POLARIZED DISTRIBUTION OF THE NA+/H+ EXCHANGE SYSTEM IN A RENAL-CELL LINE (LLC-PK1) WITH CHARACTERISTICS OF PROXIMAL TUBULAR CELLS

  • 5 March 1986
    • journal article
    • research article
    • Vol. 261  (7) , 3252-3258
Abstract
Studies of Na+ and H+ transport by confluent monolayers of the epithelial cell line LLC-PK1 were performed to verify the presence of a Na+/H+ exchange system. The presence of an outwardly directed H+ gradient produced a large stimulation of Na+ influx measured under net flux conditions. Amiloride (10-3 M) completely inhibited Na+ influx stimulated by the H+ gradient and part of the Na+ influx measured in the absence of a pH gradient. Half-maximal inhibition of the Na+ influx stimulated by a pH gradient at 143 mM Na was observed at 5 .mu.M amiloride. The presence of an inwardly oriented proton gradient also stimulated Na+ efflux from Na+-loaded cells. The stimulation was completely inhibited by the presence of 10-3 M amiloride in the washout medium. These results indicate that this system could operate in the opposite direction depending on the orientation of the Na+ and H+ gradient. Incubation in Na+-free medium or in the presence of 10-3 ouabain resulted in a dramatic decrease of H+ release from LLC-PK1 cells. This H+ release was largely, although not completely, inhibited by 10-4 M amiloride. Neither chloride substitution by the impermeable anion isethionate nor incubation in the presence of the ionophore valinomycin in high K+ medium affected Na+ influx stimulated by a pH gradient. Inhibition of the Na+ influx by amiloride occurred only from the apical side of the monolayer. These results indicate that the Na+/H+ exchange system in LLC-PK1 monolayers is specifically localized in the apical membrane of the epithelial cells.