Dual effect of calcium on erythrocyte membrane contraction.

Effect of different calcium concentrations on erythrocyte ghost shape and size has been studied. Erythrocyte ghosts are smallest at approximately 10 microM of total calcium content and in the absence of EDTA; both decrease and increase of free Ca2+ results in expansion of ghosts. Changes in erythrocyte ghost size occur at equal Ca2+ concentrations which, as it has been already found, alter ATPase activity of contractile membrane protein. The possibility is discussed that under physiologic conditions contractile membrane protein causes weak membrane contraction which differs from aphysiologic membrane shrinkage brought about particularly by cross-bridges at high Ca2+ concentrations.