A physiological, biochemical and histological study of goose tracheal mucin and its secretion

Abstract

Tracheal mucin secretion has been measured from a segment of trachea, isolated in situ , in anaesthetized geese by a method that involves radioactive labelling of tracheal mucus glycoproteins (Gallagher et al. 1975). Goose tracheal mucus comes entirely from goblet cells, since the goose trachea does not contain submucosal mucous or serous glands, and this method has been used to investigate the nervous and pharmacological control of the mucin secretion from these epithelial goblet cells. The mucins secreted have been collected, fractionated, and chemically analysed. Intracellular mucin has been examined histochemically, and the results of electron microscopic observations of epithelial cells and nerves are presented. Acetylcholine increased tracheal mucin secretion, and this effect was completely blocked by atropine. Neither α- nor β-stimulant sympathomimetic amines affected tracheal mucin secretion. Stimulation of the peripheral cut ends of the descending oesophageal nerves increased tracheal mucin secretion and the majority of this response, approximately three-quarters, appeared to be cholinergic since this proportion was blocked by atropine. The mediator for the atropine-resistant part of the response is not known, but it appears not to be a β-adrenoreceptor stimulant since the response to nerve stimulation was unaffected by propranolol given at 34 μm intrasegmentally. Other possibilities are discussed. Atropine itself decreased the resting level of tracheal mucin secretion. The local anaesthetic, lignocaine, increased tracheal mucin secretion, while at the same time blocking the responses to acetylcholine and descending oesophageal nerve stimulation. The implications of this are discussed. The electrophoretic, gel filtration and ion-exchange properties of goose tracheal mucins showed that they represented high molecular mass, negatively charged glycoproteins which could be labelled biosynthetically with [ ³⁵ S]sulphate, [ ³ H]- and [ ¹⁴ C]glucose. These mucins could be stained with Alcian blue or periodic acid Schiff reagent. The carbohydrate composition was unusual for an epithelial glycoprotein in that fucose was absent and mannose was present in small quantities. The monosaccharides present in larger quantity were galactose, N -acetylglucosamine, N -acetylgalactosamine and sialic acid. Histochemical analysis of tissue sections of gosling tracheas demonstrated that nearly all of the glycoprotein in epithelial goblet cells contained both sialic acid and sulphate residues. Sialated mucin was present also, but to a lesser extent, and many cells contained a mixture of sialated and sulphated mucins. The adult goose trachea had a high proportion of sialated glycoprotein. Electron microscopy showed a range of epithelial cell types and intra-epithelial nerves also. Many of the nerves had neurosecretory vesicles suggestive of motor function and some were near to goblet cells.