Multifocal Stimulation of the Binding of Iodothyronines by Tissues

tration of protein-bound iodine in plasma was decreased, and the binding of the hormones by plasma proteins was increased. The enhanced rate of metabolism of iodothyronines was associated with stimulation of the binding of these hormones by diverse tissues, suggesting the participation of extrahepatic degradative foci in the increased hormonal deiodination observed in vivo. Increased hepatocellular binding and a significantly enlarged hepatic distribution space of thyroxine were noted. Hepatocellular binding of triiodothyronine was similarly augmented, and a smaller but signiificant increase in the hepatic space of this iodothyronine was detected. Analysis of the hepatic subcellular partition of iodothyronines 35 min after the intravenous administraDr. Balsam's present address is the Department of Medicine, Veterans Administration Hospital, Dallas, Texas 75216. The research reported in this paper was conducted by personnel of the Environmental Sciences Division, USAF School of Aerospace Medicine, Aerospace Medical Division, AFSC, United States Air Force, Brooks AFB, Texas. Received for publication 24 July 1973 and in revised form 15 October 1973. tion of isotopically labeled thyroid hormones disclosed increased hormonal binding by the microsomal fraction in cold-adapted animals and an attendant increase in the microsomal protein concentration. Partial microsomal subfractionation in a discontinuous sucrose gradient indicated that the observed stimulation of microsomal hormonal binding was associated with proliferation of the smooth endoplasmic reticulum.