EXTRACELLULAR CALCIUM-DEPENDENT REGULATION OF TRANSMEMBRANE CALCIUM FLUXES IN MURINE KERATINOCYTES

Because the level of extracellular Ca2+ is an important stimulus for differentiation of epidermal cells in vitro, we characterized the extracellular Ca2+-dependent transmembrane Ca2+ fluxes in BALB/MK mouse keratinocytes. Increasing levels of extracellular Ca2+, ranging from 0.07 to 1.87 mM, stimulated the rate of Ca-45(2+) uptake into these cells 10- to 70-fold and doubled the rate of Ca-45(2+) efflux. The divalent cations, Ni2+ and Co2+, were able to block the influx of Ca2+, but dihydropyridines and verapamil were not. Furthermore, 10 to 100-mu-M of the trivalent cation La3+ induced a dose-dependent 2- to 100-fold increase of Ca2+ uptake, independently of the level of extracellular Ca2+. These observations suggest that keratinocytes possess a cell-surface "Ca2+-receptor," activation of which stimulates the influx of Ca-45(2+) through a type of voltage-independent, receptor-operated Ca2+ channels. Epidermal growth factor induced an accumulation of Ca-45(2+) of a much smaller magnitude than elevations of the level of extracellular Ca2+, without a detectable increase of Ca2+ efflux. Thus, the divergent cellular responses of keratinocytes to EGF and extracellular Ca2+ may be due, in part, to the distinct changes in transmembrane Ca2+ fluxes that these two stimuli generate. Treatment of cells with type beta transforming growth factor led to a gradual 6-fold increase of the Ca2+-activated rate of Ca2+ uptake over a period of 4 hours, but reduced the Ca2+ efflux by approximately 50% within 10 minutes. Thus, type beta transforming growth factor apparently stimulates Ca2+ influx indirectly, but may control the differentiation of keratinocytes by direct inhibition of Ca2+ efflux pumps.