DIDS inhibition of deformation-induced cation flux in human erythrocytes

Sickle cell dehydration: Pathophysiology and therapeutic applications

Cell dehydration is a distinguishing characteristic of sickle cell disease and an important contributor to disease pathophysiology. Due to the unique dependence of Hb S polymerization on cellular Hb S concentration, cell dehydration promotes polymerization and sickling. In double heterozygosis for Hb S and C (SC disease) dehydration is the determining factor in disease pathophysiology. Three major ion transport pathways are involved in sickle cell dehydration: the K-Cl cotransport (KCC), the Gardos channel (KCNN4) and Psickle, the polymerization induced membrane permeability, most likely mediated by the mechano-sensitive ion channel PIEZO1. Each of these pathways exhibit unique characteristics in regulation by oxygen tension, intracellular and extracellular environment, and functional expression in reticulocytes and mature red cells. The unique dependence of K-Cl cotransport on intracellular Mg and the abnormal reduction of erythrocyte Mg content in SS and SC cells had led to clinical studies assessing the effect of oral Mg supplementation. Inhibition of Gardos channel by clotrimazole and senicapoc has led to Phase 1,2,3 trials in patients with sickle cell disease. While none of these studies has resulted in the approval of a novel therapy for SS disease, they have highlighted the key role played by these pathways in disease pathophysiology.

Effect of chloride channel inhibitors on cytosolic Ca2+ levels and Ca2+-activated K+ (Gardos) channel activity in human red blood cells

DIDS, NPPB, tannic acid (TA) and AO1 are widely used inhibitors of Cl(-) channels. Some Cl(-) channel inhibitors (NPPB, DIDS, niflumic acid) were shown to affect phosphatidylserine (PS) scrambling and, thus, the life span of human red blood cells (hRBCs). Since a number of publications suggest Ca(2+) dependence of PS scrambling, we explored whether inhibitors of Cl(-) channels (DIDS, NPPB) or of Ca(2+)-activated Cl(-) channels (DIDS, NPPB, TA, AO1) modified intracellular free Ca(2+) concentration ([Ca(2+)]i) and activity of Ca(2+)-activated K(+) (Gardos) channel in hRBCs. According to Fluo-3 fluorescence in flow cytometry, a short treatment (15 min, +37 °C) with Cl(-) channels inhibitors decreased [Ca(2+)]i in the following order: TA > AO1 > DIDS > NPPB. According to forward scatter, the decrease of [Ca(2+)]i was accompanied by a slight but significant increase in cell volume following DIDS, NPPB and AO1 treatments. TA treatment resulted in cell shrinkage. According to whole-cell patch-clamp experiments, TA activated and NPPB and AO1 inhibited Gardos channels. The Cl(-) channel blockers further modified the alterations of [Ca(2+)]i following ATP depletion (glucose deprivation, iodoacetic acid, 6-inosine), oxidative stress (1 mM t-BHP) and treatment with Ca(2+) ionophore ionomycin (1 μM). The ability of the Cl(-) channel inhibitors to modulate PS scrambling did not correlate with their influence on [Ca(2+)]i as TA and AO1 had a particularly strong decreasing effect on [Ca(2+)]i but at the same time enhanced PS exposure. In conclusion, Cl(-) channel inhibitors affect Gardos channels, influence Ca(2+) homeostasis and induce PS exposure of hRBCs by Ca(2+)-independent mechanisms.

Monovalent cation leaks in human red cells caused by single amino-acid substitutions in the transport domain of the band 3 chloride-bicarbonate exchanger, AE1

We identified 11 human pedigrees with dominantly inherited hemolytic anemias in both the hereditary stomatocytosis and spherocytosis classes. Affected individuals in these families had an increase in membrane permeability to Na and K that is particularly marked at 0 degrees C. We found that disease in these pedigrees was associated with a series of single amino-acid substitutions in the intramembrane domain of the erythrocyte band 3 anion exchanger, AE1. Anion movements were reduced in the abnormal red cells. The 'leak' cation fluxes were inhibited by SITS, dipyridamole and NS1652, chemically diverse inhibitors of band 3. Expression of the mutated genes in Xenopus laevis oocytes induced abnormal Na and K fluxes in the oocytes, and the induced Cl transport was low. These data are consistent with the suggestion that the substitutions convert the protein from an anion exchanger into an unregulated cation channel.

Enantioselective uptake of BOF-4272, a xanthine oxidase inhibitor with a chiral sulfoxide, by isolated rat hepatocytes

The transport mechanisms of the enantiomers of BOF-4272, a new drug for the treatment of hyperuricemia, were studied using freshly prepared rat hepatocytes. BOF-4272 consists of S(-) and R(+) enantiomers due to a chiral center in the sulfoxide moiety. The uptake of these BOF-4272 enantiomers by hepatocytes was found to be temperature and dose dependent. The temperature-dependent uptake of the S(-) and R(+) enantiomers showed saturation kinetics. The Km values for the S(-) and R(+) enantiomers were 59.3 and 25.7 microM, respectively, which was a significant difference (p < 0.05). However, the maximal uptake rate was comparable for both enantiomers. Metabolic inhibitors such as antimycin, oligomycin, rotenone, carbonylcyanide m-chlorophenyl hydrazone, and carbonyl cyanide-p-(trifluromethoxy)-phenylhydrazone significantly inhibited uptake of the R(+) enantiomer, but had little effect on uptake of the S(-) enantiomer. Ouabain (an inhibitor of Na+/K(+)-ATPase) and p-nitrobenzylthioinosine (NBMPR, a nucleoside transporter inhibitor) showed no significant effects on the uptake of either enantiomer. Organic anions such as taurocholate and cholate reduced the uptake of both enantiomers. These results suggest that the hepatic uptake of both BOF-4272 enantiomers is not due to simple diffusion but also involves carrier-mediated uptake. We suggest that the carrier-mediated uptake of BOF-4272 enantiomers includes both NBMPR-insensitive facilitated diffusion and an active transport system in liver plasma membrane, and that the enantioselective uptake of BOF-4272 is due to differences in affinity for the active transporter.

Dipyridamole inhibits sickling-induced cation fluxes in sickle red blood cells

Sickling-induced cation fluxes contribute to cellular dehydration of sickle red blood cells (SS RBCs), which in turn potentiates sickling. This study examined the inhibition by dipyridamole of the sickling-induced fluxes of Na(+), K(+), and Ca(++) in vitro. At 2% hematocrit, 10 microM dipyridamole inhibited 65% of the increase in net fluxes of Na(+) and K(+) produced by deoxygenation of SS RBCs. Sickle-induced Ca(++) influx, assayed as (45)Ca(++) uptake in quin-2-loaded SS RBCs, was also partially blocked by dipyridamole, with a dose response similar to that of Na(+) and K(+) fluxes. In addition, dipyridamole inhibited the Ca(++)-activated K(+) flux (via the Gardos pathway) in SS RBCs, measured as net K(+) efflux in oxygenated cells exposed to ionophore A23187 in the presence of external Ca(++), but this effect resulted from reduced anion conductance, rather than from a direct effect on the K(+) channel. The degree of inhibition of sickling-induced fluxes was dependent on hematocrit, and up to 30% of dipyridamole was bound to RBC membranes at 2% hematocrit. RBC membrane content of dipyridamole was measured fluorometrically and correlated with sickling-induced flux inhibition at various concentrations of drug. Membrane drug content in patients taking dipyridamole for other clinical indications was similar to that producing inhibition of sickling-induced fluxes in vitro. These data suggest that dipyridamole might inhibit sickling-induced fluxes of Na(+), K(+), and Ca(++) in vivo and therefore have potential as a pharmacological agent to reduce SS RBC dehydration. (Blood. 2001;97:3976-3983)

The influence of merocyanine 540 and protoporphyrin on physicochemical properties of the erythrocyte membrane

The interaction of the red cell membrane with merocyanine 540 or protoporphyrin led to four phenomena, most probably interrelated. (i) The morphology changed from the normal discoid to an echinocytic form. This morphological change persisted when followed over a period of 24 h. (ii) Simultaneously, cell deformability was decreased, as revealed by viscosity measurements and a cell-filtration technique. (iii) Both drugs caused swelling of the erythrocytes in isotonic medium, due to a very-short-term increased permeability of the membrane, also for larger molecules such as lactose. The pathway of this temporary leak seems to be unrelated to the Na+/K+ -ATPase, the K+/Cl- and the Na+/K+/Cl- cotransport systems, the Ca2+-activated Gardos pathway, the oxidation/deformation-activated leak pathway and the so-called residual transport route. Despite the morphological changes, K+-leakage induced by mechanical stress was not increased. (iv) During osmotic swelling, the critical hemolytic volume was found to be increased in the presence of either merocyanine 540 or protoporphyrin. The increase critical volume protected erythrocytes against osmotic hemolysis.

Deoxygenation-induced cation fluxes in sickle cells. IV. Modulation by external calcium

Net cation movements were measured in low-density sickle red blood cells (SS RBC) in the presence and absence of oxygen. External Ca2+ (Ca2+o) partially inhibited deoxygenation-induced fluxes of both Na+ and K+. Deoxygenation-induced Na+ influx was reduced by 2 mM Ca2+o to 0.71 +/- 0.04 (SE) of its value in Ca(2+)-free solutions, whereas this ratio was 0.90 +/- 0.05 for K+ efflux (P < 0.01 by paired t-test). Because Ca2+o inhibited Na+ influx more than K+ efflux, net cation loss in deoxygenated SS RBC was higher in the presence of Ca2+o. In separate experiments, Ca2+o reduced deoxygenation-induced Na+ influx to 0.66 +/- 0.03 of its Ca(2+)-free value compared with 0.77 +/- 0.03 for Rb+ influx (P < 0.001), indicating relative selectivity of this effect for Na+ over Rb+. However, this effect is not specific for Ca2+ because other divalent cations also inhibited deoxygenation-induced Na+ and K+ fluxes. Under the conditions of these experiments, no evidence for K+ channel activation was found, indicating that K+ loss measured in deoxygenated SS RBC was mediated by the deoxygenation-induced pathway. These studies show that in the presence of Ca2+o deoxygenation-induced Na+ influx and K+ efflux are unbalanced. This pathway can, therefore, mediate cation loss and contribute directly to cellular dehydration in SS RBC.

The effect of protoporphyrin on the susceptibility of human erythrocytes to oxidative stress: exposure to hydrogen peroxide

Binding of protoporphyrin caused a perturbation of the erythrocyte membrane, as reflected by a change in cell shape from discoid to echinocyte, and a concomitant increase in mean cellular volume and K(+)-loss. Protoporphyrin-induced changes could be prevented by the presence of BaCl2, whereas binding of protoporphyrin was not affected. Exposure of erythrocytes to hydrogen peroxide leads to K(+)-leakage and lipid peroxidation. In de presence of protoporphyrin, H2O2-induced K(+)-leakage was enhanced, whereas lipid peroxidation was inhibited. The increase in H2O2-induced K(+)-leakage by protoporphyrin was not affected by diamide or various K+ channel blockers, but could be prevented by the addition of BaCl2. The inhibition of lipid peroxidation, on the other hand, was not affected by BaCl2. These results indicate that the enhancement of H2O2-induced K(+)-leakage was most likely caused by the change in cell shape. Addition of chlorpromazine and promethazine, positively charged molecules that induce stomatocytosis, did not cause an enhancement of H2O2-induced K(+)-leakage.

Membrane stress increases cation permeability in red cells

The human red cell is known to increase its cation permeability when deformed by mechanical forces. Light-scattering measurements were used to quantitate the cell deformation, as ellipticity under shear. Permeability to sodium and potassium was not proportional to the cell deformation. An ellipticity of 0.75 was required to increase the permeability of the membrane to cations, and flux thereafter increased rapidly as the limits of cell extension were reached. Induction of membrane curvature by chemical agents also did not increase cation permeability. These results indicate that membrane deformation per se does not increase permeability, and that membrane tension is the effector for increased cation permeability. This may be relevant to some cation permeabilities observed by patch clamping.