Amongst oxidants that we have studied PMS
Amongst oxidants that we have studied, PMS is unique in having a greater effect in deoxygenated cells. The resulting phenotype shows some similarities with the increased cation permeability shown by deoxygenated sickle adenosine receptor agonist . It is interesting to consider why PMS has this effect. PMS functions as a hydrogen acceptor and donor , . It is therefore able to generate oxygen free radicals in the presence of hydrogen donors, such as ascorbate, GSH, NADH and NADPH. Of these, GSH is absent whilst ascorbate is not obviously affected by O2 tension. On deoxygenation, however, the red cell metabolism switches from flux of glucose through the pentose phosphate pathway to increased flux through the glycolytic pathway . As this occurs, (re)generation of NADH will increase. We speculate that increased availability of NADH, in the presence of PMS, leads to greater formation of oxygen free radicals and hence a greater effect on K+ permeability. The increased formation of metHb in deoxygenated cells treated with PMS is consistent with this hypothesis. We are currently investigating the action of these oxidants on different cell populations and the ability of antioxidants to protect cells against damage.
Introduction Circulating red blood cells (RBCs) are always subject to oxidative attacks. Their interaction with oxygen leads to hemoglobin autooxidation, resulting in the formation of superoxide radical (О2ˉ) and other reactive oxygen species (ROS), mostly hydrogen peroxide (H2O2) and hydroxyl radical (OH˙) , . RBCs can also be attacked by exogenous ROS, originating from other blood cells (platelets, monocytes, neutrophils, and macrophages), as well as from vessel endothelium , , . Imbalance between ROS production and antioxidant cell defence has been reported to result in oxidative stress (oxidation of protein thiol groups, depletion of nonenzymatic antioxidants, and lipid peroxidation of membrane phospholipids) . ROS generation and oxidative stress in RBCs are augmented in various disease conditions, such as sepsis, shock, burns, ischemia–reperfusion, and certain enzymo- and hemoglobinopathies , . In some cases (sickle cell disease, thalassemia, and renal insufficiency), pathology-related redox alterations in RBCs go along with anemia, the appearance of an abnormally high-density RBC subset, and reduced intracellular K+ , , . In addition, loss of deformability and increased adhesiveness are observed, which lead to circulation disorders and may contribute to the development of ischemia and hypercoagulation . Evidence exists suggesting that Ca2+-activated K+ channels (Gardos channels) are involved in the damaging RBC changes , . For example, the specific Gardos channel inhibitor clotrimazole is used in vivo to normalize the properties of RBCs in patients and various animal models . On the other hand, in certain pathological situations, Gardos channel activation may be beneficial, preventing RBC lysis , , . Oxidative stress contributes greatly to aging of normal RBCs in the circulation. With aging, intracellular ionic calcium progressively increases; cell volume and surface area become smaller (although the surface-to-volume ratio remains constant). It is not unlikely that cell aging also involves Gardos channel activation , . So, oxidative stress-induced changes in the properties of RBCs have not only clinical, pathophysiological relevance, but also physiological, regulatory relevance . However, the physiological and pathophysiological functions of oxidative stress activation of Gardos channels remain obscure. The response to oxidative stress at cellular and subcellular levels is studied using a variety of oxidative systems. For example, phenazine methosulfate (PMS) and t-butyl-hydroperoxide (t-BHP) are often employed. When applied to deoxygenated RBCs or RBCs treated with ascorbate at millimolar concentrations, PMS causes intracellular Са2+ to rise and dramatically increases membrane permeability for K+ by activating Gardos channels, which results in RBC dehydration , . The response to t-BHP is similar in general; however, unlike PMS, t-BHP (1–3 mM) also considerably raises intracellular Na+, which causes RBCs to swell and lose filterability , . On the other hand, Lang et al. reported that t-BHP induced RBC shrinkage .