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.