Sympathetic and thyroid influences on metabolic rate in fed, fasted, and refed rats

Twice daily subcutaneous injections of norepinephrine (NE, 25 micrograms/100 g body wt) for 16 days, in adults rats, produced significant increases in resting oxygen consumption (VO2), thermogenic responses to NE, plasma triiodothyrone (T3) levels, interscapular brown fat mass, and hepatic mitochondrial alpha-glycerophosphate dehydrogenase activity. These responses were absent in hypothyroid rats. Rats fasted for 72 h showed significant decreases in VO2 and plasma T3. Refeeding with carbohydrate (CHO, 40 kJ) produced a rapid rise in plasma T3 within 3 h and a delayed rise in VO2 (peak rise at 24 h). Treatment with the beta-adrenergic antagonist propranolol (2 mg/100 g body wt, sc) at the time of the meal blocked the rises in T3 and VO2. Injecting fasted rats with T3 (365 ng/100 g body wt, ip) had no effect on subsequent VO2 but NE (40 micrograms/100 g body wt, sc) produced similar increases in plasma T3 and VO2 to CHO refeeding. The delayed rise in VO2 after CHO refeeding was blocked by propranolol (0.5 mg/100 g body wt, sc) when given 24 h after the meal. These results provide evidence for catecholamine-induced changes in thyroid hormone metabolism and for a sympathetic involvement in thyroid-dependent responses to nutritional manipulations.