Extracellular calcium‐dependent regulation of transmembrane calcium fluxes in murine keratinocytes

Because the level of extracellular Ca 2+ is an important stimulus for differentiation of epidermal cells in vitro, we characterized the extracellular Ca 2+ ‐dependent transmembrane Ca 2+ fluxes in BALB/MK mouse keratinocytes. Increasing levels of extracellular Ca 2+ ranging from 0.07 to 1.87 mM, stimulated the rate of 45 Ca 2+ uptake into these cells 10‐ to 70‐fold and doubled the rate of 45 Ca 2+ efflux. The divalent cations, Ni 2+ and Co 2+ , were able to block the influx of Ca 2+ , but dihydropyridines and verapamil were not. Furthermore, 10 to 100 μM of the trivalent cation La 3+ induced a dose‐dependent 2‐ to 100‐fold increase of Ca 2+ uptake, independently of the level of extracellular Ca 2+ . These observations suggest that keratinocytes possess a cell‐surface “Ca 2+ ‐receptor,” activation of which stimulates the influx of 45 Ca 2+ through a type of voltage‐independent, receptor‐operated Ca 2+ channels. Epidermal growth factor induced an accumulation of 45 Ca 2+ of a much smaller magnitude than elevations of the level of extracellular Ca 2+ , without a detectable increase of Ca 2+ efflux. Thus, the divergent cellular responses of keratinocytes to EGF and extracellular Ca 2+ may be due, in part, to the distinct changes in transmembrane Ca 2+ fluxes that these two stimuli generate. Treatment of cells with type β transforming growth factor led to a gradual 6‐fold increase of the Ca 2+ ‐activated rate of Ca 2+ uptake over a period of 4 hours, but reduced the Ca 2+ efflux by approximately 50% within 10 minutes. Thus, type β transforming growth factor apparently stimulates Ca 2+ influx indirectly, but may control the differentiation of keratinocytes by direct inhibition of Ca 2+ efflux pumps.