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 half-maximum concentration (K(h)) of cytoplasmic calcium (Ca(i)) for activation of the isolated inward exchange current decreased as the extracellular sodium (Na(o)) concentration was decreased. The K(h) of cytoplasmic sodium (Na(i)) 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 (twofold change in approximately 100 mV) and a slight saturation tendency at very positive potentials. The outward current gained in steepness as the Na(i) 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(alpha.Em)/26.6) with a slope (a) 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 (twofold change in approximately 100 mV) and showed some saturation tendency with hyperpolarization as the Ca(i) concentration was decreased. The K(h) 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. concentration. (e) In the presence of both Na and Ca on both membrane sides, the I-V relations with saturating Na(i) 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 voltage-dependent 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 Na(i) 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 muM 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.