1. The effect of low ionic strength media on the residual, i.e. (ouabain + bumetanide + Ca2+)-insensitive, K+ influx was characterized in human red blood cells. 2. This K+ flux was enhanced significantly in isotonic solutions of low ionic strength using sucrose to maintain constant osmolarity. This effect was found for fresh red blood cells as well as for stored (bank) red blood cells. However, the absolute magnitude of K+ influx in solutions of low ionic strength was halved for stored red blood cells. 3. Anion replacement of Cl- by CH3SO4- did not affect residual K+ fluxes, showing that Cl- -dependent transport pathways (e.g. the KCl co-transporter) are not involved in the low ionic strength effect. 4. The enhanced K+ influx in low ionic strength media was reversible when the cells were resuspended in a solution of physiological ionic strength. 5. K+ influx measured in light and dense fractions of erythrocytes (separated by centrifugation and corresponding to samples enriched with either 'young' or 'mature' red cells) showed that the low ionic strength effect does not change markedly with cell age. 6. Low ionic strength media elevated residual, i.e. (ouabain + bumetanide + Ca2+)-insensitive, influx of both K+ and Na+ by about the same amount. In both cases the flux was linear with concentration in the range investigated (0.25-10 mM). No significant increase in the uptake of the cations Ca2+ and lysine in low ionic strength solutions could be found. 7. In CH3SO4- -containing solutions of physiological ionic strength the residual K+ influx was almost independent of cell volume, whereas this flux in CH3SO4- -containing solutions of low ionic strength declined as cell volume was increased. 8. K+ flux measurements in solutions of different external pH, where NaCl was replaced by sodium gluconate or sodium glucuronate, showed that the reduced ionic strength is of more importance for the enhanced residual K+ influx than the changed transmembrane potential or the changed intracellular pH. However, a small pH dependence could be found, the K+ flux passing through a minimum around pH(i) 7.3. 9. Hydrostatic pressure enhanced the residual K+ flux in media of low ionic strength synergistically, so that very large fluxes (> 10 mmol (1 cells)-1 h-1) were obtained at 40 MPa. The apparent activation volumes (DELTA-V*) for the pressure-sensitive K+ flux were - 108 and - 69 ml mol-1 in low ionic strength or physiological ionic strength solutions respectively. 10. In solutions of physiological ionic strength the residual K+ influx did not vary greatly with temperature (0-37-degrees-C). In low ionic strength solutions this K+ flux was elevated and showed a minimum at about 8-degrees-C. The addition of 1 mM-LiCl inhibited the low temperature-induced K+ influx, the curve falling continuously with reducing temperature. 11. Taking into account the Goldman flux equation, one would expect a threefold decrease of the K+ influx in low ionic strength solution. However, a tenfold increase was observed which cannot be explained on the basis of classical electrodiffusion. 12. It is concluded that the low ionic strength-induced increase of the K+ influx is protein-mediated (which can be influenced by certain manoeuvres, i.e. temperature, pressure) or can occur through regions which allow ion translocation at instabilities between protein and lipid molecules.
