Exposure to reduced activity induces skeletal muscle atrophy. Oxidative stress might contribute to muscle wasting via proteolysis activation. This study aimed to test two hypotheses in rats. First, supplementation of the antioxidant vitamin E, prior and during the phase of unloading, would partly counteract unloading-induced soleus muscle atrophy. Secondly, vitamin E supplementation would decrease the rate of muscle proteolysis by reducing expression of calpains, caspases-3, -9, and -12, and E3 ubiquitin ligases (MuRF1 and MAFbx). Soleus muscle atrophy (-49%) induced by 14 days of hindlimb unloading was reduced to only 32% under vitamin E. Vitamin E partly prevented the decrease in type I and IIa fiber size. Supplementation increased HSP72 content and suppressed the rise in muscle level of thiobarbituric acid-reactive substance caused by unloading but failed to modify the lower ratio of reduced vs oxidized glutathione, the higher uncoupling proteins mRNA, and the antioxidant enzyme activities (superoxide dismutase, catalase, and glutathione peroxidase) observed after unloading. Vitamin E treatment abolished the large upregulation of caspases-9 and -12 and MuRF1 transcripts in unloaded muscle and greatly decreased the upregulation of mu-calpain, caspase-3, and MAFbx mRNA. In conclusion, the protective effect of vitamin E might be due to modulation of muscle proteolysis-related genes rather than to its antioxidant function.

Juvenile king penguins develop adaptive thermogenesis after repeated immersion in cold water. However, the mechanisms of such metabolic adaptation in birds are unknown, as they lack brown adipose tissue and uncoupling protein-1 (UCP1), which mediate adaptive non-shivering thermogenesis in mammals. We used three different groups of juvenile king penguins to investigate the mitochondrial basis of avian adaptive thermogenesis in vitro. Skeletal muscle mitochondria isolated from penguins that had never been immersed in cold water showed no superoxide-stimulated proton conductance, indicating no functional avian UCP. Skeletal muscle mitochondria from penguins that had been either experimentally immersed or naturally adapted to cold water did possess functional avian UCP, demonstrated by a superoxide-stimulated, GDP-inhibitable proton conductance across their inner membrane. This was associated with a markedly greater abundance of avian UCP mRNA. In the presence (but not the absence) of fatty acids, these mitochondria also showed a greater adenine nucleotide translocase-catalysed proton conductance than those from never-immersed penguins. This was due to an increase in the amount of adenine nucleotide translocase. Therefore, adaptive thermogenesis in juvenile king penguins is linked to two separate mechanisms of uncoupling of oxidative phosphorylation in skeletal muscle mitochondria: increased proton transport activity of avian UCP (dependent on superoxide and inhibited by GDP) and increased proton transport activity of the adenine nucleotide translocase (dependent on fatty acids and inhibited by carboxyatractylate).

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.

Despite their lack of brown adipose tissue, 6-wk-old cold-acclimated muscovy ducklings (4 degrees C; CA) exhibit nonshivering thermogenesis (NST) in the cold. To determine the site of this NST, the regional distribution of blood flow was measured by the microsphere method in the thermoneutral zone (25 degrees C) and during acute exposure to cold (8 degrees C). Ducklings reared at thermal neutrality (TN), which use shivering to produce extra heat in the cold, were compared with CA ducklings, which substitute NST for shivering. Further, the contribution of skeletal muscle thermogenesis to the increased heat production in the cold was estimated by measuring leg muscle blood flow and arteriovenous difference in oxygen content [(a-v)O2] across the leg, enabling an estimation of muscle O2 consumption. During cold exposure, a similar increase in total leg muscle blood flow occurred in TN and CA ducklings (+127 and +130% respectively), while hepatic arterial blood flow increased less (+56 to +37%, respectively). This rise in blood flow was accounted for by an increase in cardiac output, which was smaller in CA than in TN ducklings, and in both groups by a redistribution of blood flow to the most thermogenic organs (skeletal muscles and liver). The (a-v)O2 across the leg was not changed by cold exposure, indicating that the increase in leg muscle O2 consumption resulted mainly from the increase in blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)