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The cDNA of an uncoupling protein (UCP) homologue was obtained by screening a chicken skeletal-muscle library. The predicted 307-amino-acid sequence of avian UCP (avUCP) is 55, 70, 70 and 46% identical with mammalian UCP1, UCP2 and UCP3 and plant UCP respectively. avUCP mRNA expression is restricted to skeletal muscle and its abundance was increased 1.3-fold in a chicken line showing diet-induced thermogenesis, and 3.6- and 2.6-fold in cold-acclimated and glucagon-treated ducklings developing muscle non-shivering thermogenesis respectively. The present data support the implication of avUCP in avian energy expenditure.

The effect of long-term hindlimb unloading (2 or 5 week) on the expression of uncoupling protein-3 (UCP3) gene was investigated in rat skeletal muscles. The interaction of hindlimb unloading and thyroid status was also investigated at 2 weeks. Whatever the duration, mechanical unloading induced a similar increase in UCP3 mRNA relative abundance in the slow-twitch soleus (SOL) muscle (+80%, P < 0.05), whereas no effect was observed in the fast-twitch extensor digitorum longus (EDL) muscle. Hypothyroidism down-regulated while hyperthyroidism up-regulated UCP3 mRNA relative abundance in both SOL and EDL muscles, but thyroid status did not prevent the up-regulation of UCP3 induced by 2 weeks of suspension. These data therefore indicate for the first time that long-term hindlimb unloading up-regulates muscle UCP3 gene expression in a muscle-specific manner which is independent of thyroid status., (Copyright 1999 Academic Press.)

The control of uncoupling protein-1, -2 and -3 (UCP-1, UCP-2, UCP-3) mRNA levels by sympathetic innervation in rats was investigated by specific and sensitive RT-PCR assays. In rats reared at thermoneutrality (25 degrees C), unilateral surgical sympathetic denervation of interscapular brown adipose tissue (BAT) markedly reduced the UCP-1 mRNA level (-38%) as compared with the contralateral innervated BAT pad, but was without significant effect on UCP-2 and -3 mRNA levels. Cold exposure (7 days, 4 degrees C) markedly increased UCP-1 (+180%), UCP-2 (+115%) and UCP-3 (+195%) mRNA levels in interscapular BAT. Unilateral sympathetic denervation prevented the cold-induced rise in BAT UCP-1 and UCP-2 mRNAs, but not that in BAT UCP-3 mRNA. Results were confirmed by Northern blot analysis. These data indicate a differential endocrine control of UCP-1, UCP-2 and UCP-3 gene expression in rat BAT both at thermoneutrality and during prolonged cold exposure.