NITRIC-OXIDE INHIBITS NA-II MONOLAYERS( ABSORPTION ACROSS CULTURED ALVEOLAR TYPE)

We examined the mechanisms by which nitric oxide (. NO) decreased vect orial Na+ transport across confluent monolayers of rat alveolar type I I (ATII) cells grown on permeable supports. Amiloride (10 mu M) applie d to the apical side of monolayers inhibited similar to 90% of the equ ivalent (I-eq) and the short-circuit (I-sc) current, with an half-maxi mal inhibitory concentration (IC50) of 0.85 mu M, indicating that Naentry into ATII cells occurred through amiloride-sensitive Na+ channel s. . NO generated by spermine NONOate and papa NONOate added to both s ides of the monolayers decreased I-eq and increased transepithelial re sistance 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'-c yclic monophosphate (400 mu M) had no effect on transepithelial Na+ tr ansport. When the basolateral membranes of ATII cells were permeabiliz ed with amphotericin B (10 mu M) in the presence of a mucosal-to-seros al 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 inhibitin g both the amiloride-sensitive Na+ channels and Na+-K+-ATPase through guanosine 3',5'-cyclic monophosphate-independent mechanisms.