1. The effects of pH and O-2 tension on the isotonic ouabain-resistant K+ (Rb+) flux pathway and on haemoglobin O-2 binding were studied in trout red blood cells (RBCs) in order to test for a direct effect of haemoglobin O-2 saturation on K+ transport across the RBC membrane. 2. At pH values corresponding to in vivo control arterial plasma pH and higher, elevation of the O-2 partial pressure (P-O2) from 7.8 to 157 mmHg increased unidirectional K+ influx across the RBC membrane several-fold. At lower extracellular pH values, stimulation of Kf influx by O-2 was depressed, exhibiting an apparent pK(a) (pK'(a)) for the process of 8.0. Under similar conditions the pK'(a) for acid-induced deoxygenation of haemoglobin (Hb) was 7.3. 3. When trout RBCs were exposed to P-O2 values between 0 and 747 mmHg, O-2 equilibrium curves typical of Hb 0, saturation were also obtained for K+ influx and efflux. However, at pH 7.9, the P-O2 for half-maximal K+ efflux and K+ influx (P-50) was about 8- to 12-fold higher than the P-50 for Hb-O-2 binding. While K+ influx and efflux stimulation by O-2 was essentially non-cooperative, Hb-O-2 equilibrium curves were distinctly sigmoidal (Hill parameters close to 1 and 3, respectively). 4. O-2-stimulated K+ influx and efflux were strongly pH dependent. When the definition of the Bohr factor for respiratory pigments (Phi = Delta logP(50) x Delta pH(-1)) was extended to the effect of pH on O-2-dependent K+ influx and efflux, extracellular Bohr factors (Phi(0)) of -2.00 and -2.06 were obtained, values much higher than that for Hb (Phi(0) = -0.49). The results of this study are consistent with an O-2 sensing mechanism differing markedly in affinity and cooperativity of O-2 binding, as well as in pH sensitivity, from bulk Hb.
