Indium-111 oxine labeled erythrocytes: cellular distribution and efflux kinetics of the label

Indium-111 oxine label erythrocytes are useful in scintigraphic studies of splenic function because of the high yield of gamma-photons [172(90%) and 247(94%) keV] of indium-111. However, the effects of indium-111 oxine on the structural and functional integrity of erythrocytes which might influence their reticulo-endothelial (RE) sequestration are unknown. We examined the morphology of human and rat indium-111 labeled erythrocytes by SEM, the distribution of the label within the cell by analysis of the membrane and cytosol (hemoglobin solution) and the kinetics of efflux of indium-111 from erythrocytes incubated at 37 degrees C in plasma or physiological buffer. Indium-111 oxine labeled red cells retain their discocytic morphology and the cell indices, and density characteristics on phthalate ester are similar to those of the control cells. The efficiency of labeling may be as high as 97%. Human or rat erythrocyte membranes retain 33 and 41% of indium-111, and the cytosol contains 67 and 59%, respectively. About 98% of the indium-111 is bound to the membrane proteins and 1% to the lipid bilayer. Efflux of indium-111 from cells in autologous plasma showed a multiphasic release resulting in about 4-5% release of the label in 2 h and 11.5% in 20 h. Cells in PBS showed 1-5% release of the label during the incubation period. These findings suggest that indium-111 oxine labeling of erythrocytes does not grossly alter the structural and deformability integrity of the cells to induce selective RE sequestration, unless the cells have been damaged prior to or during the labeling procedure, or the spleen is hyperactive.