STUDIES ON UREA FORMATION IN HOMING PIGEONS

After several hours of flight, plasma urea concentrations in homing pi geons increases to levels normally observed in ureotelic mammals. To i nvestigate urea formation in these birds we studied the influence of d ietary protein and/or arginine contents on excretion of uric acid, ure a, and NH4+ for 14 h after feeding and on plasma levels of these compo unds at the end of this interval. A protein enriched diet significantl y increased the quantity of excreted uric acid and had a weak effect o n plasma uric acid levels. The amounts of excreted urea were significa ntly correlated with arginine, but not with protein content of the die ts. The dietary arginine content also influenced plasma urea levels. T here were no significant effects of the different diets on plasma NH4 levels and on excreted NH4+. Also, we measured the activities of the enzymes of the ornithine-urea cycle in pectoral muscle, liver, and kid ney. In these organs carbamyl phosphate synthetase, ornithine transcar bamylase, argininosuccinate synthetase, and argininosuccinate lyase ac tivities were undetectable or very low. Arginase activity was moderate in the liver and pectoral muscle and high in the kidney. Thus, in hom ing pigeons the contribution of the ornithine-urea cycle to urea forma tion is negligible and urea formation occurs by argininolysis.