Human tracheobronchial epithelial cells have been serially passaged in serum-free medium. This serum-free model was employed to investigate the effects of different concentrations of Ca2+ (0.1, 1.0 and 2.0 mM) on multiplication and morphology of the cells. The responses were analysed in terms of growth kinetics, histochemical and ultrastructural alterations. Culturing of the cells in high Ca2+ (1.0-2.0 mM) medium stimulated cell multiplication characterized by increased colony forming efficiency, greater number of cells per colony and cell population doublings per day. Additionally, the high Ca2+ concentrations induced proliferation in cultures grown to confluency in low Ca2+ (0.1 mM) medium. Cells propagated in low Ca2+ medium consisted of relatively heterogeneous cell populations, with most cells staining positive with periodic acid-Schiff (PAS) reagent. Ultrastructurally the cells exhibited secretory vesicles and microvilli on their surfaces, small desmosomes and intercellular interdigitation between cells and numerous large secretory vesicles in the cytoplasm. The cells grown in high Ca2+ medium acquired characteristics of a highly proliferative phenotype. The cultures consisted of closely packed, relatively homogeneous cells that did not stain with PAS reagent. Their characteristic features were: absence of surface secretory vesicles, reductions of microvilli and intercellular interdigitations, and increases in size and number of desmosomal junctions. The results show that low Ca2+ in the culture medium inhibits cell multiplication and favors the secretory cell phenotype, while high Ca2+ levels stimulate cell multiplication and inhibit the secretory cell phenotype.
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