Extracellular calcium ion depletion induces release of glycoproteins by canine trachea

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Abstract
The effect of depletion of extracellular Ca2+ on the release by canine trachea in vitro of [14C]glucosamine-labeled trichloroacetic-phosphotungstic acid-precipitable glycoproteins was evaluated. Incubation in Ca2+,Mg2+-free medium containing ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA, 10(-3) to 10(-5) M) induced a time-dependent increase in base-line release of high molecular weight, radiolabeled glycoprotein that was not blocked by 10(-5) M atropine, propranolol, or phentolamine. Maximal increase was proportional to EGTA concentration and was augmented by preincubation with methacholine chloride. The secretory response was reversed by reincubation of explants in medium containing Ca2+, Sr2+, or Ba2+ but not Mg2+. Assessment of explants denuded of epithelium or cartilage indicated that the secretory response to depletion of Ca2+ originated in the tracheal submucosa, probably the submucosal glands. Quantitative autoradiographic analyses showed, however, that incubation in Ca2+,Mg2+-free medium had no effect on radiolabel release from mucous or serous cells in the tracheal submucosal glands. Increased radiolabeled glycoprotein release in Ca2+,Mg2+-free medium was accompanied by exfoliation of the surface epithelium, though the two effects were apparently unrelated. The secretory response was not due to cell lysis or increased release of radiolabeled glycosaminoglycans, and the finding that the specific activity (ratio of bound radiolabel to protein content) of the glycoprotein released was not changed in Ca2+,Mg2+-free medium showed that it was not due to a change in the rate of glycoprotein synthesis. A model is proposed in which depletion of Ca2+ increases the rate of flow of mucus from the duct lumens of the tracheal glands.