Sodium-hydrogen exchange system in LLC-PK₁ epithelium

Sodium influx into LLC-PK₁ cells has been characterized. The main amiloride-sensitive pathway for sodium entry through the apical membrane in this cell line is sodium-hydrogen exchange with an apparent K(m) for sodium of 19 mM. Influx is pH dependent and is amiloride sensitive with K( 1/2 ) of 30 μM. Inhibition of the sodium transport by protons (at 1 mM external sodium) is consistent with an interaction of H⁺ ions at a single site having an apparent pK(H) of 7.2. These data are similar to those reported previously for brush border membrane (BBM) isolated from kidney proximal tubule. However, in contrast to previously published reports, H⁺ does not compete with amiloride in blocking Na⁺ influx and exhibits a mixed inhibition with sodium ions. The latter, together with a direct and independent assessment of amiloride and sodium interaction, indicates that amiloride does not compete with sodium for the same site. Possible explanations for the discrepancy in the literature concerning the interaction of Na⁺, H⁺, and amiloride with the Na⁺-H⁺ exchanger and the characteristics of the Na⁺-H⁺ exchange system in LLC-PK₁ are discussed.