1. Canine tracheal explants, cultured in medium 199, actively incorporated radioactive precursors into secreted macromolecules in vitro. 2. Puromycin, 6-diazo-5-oxo-l-norleucine and ouabain markedly inhibited the incorporation of these precursors. 3. Exogenous glucosamine at concentrations above 20mm caused a greater than 50% inhibition of the incorporation of l-[G-(3)H]fucose and l-[U-(14)C]serine. 4. Carbohydrate content of the purified secretions was approximately 50% and consisted principally of galactose, N-acetylglucosamine, N-acetylgalactosamine, fucose and sialic acids. 5. Chromatography on DEAE-cellulose and Bio-Gel A-150m and equilibrium density-gradient centrifugation in a CsCl gradient confirmed the presence of mucous glycoproteins. 6. Electrophoresis on 1% agarose gels gave profiles that were identical with canine respiratory mucus obtained in vivo. 7. These results support the utility of the explant system for studies of respiratory secretions.

1. A modified canine tracheal organ culture system was used to investigate differences between mucous secretions of epithelial goblet cells and the submucosal glands. 2. Denuded explants were prepared by removing goblet, ciliated and basal cells from the surface epithelium leaving an intact basement membrane and viable submucosa. 3. Denuded explants actively incorporated radioactive precursors into secreted macromolecules when cultured in medium 199 containing label. 4. Chromatography on Bio-Gel A-150m and electrophoresis on 1% agarose gels indicated that epithelial goblet cell secretions were relatively more sulphated than submucosal glandular secretions. 5. The glandular structures were shown to respond to a parasympathomimetic agent.

No abstract

PROCEEDINGS OF THE BIOCHEMICAL SOCIETY common and resemble metallothionein, a Cd2+_ containing protein of unknown function isolated by Kagi & Vallee (1960, 1961) from horse kidney. The subsequent isolation of 'metallothioneins' from human kidney (Pulido, Kagi & Vallee, 1965), from the livers of Cd2+-injected rabbits (Piscator, 1964) and from the livers and kidneys of rats after intravenous injection of 203HgCl2 (Jakubowski, Piotrowski & Trojanowska, 1970) suggests that these proteins constitute detoxifying mechanisms against certain heavy metals. At least in the male rat, the Cd2+-binding protein does not appear to be a normal component of the liver, but is induced in response to Cd2+. It is synthesized also in response to excess of Zn2+ and, in livers of animals pre-injected 24-48h previously with Zn2+, parenterally administered Cd2+ accumulates, and is thus immobilized, more rapidly than in the normal liver. This provides a possible explanation for the protection by Zn2+ against Cd2+ injury to the testes, particularly as it is known that Zn2+ has maximal activity when administered at least 24h before Cd2+ (Mason & Young, 1967).