Nitric oxide inhibits Na+ absorption across cultured alveolar type II monolayers

We examined the mechanisms by which nitric oxide (. NO) decreased vectorial Na+ transport across confluent monolayers of rat alveolar type II (ATII) cells grown on permeable supports. Amiloride (10 mu M) applied to the apical side of monolayers inhibited similar to 90% of the equivalent (I-eq) and the short-circuit (I-sc) current, with an half-maximal inhibitory concentration (IC50) of 0.85 mu M, indicating that Na+ entry into ATII cells occurred through amiloride-sensitive Na+ channels. . NO generated by spermine NONOate and papa NONOate added to both sides of the monolayers decreased I-eq and increased transepithelial resistance in a concentration-dependent fashion (IC50 = 0.4 mu M . NO). These changes were prevented or reversed by addition of oxyhemoglobin (50 mu M). Incubation of ATII monolayers with 8-bromoguanosine 3',5'-cyclic monophosphate (400 mu M) had no effect on transepithelial Na+ transport. When the basolateral membranes of ATII cells were permeabilized with amphotericin B (10 mu M) in the presence of a mucosal-to-serosal Na+ gradient (145:25 mM), . NO (generated by 100 mu M papa NONOate) inhibited similar to 60% of the amiloride-sensitive I-sc. In addition, after permeabilization of the apical membranes, . NO inhibited the I-sc [a measure of Na+-K+-adenosinetriphosphatase (ATPase) activity] by similar to 60%. We concluded that . NO at noncytotoxic concentrations decreased Na+ absorption across cultured ATII monolayers by inhibiting both the amiloride-sensitive Na+ channels and Na+-K+-ATPase through guanosine 3',5'-cyclic monophosphate-independent mechanisms.