title: Hysteretic activation of the Ca2+ pump revealed by calcium transients in human red cells.
abstract: 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.
mesh_terms: D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000097:blood*; D000252:Calcium-Transporting ATPases / Q000097:blood*; D004912:Erythrocytes / Q000187:drug effects / Q000201:enzymology*; D006801:Humans; D007700:Kinetics; D008433:Mathematics; D008954:Models, Biological
chemical_list: D000001:Calcimycin; D000252:Calcium-Transporting ATPases; D002118:Calcium