A detailed examination of pulmonary uptake of (99m)Tc-Tin colloid in healthy mature miniature pigs.

(99m)Tc-Tin colloid is a commonly used colloidal radiopharmaceutical in human medicine for evaluating liver function and morphology. (99m)Tc-Tin colloid is taken up in the liver by the phagocytic activity of Kupffer cells, the reticuloendothelial cells of the liver. Unlike what occurs in human beings, we demonstrated (99m)Tc-Tin colloid uptake within the lungs and liver in healthy, mature, miniature pigs. Our observations may be explained by the presence of pulmonary intravascular macrophages (PIMs) closely apposed to the endothelium of the pulmonary capillaries in several animal species, such as the sheep, horse, goat, cat and pig. In the current study, we compared scintigraphic images using (99m)Tc-Tin colloid in rats with those in mature, miniature pigs, and identified the presence of PIMs, reticuloentothelial cells similar to Kupffer cells, by immunohistochemistry in pigs. Pulmonary uptake of (99m)Tc-Tin colloid occurred only in pigs, and PIMs in the pulmonary capillaries stained positively for mouse monoclonal MAC387 antibodies to macrophages in lung sections, as well as Kupffer cells in liver sections. Therefore, we conclude that the uptake of intravenously injected (99m)Tc-Tin colloid within both Kupffer cells and PIMs results in scintigraphic imaging of the lung and liver in miniature pigs.