The Na+/H+ antiport in renal mitochondria

In isolated renal mitochondria, Na+ and Li+ stimulated H+ efflux from the mitochondrial matrix. In submitochondrial particles (SMP), Na+ flux was also coupled to H+ transport in the opposite direction. The overshoot of Na+ uptake in SMP with an outwardly directed H+ gradient indicated that downhill efflux of H+ through the mitochondrial membrane induced uphill transport of Na+. Similar to the Na+/H+ antiport in other types of mitochondria, the antiport in renal mitochondria was more sensitive to amiloride derivatives than to amiloride itself. Benzamil and ethylisopropylamiloride (EIPA), but not amiloride, inhibited the antiport, with 50% inhibition of 10(-4) M for both benzamil in mitochondria and EIPA in SMP. The Na+/H+ antiport in renal mitochondria had simple saturation kinetics for external Na+ [Michaelis-Menten constant (Km) = 3.27 +/- 0.63 mM; maximal velocity (Vmax) = 0.022 +/- 0.002 pH units/s] and Li+ (Km = 3.62 +/- 0.75 mM; Vmax = 0.022 +/- 0.002 pH units/s). NH4Cl and NH4 acetate stimulated Na+ efflux and inhibited Na+ uptake in SMP. Comparable results with NH4 acetate and chloride suggested that NH4+ modified Na+ transport through its direct interaction with the Na+/H+ antiport, rather than through the alkalinization of intra-SMP space from non-ionic diffusion of NH3. These results suggested that the Na+/H+ antiport may be a factor in the exit of NH4+ from renal mitochondria.