Developmental differences in rabbit lung epithelial cell Na⁺-K⁺-ATPase

Previous studies showed that ouabain-sensitive rubidium (⁸⁶Rb⁺) uptake by rabbit lung epithelial cells increases at birth, followed by a more gradual postnatal increase, reaching adult values by age 30 days. To see whether these changes in ouabain-sensitive cation transport were the result of changes in Na⁺ pump number or turnover rate, we measured binding of [³H]ouabain and ouabain-sensitive ⁸⁶Rb⁺ uptake by freshly isolated lung epithelial cells harvested from near-term fetal, newborn, and adult rabbits. Ouabain-sensitive ⁸⁶Rb⁺ uptake by fetal cells was 1/4 that of newborn cells and 1/10 that of adult cells. The maximal number of ouabain binding sites (U(max)) was the same for fetal and newborn cells but almost threefold greater for adult cells. Na⁺ pump turnover rate, determined from ouabain-sensitive ⁸⁶Rb⁺ uptake and U(max), was four times greater in newborn and adult cells than it was in fetal cells. Thus the increase in ⁸⁶Rb⁺ uptake at birth could be explained by an increase in Na⁺-K⁺-adenosinetriphosphatase (ATPase) turnover rate, whereas the postnatal increase in ⁸⁶Rb⁺ uptake could be accounted for by an increase in the amount of Na⁺-K⁺-ATPase per cell.