A test of alternative models for increased tissue nitrogen isotope ratios during fasting in hibernating arctic ground squirrels

Abstract

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 (δ15N). 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 δ15N 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 and then collected organ and muscle tissues for analysis of δ15N values. The catabolic model predicts increased δ15N values in both liver and muscle, as 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 showed an increase 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. © 2012. Published by The Company of Biologists Ltd.