ONTOGENY OF ION-TRANSPORT ACROSS FETAL PULMONARY EPITHELIAL-CELLS IN MONOLAYER-CULTURE

The transition of the fetal lung from a fluid-secreting to a fluid-absorbing organ is dependent on ion transport across the pulmonary epithelium. This study examined the ion transport characteristics of distal pulmonary epithelial cells isolated from rat fetuses in late gestation and maintained in differentiation-arrested monolayer cultures. The response to inhibitors of active ion transport suggested the presence of apical to basolateral Na+ transport in monolayers derived from each gestational age. However, amiloride inhibition of short-circuit current (I(sc)) varied with gestational age, decreasing I(sc) by 30% in monolayers derived from day 18 fetuses and by 55% in monolayers from day 21 fetuses. A portion (10%) of the residual I(sc) remaining after amiloride addition to monolayers from day 18 fetuses could be inhibited by bumetanide, suggesting the induction of net Cl- transport. Ion-substitution experiments confirmed the presence of Na+ and inducible Cl- transport mechanisms in monolayers from day 18 fetuses and only Na+ transport mechanisms in monolayers from day 21 fetuses. Beta-Adrenergic stimulation increased I(sc) but maintained the age-dependent characteristics of Na+- and Cl--dependent ion transport. In summary, monolayer cultures of fetal pulmonary epithelial cells exhibit age-dependent differences in ion transport properties that are consistent with a transition or maturation of the distal pulmonary epithelium from an epithelium capable of Na+ absorption and Cl- secretion preterm to one capable of only Na+ absorption at term.