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[3H]Norepinephrine ([3H]NE) was injected intravenously to euthyroid, hypothyroid, and hyperthyroid rats kept at 23 °C, acutely exposed to 4 °C and acclimated to 4 °C. The decline of specific activity in heart muscle (taken as an index of turnover) was measured at 10 min and [Formula: see text], 6, 12, and 18 h after injection. Turnover was significantly higher in each treatment group at 4 °C than at 23 °C. At 23 °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 °C, the same relationship was found (half times 1.9, 3.5, and 4.3 h respectively (P < 0.001). After acclimation to 4 °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 °C. The highest turnover rates were in rats with lowest serum thyroxine (T4). Nevertheless hyperthyroid rats exposed to 4 °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 °C, 4 °C, and acclimated to 4 °C were 35, 19, and 17 h. The latter two 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 °C and a partial reciprocal relationship exists between thyroid hormonal activity and turnover of NE.

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Since thyroid hormones influence urinary excretion of catecholamines after exposure to cold, the effects of hyper- and hypo-thyroidism on adrenal tyrosine hydroxylase (TH) (EC 1.14.16.2), phenylethanolamine-N-methyl transferase (PNMT) (EC 2.1.1.28), and serum dopamine-beta-hydroxylase (DbetaH) (EC 1.14.17.1) of rats of 23 and 4 degrees C were studied. TH changes resembled the urinary excretion pattern at 4 degrees C in being higher after 8 days than after 1 day of exposure, and in declining as acclimation occurred. At 23 degrees C, TH activity of hypothyroid rats was significantly higher than in euthyroid or hyperthyroid animals, and after 1 day at 4 degrees C the value increased even more. While in the hypothyroid animals at 4 degrees C the concentration of adrenal catecholamines was less, the epinephrine to norepinephrine ratio was higher than at 23 degrees C. Very high TH activity with a decline in catecholamine concentration suggests that the capacity of TH had been exceeded. PNMT activity was significantly elevated in this group. TH activity was not decreased in the hyperthyroid group at 23 degrees C, and was increased after 8 days at 4 degrees C, suggesting that circulating thyroid hormones have no direct inhibitory effect on TH. Serum DbetaH was elevated after exposure to 4 degrees C, regardless of thyroid hormonal status. The activation of adrenal TH in hypothyroid rats at 23 degrees C and of TH, PNMT, and serum DbetaH at 4 degrees C is probably the result of increased activity of the sympathetic nervous system.