Inhibition by amiloride of sodium transport into rabbit kidney medulla microsomes

Sodium transport into rabbit kidney medulla microsomes was 50% inhibited by amiloride. This Na⁺ uptake was shown to represent transport when the uptake process was reversed by the ionophore nigericin. The transport was complete within 60 min and proportional to the microsomal protein concentration. The effect of amiloride on transport was specific since the similar compound sulfaguanidine failed to affect microsomal Na⁺ transport. Amiloride-sensitive Na⁺ transport into microsomes was inhibited 70% by decreasing the pH (from 7.0 to 5.9), but was unaffected by the presence of a pH gradient. The kinetics of Na⁺ transport could be explained by a simple model, assuming that amiloride lowered the rate of Na⁺ entrance into the vesicles but had no effect on the rate of efflux. The failure of amiloride to effect efflux from the vesicles was also demonstrated directly.