Physiological shear stresses enhance the Ca2+ permeability of human erythrocytes.

Abstract

A membrane-bound Ca2+ pump, capable of extruding Ca2+ at a rate of 4−30 × 10−3 mol Ca2+ per l of cells per h in optimal conditions1, enables mammalian erythrocytes to maintain a large transmembrane Ca2+ concentration gradient (10−3M outside compared with 10−6M inside)2. This rate is much greater than the passive Ca2+ influx, estimated3 to be 6−10 × 10−6 mol per l of cells per h. It has been suggested that the apparently large excess capacity of the Ca2+ pump both gives the required Ca2+ sensitivity to a pump incapable of regulating its Ca2+ affinity4 and ensures against Ca2+ toxicity in certain red cell activities5. The results presented here, however, show that circulatory shear stresses enhance the Ca2+ permeability of the membrane, and suggest that greater demands are imposed on the Ca2+ pump in vivo than had been previously thought. We find that the passive permeability to Ca2+ is increased by up to an order of magnitude and the Ca2+-stimulated ATPase activity is strongly enhanced when red cells are sheared at physiological rates in a rotational viscometer.