Figure 4. Fitting of Na⁺–H⁺ exchanger (NHE) model (lines) to data (points) on NHE fluxes.

The initial rates of H⁺ efflux or Na⁺ influx were measured: A, with varying external buffer [Na⁺] and fixed external buffer pH = 7.0 and matrix pH = 6.95 in purified mitochondria from rat heart (the filled circles are the measured H⁺ efflux rates while the open circles are the calculated H⁺ efflux rates from the measured rates of decreased matrix pH with a fixed matrix buffering capacity of 36 nmol H⁺ mg⁻¹ (pH unit)⁻¹) (Kapus et al. 1989), and B, with varying external buffer pH and fixed external buffer [Na⁺] = 9.6 mm and 15 mm and matrix pH = 7.0 in purified mitochondria from rat liver (Kapus et al. 1988). The model fits to these data sets with K_Na,NHE = 22 mm and X_NHE = 12 nmol mg⁻¹ s⁻¹ (for both Na⁺ influx and H⁺ efflux). C and D, the fitting of the same kinetic model to another data set is shown where the initial rates of decrease in matrix pH or H⁺ efflux in rat heart mitochondria were measured with varying matrix pH and fixed external buffer pH = 7.0 and [Na⁺] = 50 mm (Kapus et al. 1989). For the kinetic model to fit this data set, the X_NHE value is adjusted to 18 nmol mg⁻¹ s⁻¹ while keeping the K_Na,NHE value fixed at 22 mm.