Regulation of amiloride-sensitive Na(+) transport by basal nitric oxide

We investigated the mechanisms of endogenous nitric oxide (NO) modulation of lung sodium (Na(+)) transport. C57BL/6 mice injected intraperitoneally with the specific inducible NO synthase (iNOS) inhibitor 1400W (10 mg/kg every 8 h for 72 h) exhibited decreased alveolar nitrite levels and Na(+)-dependent amiloride-sensitive alveolar fluid clearance as compared with mice injected with vehicle. Similarly, pretreatment of mouse tracheal epithelial cells with 1400W abolished the inhibitory effects of amiloride on their Na(+) short circuit currents. On the other hand, mouse tracheal epithelial cells pretreated with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, a specific inhibitor of guanylate cyclase, had lower levels of cGMP, but normal values of amiloride-sensitive Na(+) currents. Amiloride also inhibited whole-cell Na(+) currents across A549 cells treated with vehicle (K(i) = 249 nM), but had no effect in A549 cells treated with 1400W. Western blotting studies showed significantly lower levels of alpha and gammaENaC in lung tissues and alveolar type II (ATII) cells from iNOS(-/-) as well as iNOS(+/+) mice treated with 1400W, as compared with the corresponding values from vehicle-treated iNOS(+/+) mice. Similar values for ratios of alpha, beta, and gammaenac to gapdh were obtained by real-time polymerase chain reaction for iNOS(+/+) mice and iNOS(-/-) mice. We concluded that NO derived from iNOS under basal conditions is necessary for amiloride-sensitive Na(+) transport across lung epithelial cells and modulates the amount of alpha and gammaENaC via post-transcriptional, cGMP-independent mechanisms.