Biochemical identification of two types of phenamil binding sites associated with amiloride-sensitive Na+ channels

The existence of distinct forms of the epithelium Na+ channel that differ in their sensitivity to amiloride has been repeatedly suggested by physiological data. The biochemical basis for these differences was analyzed by using phenamil, the most potent inhibitor known so far for the epithelium Na+ channel. [3H]Phenamil of high radioactive specific activity (30 Ci/mmol) was prepared and used to titrate [3H]phenamil binding sites in pig kidney membranes. Kinetic experiments, equilibrium binding studies, and competition experiments indicated the presence in crude membrane preparations of two classes of independent binding sites. A first binding site was characterized by a high affinity for phenamil (Kd1 = 0.4 nM) and for amiloride (Kd1 = 0.1 microM). A second binding site recognized phenamil and amiloride with lower affinities [Kd2(phenamil) = 28 nM, Kd2(amiloride) = 4 microM]. The ratio of the respective amounts of low- and high-affinity binding sites was 14 +/- 2 in different membrane preparations (range: 6-22). The two types of binding sites for [3H]phenamil copurified and were still observed after purification of the epithelium Na+ channel to homogeneity. These results indicate that at least two types of pharmacologically distinguishable Na+ channels exist in the kidney. They correspond either to two isoforms of the apical Na+ channel or to one single type of channel under two different states of covalent regulation.