Intracellular Distribution of Iodine-Containing Compounds in Rat Liver, Kidney and Heart

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Abstract
The intracellular distribution and quantitation of endogenously labeled iodine-containing compounds in normal rat liver, kidney and heart was determined under steady state conditions. An isotope equilibrium method based on chronic feeding of I125-labeled diet of known specific activity was employed. Virtually all of the label appeared as T4, T3 and iodide. T4 was more abundant than T3 in all fractions of liver, and in all fractions except nuclei of kidney and heart. T4 tended to concentrate in mitochondria, microsomes and soluble fraction and to be lowest in nuclei in all 3 tissues. This may be due to the available T4 binding sites in these fractions. T3 was more uniformly distributed among the 4 fractions. Quantities found per mg nitrogen were in the range of 0.1-0.7 m[mu]g of T4 and 0.02-0.35 m[mu]g of T3. Comparison of the subcellular distribution of endogenous hormone and exogenous hormone added in vitro to tissue homogenates indicated that the 2 methods of labeling gave different results. It would seem desirable to employ endogenously labeled hormone prepared under steady state conditions in biologic studies of thyroid hormone in cell free systems.