The effects of thyroxine (T
4) and thiouracil (TU) on growth and protein metabolism were examined in male mice given diets containing different levels of protein (casein) at two different growth stages (25 and 60 days old). Changes in protein metabolism were assessed from the expiratory
14CO
2 from [
U-
14C]leucine injected, the liver nucleic acid contents and the rates of synthesis and degradation of liver protein estimated by single injection method using [6-
14C]arginine. Each mouse, excluding the control group, received daily intraperitoneal injection of 10μg of L-thyroxine sodium salt per 100 g b.w. (T
4 group) or were given a diet containing 0.05% 2-thiouracil (TU group). In the 25-day-old mice, growth of the T
4 group was accelerated at protein levels above 15% and that of the TU group was severely retarded at protein levels below 10%. On the other hand, in the 60-day-old mice, growth of the TU group tended to be accelerated at protein levels from 10% to 25%, while it was significantly retarded at the 5%-protein level. The expiratory
14CO
2 increased when the growth was retarded, and decreased when growth was accelerated by T
4 or TU in both age groups, but was not significant in either case. The nucleic acid content of the liver was increased by both T
4 and TU when the dietary protein level was above 15%. The rate of protein synthesis was increased, but not significantly, by T
4, while it was not affected by TU. The rate of protein degradation was increased, but not significantly, by TU, while it was not affected by T
4 in the 25-day-old mice. In the 60-day-old mice, the rates of both liver-protein synthesis and degradation were significantly increased by TU, while they were not affected by T
4. These results definitely indicate that the growth stage and the dietary protein level change the effects of thyroid function on growth and protein metabolism of mice.
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