Bioelectric properties and ion transport across excised canine fetal and neonatal airways

Knowledge of liquid secretion by fetal lung stems from studies of sheep. We extended these studies to dogs and examined the persistence of the fetal pattern of airway epithelial permeability and ion transport in the neonatal animal. Plasma and lung liquid from fetal dogs were analyzed for Na+, K+, Cl-, HCO3-. Only the Cl- concentration of fetal lung liquid (129 meq/l) was significantly different from that of fetal plasma (111 meq/l). Segments of trachea from fetal and neonatal (< 1, 7-10, and 21-46 days after birth) dogs were excised and mounted in flux chambers. The transepithelial potential difference (PD) of all tissues was oriented lumen negative (9.8-14.8 mV). Under short-circuit conditions, unidirectional Na+ flows were symmetrical. Cl- was secreted, and the secretion was equivalent to short-circuit current (I(sc)). Cl- secretion persisted under open-circuit conditions. Lobar bronchi from 21- to 46-day neonates absorbed Na+ (1.9 μeq · cm-2 · h-1), but unidirectional flows of Cl- were symmetrical. Amiloride (10-4 M) reduced I(sc) of neonatal bronchi by 47% but did not affect fetal bronchi. Isoproterenol increased I(sc) of both fetal (33%) and neonatal (40%) bronchi. These responses suggest that fetal bronchi do not absorb Na+ but can be stimulated to secrete Cl-. We conclude that Cl- secretion by epithelium of large airways may contribute to fetal lung liquid production, but it is unlikely that the tracheal epithelium is involved in fluid absorption at birth. Whereas fetal bronchi appear to secrete Cl-, neonatal bronchi absorb Na+. This transition appears to be too slow to assist with rapid lung liquid clearance at birth but may help maintain liquid balance across the bronchial epithelium of the air-filled lung in neonates > 2 wk of age.