Effects of noradrenaline and the blood perfusion rate on the oxygen consumption of intact and isolated muscles of cold-acclimated rats.

The authors studied the effect of the blood perfusion rate and of noradrenaline (NA) on the oxygen consumption of the isolated hind limb and on the partly - and vascularly completely - isolated cranial gracilis muscle of cold-acclimated rats. Oxygen consumption of the limb was stimulated by a raised perfusion rate together with growth of the oxygen extraction coefficient and by NA, which also raised oxygen consumption when the perfusion rate was constant. Oxygen consumption of the partly isolated muscle was likewise stimulated by a raised perfusion rate, but without a simultaneous increase in the oxygen extraction coefficient. In the vascularly completely isolated muscle, a raised perfusion rate had only a transient stimulant effect on oxygen consumption. In the partly and the completely isolated muscle, NA raised the arteriovenous difference in the blood oxygen content and organ resistance independently of each other. The calorigenic effect of NA, which was determined by the ratio of the two effects, did not exceed 34% above the resting level. The conclusions that the thermogenesis of resting muscle can be controlled by the blood flow on the basis of a mechanism other than the limitation of oxygen or substrates supply, and that NA acts independently of oxygen extraction from the blood and of the blood flow, show the blood flow to be a mechanism at organ level, which participates in the control of nonshivering thermogenesis in skeletal muscle.