Effects of vanadate, menadione and menadione analogs on the Ca2+-activated K+ channels in human red cells. Possible relations to membrane-bound oxidoreductase activity

Endothelin-1 receptor antagonists regulate cell surface-associated protein disulfide isomerase in sickle cell disease

Increased endothelin-1 (ET-1) levels, disordered thiol protein status, and erythrocyte hydration status play important roles in sickle cell disease (SCD) through unresolved mechanisms. Protein disulfide isomerase (PDI) is an oxidoreductase that mediates thiol/disulfide interchange reactions. We provide evidence that PDI is present in human and mouse erythrocyte membranes and that selective blockade with monoclonal antibodies against PDI leads to reduced Gardos channel activity (1.6±0.03 to 0.56±0.02 mmol·10(13) cell(-1)·min(-1), P<0.001) and density of sickle erythrocytes (D50: 1.115±0.001 to 1.104±0.001 g/ml, P=0.012) with an IC50 of 4 ng/ml. We observed that erythrocyte associated-PDI activity was increased in the presence of ET-1 (3.1±0.2 to 5.6±0.4%, P<0.0001) through a mechanism that includes casein kinase II. Consistent with these results, in vivo treatment of BERK sickle transgenic mice with ET-1 receptor antagonists lowered circulating and erythrocyte associated-PDI activity (7.1±0.3 to 5.2±0.2%, P<0.0001) while improving hematological parameters and Gardos channel activity. Thus, our results suggest that PDI is a novel target in SCD that regulates erythrocyte volume and oxidative stress and may contribute to cellular adhesion and endothelial activation leading to vasoocclusion as observed in SCD.

The hSK4 (KCNN4) isoform is the Ca2+-activated K+ channel (Gardos channel) in human red blood cells

The question is, does the isoform hSK4, also designated KCNN4, represent the small conductance, Ca2+-activated K+ channel (Gardos channel) in human red blood cells? We have analyzed human reticulocyte RNA by RT-PCR, and, of the four isoforms of SK channels known, only SK4 was found. Northern blot analysis of purified and synchronously growing human erythroid progenitor cells, differentiating from erythroblasts to reticulocytes, again showed only the presence of SK4. Western blot analysis, with an anti-SK4 antibody, showed that human erythroid progenitor cells and, importantly, mature human red blood cell ghost membranes, both expressed the SK4 protein. The Gardos channel is known to turn on, given inside Ca2+, in the presence but not the absence of external Ko+ and remains refractory to Ko+ added after exposure to inside Ca2+. Heterologously expressed SK4, but not SK3, also shows this behavior. In inside-out patches of red cell membranes, the open probability (Po) of the Gardos channel is markedly reduced when the temperature is raised from 27 to 37 degrees C. Net K+ efflux of intact red cells is also reduced by increasing temperature, as are the Po values of inside-out patches of Chinese hamster ovary cells expressing SK4 (but not SK3). Thus the envelope of evidence indicates that SK4 is the gene that codes for the Gardos channel in human red blood cells. This channel is important pathophysiologically, because it represents the major pathway for cell shrinkage via KCl and water loss that occurs in sickle cell disease.

Non-selective voltage-activated cation channel in the human red blood cell membrane

Using the patch-clamp technique, a non-selective voltage-activated Na+ and K+ channel in the human red blood cell membrane was found. The channel operates only at positive membrane potentials from about +30 mV (inside positive) onwards. For sodium and potassium ions, similar conductances of about 21 pS were determined. Together with the recently described K+(Na+)/H+ exchanger, this channel is responsible for the increase of residual K+ and Na+ fluxes across the human red blood cell membrane when the cells are suspended in low ionic strength medium.

Properties of the Ca2+ influx reveal the duality of events underlying the activation by vanadate and fluoride of the Gárdos effect in human red blood cells

The properties of the 45Ca2+ influx by human red blood cells (RBC) induced by NaVO3 or NaF were compared. The NaVO3-induced 45Ca2+ influx was slower and less extensive than that induced by NaF. Both processes were saturable with Ca2+. Substitution of Na+ by K+ inhibited the 45Ca2+ influx induced by NaVO3 but stimulated that by NaF. The NaVO3-induced Ca2+ influx was sensitive to nifedipine (IC50 = 50 mol/l), Cu2+ (IC50=9 mol/l), DTNB (5,5'-dithiobis-(dinitrobenzoic acid)) (IC50 = 12 mol/l) (maximal inhibition 16%, 18%, and 28%, respectively, if NaF was used as inducer). On the other hand, tetrodotoxin (TTX) and cyclosporin A inhibited only the NaF-induced 45Ca2+ influx (IC50 = 21 mol/l and 28 mol/l, respectively). Pig RBC, known not to display the NaVO3-induced Ca2+ influx, exhibited Ca2+ influx induced by NaF. The results show that NaVO3 activates the Ca2+ influx via a pathway homologous to the L-type Ca2+ channel while the NaF-induced Ca2+ influx is mediated via the TTX-sensitive Na+ channel in the presence of NaF with possible participation of calcineurin or cyclophilin. Thus, the Gardos effect induced by NaVO3 and NaF represents two phenomena activated by different mechanisms present in the cryptic state in the RBC membrane.

Potassium channel modulation in rat portal vein by ATP depletion: a comparison with the effects of levcromakalim (BRL 38227)

1. The effects of levcromakalim and of adenosine 5'-triphosphate (ATP) depletion on membrane potential and ionic currents were studied in freshly-dispersed smooth muscle cells of rat portal vein by use of combined voltage- and current-clamp techniques. 2. Levcromakalim (1 microM) induced a glibenclamide-sensitive, non-inactivating K-current (IKCO) and simultaneously inhibited the slow, transient outward, delayed rectifier K-current (ITO). Levcromakalim also hyperpolarized the portal vein cells by approximately 20 mV. 3. Reduction of intracellular ATP by removal of glucose and carboxylic acids from the recording pipette and of glucose from the bath fluid, induced a slowly-developing, non-inactivating and glibenclamide-sensitive K-current (Imet) within 60-300 s after breaking the membrane patch. Imet reached peak amplitude after 300-900 s, remained at a plateau for 200-800 s and then slowly ran down. At the peak of Imet, the cells were hyperpolarized by approximately 20 mV and their input conductance was increased by 42%. 4. At the time of maximum development of Imet, the delayed rectifier current, ITO, was reduced by 48%. 5. In the absence of glucose and carboxylic acids, addition of 1 microM free ATP to the recording pipette almost doubled the magnitude of Imet. At a holding potential of -10 mV, Imet was increased from 124 +/- 11 pA to 228 +/- 54 pA whereas the time-course of development and run-down of Imet was unaffected. 6. During the development and after the run-down of Imet, levcromakalim (1-10 microM) failed to induce IKCO. 7. Stationary fluctuation analysis of the current noise associated with Imet revealed a unitary conductance of between 10-20 pS in a physiological potassium gradient. A second contaminating current with an underlying unitary conductance of approximately 150 pS remained after Imet had run down. 8. It is concluded that IKCO induced by levcromakalim and Imet are carried by the same population of relatively small conductance, glibenclamide-sensitive K-channels. The open state of these is increased by procedures designed to lower intracellular ATP concentrations. 9. The simultaneous inhibition of the delayed rectifier current (ITO) by both levcromakalim and during the development of Imet is highly significant. It suggests that levcromakalim could modify the interaction of ATP with sites linked to more than one type of K-channel. This results in the opening of those channels which underlie IKCO (and which are normally inhibited by ATP binding) together with the modulation of phosphorylation-dependent channels such as those which underlie ITO.

The effect of ferricyanide with iodoacetate in calcium-free solution on passive cation permeability in human red blood cells: comparison with the Gardos-effect and with the influence of PCMBS on passive cation permeability

Freshly prepared human red blood cells incubated with 5 mM ferricyanide, 0.2 mM iodoacetate and 2 mM adenosine in the presence of 5 mM EGTA demonstrate comparable increases in Na+ and K+ permeability (ferricyanide effect). This effect is unrelated to the Ca2+-activated K+ channel (Gardos effect) since influx of Ca2+ from outside the cell is excluded. Also this effect is different from the non-specific Na+ and K+ permeability change elicited by PCMBS. These differences become obvious by using various reagents. For example, A23187 and quinidine exert opposite effects in Gardos and ferricyanide experiments, where A23187 and atebrin react oppositely in the latter and in PCMBS experiments. The ferricyanide effect described here does not involve formation of nonspecific channels. The change in Na+ permeability separately from K+ permeability under certain circumstances suggests a more specific effect.

Modulation of the Ca2+- or Pb2+-activated K+-selective channels in human red cells. I. Effects of propranolol

To study the effect of propranolol on the Ca2+- or Pb2+-activated K+ permeability in human erythrocytes, K+ effluxes were compared with single-channel currents. The results demonstrate that propranolol has a twofold effect: (1) it renders the channel protein more sensitive to Ca2+ or Pb2+; and (2) it simultaneously inhibits channel activity and slightly reduces single-channel conductance. The number of active channels is not affected.

Modulation of the Ca2+- or Pb2+-activated K+-selective channels in human red cells. II. Parallelisms to modulation of the activity of a membrane-bound oxidoreductase

Modulation of Ca2+-activable K+ permeability was compared with modulation of a membrane-bound oxidoreductase activity in human erythrocytes. Changes in the K+ permeability were monitored by flux measurements and single-channel recordings. The enzyme activity was detected by measuring reduction of ferricyanide. Pb2+, Atebrin and menadione had parallel effects on the channel protein and the enzyme. In contrast, propranolol stimulates K+ permeability, but is without effect on enzyme activity. The results demonstrate that the K+ channel and the enzyme are distinct membrane proteins but that the enzyme activity may influence channel gating.

Changes in autorhythmic heart frequency elicited by redox agents

In isolated frog heart it was established that methylene-blue (MB, an oxidizing agent) decreased, while ascorbate (ASC, a reducing agent) increased the frequency of autorhythmic heart contractions. After MB treatment, in parallel with this phenomenon, the extracellular K+ concentration [K+]o showed a slow increase, but following ASC application a slow decrease occurred. Since these correlations are in good accordance with the idea that the pacemaking ability of heart, among other properties, depends on the voltage and time-dependent decrease in potassium conductance following the spike, changes in [K+]o might be one mechanism by which oxidizing and reducing agents modulate heart frequencies. On the basis of the effect of insulin (INS) and K-strophantoside (STR) on these modulatory influences, it is presumed that the changes in slow delta [K+]o transients might result, at least partly, from the effect of redox agents on the active transport system. In light of the increase in passive K+ fluxes after oxidant treatment and the decrease in this parameter following reductant treatment an effect of redox agents on the characteristics of the K+-channel is also postulated.