title: Stable isotope analysis of CO2in breath indicates metabolic fuel shifts in torpid arctic ground squirrels.
abstract: Stable carbon isotope ratios (δ13C) in breath show promise as an indicator of immediate metabolic fuel utilization in animals because tissue lipids have a lower δ13C value than carbohydrates and proteins. Metabolic fuel consumption is often estimated using the respiratory exchange ratio (RER), which has lipid and carbohydrate boundaries, but does not differentiate between protein and mixed fuel catabolism at intermediate values. Because lipids have relatively low δ13C values, measurements of stable carbon isotopes in breath may help distinguish between catabolism of protein and mixed fuel that includes lipid. We measured breath δ13C and RER concurrently in arctic ground squirrels (Urocitellus parryii) during steady-state torpor at ambient temperatures from -2 to -26°C. As predicted, we found a correlation between RER and breath δ13C values; however, the range of RER in this study did not reach intermediate levels to allow further resolution of metabolic substrate use with the addition of breath δ13C measurements. These data suggest that breath δ13C values are 1.1‰ lower than lipid tissue during pure lipid metabolism. From RER, we determined that arctic ground squirrels rely on nonlipid fuel sources for a significant portion of energy during torpor (up to 37%). The shift toward nonlipid fuel sources may be influenced by adiposity of the animals in addition to thermal challenge.
authors.affiliation: Institute of Arctic Biology, 311 Irving I, University of Alaska Fairbanks, Fairbanks, AK 99775, USA. Electronic address: tnlee@harding.edu.
authors.forename: Trixie N
authors.initials: TN
authors.lastname: Lee
authors.affiliation: Department of Biological Sciences, University of Alaska Anchorage, 3101 Science Circle, Anchorage, AK 99508, USA.
authors.forename: Melanie M
authors.initials: MM
authors.lastname: Richter
authors.affiliation: Institute of Arctic Biology, 311 Irving I, University of Alaska Fairbanks, Fairbanks, AK 99775, USA.
authors.forename: Cory T
authors.initials: CT
authors.lastname: Williams
authors.affiliation: Institute of Arctic Biology, 311 Irving I, University of Alaska Fairbanks, Fairbanks, AK 99775, USA.
authors.forename: Øivind
authors.initials: Ø
authors.lastname: Tøien
authors.affiliation: Institute of Arctic Biology, 311 Irving I, University of Alaska Fairbanks, Fairbanks, AK 99775, USA.
authors.forename: Brian M
authors.initials: BM
authors.lastname: Barnes
authors.affiliation: Institute of Arctic Biology, 311 Irving I, University of Alaska Fairbanks, Fairbanks, AK 99775, USA.
authors.forename: Diane M
authors.initials: DM
authors.lastname: O'Brien
authors.affiliation: Department of Biological Sciences, University of Alaska Anchorage, 3101 Science Circle, Anchorage, AK 99508, USA.
authors.forename: C Loren
authors.initials: CL
authors.lastname: Buck
mesh_terms: D000818:Animals; D001831:Body Temperature; D002244:Carbon / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism*; D002247:Carbon Isotopes / Q000737:chemistry; D004734:Energy Metabolism / Q000502:physiology*; D006605:Hibernation / Q000502:physiology*; D050356:Lipid Metabolism / Q000502:physiology; D011506:Proteins / Q000378:metabolism; D012119:Respiration; D012589:Sciuridae / Q000378:metabolism
chemical_list: D002247:Carbon Isotopes; D011506:Proteins; D002245:Carbon Dioxide; D002244:Carbon
keywords: Arctic ground squirrel; Breath carbon isotopes; Hibernation; Metabolic fuel; Stable isotopes