Modeling sickle cell vasoocclusion in the rat leg: quantification of trapped sickle cells and correlation with 31P metabolic and 1H magnetic resonance imaging changes.

Abstract

We have developed an animal model to elucidate the acute effects of perfusion abnormalities on muscle metabolism induced by different density-defined classes of erythrocytes isolated from sickle cell anemia patients. Technetium-99m (9"'Tc)-labeled, saline-washed normal (AA), homozygous sickle (SS), or high-density SS (SS4) erythrocytes were injected into the femoral artery of the rat and quantitative 99fTc imaging, 31p magnetic resonance spectroscopy by surface coil at 2 teslas, and 'H magnetic resonance imaging at 0.15 tesla were performed. Between 5 and 25 p1 of SS4 cells was trapped in the microcirculation of the thigh (or 1-6 X 107 cells per cubic centimeter of tissue). In contrast, fewer SS discocytes (SS2) or AA cells were trapped (an equivalent packed cell volume of less than 6.7 p1 and 0.3 pl, respectively). After injection of SS4 cells an initial increase in inorganic phosphate was observed in the region of the thigh served by the femoral artery, intracellular pH decreased, and subsequently the proton relaxation time T, reached a broad maximum at 18-28 hr. When T, obtained at this time was plotted against the volume of cells trapped, an increase of T, over the control value of411 ± 48 msec was found that was proportional to the number of cells trapped. We conclude that the densest SS cells are most effective at producing vasoocclusion. The extent of the change detected by 'H magnetic resonance imaging is dependent on the amount ofcells trapped in the microcirculation and the magnitude of the initial increase of inorganic phosphate.

Whole-Body Imaging of Sequestration of Plasmodium falciparum in the Rat

“…The ROI images revealed that human RBC, regardless of being infected or not, ended up in the liver, spleen and kidneys, an unspecific accumulation previously noted and suggested to depend on the phagocytosis of syngeneic RBC (11). Although the dynamics of the binding may superficially look similar between rats injected with uninfected RBC or with iRBC, a more thorough analysis of the first 5 min after injection shows that the rats injected with iRBC still maintained 60 to 80% of the initial binding while those injected with uninfected cells had lost more than 60% of the binding.…”

“…While the vast majority of the injected human RBC, regardless of being infected or not, were nonspecifically absorbed in liver, spleen, and kidneys, significant differences regarding the amount of retained material were noted in the lungs, both between iRBC and RBC and between the different strains and clones of malaria parasites used. In about 50% of the animals some (1 to 5%) of the total activity ended up in the urinary bladder, most likely a consequence of free 99m Tc from the sample injected and from catabolism of labeled cells (11).…”

“…Although the dynamics of the binding may superficially look similar between rats injected with uninfected RBC or with iRBC, a more thorough analysis of the first 5 min after injection shows that the rats injected with iRBC still maintained 60 to 80% of the initial binding while those injected with uninfected cells had lost more than 60% of the binding. As human erythrocytes previously have been shown to have a half-life of ϳ7 min in the rat (11), this is likely to result from a combination of continuous binding of circulating iRBC as well as higher stability of the specific binding of iRBC as compared to the passive trapping of noninfected cells. This also fits with previous observations that initial iRBC adherence is sometimes transitory, followed by progressive cell recruitment.…”

Erythrocyte density in sickle cell syndromes is associated with specific clinical manifestations and hemolysis

“…44 Although less adherent to vasculature, 45,46 DRBCs caused persistent blockage of small postcapillary venules in an ex vivo model 45,47 and were more prone to being trapped, in vivo, with acute effects, resulting in perfusion deficits followed by metabolic as shown by 99 mTc imaging and magnetic resonance spectroscopy. 48 More recently, the extent of mechanical deformation of the RBC membrane was found to control shearinduced ATP release and to regulate blood pressure by releasing ATP as a vasodilatory signaling molecule; therefore, increased RBC rigidity could be responsible for impaired ATP delivery and could favor vasoconstriction. 49 Hydrocarbamide, the only drug with clinical proven benefit in SCD, significantly decreases the % DRBCs after 6 months of therapy without a correlation with HbF % increase (Figure 1).…”

Ameliorating effects of fluorocarbon emulsion on sickle red blood cell–induced obstruction in an ex vivo vasculature

“…Adhesion of sickle red cells to vascular endothelium may narrow vessel lumen, increase red cell transit times, and allow cells with long delay times to polymerize and obstruct. 1 As observed in ex vivo 2 and in vivo models, 3 SS RBC-induced vaso-occlusion is often partial, allowing for decreased remnant flow. Hence, if oxygen is delivered to these areas, 4 decreased obstruction might be achieved.…”