Figure 5.

Dependence of the Na⁺,K⁺-ATPase transported charge-voltage relationship (ΔQ-V curve) on the extracellular Na⁺ concentration after voltage jumps from the membrane voltage V_m to −40 mV. Each symbol corresponds to the following Na⁺ concentrations: 36.3 mM (▾), 72.5 mM (○), and 145 mM (●). (Solid lines between the points have been drawn to aid the eye.) (Upper curve) Experimental results for rat heart myocytes, obtained via the voltage-clamp technique, reproduced from Fig. 3C of Peluffo (24). The transported charge, ΔQ, has been divided by the capacitance of each cell to correct for variations in cell size. The experimental conditions were cytoplasmic [Na⁺] = 120 mM, cytoplasmic [K⁺] = extracellular [K⁺] = 0 mM, and cytoplasmic [ATP] = 15 mM, T = 23°C. (Lower curve) Computer simulations of the ΔQ-V curve for mammalian heart Na⁺,K⁺-pump current based on the Albers-Post scheme described by Figs. 1 and 3 and the kinetic and equilibrium parameters given in Table 1 of Garcia et al. (23); however, a microscopic dissociation constant of Na⁺ for the nonspecific transport sites on E2P state of 180 mM and a dissociation constant of Na⁺ to the specific site on E2P of 40 mM have been used. The ion concentrations, ATP concentration, and temperature used for the simulations were the same as for the upper curve.