Relationship between K-Cl Cotransport Activity and Membrane Fragility of Density-Separated Erythrocytes from Thoroughbred Horses

Abstract

In this study, we examined the relationship between the osmotic fragility and K-Cl cotransport (KCC) activity in the density-separated erythrocytes from thoroughbred horses. Erythrocytes from four healthy thoroughbreds were collected, and separated into five fractions according to their density. The KCC activities of the cells were determined as the K+ congener Rb+ influx in the cells suspended in isotonic medium in which the main salts were constructed with N-methyl-D-glucamine (NMDG)/NO3 or NMDG/Cl and the medium pH were varied from 7.0 to 7.8. The decrease in medium pH was associated with an increase in the KCC activities, particularly the KCC activity at pH 7.0 was 4- and 50-fold higher than that at pH 7.4 and 7.8, respectively. The KCC activity in the dense cells was also significantly higher than that in the light cells. The decrease in medium pH and/or the density of erythrocytes was associated with a progressive increase in the osmotic fragility. The osmotic fragility measured in the NMDG/NO3 medium always showed a tendency to be higher than that incubated in the NMDG/Cl medium in each medium pH and/or cell fraction. These results suggest that 1) the increase in the KCC activity may cause shrinkage of the cell volume and decrease the fragility; and 2) the KCC has a functional interrelation to the change in the erythrocyte fragility with the fall in pH. The KCC of equine erythrocytes therefore plays an important role in the prevention of fragilocytes and protection of haemolysis in blood vessels with exercise.