Disposition of tritium-labelled heparin in rats

To evaluate the distribution of water soluble macromolecular compound in the body, heparin was selected as one of the model compounds. Urinary and fecal excretion, and plasma levels following intravenous administration of commercial 3H-heparin were investigated at three different dose levels. Whole-body autoradiography was also carried out periodically following intravenous administration of 3H-heparin at single dose level. The distribution of radioactivity following the administration of 3H-heparin was compared with that of 14C-polyethylene glycol 6000. The statistically significant but only very small differences in the pharmacokinetic parameters and in the urinary excretion were observed over the wide range of dose (0.20, 2.0 and 20 mg/kg). The decline of plasma radioactivity following the administration of 3H-heparin appeared to be biexponential with time and pharmacokinetic constants were calculated according to a two-compartment open model. Whole-body autoradiograms showed that the reticulo-endotherial system (R.E.S.) may play an important role in removal of the radioactivity of heparin from the circulating blood.