Steady-state and dynamic properties of cardiac sodium-calcium exchange: Ion and voltage dependencies of the transport cycle

Ion and voltage dependencies of sodium-calcium exchange current were studied in giant membrane patches from guinea pig ventricular cells after deregulation of the exchanger with chymotrypsin. (a) Under zero-trans conditions, the haft-maximum concentration (Kh) of cytoplasmic calcium (Cai) for activation of the isolated inward exchange current decreased as the extracellular sodium (Na⁺) concentration was decreased. The Kh of cytoplasmic sodium (Nai) for activation of the isolated outward exchange current decreased as the extracellular calcium (Ca.o) concentration was decreased. (b) The current-voltage (I-V) relation of the outward exchange current with saturating concentrations of Na_i and Ca_o had a shallow slope (two fold change in ~100 mV) and a slight saturation tendency at very positive potentials. The outward current gained in steepness as the Nai concentration was decreased, such that the K_h for Na_i decreased with depolarization. The decrease of K_h for Na_i with depolarization was well described by a Boltzmann equation (e^α/26.6) with a slope (ct) of -0.06. (c) Voltage dependence of the outward current was lost as the Ca_o concentration was decreased, and the K_h for Ca_o increased upon depolarization with a Boltzmann slope of 0.26. (d) The I-V relation of the inward exchange current, under zero-trans conditions, was also almost linear (two fold change in ~100 mV) and showed some saturation tendency with hyperpolarization as the Ca_i concentration was decreased. The Kh for Ca_i decreased with depolarization (Boltzmann slope, -0.10). Voltage dependence of the inward current was decreased in the presence of a high (300 mM) Na_o concentration. (e) In the presence of both Na and Ca on both membrane sides, the I-V relations with saturating Nai show sigmoidal shape and clear saturation at positive potentials. Measured reversal potentials were close to the equilibrium potential expected for a 3 Na to 1 Ca exchange. (f) Na_i and Ca_i interacted competitively with respect to the outward current, but in a mixed competitive-noncompetitive fashion with respect to the inward current. (g) Ca_i inhibited the outward exchange current in a voltagedependent manner. The half-effective concentration for inhibition (K_i) by Ca_i increased upon depolarization with a Boltzmann slope of 0.32 in 25 mM Nai and 0.20 in 100 mM Na_i. (h) Na_i also inhibited the inward exchange current voltage dependently. The K_i decreased upon depolarization (Boltzmann slope, -0.11 at 3 p, M Ca_i and -0.10 at 1.08 mM Ca_i). (i) All described exchange current characteristics were well explained by consecutive-type exchange models, assuming (1) multiple voltage- and time-dependent Na occlusion/deocclusion steps in the Na translocation pathway, (2) a small voltage dependence of Ca occlusion/deocclusion on the cytoplasmic side, and (3) the existence of a binding site configuration that can be occupied by 1 Na ion and 1 Ca ion on the cytoplasmic side.