Reverse triiodothyronine nuclear binding in rat brain.
Reverse triiodothyronine (rT3) had been believed biologically inactive, but recently we demonstrated nuclear binding sites for rT3 in human placenta. In this study we examined rT3 binding sites in rat brain. Male Wistar normal rats aged 2, 4 and 9 weeks were killed and the brain was removed for rT3 binding assay. In another series, male Wistar rats weighing about 40g were divided into two groups: group 1 (rickets group), kept on our original rachitogenic diet and group 2, kept on standard diet. After 28 days on either diet they were killed and the brain was removed for assay. Cerebral nuclear protein was extracted with 0.4M KCl buffer. In normal 2 week-old rats specific binding sites for rT3 were detected in all parts of the brain, but at 7 weeks of age the density of the binding sites was decreased and at 9 weeks it is almost 0. Scatchard analysis performed in rachitic rats showed a curvilinear pattern, suggesting two sets of receptors existing in brain tissue; in cerebral cortex one with a association constant (Ka) of 1.07 X 10(8)M-1 and a limited capacity (Bmax) of 0.75 X 10(-15) moles/mg tissue and the other with Ka = 4.93 X 10(6), Bmax = 12.1 X 10(-15), and in thalamus including hypothalamus, one with Ka = 1.00 X 10(8), Bmax = 1.00 X 10(-15) and the other with Ka = 4.57 X 10(6) and Bmax = 18.6 X 10(-15).