O2‐dependent K+ fluxes in trout red blood cells: the nature of O2 sensing revealed by the O2 affinity, cooperativity and pH dependence of transport

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Abstract
1 The effects of pH and O2 tension on the isotonic ouabain-resistant K+ (Rb+) flux pathway and on haemoglobin O2 binding were studied in trout red blood cells (RBCs) in order to test for a direct effect of haemoglobin O2 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 O2 partial pressure (PO2) from 7.8 to 157 mmHg increased unidirectional K+ influx across the RBC membrane several-fold. At lower extracellular pH values, stimulation of K+ influx by O2 was depressed, exhibiting an apparent pKa (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 PO2 values between 0 and 747 mmHg, O2 equilibrium curves typical of Hb O2 saturation were also obtained for K+ influx and efflux. However, at pH 7.9, the PO2 for half-maximal K+ efflux and K+ influx (P50) was about 8- to 12-fold higher than the P50 for Hb-O2 binding. While K+ influx and efflux stimulation by O2 was essentially non-cooperative, Hb-O2 equilibrium curves were distinctly sigmoidal (Hill parameters close to 1 and 3, respectively). 4 O2-stimulated K+ influx and efflux were strongly pH dependent. When the definition of the Bohr factor for respiratory pigments (Φ=ΔlogP50×ΔpH−1) was extended to the effect of pH on O2-dependent K+ influx and efflux, extracellular Bohr factors (Φo) of -2.00 and -2.06 were obtained, values much higher than that for Hb (Φo= -0.49). The results of this study are consistent with an O2 sensing mechanism differing markedly in affinity and cooperativity of O2 binding, as well as in pH sensitivity, from bulk Hb.