ACTIVE NA-22(+) TRANSPORT BY THE INTACT LUNG DURING EARLY POSTNATAL LIFE

The lung relies upon epithelial active transport of Na+ to aid in the clearance of fluid from its air spaces. Because it is unknown whether the rate of active Na+ transport by the distal lung epithelium varies during early postnatal age, we performed studies in young guinea pigs (7 and 30 days after birth). We used a single pass isolated perfused l ung model in which a Krebs Ringer bicarbonate solution containing Na-2 2(+), [C-14]sucrose, and FITC-dextran was placed into the air spaces o f the lungs, and apparent permeability - surface area (PS) products we re calculated after determining the changes in lung weight and the con centrations of the isotopes in the vascular effluent. The PS product f or Na-22(+), but not [C-14]sucrose, decreased significantly at both ag es when amiloride was infused (final concentration of 10(-4) M). Amilo ride also decreased the rate of fluid clearance, as assessed by change s in organ weight, at both ages. Although the absolute rate of amilori de-sensitive Na-22(+) transport increased with age, morphometric measu rement of the alveolar region demonstrated that the rate of amiloride- sensitive Na-22(+) transport per unit alveolar surface area was simila r. These data indicate that although the guinea pig lung undergoes sig nificant growth shortly after birth, the rate of amiloride-sensitive a ctive Na+ transport per unit surface area remains constant. Since a co mponent of weightless was insensitive to amiloride, these in vivo stud ies suggest that the amiloride-insensitive Na+ transport pathways prev iously identified in cultured lung epithelium exist in the intact lung .