Effects of radioactive iodine on the rat thyroid's function, regeneration and response to goitrogens

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The catabolism of plasma albumin in the rabbit: its rate and regulation

From I¹³¹-albumin studies and previously defined mathematical formulations, rates of breakdown were estimated for native plasma albumin in rabbits. These rates of catabolism per unit weight of animal were remarkably constant and were independent of variations in the steady state values of albumin concentration in the plasma. These results imply that, at least between animals, the breakdown of plasma albumin follows a kinetic process of approximately zero order. It seems plausible that the process operates similarly in individual animals, and hence that albumin is maintained at normal steady state levels in the healthy animal primarily by means of a regulated rate of synthesis.

The nature and rates of excretion of radioactive breakdown products of I131-albumin in the rabbit

When I¹³¹-albumin is given intravenously to rabbits, the radioactive breakdown products that are released into the plasma and urine can be extracted into acetone. Paper chromatography and paper electrophoresis show that about 80 per cent of these are I¹³¹-iodide and the remainder are organic I¹³¹-iodine compounds. When I¹³¹-iodide is given to rabbits taking iodide in their drinking water, the radioactivity is quantitatively excreted, without being accumulated in the tissues and without becoming attached to the plasma proteins. The rate of excretion can be defined by a first order rate process with a rate constant, a, ranging between 1 and 3day^-1. The organic I¹³¹-iodine compounds liberated during the metabolism of I¹³¹-albumin can be closely matched by a mixture of the organic I¹³¹-iodine compounds liberated during the metabolism of I¹³¹-monoiodotyrosine, I¹³¹-diiodotyrosine, and the amino acids released by digestion from I¹³¹-albumin. These organic I¹³¹-iodine compounds are not accumulated in the body and their radioactivity does not become attached to the plasma proteins. Their radioactivity is excreted as fast or faster than that of I¹³¹-iodide, and, to a satisfactory approximation, the same equations describing the excretion of I¹³¹-iodide with the same constants may be used for describing the excretion of the organic I¹³¹-iodine. These results permit improved estimates of the distribution and catabolism of I¹³¹-albumin.

The kinetics of the distribution and breakdown of 1131-albumin in the rabbit. Observations on several mathematical descriptions

Rabbit plasma albumin was labelled with I¹³¹, injected intravenously, and measurements were made of the radioactivity in plasma, urine, and feces over many days. In some experiments plasma radioactivity was fractionated into I¹³¹-albumin activity and that of labelled breakdown products. Curves of these radioactivities were compared with corresponding curves predicted by four mathematical models. Each model included a vascular and extravascular albumin compartment in transfer equilibrium, a radioactive breakdown products compartment, and an excretion compartment; but model A supposed I¹³¹-albumin catabolism to occur within the vascular system, model B within the extravascular compartment, model C within both, and model D within a separate compartment receiving albumin for catabolism from the plasma. The experimental data were reasonably well predicted by models A and C. However, model D, though data were insufficient for its complete validation, gave the best predictions and agrees with present knowledge of albumin catabolism. Various methods for calculating the rate of albumin breakdown are discussed. When calculations are based solely on the plasma radioactivity data, identical rates are predicted by models A, C, and D. When, as a valuable independent method, catabolism is calculated from plasma and excreted radioactivities, an error (ordinarily small) is incurred unless account is taken of the rate of passage of I¹³¹-albumin to the breakdown sites, and of the rate of excretion of the radioactive breakdown products.

Time-concentration curves and dilution spaces of T-1824 and I-1824 and I-131-labeled proteins in dogs

Five-hour time-concentration curves were obtained after injection of T-1824, I131-labeled human albumin, and I131-labeled dog fibrinogen. The albumin and fibrinogen curves can be roughly divided into 3 segments (0–20 min, 20–70 min, and 70–310 min), each showing a different mean slope. The T-1824 curves show only one change in slope, at 20 min. After at least 70 min the dye left the circulation at a much greater rate than either of the iodinated proteins. Thoracic duct lymph return indicated that 12.9% of the injected iodinated albumin, 8.3% of the T-1824, and 6.3% of the iodinated fibrinogen was returned over the 310-min period. The spaces measured by the T-1824 and the iodinated albumin were not significantly different, whereas the fibrinogen measured a significantly smaller space than either of the former. It is inferred that the dye-protein complex might be broken within the vascular system. Also, the loss of fibrinogen from the capillary is less han for the iodinated albumin.