Seasonal changes in thermogenic properties of liver and muscle in tree sparrows Passer montanus

Seasonal changes in avian physiology and behavior are considered to be a part of an adaptive strategy for survival, and basal metabolic rate (BMR) is important component of energy budget in endotherms. In order to understand seasonal changes in cellular and hormonal thermogenic properties in tree sparrows, we measured body mass and BMR, mitochondrial protein contents, mitochondrial state 4 respiration, cytochrome C oxidase (COX) activity in liver and muscle, as well as serum triiodothyronine (T3) and thyroxine (T4) concentration in the summer (late June and July) and winter (late December and January). Oxygen consumption was measured using a closed-circuit respirometer, state 4 respiration in liver and muscle mitochondria was measured with a Clark electrode. Likewise, activities of COX of liver and muscle were measured using a Clark electrode. Both body mass and BMR were higher in winter than in summer, and serum T3 concentration was higher in winter than in summer. The mitochondrial state 4 respiration in liver, and COX activity in liver and muscle exhibited higher levels in winter and lower levels in summer. These data indicate that increase in thermogenic capacity at both organismal and cellular levels are important adjustments to cope with cold stress in wild tree sparrows [Acta Zoologica Sinica 54 (5): 777– 784, 2008].