Different sensitivities of rat and human red cells to exogenous Ca2+

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Abstract
During an examination of the effects of shear and of the Ca2+ ionophore A23187 on Ca2+ entry into erythrocytes of rats and humans, we noted that rat erythrocytes were much more sensitive to Ca2+‐induced hemolysis than the human cells. An examination of the effect of Ca2+ on transglutaminase, a cytosolic enzyme in the erythrocyte which crosslinks membrane proteins and renders cells less deformable, demonstrated a correlation between enzyme activity and Ca2+‐induced hemolysis. Both rat and human cells subjected to shear‐induced Ca2+ entry exhibited increased enzyme activity and altered membrane protein SDS‐PAGE patterns. Twenty micromolar A23187 with Ca2+ at concentrations above 80 μM caused hemolysis of rat erythrocytes. In contrast to human erythrocytes, under these conditions no membranes were recoverable from rat erythrocytes. At lower concentrations of Ca2+ (25 and 50 μM), however, rat erythrocytes maintained integrity, and exhibited enhanced transglutaminase activity and cross‐linking of membrane proteins. The rat enzyme can be activated 30% by 10 μM Ca2+, while 50 μM Ca2+ was necessary to achieve a similar activation of the enzyme from human red blood cells. In studies of shear‐stimulated Ca2+ uptake by erythrocytes the rat red cell enzyme was more readily activated. The SDS‐PAGE pattern of rat red cell membranes after a 30 sec shear showed specific changes in protein banding, including the appearance of bands >330 kDa. Changes in protein banding were also apparent in cytosolic proteins. This work supports the view that shear‐induced Ca2+ entry activates transglutaminase that leads to cross‐linking of membrane components, a loss of cell integrity, and eventual cell death.