Renal acid excretion in the domestic fowl

1. In order to assess the role of uricotelism in net renal acid excretion, blood and ureteral urine samples were collected from five hens fed a commercial poultry feed (Diet A) and five hens fed a protein-rich, Na-poor feed (Diet B). All samples were analysed for pH, PCO2, ammonium, phosphate, uric acid and urates (UA + U) and inulin. 2. On Diet A, average pH in venous blood was 7.42, while urinary pH (pHu) ranged from 4.74 to 7.25. At average pHu (6.10), uric acid accounted for 52% of total acid excreted, H2PO4 for 20% and NH4 for 28%. Net acid excretion in ureteral urine was 345 muequiv h-1 kg body weight-1, or 5-10 times that observed in ureotelic vertebrates (amphibians and mammals). 3. The relative contributions of these urinary buffers to net renal acid excretion changed with pHu. Significant negative correlations exist between pHu and both total phosphate and ammonium excretion rates (P less than 0.001). Excretion rates of (UA + U) showed a positive correlation (P less than 0.05) with pHu. 4. Feeding on Diet B revealed the homeostatic power of the avian kidney. Blood pH and PCO2 were not changed relative to values in hens fed the control diet while striking increases in excretion rates of all urinary buffers (except HCO3) were observed. Average pHu fell to 5.12, and the average net renal acid excretion rate doubled.