Effects of thyroid hormones and cold exposure on turnover of norepinephrine in cardiac and skeletal muscle

Abstract

[3H]Norepinephrine ([3H]NE) was injected i.v. into euthyroid, hypothyroid and hyperthyroid rats kept at 23.degree. C, acutely exposed to 4.degree. C and acclimated to 4.degree. C. The decline of specific activity in heart muscle (taken as an index of turnover) was measured at 10 min and 1 1/2, 6, 12, and 18 h after injection. Turnover was significantly higher in each treatment group at 4.degree. C than at 23.degree. C. At 23.degree. C turnover in the hypothyroid group was higher than in the euthyroid, which in turn was higher than in the hyperthyroid group (half times 5, 8.3 and 10.8 h, respectively (P < 0.001). After 12 h at 4.degree. C, the same relationship was found (half times 1.9, 3.5, and 4.3 h, respectively (P < 0.001). After acclimation to 4.degree. C, turnover in the hypothyroid group was higher than in the euthyroid or hyperthyroid groups (P < 0.001) with half times of 2.1, 4.1 and 3.9 h. These values were not significantly different from those of the same treatment groups acutely exposed to 4.degree. C. The highest turnover rates were in rats with lowest serum thyroxine (T4). Nevertheless hyperthyroid rats exposed to 4.degree. C had increased turnover rates in spite of high serum T4 concentration. The decline in specific activity of [3H]NE is attributable to dilution by newly synthesized NE and hence it is unlikely that the effect of T4 was produced by inhibition of biosynthesis. Decline of specific activity of [3H]NE in gastrocnemius-soleus muscle of euthyroid rats was measured at 10 min and 6 and 12 h after injection. Half times for animals at 23.degree. C, 4.degree. C, and acclimated to 4.degree. C were 35, 19, and 17 h. The latter 2 values (as with heart) did not differ significantly. The findings afford direct evidence that increased synthesis and secretion of NE occur in rats exposed to 4.degree. C and a partial reciprocal relationship exists between thyroid hormonal activity and turnover of NE.