Restraint-induced thermogenesis blunted by fasting in rats.

Physical restraint was performed in fasted (F3, fasted for 3 days; F7, fasted for 7 days), and fed control (C) rats. In contrast to the C group, the F3 and F7 groups showed a low resting metabolism and a low calorigenic response to restraint. During restraint, plasma levels of norepinephrine (NE) and epinephrine (E) were markedly increased but were not significantly different between the C and F7 groups. Calorigenic responses to NE and E (6.0 and 12.0 nmol X kg-1 X min-1) were significantly less in the F3 and F7 groups than in the C group. Plasma concentrations of glucose were increased by the infusion of catecholamines in the C and F7 groups. The values were not different between the two groups. Plasma levels of free fatty acid in the F7 group were significantly higher than in the C group before and during catecholamine infusion. Plasma concentrations of thyroxine and triiodothyronine were significantly less in the F7 group than in the C group. The blunted restraint-induced thermogenesis in fasted rats may be mainly attributed to the decreased calorigenic response to catecholamines. The decreased calorigenic response may be due to suppressed utilization of energy substrates in calorigenic tissues produced by the lowered activity of thyroid hormone.