Autoradiographic localization of slow turnover iodocompounds within the follicular cells of the rat thyroid gland.

In addition to thyroglobulin, several low molecular wt iodinated glycopeptides are present in the thyroid gland. Some of these compounds are considered to be slowly metabolized remnants of intracellular lysis of engulfed colloid droplets. In this paper we present autoradiographic evidence obtained by light microscopy, suggesting that breakdown products of thyroglobulin digestion can indeed be located within the thyroid cells. The thyroid glands of rats were labeled with 125I to approach isotopic equilibrium. The tracer was then withdrawn from the otherwise unchanged high-iodine diet, and thyroidal radioactivity was washed out for 80 days. Autoradiographs of glands containing 10% or less of the tracer present at equilibrium showed the bulk of 125I within the follicular cells while the colloid was only slightly labeled. An additional finding was the presence of dense radioactive inclusions in the lumina of many follicles. They are assumed to represent clusters of thyroglobulin molecules not readily available to pinocytosis. In addition to autoradiographic evidence, the identity of intracellularly located radioactivity with slow-turnover enzyme-resistant iodocompounds is supported by double labeling experiments, using 125I as the "old" tracer, accumulating within the cells, and 131I as the "new" label representing intraluminal 19 S thyroglobulin. Experimental manipulations which tended to deplete the thyroid of thyroglobulin, increased the 125I/131I ratio in the gland as predicted by the working hypothesis. Thus, this paper describes a new kind of autoradiographic ring reaction due to a hitherto barely considered intracellular organic iodine pool.