Uptake and binding of thyroxine and triiodothyronine by rat diaphragm in vitro
- 30 April 1961
- journal article
- research article
- Published by American Physiological Society in American Journal of Physiology-Legacy Content
- Vol. 200 (5), 1029-1031
- https://doi.org/10.1152/ajplegacy.1961.200.5.1029
Abstract
Hemidiaphragms from weanling Sprague-Dawley rats were incubated in Krebs-Ringer buffer containing 1.3 x 10–7 m l-thyroxine or l-triiodothyronine labeled with radioiodine. Triiodothyronine uptake was about twice as rapid as thyroxine uptake. The data were consistent with a kinetic analysis based on a two-step process: 1) simple diffusion into the tissue (rate-limiting step) and 2) binding to cellular proteins (a process that occurs very rapidly compared to diffusion). The diffusion constants were calculated to be kd(T4) = 0.153 cm3/min and kd(T3) = 0.352 cm3/min. The data suggest a single species of tissue-binding site with identical tissue-binding capacity and binding affinity for both triiodothyronine and thyroxine. Maximum binding was found to be 28 x 10–4 µm/g tissue and the affinity constant for binding was 1.4 x 104 g/µm. The difference in the rate of accumulation of thyroxine and triiodothyronine appears to be due only to the difference in the diffusion rate. A direct rate equation for the accumulation of thyroid hormone was derived. Points calculated by means of this equation fit remarkably well with the experimental data.Keywords
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