Sodium-calcium exchange in mammalian heart: current-voltage relation and intracellular calcium concentration

Membrane currents and changes in intracellular calcium ion concentration (Ca²⁺]_i) have been recorded that can be attributed to the operation of an electrogenic, voltage-dependent sodium-calcium (Na-Ca) exchanger in mammalian heart cells. Single guinea-pig ventricular myocytes under voltage clamp were perfused internally with the fluorescent Ca²⁺-indicator, fura-2, and changes in Ca²⁺]_i and membrane current that resulted from Na-Ca exchange were isolated through the use of various organic channel blockers (verapamil, TTX), impermeant ions (Cs⁺, Ni²⁺), and inhibitors of sarcoplasmic reticulum (ryanodine). The I-V relation of Na-Ca exchange was obtained from the Ni²⁺-sensitive current elicited by ramp repolarization from +90 mV to –80 mV. Ramps were sufficiently rapid that little change in Ca²⁺]_i occured during the ramp. The (constant) Ca²⁺]_i during the ramp was varied over the range 100 nM to 1000 nM by varying the amplitude and duration of a pre-pulse to the ramp. The reversal potential of the Ni²⁺-sensitive ramp current varied linearly with 1n(Ca²⁺])_i. The I-V relations at different Ca²⁺]_i over the range –60 mV to +140 mV were in reasonable accord with the predictions of a simple, simultaneous scheme of Na-Ca exchange, on the basis that only Ca²⁺]_i had changed. The relationship between Ca²⁺]_i and current at a constant membrane voltage was also in accord with this scheme. We suggest that Ca²⁺-fluxes through the exchanger during the cardiac action potential can be understood quantitatively by considering the binding of Ca²⁺ to the exchanger during the Ca²⁺]_i-transient and the effects of membrane voltage on the exchanger.