VOLUME-SENSITIVE K-CL COTRANSPORT IN INSIDE-OUT VESICLES MADE FROM ERYTHROCYTE-MEMBRANES FROM SHEEP OF LOW-K PHENOTYPE

Unidirectional K ion effluxes were measured from inside-out vesicles prepared from erythrocyte membranes from sheep of the low-K phenotype. Total K efflux was 150 nmol per mg of protein per hr in a Cl medium of 295 mosmol/kg (with the Na/K pump inhibited). Cl-dependent K efflux (determined with methanesulfonate replacing Cl) was 54 nmol/(mg·hr). Cl-dependent K efflux (K-Cl cotransport) increased to 77 nmol/(mg·hr) with osmotic swelling of ≃30% in 230-mosmol/kg medium and decreased to 13 nmol/(mg·hr) after shrinkage of ≃60% in 430-mosmol/kg medium. Osmotically induced changes in transport and vesicle volume were reversible. K-Cl cotransport was enhanced by ATP. Nonhydrolyzable ATP analogues failed to substitute for ATP, indicating that phosphorylation is involved. However, in the absence of added ATP there was significant K-Cl cotransport, suggesting that phosphorylation is not essential for function. The results provide clues about the nature of the signals detected by the sensor of cell volume changes and demonstrate that inside-out vesicles from sheep erythrocyte membranes provide an advantageous experimental system for investigation of the volume sensor.