Enhanced heat production in physically restrained rats in hypoxia

Combined effects of restraint and hypoxia (9.5% O2 in N2) on heat production (M), heat loss (HL), colonic temperature (Tcol), and cardiovascular activity were studied in unanesthetized rats. In freely moving (F) rats, hypoxia decreased M, HL, Tcol, and heart rate (HR). In restrained (R) rats, however, hypoxia increased M, HL, Tcol, and HR. The increase in these parameters was greatest within 30 min of hypoxia, and the peak value of M was 10.4 W X m-2 higher than in normoxia. After chemical sympathectomy with 6-hydroxydopamine hydrobromide, no such increase was observed in R rats. l-Norepinephrine bitartrate (0.4 mg X kg-1) increased M and HR in F rats in hypoxia. The increase in M was, however, 9.5 W X m-2 and significantly less than that observed in normoxia. Hypoxia significantly reduced blood flow to the brown adipose tissues (BAT) in R rats. The BAT may not be responsible for the increased metabolism in restrained hypoxia-exposed rats. Hypoxia significantly increased blood flow to the heart and the diaphragm, and to a lesser extent to the limb muscles. The increased metabolism in the cardiac and respiratory muscles may be a cause of hyper-metabolism in restrained rats. The limb muscles may also play some role in increasing metabolism in these rats.