Transmural Calcium Fluxes and Role of Mucins As Cellular Calcium-transport Vehicles in Chicken Trachea in Vitro

1. Transmural Ca2+ fluxes in tracheal tissue under physiological [Ca2+] conditions and the effect of altered serosal and luminal [Ca2+] on Ca2+ movements were investigated using chicken tracheal preparations in vitro. 2. In the presence of physiological [Ca2+] (1.8 mM), there was unidirectional Ca2+ flux with a small but steady uptake of Ca2+ from the serosal side into the submucosa followed by Ca2+ transport into the mucosa and then Ca2+ efflux into the tracheal lumen. The Ca2+ uptake by the tracheal tissue was via a diffusion process. There was no evidence of Ca2+ uptake via slow Ca2+ channels or Na+-Ca2+ exchange pathways. On the other hand, Ca2+ uptake from the lumen into the mucosa and Ca2+ efflux from the submucosa into the serosal side were almost negligible. 3. High serosal [Ca2+] (18.0 mM) and/or low luminal [Ca2+] (0.18 mM) increased significantly both Ca2+ uptake by the tissue from its serosal side and Ca2+ efflux into the lumen. Directional Ca2+ effect appeared to increase Ca2+ uptake via a diffusion process. 4. Transport of Ca2+ from the mucosa into the lumen comprised efflux of both filterable and mucin-bound forms of Ca2+. Under physiological [Ca2+] conditions, whilst initial efflux rates of both filterable and mucin-bound Ca2+ were almost equal, the net efflux of Ca2+ in mucin-bound form after 10 min was about 33% higher than that of filterable Ca2+. Similarly, the increase in Ca2+ efflux as a result of high serosal [Ca2+] involved a significant increase in the efflux of mucin-bound Ca2+ only, whereas the increased Ca2+ efflux as a result of low luminal [Ca2+] involved a significant increase in efflux of both filterable and mucin-bound Ca2+. 5. The transport of Ca2+ from the mucosa into the lumen in the form of mucin-bound Ca2+ appeared to play a significant role in the regulation of Ca2+ efflux from the tissue under increased Ca2+ influx or efflux conditions resulting from interventions with serosal and luminal [Ca2+]. 6. A concurrent stimulation of secretion of unique low molecular weight sulphate-rich components and high molecular weight mucin complexes with increased Ca2+ influx into and efflux from the tracheal tissue in response to high serosal and low luminal [Ca2+] allude to a plausible role of these secretory macromolecular mucin complexes as cellular Ca2+ transport vehicles.