The regulation of selective and nonselective Na+ conductances in H441 human airway epithelial cells

Analysis of membrane currents recorded from hormone-deprived H441 cells showed that the membrane potential (V-m) in single cells ( approximately -80 mV) was unaffected by lowering [Na+](o) or [Cl-](o), indicating that cellular Na+ and Cl- conductances (G(Na) and G(Cl), respectively) are negligible. Although insulin ( 20 nM, similar to 24 h) and dexamethasone (0.2 mu M, similar to 24 h) both depolarized Vm by similar to 20 mV, the response to insulin reflected a rise in GCl mediated via phosphatidylinositol 3-kinase (PI3K) whereas dexamethasone acted by inducing a serum- and glucocorticoid-regulated kinase 1 (SGK1)-dependent rise in GNa. Although insulin stimulation/PI3K-P110 alpha expression did not directly increase G(Na), these maneuvers augmented the dexamethasone-induced conductance. The glucocorticoid/SGK1-induced GNa in single cells discriminated poorly between Na+ and K+ (P-Na/P-K similar to 0.6), was insensitive to amiloride ( 1 mM), but was partially blocked by LaCl3 (La3+; 1 mM, similar to 80%), pimozide ( 0.1 mM, similar to 40%), and dichlorobenzamil ( 15 mu M, similar to 15%). Cells growing as small groups, on the other hand, expressed an amiloride-sensitive ( 10 mu M), selective GNa that displayed the same pattern of hormonal regulation as the nonselective conductance in single cells. These data therefore 1) confirm that H441 cells can express selective or nonselective GNa ( 14, 48), 2) show that these conductances are both induced by glucocorticoids/SGK1 and subject to PI3K-dependent regulation, and 3) establish that cell-cell contact is vitally important to the development of Na+ selectivity and amiloride sensitivity.