Hysteretic activation of the Ca2+ pump revealed by calcium transients in human red cells

The enzymatic basis for the Ca2+ pump in human red cells is an ATPase with hysteretic properties. The Ca2+-ATPase shifts slowly between a ground state deficient in calmodulin and an active state saturated with calmodulin, and rate constants for the reversible shifts of state were recently determined at different Ca2+ concentrations (Scharff, O. and Foder, B. (1982) Biochim. Biophys. Acta 691, 133-143). In order to study whether the Ca2+ pump in intact red cells also exhibits hysteretic properties we have analysed transient increases of intracellular calcium concentrations (Cai), induced by the divalent cation ionophore A23187. The time-dependent changes of Cai were measured by use of radioactive calcium (45Ca2+) and analysed with the aid of a mathematical model, based partly on the Ca2+-dependent parameters obtained from Ca2+-ATPase experiments, partly on the A23187-induced Ca2+ fluxes determined in experiments with intact red cells. According to the model a delay in the activation of the Ca2+ pump is a prerequisite for the occurrence of A23187-induced calcium transients in the red cells, and we conclude that the Ca2+ pump in human red cells responds hysteretically. It is suggested that Ca2+ pumps in other types of cell also have hysteretic properties.