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Human Tracheobronchial Secretions: Development of Mucous Glycoprotein and Lysozyme-secreting Systems

Summary: Baseline rates for secretion of mucous glycoprotein were similar (680–830 μg/g tissue/24 hr) for cultured tracheal epithelium from new horns of 26–32 weeks' gestation, full term newborns, and older children. Addition of methacholine to culture medium augmented secretory rates of glycoprotein from all tissue sources 3–5-fold. The overall composition of secreted mucous glycoproteins changed little with increasing age. A trend toward less sulfation and toward increased sialic acid and fucose content was noted in secreted glycoproteins from explants of older subjects. Histochemical observations of stored glycoprotein in tracheal tissue, which was subsequently used for organ culture experiments, confirmed that a modest, but consistent sulfate to sialic acid shift occurs during early life. In contrast, baseline secretory rates for lysozyme from tracheal epithelium of preterm infants were one-half as large as rates from epithelium of full term babies and were refractory to cholinergic stimulation. Stimulation of lysozyme secretion by a cholinergic agonist was achieved in all cases by 40 weeks' gestation. We conclude that basal glycoprotein secretion and the mechanism for glycoprotein response to cholinergic stimulation have developed by the earliest age of viability, but that basal lysozyme secretion is deficient and is unresponsive to cholinergic stimulation in tracheal tissue from preterm newborns.Speculation: The lungs of preterm inants may be more susceptable to bactrial invasion than lungs of full term infants and older children as a result of deficient lysozyme secretion by tracheobronchial epithelium.