Thyroxine uptake by liver mitochondria of the cold-exposed rat.

Abstract

Mitochondrial binding of thyroxine as a function of thyroxine concentration (1.0×10−5M to 6.0×10−5M) was studied in the livers of normal control, hyperthyroid and hypothyroid rats with and without cold exposure at 2–4° C. Binding studies were carried out using non-labeled L-thyroxine and measuring the ultra-violet absorbancy of thyroxine left after the separation of mitochondria from the incubating medium. In mitochondria from the normal rat, a linear relationship between the amount of thyroxine bound and the concentration in the medium was observed. In mitochondria of rats cold-exposed for 5 hours, there appeared to be a smaller rate of increase in thyroxine binding at the higher concentrations. In the case of rats refrigerated for 8 hours, the uptake of thyroxine was maximal at 3×10−5M. The same linear relationship as for normal controls was found with mitochondria from hyperthyroid rats and 5 hours of cold-exposure did not exert any influence on this relationship. In mitochondria from hypothyroid rats,...