Seasonal changes in enzymes of aerobic heat production in the white-footed mouse

Abstract
During seasonal acclimatization in white-footed mice (Peromyscus leucopus), there is a substantial increase in the capacity for aerobic heat production under cold stress (Mmax) in winter animals. The possibility that increases in levels of enzymes involved in aerobic heat production could be responsible for the increase in Mmax was investigated in mice captured in summer and winter. Activities of citrate synthase (CS) and beta-hydroxyacyl-CoA dehydrogenase (HOAD) and concentrations of cytochrome c (cyt c) were measured in the two primary thermogenic tissues of small mammals, skeletal muscle and brown adipose tissue (BAT). Additionally, cyt c was measured in heart, liver, and whole-body samples. CS and cyt c were used as indicators of aerobic capacity, and HOAD was used to indicate the capacity for beta-oxidation. In winter CS, cyt c, and HOAD increased (expressed per g wet mass) in skeletal muscle and BAT. There was an increase in cyt c of whole-body samples, liver, and skeletal muscle of between 55 and 78%, but no change was observed in cardiac tissue. There was an approximately 80% increase in CS and HOAD in skeletal muscle. The highly aerobic nature of BAT and its primary role in heat production are supported by the high activities in summer animals and the increase observed in winter (200, 1,570, and 220% increase in CS, HOAD, and cyt c, respectively).