Sodium transport and intracellular sodium activity in cultured human nasal epithelium

Human airway epithelia are predominantly Na+-absorbing epithelia. To investigate the mechanisms for Na+ absorption across airway epithelia, the driving forces and paths for Na+ translocation across each membrane were examined with double-barreled Na+-selective microelectrodes in cultured human nasal epithelium (HNE). Under control conditions, intracellular Na+ activity (a(c)(Na)) was 23 ± 1 mM (n = 44 preparations, 393 impalements). Amiloride (10-4 M) hyperpolarized the apical membrane and increased the fractional apical membrane resistance but did not affect a(c)(Na). Exposure to Na+-free luminal solution induced bioelectric responses similar to amiloride but also reduced a(c)(Na) to 8 ± 1 mM. Reduction of luminal Na+ concentration ([Na+]) in the presence of amiloride also reduced a(c)(Na) without further changes in bioelectric parameters. Reduction of serosal [Na+] decreased a(c)(Na), a response blocked by bumetanide (10-4 M). Ouabain (10-4 M, serosal) led to a reduction in equivalent short-circuit current (I(eq)) and increase in a(c)(Na). We conclude that 1) a(c)(Na) is higher in HNE than in most mammalian epithelial cells, 2) the apical membrane expresses a conductive Na+ path, and 3) the basolateral membrane transports Na+ via the Na+-K+-adenosinetriphosphatase and a Na+-K+-2Cl- cotransport system.