Voltage and Ca2+ dependence of pre-steady-state currents of the Na-Ca exchanger generated by Ca2+ concentration jumps

The Ca2+ concentration and voltage dependence of the relaxation kinetics of the Na-Ca exchanger after a Ca2+ concentration jump was measured in excise d giant membrane patches from guinea pig heart. Ca2+ concentration jumps on the cytoplasmic side were achieved by laser flash-induced photolysis of DM -nitrophen. In the Ca-Ca exchange mode a transient inward current is genera ted. The amplitude and the decay rate of the current saturate at concentrat ions > 10 muM. The integrated current signal, i.e., the charge moved is fai rly independent of the amount of Ca2+ released. The amount of charge transl ocated increases at negative membrane potentials, whereas the decay rate co nstant shows no voltage dependence. It is suggested that Ca2+ translocation occurs in at least four steps: intra- and extracellular Ca2+ binding and t wo intramolecular transport steps. Saturation of the amplitude and of the r elaxation of the currrent can be explained if the charge translocating reac tion step is preceded by two nonelectrogenic steps: Ca2+ binding and one co nformational transition. Charge translocation in this mode is assigned to o ne additional conformational change which determines the equilibrium distri bution of states. In the Na-Ca exchange mode, the stationary inward current depends on the cytoplasmic Ca2+ concentration and voltage. The Km for Ca2 is 4 muM for guinea pig and 10 muM for rat myocytes. The amplitude of the pre-steady-state current and its relaxation saturate with increasing Ca2+ c oncentrations. In this mode the relaxation is voltage dependent.