SODIUM-CHANNEL BUT NEITHER NA+-H+ NOR NA-GLUCOSE SYMPORT INHIBITORS SLOW NEONATAL LUNG WATER CLEARANCE

Normal clearance of alveolar liquid following birth requires active Na transport; however, the contribution of Na channels, Na-H antiports, and Na-glucose symports is unknown. We demonstrated that intraalveolar instillation of amiloride (n = 6) or the more specific Na channel blockers benzamil (n = 13) or phenamil (n = 12) before the first breath impaired lung water clearance relative to control newborns (n = 34). Benzamil and phenamil were more potent than amiloride (P < 0.05). Neither the Na-H antiport inhibitor dimethyl amiloride (n = 7) nor the Na-glucose symport inhibitor phloridzin (n = 7) impaired lung water clearance. Ion substitution experiments with fetal rat type II alveolar epithelia demonstrated that more than 95% of their resting or terbutaline-stimulated short circuit current (I(sc)) depended upon Na bathing their apical membrane. I(sc) was decreased by amiloride (IC50 of amiloride-sensitive I(sc) = 0.3 x 10(-6) M) and benzamil (IC50 of benzamil-sensitive I(sc) = 0.3 x 10(-7) M) but was unaffected by dimethyl amiloride (10(-4) M). We conclude that in vivo postnatal clearance of fetal lung liquid can be impaired by Na channel blockers and is unaffected by blockers of Na-H antiports and Na-glucose symports. Na transport in fetal type II cells has high affinity for amiloride, and these cells likely contribute to normal neonatal lung liquid clearance.