Long-term Cold Exposure Research Articles

Potential of brown adipose tissue type II thyroxine 5'-deiodinase as a local and systemic source of triiodothyronine in rats.

Previous reports suggest that a type II iodothyronine 5'-deiodinase may become the main enzymatic pathway for extrathyroidal triiodothyronine (T3) generation when the enzyme levels are sufficiently elevated and/or liver and kidney type I 5'-deiodinase activity is depressed. The present studies assessed the potential of brown adipose tissue (BAT) type II 5'-deiodinase to generate T3 for the plasma pool. BAT 5'-deiodination (BAT 5'D) was stimulated by either short- (4 h) or long-term (7 wk) cold exposure (4 degrees C). Long-term cold exposure increased thyroxine (T4) secretion 40-60% and extrathyroidal T3 production three-fold. In cold-adapted rats treated with propylthiouracil (PTU), extrathyroidal T3 production was 10-fold higher than in PTU-treated rats maintained at room temperature. Cold did not stimulate liver or kidney 5'D, but the cold-adapted rats showed a six- to eightfold higher BAT 5'D content. PTU caused greater than 95% inhibition of liver and kidney 5'D, but did not affect BAT 5'D. Thyroidectomized rats maintained on 0.8 micrograms of T4/100 g of body weight (BW) per day were acutely exposed to 4 degrees C. In rats given 10 mg of PTU/100 g of BW, 4 h of cold exposure still caused a 12-fold increase in BAT 5'D, a 2.3-fold increase in plasma T3 production, and a 4.8-fold increment in the locally produced T3 in BAT itself. All these responses were abolished by pretreatment with the alpha 1-antiadrenergic drug prazosin. Regardless of the ambient temperature, liver 5'D activity was greater than 90% inhibited by PTU. These results indicate that BAT can be a major source of plasma T3 under suitable circumstances such as acute or chronic exposure to cold. Furthermore, BAT 5'D activity affects BAT T3 content itself, suggesting that thyroid hormone may have a previously unrecognized role in augmenting the thermogenic response of this tissue to sympathetic stimulation. Such interactions may be especially important during the early neonatal period in humans, a time of marked thermogenic stress.

Changes in food intake in response to alterations in the ambient temperature: modifications by previous thermal and nutritional experience.

1. The influence of ambient temperature on ad libitum food intake has been investigated in piglets weaned at 2 weeks and kept in the hot (35 degrees C) or the cold (10 degrees C) on a high or low energy intake for 6 weeks. Further, the extent to which food intake was correlated with fasting body temperatures and resting metabolic rate was also examined. 2. The amount of food eaten in a single meal was recorded daily in two series of tests. In Test I, measurements were made both at the normal living temperature and after 2 and 24 h at thermal neutrality (25 degrees C). In Test II, the animals were examined at 25 degrees C over a 4 day period. Resting metabolic rate and body temperatures were also measured, immediately before the measurement of food intake. 3. Food intake was much greater at 10 than at 35 degrees C, but even at thermal neutrality it was significantly higher for those which had been in the cold. This was true for periods of between 2h and 4 days at 25 degrees C. Although food intake was not correlated with skin or core temperatures, there was an indication of a correlation with metabolic rate, particularly at thermal neutrality. 4. The results show that long-term cold exposure has a lasting effect on food intake, even at thermal neutrality, and it is suggested that this may be related to hormonal changes.

Studies on the lipotropic effect of cold exposure in choline-deficiency fatty liver

Sellers and You (1,2) succeeded in preventing choline-deficiency fatty liver by long-term cold exposure. This co-called “pseudolipotropic effect of cold exposure” has been confirmed by Treadwell and coworkers (3–5), with the unique exception represented by the findings of Yoshida and Ashida (6) obtained by treating the animals under milder experimental conditions.

Cold stress of the mammal, bird and reptile

1. (1) A rise in plasma sodium and haemodilution occurs on short-term (48 hr) cold exposure (0°C) in the pigeon. These shifts are similar to those recorded for the intact rat but more rapidly to normal. The changes are in agreement with a theory of increased mineralocorticoid activity resulting from the acute cold stress and the more rapid adaptation of the pigeon compared with the rat is in line with its greater resistance to cold. 2. (2) There is a haemodilution in reptiles on similar short-term cold exposure but both plasma sodium and potassium levels fall. This contrasts with the response of the rat and pigeon. 3. (3) In the rat on long-term (4–20 weeks) cold exposure (0°C) the plasma sodium and potassium levels are high but associated with haemodilution. These changes are accompanied by a slight fall in body temperature and pathological changes including tissue breakdown. There is a doubling of adrenal weight and histological signs of kidney and adrenal cortical damage. 4. (4) The pigeon on long-term (2–10 weeks) cold exposure (0°C) does not show similar changes. Plasma sodium and potassium are just below normal but again there is haemodilution. There is no fall in body temperature and the birds maintain good health with no gross pathological or histological abnormalities. The adrenal cortex shows no increase in size or weight. 5. (5) These results suggest that the long-term cold exposure of the intact rat may be associated with an elevated mineralocorticoid level and the cold stress produces changes similar to long-term dosage of sensitized animals with mineralocorticoids. The pigeon by contrast does not respond to a similar degree of long-term cold stress, but only shows haemodilution.