THE CONCENTRATION AND BINDING OF THYROXINE AND TRIIODOTHYRONINE BY RAT DIAPHRAGM 1

Abstract

Studies of the uptake of I131-labeled thyroxine by rat diaphragm as a function of incubation time, hormone concentration in the medium and duration of rinsing, as well as equilibrium studies, are presented. When rat diaphragm was incubated in medium containing varying concentrations of thyroxine for increasing periods of time, the amount of the hormone bound was linearly related to the time of incubation at all concentrations. Similarly, a direct linear relationship between the amount bound to tissue was demonstrated. Washout studies showed that, with prolonged rinsing, little thyroxine was removed from rat muscle after the first 15 minutes of rinsing. Equilibrium studies, wherein hemidiaphragms were incubated first with labeled thyroxine and then transferred to solutions containing unlabeled material, showed no significant loss of initial radioactivity during incubation with the non-radioactive thyroxine. Observations indicate that thyroxine is specifically bound to rat muscle tissue in concentrations exceeding those in the incubating medium. Similar incubation and washout studies were carried out with triiodothyronine. Triiodothyronine was also specifically bound to rat muscle tissue; this binding took place much more rapidly than in the case of thyroxine. When the hormone concentration was 1.3 x 10-4 [mu][image]/ml triiodothyronine-I131 was bound to rat diaphragm 2.5 times as rapidly as was thyroxine-I131. This finding is in accord with previous observations regarding the relative rates of action of these two compounds in vivo. Comparison studies with NaI131 showed that, in equimolar concentrations, relatively insignificant amounts of this compound were taken up by rat diaphragm and that which was absorbed was readily removed by washing. These investigations are discussed in relation to the mechanism of action of thyroxine and triiodothyronine.