Distribution and elimination of 131I- and 14C-labelled plasma proteins in the rabbit

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Abstract
Jet-iodinated plasma protein was given to 14 adult male rabbits intravenously. Elimination rate of Il31-labeled protein and its distribution between intra- and extra-vascular pools showed considerable individual variation (- 20%). This is due more to animal variation than to differences between batches of labeled protein. Homologous and autogenous plasma proteins were indistinguishable in regard to distribution and rate of elimination. Comparison of the fate of Il31- and C14-labeled plasma proteins given to the same animals revealed that (a) equilibration times were closely similar, the equilibration of globulin (65-140 hours) being generally slower than that of albumin (40-65 hours), (b) Equilibration was followed by a decline in plasma specific activity which maintained a constant exponential rate during 3-4 weeks of observation. Biological half-lives of plasma proteins were closely similar when measured with C14 and Il31. In general, albumin (160-230 hours) was eliminated more slowly than globulin (140-170 hours) and both much more slowly than fibrinogen (52-85 hours), (c) When equilibration was complete, the mass of protein in the extra -vascular compartment was about 1.5 times that in the blood stream. Albumin had a lower extra-intravascular distribution ratio than globulin, while that of fibrinogen was considerably less. The complex significance of these ratios is discussed. The presence of denatured, rapidly eliminated components in the injected protein was associated with a high urinary output of I131 in the first 72 hours and with unusually low plasma specific activity values subsequently, the latter giving rise to incorrectly high estimates of pool size. I131 detached from plasma protein was rapidly and almost completely excreted in the urine and provided an accurate measure of the protein elimination rate. There was no detectable re-incorporation of I131 to newly synthesized plasma protein. Retention of I131 by the thyroid gland was negligible in animals receiving iodide in the drinking water. It is concluded that rabbit plasma can be labeled with radioactive iodine in such a way as to provide data concerning the in vivo distribution and elimination of albumin and globulin closely similar to that obtained with the corresponding C14-labeled proteins.