The calcium permeability of cold-stored erythrocytes

Vanadate-induced Ca(2+) and Co(2+) uptake in human red blood cells

The vanadate-induced increase in passive uptake of calcium and cobalt and their interference were studied in human red cells using (45)Ca and (57)Co as tracers. Vanadate is a potent inhibitor of the Ca-pump in red cells, although in fed cells a residual pump activity remains that is highly significant compared to the passive influx, and even in cells that are both ATP-depleted and vanadate-treated the pump arrest is not complete. In the presence of vanadate the Ca(2+) uptake is increased due to inhibition of Ca-pump extrusion, but is further increased due to a vanadate-induced increment in passive influx. In order to measure the vanadate-induced increment in Ca(2+) influx, the total uptake in vanadate-treated cells is corrected for the basal influx, as recorded in ATP-depleted cells in the presence of tetrathionate (5mM) that has been shown to eliminate the residual Ca-pump activity in ATP-depleted cells. The (57)Co uptake is also increased by vanadate. (57)Co is not transported by the Ca-pump, and hence the uptake in vanadate-treated cells can be directly compared to the basal uptake, both in fed and in ATP-depleted cells. The vanadate effect shows rapid onset and appears to be irreversible. The vanadate-induced increment in uptake of both (45)Ca and (57)Co is reduced by about 50% in ATP-depleted cells compared to fed cells, suggesting a metabolism- or SH-group-dependent component. The influx of both (45)Ca (in ATP-depleted cells) and (57)Co (in fed cells) increases with the vanadate concentration, with a similar K(½) (0.4 and 0.3mM, respectively), and is nearly maximal at 5mM vanadate. The vanadate-induced increment in influx of both (45)Ca and (57)Co increases with the extracellular concentration as a saturable function, with K(½) estimated at, respectively, 700 and 80μM. In the case of (57)Co K(½) is similar in fed and in ATP-depleted cells. The vanadate-induced uptake of (45)Ca and of (57)Co shows interference. The uptake of (45)Ca is inhibited by Co(2+), and the uptake of (57)Co is inhibited by Ca(2+), although with an unexplained time course. The vanadate-induced uptake of (45)Ca and (57)Co are both inhibited, and to a similar degree, by the 1,4-dihydropyridine Ca(2+)-channel blocker nifedipine, although only at concentrations much higher than IC(50) for classical Ca-channels. The vanadate-induced increment in (57)Co uptake is electroneutral, in contrast to the basal uptake that is at least partially electrogenic. In experiments with resealed ghosts a vanadate-induced (57)Co uptake could not be detected. The vanadate-induced increment in (57)Co uptake amounts to nearly half the increment in (45)Ca uptake, both in fed and in ATP-depleted cells. It is speculated that the vanadate-induced Ca(2+) and Co(2+) uptake could be mediated by a putative common transporter, which appears to be separate and distinct from the putative common transporter for basal Ca(2+) and Co(2+) uptake.

Passive transport pathways for Ca(2+) and Co(2+) in human red blood cells. (57)Co(2+) as a tracer for Ca(2+) influx

The passive transport of calcium and cobalt and their interference were studied in human red cells using (45)Ca and (57)Co as tracers. In ATP-depleted cells, with the ATP concentration reduced to about 1μM, the progress curve for (45)Ca uptake at 1mM rapidly levels off with time, consistent with a residual Ca-pump activity building up at increasing [Ca(T)](c) to reach at [Ca(T)](c) about 5μmol(lcells)(-1) a maximal pump rate that nearly countermands the passive Ca influx, resulting in a linear net uptake at a low level. In ATP-depleted cells treated with vanadate, supposed to cause Ca-pump arrest, a residual pump activity is still present at high [Ca(T)](c). Moreover, vanadate markedly increases the passive Ca(2+) influx. The residual Ca-pump activity in ATP-depleted cells is fuelled by breakdown of the large 2,3-DPG pool, rate-limited by the sustainable ATP-turnover at about 40-50μmol(lcells)(-1)h(-1). The apparent Ca(2+) affinity of the Ca-pump appears to be markedly reduced compared to fed cells. The 2,3-DPG breakdown can be prevented by inhibition of the 2,3-DPG phosphatase by tetrathionate, and under these conditions the (45)Ca uptake is markedly increased and linear with time, with the unidirectional Ca influx at 1mM Ca(2+) estimated at 50-60μmol(lcells)(-1)h(-1). The Ca influx increases with the extracellular Ca(2+) concentration with a saturating component, with K(½(Ca)) about 0.3mM, plus a non-saturating component. From (45)Ca-loaded, ATP-depleted cells the residual Ca-pump can also be detected as a vanadate- and tetrathionate-sensitive efflux. The (45)Ca efflux is markedly accelerated by external Ca(2+), both in control cells and in the presence of vanadate or tetrathionate, suggesting efflux by carrier-mediated Ca/Ca exchange. The (57)Co uptake is similar in fed cells and in ATP-depleted cells (exposed to iodoacetamide), consistent with the notion that Co(2+) is not transported by the Ca-pump. The transporter is thus neither SH-group nor ATP or phosphorylation dependent. The (57)Co uptake shows several similarities with the (45)Ca uptake in ATP-depleted cells supplemented with tetrathionate. The uptake is linear with time, and increases with the cobalt concentration with a saturating component, with J(max) about 16μmol(lcells)(-1)h(-1) and K(½(Co)) about 0.1mM, plus a non-saturating component. The (57)Co and (45)Ca uptake shows mutual inhibition, and at least the stochastic Ca(2+) influx is inhibited by Co(2+). The (57)Co and (45)Ca uptake are both insensitive to the 1,4-dihydropyridine Ca-channel blocker nifedipine, even at 100μM. The (57)Co uptake is increased at high negative membrane potentials, indicating that the uptake is at least partially electrogenic. The (57)Co influx amounts to about half the (45)Ca influx in ATP-depleted cells. It is speculated that the basal Ca(2+) and Co(2+) uptake could be mediated by a common transporter, probably with a channel-like and a carrier-mediated component, and that (57)Co could be useful as a tracer for at least the channel-like Ca(2+) entry pathway in red cells, since it is not itself transported by the Ca-pump and, moreover, is effectively buffered in the cytosol by binding to hemoglobin, without interfering with Ca(2+) buffering. The molecular identity of the putative common transporter(s) remains to be defined.

Peroxiredoxin-2 as a candidate biomarker to test oxidative stress levels of stored red blood cells under blood bank conditions

BACKGROUND

Several researches on aging red blood cells (RBCs)--performed both in vivo and under blood bank conditions--revealed that RBC membrane proteins undergo a number of irreversible alterations, mainly due to oxidative stress. The individuation of proteins to be used as indicators of irreversible RBC injury and to be proposed as candidate biomarkers of oxidative damage or aging status during blood storage is therefore of great interest.

STUDY DESIGN AND METHODS

Based on this purpose we performed proteomic analysis of the membranes of RBCs during various storage periods under blood bank conditions. Changes in protein composition of RBC membranes were monitored as a function of the storage period by means of polyacrylamide gel electrophoresis coupled with immunoblotting and mass spectrometry analyses.

RESULTS

During storage, a progressive linkage of typical cytosolic proteins to the membrane was detected, including both antioxidant and metabolic enzymes (such as catalase, peroxiredoxin-2 [Prx2], and 2,3-bisphosphoglycerate-mutase), as well as nonreducible cross-linkings of probably oxidized or denatured hemoglobin. This phenomenon was unequivocally related to oxidative stress, since storage of RBCs under anaerobic conditions showed a suppression of these protein recruitments to the membrane.

CONCLUSION

The detailed analysis of these protein associations to the membrane of aged RBCs allowed Prx2 to be suggested as a potential RBC oxidative stress marker for the sake of developing new approaches in quality assurance of blood components.

Potassium loss and cellular dehydration of stored erythrocytes following incubation in autologous plasma: role of the KCl cotransport system

We studied the regulation of cell volume and cation content in erythrocytes stored at 4 degrees C under blood bank conditions for various lengths of time and subsequently incubated in autologous plasma at 37 degrees C for 4 or 24 h. Cell swelled during storage at 4 degrees C whereas marked K+ loss and cell shrinkage were observed when erythrocytes were incubated at 37 degrees C in autologous plasma. The cell shrinkage was inhibited only by the K+ Cl- cotransport-specific inhibitor, [(dihydroindenyl)oxy] alkanoic acid, and not by other specific inhibitors of cation transport systems such as ouabain (Na(+)-K+ ATPase pump), bumetanide (Na(+)-K(+)-Cl- cotransport) or carbocyanine (Ca+(+)-activated K+ channel). Acidification and swelling of the erythrocytes are well known to be able to activate the K+ Cl cotransport; such conditions, which were demonstrated to occur during the storage, could lead to activation of the K+ Cl- cotransport in reinfused cells. These data strongly support the evidence that K+ Cl- cotransport plays a role in K+ loss and dehydration of stored erythrocytes, when incubated in autologous plasma.