Meeting Abstract

108.1  Tuesday, Jan. 7 08:15  A test of alternative models for increased tissue nitrogen isotope ratios during fasting in hibernating arctic ground squirrels LEE, T.N.*; BUCK, C.L.; BARNES, B.M.; O’BRIEN, D.M.; Harding University, Searcy, AR; University of Alaska, Anchorage; University of Alaska, Fairbanks; University of Alaska, Fairbanks tnlee@harding.edu

We describe two models explaining the increase in tissue nitrogen isotope ratios (δ¹⁵N) that occurs during fasting in animals. The catabolic model posits that protein breakdown selectively removes the lighter isotope of nitrogen (¹⁴N) from catabolized tissues, causing an increase in the proportion of heavy nitrogen isotope (¹⁵N). The anabolic model posits that protein synthesis during fasting results in elevated δ¹⁵N values, as the unreplaced loss of ¹⁴N to urea results in a higher proportion of ¹⁵N in plasma amino acids used for protein synthesis. We effected a range of lean mass loss in arctic ground squirrels (Urocitellus parryii) fasting during hibernation before collecting organ and muscle tissues for analysis of δ¹⁵N values. The catabolic model predicts increased δ¹⁵N values in both liver and muscle, since these tissues undergo significant catabolism during hibernation. The anabolic model predicts no change in muscle, but an increase in δ¹⁵N values in liver, which has high levels of protein synthesis during euthermic phases of hibernation. We found a significant increase in liver δ¹⁵N values and no change in muscle δ¹⁵N values with lean mass loss, which supports the anabolic model. Heart, small intestine, and brown adipose tissue also increased in δ¹⁵N values, indicating protein synthesis in these organ tissues during hibernation. Urine was 3.8‰ lighter than plasma, and both urine and plasma increased in δ¹⁵N values with lean mass loss. This study helps clarify the mechanisms causing δ¹⁵N change during nutritional stress, thus increasing its utility for physiological research and reconciling previously contradictory results.