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−.