Effects of low ionic strength media on passive human red cell monovalent cation transport.

Abstract

SUMMARY1. 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 (025-10 mM). No significant increase in the uptake of the cations Ca2+ and lysine in low ionic strength solutions could be found.7. In CH3S04--containing solutions of physiological ionic strength the residual K+ influx was almost independent of cell volume, whereas this flux in CH3S04--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 pHi 7.3.9.

Cryohydrocytosis: increased activity of cation carriers in red cells from a patient with a band 3 mutation

“…Characterization of the K + influx via KCC and KNHE in the CHC patient's erythrocytes showed that the unusually high cation permeability in these cells at low temperature is, at least in part, a cumulative effect of an abnormal activation of two independent ion transport systems: KCC and KNHE. As follows from the Table 1, and in agreement with former reports, 22,30 both ion transporters are almost silent in RBC from healthy donors (and the patient's family members), particularly during cold storage. Several lines of evidence point towards an abnormally high activity of the cation-proton exchanger in RBC of the CHC patient at 37°C and even more so at 0°C.…”

“…A minor increase in residual K + leak at low temperature had already been reported and was especially pronounced under conditions of low ionic strength. 22,31 This cold-induced response attributed to the activation of the KNHE 16 is amplified many fold in the patient's RBC. When the patient's RBC were suspended in cold, NaCl-containing solution, they showed minor changes in cell volume/density ( Figure 1B) because the Na + accumulation almost matched the K + loss ( Figure 2B and C).…”

“…KCC activity was assessed using 86 Rb as a tracer for K + , as described elsewhere. 22 The activity of KNHE was measured as a unidirectional K + influx component sensitive to HOE-642 [cariporide, (4-isopropyl-3-methylsulfonyl-benzoyl)-guanidine methanesulfonate, a gift from Sanofi-Aventis, Germany]. 15 In all experiments ouabain (0.1 mM), bumetanide (0.1 mM), and EGTA (0.1 mM) were present in the incubation medium during the flux measurement.…”

“…Either 100 µM DIDS (a known inhibitor of band 3-mediated anion exchange at >10 µM) or 500 µM HOE-642 (an inhibitor of the cation-proton exchanger) were added to whole blood of the patient and a healthy subject 10 min prior to cold exposure. The high concentration of DIDS (the free concentration may be less than the nominal due to DIDS binding to plasma albumin) and HOE-642 decreased the steady-state plasma K + concentration in cold-stored blood to 22 Figure 2C). These data are in line with those obtained from the long-term MCV and plasma K + measurements ( Figure 2A) and indicate that the observed cold-induced response is mediated by at least two independent processes.…”

Epithelia & Membrane Transport

“…Incubating human red blood cells in low ionic strength media induces a transport pathway selective for monovalent cations (Na+, K+), but not Ca2" or lysine (Bernhardt et al 1991), which shows properties reminiscent of the volume-activated organic substrate transport pathway in fish erythrocytes (Kirk et al 1992) and malariainfected human erythrocytes . To investigate the characteristics of the low ionic strengthinduced pathway further we have compared taurine transport in human red cells incubated in a buffered sucrose medium (LIS) with cells in Mops-buffered physiological saline.…”