Labelled polyvinylpyrrolidone as an in vivo indicator of reticuloendothelial activity

It is shown that after a single i.v. dose of [131I]-polyvinylpyrrolidone ([131I]-PVP) the total body and plasma radioactivities of rabbits decrease at distinctly different rates. The difference between these two rates is utilized to calculate the phagocytic rate of [131I]-PVP by reticuloendothelial cells.A number of experimental conditions are reported in which enhanced reticuloendothelial uptake of [131I]-PVP is readily demonstrable. They include the injection of small quantities of heterologous plasma, certain proteolytic fragments of the fibrinogen molecule, the clearance of antigen-antibody complexes, and the acute phase reaction (inflammatory response) as brought about by serum sickness, sterile abscess and vaccination. Based on these observations it is suggested that [131I]-PVP may provide a convenient technique for the long-term monitoring of the activity of reticuloendothelial cells, presumably mainly that of the histiocytes. The pronounced polydispersity of commercially available [131I]-PVP is a serious problem in this respect which can be largely overcome, but not completely abolished, by the screening techniques described herein. Post-mortem analyses of rabbit tissues showed most of the [131I]-PVP to be present in the skin (20%), followed by the liver (14%), bone marrow (10%), muscle (7%) and kidney (5%). Gel filtration studies with [131]-PVP in the presence and in the absence of plasma proteins failed to demonstrate any association between PVP and the proteins. [131I]-PVP kept at physiological pH and 37 degrees C lost less than 5% of its radioactivity over one month due to spontaneous deiodination.