The role of urea in nitrogen excretion and caecal nitrogen metabolism in Willow ptarmigan
In response to fasting or fasting combined with water deprivation urine urea excretion declined sharply in normothermic ground squirrels (Spermophilus lateralis) when compared with laboratory rats. Intraperitoneal injection of urea equivalent to 0.3% of body weight failed to increase blood urea in 12-h fasted ground squirrels, and led to little urine urea excretion when compared with similarly treated rats. Collection of expired CO2 from fasted or fasted water-deprived ground squirrels that were urea loaded and injected with a tracer dose of 14C-tagged urea revealed a 1,000- to 1,500-fold greater 14CO2 expiration than controls. Treatment of fasted squirrels with antibiotics (penicillin, tetracycline, or sulfamethazine) to reduce intestinal microflora reduced expired 14CO2 to control levels. In fasted squirrels, drug treatment also enhanced urine urea excretion with values approaching those of controls. The results of these experiments point to the possibility of urea nitrogen recycling in nutritionally stressed ground squirrels.