Acute effects of food, 2-deoxy-D-glucose and noradrenaline on metabolic rate and brown adipose tissue in normal and atropinised lean and obese (fa/fa) Zucker rats.

1. Intragastric feeding (40 kJ) produced a 17% rise in metabolic rate in lean Zucker rats but only an 8% increase in obese (fa/fa) rats, and both of these responses were significantly reduced by beta-adrenergic blockade with propranolol (10 mg/kg, s.c.). 2. Parasympathetic blockade with atropine (0.5 mg/kg, s.c.) caused a doubling of the response to food in lean rats and a threefold increase in the obese mutants, such that all atropinised animals showed the same increase in metabolic rate after food. 3. Feeding also caused a significant rise in interscapular brown adipose tissue temperature, which was greatest in the lean animals and was enhanced by atropine in both groups. 4. Injection of noradrenaline (250 micrograms/kg, s.c.) caused a similar (40%) rise in metabolic rate in lean and obese animals but this response was unaffected by atropine. 5. 2-Deoxy-D-glucose injection (360 mg/kg, s.c.) depressed oxygen consumption by 25 and 8% in lean and obese rats respectively and this effect was totally abolished by atropine. 6. These results suggest that the rise in metabolic rate after a meal is partly due to sympathetic activation of brown adipose tissue. The reduced thermic response in obese Zucker rats is not due to insensitivity to noradrenaline, but may be partly due to parasympathetic inhibition of thermogenesis and partly to insensitivity to glucose availability.