The metabolism of I 131-labeled iodine, thyroxine, and triiodothyronine in the mammary gland of the lactating rat.

It has been known for many years that administered iodide appears in the milk of lactating animals. The early work on this subject was reviewed in 1937 by Elmer (1). Since that time, ample confirmation that the mammary gland secretes iodine has been obtained through experiments with radioactive iodide in man (Z-5) and a variety of animals (6-14). In rabbits (6, 8, 9) and goats (6), it has been shown that as much as half of an administered dose of I’s1 can be recovered in the milk produced. The mammary secretion of I’s1 proceeds so readily that thyroidal accumulation of administered 1’3l in lactating animals (8, 12, 13) has been found to be drastically reduced. The iodine in milk, as studied by labeling with P31, has been reported to be mostly in the form of inorganic iodide in the case of man (3, 5), goats (6), rabbits (6, 8), and mice (13). In the dog, however, 30 to 60 per cent of the I1sl in milk was found in protein-bound form (7, 14). In a preliminary note (12), we showed that as much as 86 per cent of the I’s1 present in rat milk collected 24 hours after an Im-iodide injection was proteinbound. A value of 40 per cent was reported recently by BrownGrant and Galton (15) for milk taken from rats 3 to 4 hours after I’s1 administration. Chromatographic analysis of an enzymatic hydrolysate of the milk revealed that the major P31-component in the protein was monoiodotyrosine. A small amount of P31diiodotyrosine was also present, but no 1131-thyroxine was detected at various intervals up to 24 hours after the P31 injections. The present report deals with the following aspects of iodine metabolism in the rat mammary gland: (a) the transport of administered inorganic iodine into milk, and the form in which this iodine appears; (b) the effects of thyroidectomy and of thyroid inhibitors on the transport and organic binding of radioiodide by the mammary gland; and (c) the appearance of injected P31-thyroxine and P3’-triiodothyronine in milk.