conditions chosen in order to stimulate the K+–Cl− cotransporter (KCC). 86Rb+ was added in 150 mM KNO3 to give a final [K+] of 7.5 mM in all experiments except those with HK saline and A23187-treated RBCs. After incubation with radioisotope for 10 min, RBCs were washed to remove extracellular 86Rb+, five-times in an ice-cold MgCl2 wash solution. For K+ efflux experiments, RBCs were loaded overnight at 4 °C by addition of 86Rb+ after which cells were washed five times in an ice-cold wash solution. RBCs were then suspended at 2% haematocrit
(Hct) in standard saline at 37 °C. Aliquots were taken at 5 min intervals for 30–60 min and spun through phthalate oil. The cell pellet was lysed with detergent, deproteinised with TCA, and counted by liquid scintillation (cpm). A semilog plot (of cpm at time = t/cpm at time = 0) was used to determine the rate constant for K+ efflux. Except for experiments to measure Na+/K+ pump activity, ouabain (100 μM) and bumetanide (10 μM) were present in all experiments to obviate any K+ transport through the Na+/K+ pump and the Na+–K+–2Cl− cotransporter, respectively. Either microhaematocrit determination or the cyanohaemoglobin method was used to measure the final Hct. KCC activity was assayed as
Cl−-dependent K+ influx; Gardos channel activity as the CLT-sensitive (5 μM) K+ influx; Na+/K+ pump activity as the ouabain-sensitive (100 μM) K+ influx and Psickle as the deoxygenation-induced K+ influx measured in the absence of Cl−.