1. Hypoxia is known to decrease thermogenesis. We set out to determine whether this is accompanied by alterations in the brown adipose tissue, which is a major source of non-shivering thermogenesis.

2. Measurements were performed on 25- and 64-day-old rats, after 4 days of hypoxia (10% inspired O2), and on ∼3.5-month-old rats in hypobaric hypoxia since birth, at an ambient temperature of 25°C.

3. All hypoxic rats had higher haematocrit and lower body mass than corresponding controls.

4. In the 25-day-old rats, hypoxia had minimal and non significant effects on brown adipose tissue mass, proteins and DNA concentration. The content of the mitochondrial uncoupling protein thermogenin, evaluated by immunoblot after electrophoretic separation, relative to the cytoskeleton actin (UCP/Act), was not significantly altered.

5. In 25-day-old rats exposed for 4 days to cold (ambient temperature = 7–9°C), brown adipose tissue was hyperplastic, with increased UCP/Act; hypoxia did not appreciably alter the response to cold.

6. In the 2-month-old rats, after 4 days of hypoxia UCP/Act was reduced to about 40% of control.

7. In the 3.5-month-old rats maintained in hypoxia since birth, brown adipose tissue mass was reduced in proportion to body mass, with little effect on total proteins and DNA; UCP/Act was decreased to about 50% of control.

8. We conclude that chronic hypoxia had a minimal effect on brown adipose tissue total proteins and DNA content. However, the uncoupling protein content can be greatly reduced, depending upon age and duration of hypoxia.

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