BIDIRECTIONAL REGULATION OF UNCOUPLING PROTEIN-3 AND GLUT-4 MESSENGER-RNA IN SKELETAL-MUSCLE BY COLD

To elucidate the possible role of the mitochondrial uncoupling protein (UCP)-3 in skeletal muscle as a regulator of adaptive thermogenesis a nd energy balance, we examined the modulation by cold exposure (5 degr ees C) of UCP-3 and glucose transporter isoform GLUT-4 mRNAs in male S prague-Dawley rats. In skeletal muscle, UCP-3 and GLUT-4 mRNAs increas ed two- to three-fold between 6 and 24 h of cold exposure and then dec reased to 50% of the control value after 6 days in the cold. In contra st, skeletal muscle UCP-2 mRNA showed a small increase on day 3 and re turned to normal after 6 days. The bidirectional regulation of UCP-S a nd GLUT-4 mRNAs in skeletal muscle by cold suggests that UCP-3 may be a major mediator of acute adaptive thermogenesis but then is downregul ated, along with GLUT-4, in the chronic state to preserve energy. In c ontrast, cold exposure caused only transient changes of UCP-2 and GLUT -LL mRNA in heart. These data are consistent with the necessity of the heart to continuously expend energy to maintain blood circulation, re gardless of environmental conditions.