The calcium dependency of mucus glycoconjugate secretion by canine tracheal explants

Canine tracheal explants, incubated overnight with [14C]-glucosamine, elicited an enhanced secretion of ethanol-precipitated 14C-labelled glycoconjugate when challenged with methacholine, 10 microM. Explants were rendered deficient in total calcium content and unresponsive to methacholine, 10 microM, by incubating them in calcium-free medium for 18 to 22 h; however, the secretory response to the cholinergic agonist was restored with the addition of calcium to the medium. A dose-response relationship resulted when explants were challenged with methacholine in nutrient medium containing varied calcium concentrations (0.45 to 7.2 mM); alterations in the calcium concentration in the absence of methacholine had no significant effect on the basal secretion of 14C-labelled glycoconjugate. The calcium-selective ionophore A23187, 10 microM, stimulated [14C]-glycoconjugate secretion and induced the most significant effect in the presence of nutrient medium containing calcium. Verapamil, 10 microM, a calcium-entry blocker failed to inhibit basal or stimulated [14C]-glycoconjugate secretion; however, the intracellular calcium antagonist TMB-8, 10 to 100 microM, inhibited methacholine-induced [14C]-glycoconjugate secretion in a dose-dependent manner. These data suggest that respiratory mucus secretion is a calcium-dependent process and that intracellular calcium is more vital than extracellular calcium in supporting this phenomenon.