Studies on the cation permeability of human red cell ghosts. Characterization and biological significance of two membrane sites with high affinities for Ca.

Abstract

Net K movements in reconstituted human red cell ghosts and the resealing of ghosts to cations after osmotic hemolysis of red cells have been studied as functions of the free Ca ion concentration. The Ca-dependent specific increase in K permeability was shown to be mediated by a site close to the internal surface of the membrane with an apparent dissociation constant ap pH 7.2 for Ca (K'p1) of 3-5 X 10(-7) M, for Sr of 7 X 10(-6) M. Ba and Mg did not increase the K-permeability of the membrane but inhibited the Ca-mediated permeability changes. K'D1 decreased in a nonlinear fashion when the pH was increased from 6.0 to 8.5. Two different pK' values of this membrane site were found at pH 8.3 and 6.3. The Ca-activated net K efflux into a K-free medium was almost completely inhibited by an increase in intracellular Na from 4 to 70mM. Extracellular K antagonized this Na effect. Changes in the extracellular Na (0.1-140 mM) or K (0.1-6 mM) concentrations had little effect and did not change K'p1. The Ca-stimulated recovery of a low cation permeability in ghost cells appeared to be mediated by a second membrane site which was accessible to divalent cations only during the process of hemolysis in media of low ionic strength. The apparent dissociation constant for Ca at this site (K'p2) varied between 6 X 10(-7) and 4 X 10(-6) M at pH 7.2 Mg, Sr, and Ba could replace Ca functionally. The selectivity sequence was Ca greater than Sr greater than Ba greater than Mg. K'p2 was independent on the pH value in the range between 6.0 and 8.0 Hill coefficients of 2 were observed for the interaction of Ca with both membrane sites suggesting that more than one Ca ion is bound per site. The Hill cofficients were affected neither by the ion composition nor by the Ph values of the intra-and extracellular media. It is concluded that two different pathways for the permeation of cations across the membrane are controlled by membrane sites with high affinities for Ca: One specific for K, one unspecific with respect to cations. The K-specific "channel" has properties similar to the K channel in excitable tissues.