Modulation of K(+)-Cl- cotransport in equine red blood cells

Potassium transport was measured in equine red blood cells, using 86Rb+ influx as a convenient assay. A significant component of volume- and pH-sensitive K(+)-Cl- cotransport to the overall K+ flux was observed in all blood samples studied, although fluxes were variable between animals, and within individuals when measured at intervals over a period of weeks. The aryloxyacetic acid [(dihydroindenyl)oxy]alkanoic acid (DIOA), at a final concentration of 100 microM, inhibited most (> 95%) of the Cl(-)-dependent K+ flux, and DIOA sensitivity was therefore used to define the activity of the K(+)-Cl- cotransport. K(+)-Cl- cotransport was also sensitive to protein phosphatase inhibition with calyculin A or okadaic acid, with inhibition constants of 9 +/- 1 nM for calyculin and about 100 nM for okadaic acid. Peak fluxes were observed at an external pH of 6.7-7.0, with inhibition at higher and lower values. Volume-sensitive K+ fluxes assayed in autologous plasma, controlled for osmolaity, pH and potassium concentration, were significantly lower (28 +/- 8% of control values, n = 6) than those measured in saline. This inhibition was mimicked by the culture medium RPMI, but disappeared following dialysis of the plasma. Phosphate (5.6 mM) inhibited volume-sensitive K+ fluxes by 48 +/- 2%, n = 3; no significant effect was observed by increasing external magnesium concentrations to 0.5 or 2 mM. Thus, inhibition by RPMI, but not that by plasma, may be due to phosphate. Finally, volume- and pH-sensitive K+ fluxes were sensitive to oxygen tension and were abolished reversibly by equilibrating solutions with nitrogen, as opposed to air. Use of solutions equilibrated with different values of Po2 may account for some of the variability in equine red blood cell KCl fluxes. The importance of these observations to equine red blood cell homeostasis and haemodynamics is discussed.