Effects of calcium on potassium and water transport in human erythrocyte ghosts

Bulk water transport in reconstituted ghosts is statistically comparable to that in the parent red cells, and is unaffected by incorporation of Ca²⁺ over the range of 0.01 to 1 mM. Brief exposure of ghosts to p-chloromercuribenzene sulfonate results in a supression of osmotic water flow but leaves K⁺ permeability unchanged. Incorporation of p-chloromercuribenzene sulfonate provokes extremely rapid K⁺ loss which can be counteracted by simultaneous inclusion of Ca²⁺.Erythrocyte ghosts, when prepared with a small amount of Ca²⁺, demonstrate recovery of normal impermeability to choline, sucrose, Na⁺ and inulin and have an improved K⁺ retention over Ca²⁺-free preparations.The rate of passive transport of K⁺ from unwashed erythrocyte ghosts was measured during the initial few minutes of efflux. The initial rates vary in a bimodal fashion with the concentration of Ca²⁺ incorporated at the time of hemolysis. In low concentrations (0.01–0.1 mM), Ca²⁺ protects the K⁺ barrier while at higher concentrations (0.1–1.0 mM) it provokes a K⁺ leakage ranging from 7 to 50 times the normal rate of passive K⁺ loss. The Ca²⁺-induced K⁺ leak is thus a graded response rather than a discrete membrane transport state. The transition from a Ca²⁺-protected to a Ca²⁺-damaged membrane occurs upon an increase in Ca²⁺ concentration of less than 50 μmoles/l.