The effects of pentoxifylline on rat erythrocytes of different age

Erythrocyte ion channels in regulation of apoptosis

Erythrocytes lack mitochondria and nuclei, key organelles in the regulation of apoptosis. Until recently, erythrocytes were thus not considered subject to this type of cell death. However, exposure of erythrocytes to the Ca2+ ionophore ionomycin was shown to induce cell shrinkage, cell membrane blebbing and breakdown of phosphatidylserine asymmetry with subsequent phosphatidylserine exposure at the cell surface, all typical features of apoptosis. Further studies revealed the participation of ion channels in the regulation of erythrocyte "apoptosis." Osmotic shock, oxidative stress and energy depletion all activate a Ca2(+)-permeable non-selective cation channel in the erythrocyte cell membrane. The subsequent increase of Ca2+ concentration stimulates a scramblase leading to breakdown of cell membrane phosphatidylserine asymmetry and activates Ca2+ sensitive K+ (Gardos) channels leading to KCl loss and (further) cell shrinkage. Phosphatidylserine exposure and cell shrinkage are blunted in the nominal absence of extracellular Ca2+, in the presence of the cation channel inhibitors amiloride or ethylisopropylamiloride, at increased extracellular K+ or in the presence of the Gardos channel inhibitors clotrimazole or charybdotoxin. Thus, increase of cytosolic Ca2+ and cellular loss of K+ participate in the triggering of erythrocyte scramblase. Nevertheless, phosphatidylserine exposure is not completely abrogated in the nominal absence of Ca2+, pointing to additional Ca2(+)-independent pathways. One of those is activation of sphingomyelinase with subsequent formation of ceramide which in turn leads to stimulation of erythrocyte scramblase. The exposure of phosphatidylserine at the extracellular face of the cell membrane stimulates phagocytes to engulf the apoptotic erythrocytes. Thus, sustained activation of the cation channels eventually leads to clearance of affected erythrocytes from peripheral blood. Erythropoietin inhibits the non-selective cation channel and thus interferes with erythrocyte "apoptosis." Susceptibility to scramblase activation is enhanced in thalassemia, sickle cell disease and glucose-6-phosphate dehydrogenase deficiency. Infection with Plasmodium falciparum leads to activation of the cation channel eventually triggering erythrocyte "apoptosis."

Cation channels, cell volume and the death of an erythrocyte

Similar to a variety of nucleated cells, human erythrocytes activate a non-selective cation channel upon osmotic cell shrinkage. Further stimuli of channel activation include oxidative stress, energy depletion and extracellular removal of Cl-. The channel is permeable to Ca2+ and opening of the channel increases cytosolic [Ca2+]. Intriguing evidence points to a role of this channel in the elimination of erythrocytes by apoptosis. Ca2+ entering through the cation channel stimulates a scramblase, leading to breakdown of cell membrane phosphatidylserine asymmetry, and stimulates Ca(2+)-sensitive K+ channels, thus leading to KCl loss and (further) cell shrinkage. The breakdown of phosphatidylserine asymmetry is evidenced by annexin binding, a typical feature of apoptotic cells. The effects of osmotic shock, oxidative stress and energy depletion on annexin binding are mimicked by the Ca2+ ionophore ionomycin (1 microM) and blunted in the nominal absence of extracellular Ca2+. Nevertheless, the residual annexin binding points to additional mechanisms involved in the triggering of the scramblase. The exposure of phosphatidylserine at the extracellular face of the cell membrane stimulates phagocytes to engulf the apoptotic erythrocytes. Thus, sustained activation of the cation channels eventually leads to clearance of affected erythrocytes from peripheral blood. Susceptibility to annexin binding is enhanced in several genetic disorders affecting erythrocyte function, such as thalassaemia, sickle-cell disease and glucose-6-phosphate dehydrogenase deficiency. The enhanced vulnerability presumably contributes to the shortened life span of the affected erythrocytes. Beyond their role in the limitation of erythrocyte survival, cation channels may contribute to the triggering of apoptosis in nucleated cells exposed to osmotic shock and/or oxidative stress.

The effects of extended perioperative pentoxifylline on random skin flap survival

PURPOSE

This study was designed to determine the effects of long-term perioperative pentoxifylline administration on random skin flap survival in an appropriate animal model. A secondary objective was to document bioavailability of pentoxifylline in the animal model by measuring blood levels of parent compound and metabolites at regular intervals and comparing these to levels measured in humans.

MATERIALS AND METHODS

A randomized control study of the effects of oral pentoxifylline on the survival of "random" skin flaps was conducted at the animal care facility of an academic tertiary referral center on six randomly selected Yorkshire pigs. Oral pentoxifylline was administered daily to four pigs for 3 months, and two pigs received placebo. Pentoxifylline blood levels for each experimental animal were measured after 4, 8, and 12 weeks of daily dosing. Blood viscosity, fibrinogen, and hematocrit were measured for each of the six animals on day 1, day 30, day 60, and day 91. On day 91, 12 surgical random skin flaps were elevated on each of the six animals and immediately sewn back in the donor bed. Pentoxifylline dosing was continued for 2 weeks, and placebo was continued in control animals. On postoperative day 15, all animals were killed and all flaps were measured individually for area of viability. The outcome measure was the detection of statistically significant increase in survival area in skin flaps of those animals administered perioperative pentoxifylline.

RESULTS

No significant augmentation of flap survival was noted in experimental animals when compared with those in the control group; no significant difference was noted between or within groups of experimental animals. Pentoxifylline and metabolite blood levels in all experimental animals at every interval were noted to be comparable to those documented in human studies; metabolite concentrations conformed to expected patterns as observed in humans. No significant alterations of blood viscosity, fibrinogen, or hematocrit were measured in the experimental animals when compared with those in the control group. The only animal showing mean flap survival outside the 95% confidence interval was one in the control group.

CONCLUSION

No augmentation of random skin flap survival could be shown in the pig model after a 3 month regimen of daily oral pentoxifylline. Blood levels of pentoxifylline in experimental animals were compared with those documented in humans. No alteration of blood viscosity, fibrinogen, or hematocrit was noted in any of the experimental animals when compared with each other or with those in the control group.

Generalised granuloma annulare successfully treated with pentoxifylline

Generalised granuloma annulare (GA) is a chronic disease of unknown aetiology and is recalcitrant to many treatment regimes. Some investigators have suggested that an immune medicated vasculitis may be involved in the pathogenesis of GA. We describe a patient with a ten year history of generalised GA, who showed dramatic clearing of the majority of papules after four weeks of treatment with pentoxifylline. This drug has shown promising results in the treatment of many dermatologic disorders including necrobiosis lipoidica diabeticorum, leukocytoclastic vasculitis and Raynaud's phenomenon. Pentoxifylline is thought to reduce blood viscosity via effects on all major blood components, and its clinical effectiveness in generalised GA lends support to a model of immune-medicated vasculitis in the pathogenesis of this disorder. Thus, pentoxifylline offers a well-tolerated and effective alternative to the treatment options available for patients with granuloma annulare.

The effects of pentoxifylline on the plasma membrane Ca2+ ATPase in age-separated rat and human erythrocytes

Pentoxifylline, a dimethyl xanthine derivative given to patients with peripheral vascular disorders, increases erythrocyte deformability, diminishes Ca2+ entry, inhibits Ca(2+)-dependent transglutaminase activity and elevates ATP levels. The present study examined the effects of pentoxifylline on the Ca2+ pump ATPase, an enzyme which regulates intracellular Ca2+ levels. Studies were carried out with inside-out vesicles (IOVs) prepared from young (Ey) and old (Eo) human and rat erythrocytes. The pumping of Ca2+ depends on the concomitant hydrolysis of ATP; the two processes were measured using radiolabeled substrates. The catalytic properties of IOVs from young and old erythrocytes were stimulated by pentoxifylline when added directly to the assay medium. Pentoxifylline (0.5 to 5.0 mM) significantly activated the rates of Ca2+ dependent ATP hydrolysis in Ey and Eo IOVs of rat erythrocytes. The percent of activation was greater in the IOVs from older erythrocytes. The Ca2+ translocation was also affected by pentoxifylline. The early burst of Ca2+ uptake into IOVs decreased in the presence of pentoxifylline in Ey IOVs prepared from either species whereas steady state rates of Ca2+ transport only declined at 5.0 mM pentoxifylline. This pattern was not evident in the corresponding Eo IOVs. The response of Ey and Eo IOVs to pentoxifylline may be the basis of the difference in sensitivity of young and old erythrocytes to the drug in regulating intracellular Ca2+ concentrations.

Ca2+ transport activities of inside-out vesicles prepared from density-separated erythrocytes from rat and human

The plasma membrane Ca2+ ATPase in erythrocytes is vital for the maintenance of intracellular Ca2+ levels. Since the cytoplasmic Ca2+ concentration is elevated in older erythrocytes, the properties of the Ca2+ transport ATPase were examined during cell aging using inside-out vesicles (IOVs) prepared from density-separated, young (less dense, Ey) and old (more dense, Eo) rat and human erythrocytes. The transport of Ca2+ and the coupled hydrolysis of ATP were measured using radiolabeled substrates. The calmodulin-independent Ca2+ transport activity (Ey, 38.8 vs. Eo, 23.3 nmols/min/mg IOV protein) and the Ca2+ dependent ATP phosphohydrolase activity (Ey, 53.5 vs. Eo, 48.8 nmols/min/mg protein) were greater in IOVs prepared from younger (less dense) rat erythrocytes. The calmodulin-independent Ca2+ transport activity in IOVs from human erythrocytes was 12.9 nmols/min/mg IOV protein for Ey and 10.7 nmols/min/mg IOV protein for Eo. Inside-out vesicles from older (more dense) cells exhibited a lower pumping efficiency as determined by the calculated stoichiometry, molecule of Ca2+ transported per molecule of ATP hydrolyzed (rat: Ey, 0.74 vs. Eo, 0.49; human: Ey, 1.22 vs. Eo, 0.77). The enzymatic activity of rat and human Ey IOVs was labile. The Ca2+ transport activity in Ey but not Eo IOVs rapidly declined during cold storage (4 degrees C). The decrease in Ca2+ transport activity during aging may accentuate the age-related decline in several erythrocytic properties.

Effects of pentoxifylline on Ca2+-dependent transglutaminase in rat erythrocytes

We have examined the effects of pentoxifylline, a drug used to improve peripheral blood flow in patients with chronic vascular disorders, on shear-induced Ca2+ entry and Ca2+-dependent protein crosslinking in the erythrocyte. Cells were washed in isosmotic bicarbonate buffer, pH 7.5, and brought to a 20% suspension. 5 ml samples were sheared by swirling them for seconds; 45Ca2+ uptake, activation of Ca2+-dependent transglutaminase, and products of the crosslinking reaction were measured. A 5 second shear promoted 45Ca2+ entry, activation of Ca2+-dependent transglutaminase and crosslinking of proteins in the membrane-cytoskeletal fraction. Pentoxifylline, a drug that promotes erythrocyte deformability, diminished Ca2+ entry and inhibited activation of Ca2+-dependent transglutaminase, and had a prophylactic role on the effects of Ca2+ entry due to shear. Incubation of cells with 2.5 mM pentoxifylline before swirling minimized the effects of shear on 45Ca2+ entry, Ca2+-dependent transglutaminase and crosslinking of proteins. This study indicates that pentoxifylline promotes erythrocyte flexibility by minimizing shear-induced Ca2+ entry and Ca2+-dependent crosslinking.