Global analysis of circulating metabolites in hibernating ground squirrels

Hibernation in mammals involves major alterations in nutrition and metabolism that would be expected to affect levels of circulating molecules. To gain insight into these changes we conducted a non-targeted LC–MS based metabolomic analysis of plasma using hibernating ground squirrels in late torpor (LT, Tb ~ 5 °C) or during an interbout arousal period (IBA, Tb ~ 5 °C) and non-hibernating squirrels in spring (Tb ~ 37 °C). Several metabolites varied and allowed differentiation between hibernators and spring squirrels, and between torpid and euthermic squirrels. Methionine and the short-chain carnitine esters of propionate and butyryate/isobutyrate were reduced in LT compared with the euthermic groups. Pantothenic acid and several lysophosphatidylcholines were elevated in LT relative to the euthermic groups, whereas lysophosphatidylethanolamines were elevated during IBA compared to LT and spring animals. Two regulatory lipids varied among the groups: sphingosine 1-phosphate was lower in LT vs. euthermic groups, whereas cholesterol sulfate was elevated in IBA compared to spring squirrels. Levels of long-chain fatty acids (LCFA) and total NEFA tended to be elevated in hibernators relative to spring squirrels. Three long-chain acylcarnitines were reduced in LT relative to IBA; free carnitine was also lower in LT vs. IBA. Our results identified several biochemical changes not previously observed in the seasonal hibernation cycle, including some that may provide insight into the metabolic limitations of mammalian torpor.