Urea synthesis in enterocytes of developing pigs.
Urea synthesis from ammonia, glutamine and arginine was determined in enterocytes from newborn (0-day-old), 2-21-day-old suckling, and 29-58-day-old post-weaning pigs. Pigs were weaned at 21 days of age. Cells were incubated for 30 min at 37 degrees C in Krebs-Henseleit bicarbonate buffer (pH 7.4) containing (i) 0.5-2 mM NH4Cl plus 0.05-2 mM ornithine and 2 mM aspartate, (ii) 1-5 mM glutamine, or (iii) 0.5-2 mM arginine. In enterocytes from newborn and suckling pigs, there was no measurable synthesis of urea from ammonia, glutamine or arginine, and analysis of amino acids by a sensitive fluorimetric HPLC method revealed the formation of negligible amounts of ornithine from arginine. In contrast, in cells from post-weaning pigs, relatively large amounts of urea and ornithine were produced from ammonia, glutamine and arginine in a dose-dependent manner. To elucidate the mechanism of the developmental change of urea synthesis in pig enterocytes, the activities of urea-cycle enzymes were determined. The activities of enterocyte carbamoyl phosphate synthase I and ornithine carbamoyltransferase were lower in post-weaning pigs than in suckling ones, whereas there was no difference in arginino-succinate lyase. The activities of argininosuccinate synthase and arginase were increased by 4-fold and 50-100-fold, respectively, in enterocytes from post-weaning pigs compared with suckling pigs. The induction of arginase appears to be sufficient to account for the formation of urea from ammonia, glutamine and arginine in post-weaning pig enterocytes. These results demonstrate for the first time the presence of synthesis of urea from extracellular or intramitochondrially generated ammonia in enterocytes of post-weaning pigs. This hitherto unrecognized urea synthesis in these cells may be a first line of defence against the potential toxicity of ammonia produced by the extensive intestinal degradation of glutamine (a major fuel for enterocytes) and derived from diet and luminal micro-organisms.