Carbonate dehydratase (carbonic anhydrase) and the fetal lung

Carbonic anhydrase activity (carbonic dehydratase, EC 4.2.1.1) has been detected in the fetal lungs of stillborn human infants and rhesus monkeys, but a role for this enzyme in the fetal lung has not been elucidated. In utero the mammalian lung develops as a liquid-filled structure, the liquid being secreted by the lung. In the fetal lamb this liquid, when compared with plasma, has a high chloride and a low bicarbonate concentration, suggesting a possible role for carbonate dehydratase. Studies on 10 fetal lambs confirmed the presence of carbonate dehydratase in the lung. Levels at 60-66 days were negligible and rose to 0.30 Meldrum Roughton units/mg protein at about 140 days (term 147 days), with little change after birth. In another six fetal lambs at 135-136 days, inhibition of this enzyme with 100 mg acetazolamide suppressed the mean rate of secretion of lung liquid by 64.5% (P less than 0.005), which correlated with a significant drop in chloride concentration (P less than 0.001). This magnitude of changes in secretion after acetazolamide is of the same order as that occurring in the secretion of cerebrospinal fluid when carbonate dehydratase is inhibited. This observation supports the hypothesis that carbonate dehydratase in fetal lung affects the secretion of lung liquid, although its mechanism is as yet unknown.