In vivo localization of Ga-citrate in rat liver as determined by cell fractionation with isopycnic rate-zonal ultracentrifugation

The in vivo behaviour of Ga-67 citrate was studied, using a rat liver model, and the mechanisms of cellular uptake were investigated by observing the time course of the radionuclide in subcellular liver cell fractions. Modified continuous ultracentrifugation was used to successively fractionate the homogenates according to isopycnic and rate-zonal principles. To collect the cell nuclei, a special technique, which consisited of supplementing the outer sucrose layer with cesium chloride to increase density, was applied to trap nuclei with isopycnic equilibrium. We isolated nuclei, mitchondria, lysosomes and peroxysomes, microsomes, and the cell supernatant in a sufficiently purified state. By this method, the in vivo localization of Ga-67 was studied. At 30 min after intraveneous injection, radioactivity was found mainly in the cell supernatant, the radioactivity peak corresponded to 5 S. However, after 24 h, the radioactivity was localized in the heavier fraction containing lysosomes and heavy endoplasmic reticulum. No intranuclear localization was observed. In spite of the difficulty in completely separating the lysosomal fraction from heavy endoplasmic reticulum, our results suggest the participation of heavy endoplasmic reticulum in gallium localization. (orig.)