Activity of antioxidant enzymes in rat skeletal muscle and brown fat: effect of cold and propranolol

1. Effects of acute (6 h) and chronic (21 day) cold (6°C) exposure, as well as propranolol (15 mg/kg) on the activities of CuZnSOD, MnSOD and catalase in the rat skeletal muscle (SM) and brown adipose tissue (BAT), which are important sites of cold-induced thermogenesis, were investigated.

2. The changes in the activity of antioxidant enzymes were tissue specific and dependent on the duration of cold exposure. Thus, in the SM of acutely cold exposed rats, the activity of all antioxidant enzymes studied was elevated, whereas in the BAT the activity of both SOD_s decreased and that of catalase remained unchanged. In cold acclimated rats, the activity of all the three enzymes was increased in the BAT whereas in the SM, CuZnSOD activity was enhanced, MnSOD activity decreased and catalase activity returned to the control level.

3. Propranolol also differently altered the antioxidant enzyme activity in SM and BAT, alterations being dependent on the acclimation temperature. Thus, in room acclimated rats propranolol decreased the activity of all antioxidant enzymes in SM but did not affect those in BAT. However, in the SM propranolol prevented the elevation of MnSOD and catalase activities, induced by acute cold. In cold acclimated rats propranolol inhibited CuZnSOD activity in both SM and BAT but increased that of MnSOD.

Autophagy in Myf5+ progenitors regulates energy and glucose homeostasis through control of brown fat and skeletal muscle development

Macroautophagy (MA) regulates cellular quality control and energy balance. For example, loss of MA in aP2‐positive adipocytes converts white adipose tissue (WAT) into brown adipose tissue (BAT)‐like, enhancing BAT function and thereby insulin sensitivity. However, whether MA regulates early BAT development is unknown. We report that deleting Atg7 in myogenic Myf5+ progenitors inhibits MA in Myf5‐cell‐derived BAT and muscle. Knock out (KO) mice have defective BAT differentiation and function. Surprisingly, their body temperature is higher due to WAT lipolysis‐driven increases in fatty acid oxidation in ‘Beige’ cells in inguinal WAT, BAT and muscle. KO mice also present impaired muscle differentiation, reduced muscle mass and glucose intolerance. Our studies show that ATG7 in Myf5+ progenitors is required to maintain energy and glucose homeostasis through effects on BAT and muscle development. Decreased MA in myogenic progenitors with age and/or overnutrition might contribute to the metabolic defects and sarcopenia observed in these conditions.     

Acceptor activity, isoacceptor profiles and function in protein synthesis of transfer RNAs from regenerating skeletal muscle

Transfer RNAs have been prepared from control and regenerating rat skeletal muscle. The yield of tRNA is highest during the early stages of the regeneration process (5 and 8 days following the induction of regeneration) and decreases to near control values thereafter. The amino acid acceptor activity (extent of aminoacylation) of tRNA from regenerating muscle was also found to be higher for some amino acids than the activity of control tRNA, and the maximum increase in activity was observed between 5 and 8 days following the initiation of regeneration with a decrease to control levels through 15 and 30 days. The isoacceptor pattern, determined by RPC-5 chromatography, for methionyl-tRNAs from control muscle and 5-day regenerating muscle were essentially indistinguishable, while a minor peak of prolyl-tRNA was observed in the population from 5-, 8- and 15-day regenerates which was apparently absent from the control tRNA. Lysyl-tRNAs from control muscle contain two major isoacceptors while a third isoacceptor is observed in the tRNA preparations from 5-, 8- and 15-day regenerating muscle. The relative amount of this third isoacceptor is highest in the 8-day population and decreases in amount in tRNAs from 15- and 30-day regenerates. Control muscle also contains two major glutamyl-tRNA species while a third isoacceptor can be detected in regenerates. The relative amount of this species increases during the early course of the regeneration process but is present at near control levels by 30 days following Marcaine injection. Cell-free protein synthesis using muscle polyribosomes showed that tRNAs from regenerating muscle were more effective in stimulating [³⁵S]methionine incorporation than tRNAs from control muscle.