive binding of external Na and Ca

Abstract

Na-Ca exchange current was measured at various concentrations of internal Na ([Na]i) and Ca ([Ca]i) 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-Ca0 exchange beginning at ~10 nM and saturating at ~50 nM with a half maximum [Ca]i (Km[Ca]i) of 22 nM (Hill coefficient, 3.7). Measurement of Nai-Ca0 exchange current at various concentrations of [Na]i revealed an apparent Km[Na]i of 20.7 ± 6.9 mM (n - 14) with of 3.5 ± 1.2 µA/µF. For [Ca]i transported by the exchange, a Km[Ca]i of 0.60 ± 0.24 µM (n - 8) with an of 3.0 ± 0.54 µA/µF was obtained by measuring Nai-Ca0 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-Ca0 exchange current became larger by reducing [Na]0. The double reciprocal plot of the current magnitude and [Ca]0 at different [Na]0 revealed a competitive interaction between Na and Ca. In the absence of competitor [Na]o an apparent Km[Ca]0 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. © 1989, Rockefeller University Press., All rights reserved.