Ca2+-dependence of Ca2+ release from intracellular stores of intact and permeabilized Ehrlich ascites tumor cells

Comparative study of the effect of Ca2+ ions on mobilization of Ca2+ from i ntracellular stores of intact and permeabilized by digitonin (15 mu M) Ehrl ich ascites tumor cells (EATC) has been carried out. in the experiments on permeabilized cells, the dependences of the initial rate and amplitude of C a2+ mobilization evoked by the addition of 100 nM inositol 1,4,5-trisphosph ate (IP3) on preexisting [Ca2+] were bell-shaped within a [Ca2+] range 10(- 7)-10(-6) M with the maxims at [Ca2+] = 166 nM. In intact cells, different concentrations of free cytosolic Ca2+ ([Ca2+](i)) were produced using low ( up to 0,005%) concentrations of digitonin which selectively increased perme ability of the plasma membrane. Stimulation of the cells by exogenous ATP a t [Ca2+](i) = 10(-8)-10(-6) M resulted in Ca2+ mobilization, whose rate and amplitude were maximal at 102-115 nM Ca2+. The experimental Ca2+-dependenc es were approximated based on a model that suggests channel opening upon Ca 2+ binding and transition to the inactive states upon Ca2+ binding to the c losed and open channel forms. Three inactivation types (including two parti cular cases) demonstrate a slight priority of inhibitory binding of Ca2+ on ly to the open channel, but predict markedly different parameter values. We concluded that an increase in [Ca2+] can stimulate the IP3-induced mobiliz ation, but in intact EATC, deviations of [Ca2+](i) from the resting level ( about 100 nM) attenuate responses to the agonist stimulation.