Plutonium Deposition Kinetics in the Rat

Abstract

A conventional kinetic model was constructed to describe the transport and deposition of intravenously injected Pu(IV) citrate in the rat. A digital computer program (MIMIC) was used to generate numerical solutions to the differential equations representing the compartment model. Tissue data and information on the rate of Pu-protein binding in plasma were drawn from published sources. The model consisted of two blood compartments, unbound Pu (Puf) and protein-bound Pu (Pub), assumed to be bound for the most part to the Fe-carrying protein, transferrin (TF); two extracellular fluid (ECF) compartments, Put and Pub; and four effectively non-returning sinks: liver, skeleton, soft tissues and excreta. The implications of the solution of the model led to the following working hypotheses: (a) Put reacts with protein, presumably TF, in ECF as well as in plasma. The Pu-TF complex is the most likely form in which diffusible Pu is mobilized from parenteral injection sites, and the most likely form in which orally administered Pu is carried once it reaches the plasma. (b) Little, if any, Pub is excreted or deposited in the liver. Formation of the Pu-TF complex is probably not a necessary preliminary to liver deposition of diffusible Pu, (c) Both Put and Pub are sources of Pu deposited in bone. The surface of the reticulocyte (where Fe is released from the Fe-TF complex) is considered the most likely site of dissociation of the Pu-TF complex. Pu released at that site could either recombine with TF and recirculate as Pu-TF, effectively prolonging Pu circulation, or recirculate temporarily as Pup providing the necessary feedback of Puf, or diffuse to the most readily accessible bone surface.