Transcellular sodium transport in cultured cystic fibrosis human nasal epithelium

Cystic fibrosis (CF) airway epithelia exhibit raised transepithelial Na+ transport rates, as determined by open-circuit isotope fluxes and estimates of the amiloride-sensitive equivalent short-circuit current (I(eq)). To study the contribution of apical and basolateral membrane paths to raised Na+ transport in CF, CF nasal epithelial cultures were studied with double-barreled Na+-selective microelectrodes and the Ussing chamber technique. Intracellular Na+ activity (a(c)(Na)) was 24.1 ± 1.5 mM (n = 36), a value similar to a(c)(Na) of normal nasal epithelial cells. Reduction of luminal [Na+] to 3 mM abolished I(eq) and reduced a(c)(Na). Amiloride (10-4 M) abolished I(eq) but increased a(c)(Na) from 20 ± 2 to 36 ± 7 mM (n = 10). Amiloride-induced increase in a(c)(Na) was not affected by serosal [Na+] reduction but was blocked by preexposure to reduced luminal [Na+]. Amphotericin B increased I(eq) during amiloride exposure, indicating that amiloride did not inhibit Na+-K+-ATPase. Ouabain abolished I(eq) and slowly raised a(c)(Na). Reduction of serosal [Na+] led to a decrease in a(c)(Na) that was blocked by bumetanide. It is concluded that 1) CF airway epithelia exhibit an increased apical membrane Na+ permeability, 2) a(c)(Na) is regulated to a normal level in CF cells despite increased transcellular Na+ fluxes, 3) the abnormal increase in a(c)(Na) in response to amiloride is dependent on luminal Na+, 4) Na+ is transported across the basolateral membrane by a bumetanide-sensitive cotransport mechanism, and 5) ouabain inhibits the basolateral Na+-K+-ATPase, causing slow dissipation of the chemical and electrical gradients across the cell membranes.