Stimulation of the peripheral metabolism of L-thyroxine and and 3,5,3'-L-triiodothyronine in the physically trained rat.

Abstract

The effect of physical training on the peripheral metabolism of thyroxine and triiodothyronine was evaluated in rats trained on a motorized treadmill for 12 weeks. The metabolism of thyroid hormones was assessed by analysis of plasma disappearance kinetics and hepatic accumulation of injected labeled iodothyronines. Training was associated with increased metabolic clearances of the two hormones. The enhanced hormonal disposition was manifest in increased fecal clearances. Increments of lesser magnitude observed in the urinary clearances of both hormones were not significant. Analysis of the plasma protein and cellular determinants of hormonal turnover, as proposed by Oppenheimer (J Clin Invest 49: 1016, 1970), suggested that the increased hormonal degradation was attributable to stimulation of cellular turnover. The detection of increased hepatic uptakeof iodothyronines provided additional evidence of augmented cellular disposition. These current data extend observations in earlier studies in the rat and ...