Incorporation of microbially salvaged urea-nitrogen into anabolic amino acids during hibernation in arctic ground squirrels

Hibernators rely on endogenous supplies of amino acids (AAs) and other nutrients to sustain organ function and skeletal muscle mass. Gut microbiota-mediated recycling of urea-nitrogen into AAs has long been considered a mechanism to assist in conserving nitrogen; however, the relevance of urea nitrogen salvage (UNS) to overall host physiology and metabolism is debated. We hypothesized that incorporation of microbially-liberated urea-nitrogen (MLUN) into AAs would be higher in host tissues of hibernating arctic ground squirrels compared to those of summer active squirrels, and that MLUN would support synthesis of anabolic AAs that regulate protein balance. To test this, we injected [¹³C, ¹⁵N₂]-urea into summer squirrels and into squirrels hibernating at an ecologically relevant ambient temperature (-16 °C). We found greater incorporation of MLUN into non-essential AAs and specific essential AAs in several tissues of hibernating squirrels compared to summer. We also observed increased ¹⁵N enrichment in leucine-isoleucine, citrulline and glutamine, anabolic AAs known to influence protein balance and trans-organ nitrogen balance. Compared to studies in which ground squirrels hibernated at ambient temperatures above 0 °C, our results suggest that squirrels hibernating at subzero temperatures may up modulate synthesis of AAs that preserve protein and nitrogen balance during prolonged fasting and inactivity.