Stoichiometry of NA+−CA2+ exchange is 3:1 in guinea‐pig ventricular myocytes

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Abstract
In single guinea-pig ventricular myocytes, we examined the stoichiometry of Na+-Ca2+ exchange (NCX) by measuring the reversal potential (ENCX) of NCX current (INCX) and intracellular Ca2+ concentration ([Ca2+]i) with the whole-cell voltage-clamp technique and confocal microscopy, respectively. With given ionic concentrations in the external and pipette solutions, the predicted ENCX were −73 and −11 mV at 3:1 and 4:1 stoichiometries, respectively. ENCX measured were −69 ± 2 mV (n= 11), −47 ± 1 mV (n= 14) and −15 ± 1 mV (n= 15) at holding potentials (HP) of −73, −42 and −11 mV, respectively. Thus, ENCX almost coincided with HP, indicating that [Ca2+]i and/or [Na+]i changed due to INCX flow. Shifts of ENCX (ΔENCX) were measured by changing [Ca2+]o or [Na+]o. The measured values of ΔENCX were almost always smaller than those expected theoretically at a stoichiometry of either 3:1 or 4:1. Using indo-1 fluorescence, [Ca2+]i measured under the whole-cell voltage-clamp supported a 3:1 but not 4:1 stoichiometry. To prevent Ca2+ accumulation, we inhibited INCX with Ni2+ and re-examined ENCX during washing out Ni2+. With HP at predicted ENCX at a 3:1 stoichiometry, ENCX developed was close to predicted ENCX and did not change with time. However, with HP at predicted ENCX for a 4:1 stoichiometry, ENCX developed initially near a predicted ENCX for a 3:1 stoichiometry and shifted toward ENCX for a 4:1 stoichiometry with time. We conclude that the stoichiometry of cardiac NCX is 3:1.