Ornithine metabolism, but not arginine synthesis, is affected by the addition of ornithine to an arginine-deficient diet in enterally-fed piglets

In a previous study, ornithine addition to an arginine-deficient diet did not improve whole-body arginine status in enterally-fed piglets; however, the metabolic fates of the supplemental ornithine were not studied. This experiment determined the metabolic fates of the supplemental ornithine and whether ornithine metabolism was affected by the addition of α-ketoglutarate. Male piglets (n = 20, 1.8 kg), fitted with gastric catheters for diet and isotope infusion, portal vein catheters for isotope infusion and femoral vein catheters for blood sampling (d 0), received 2 d of a complete diet, followed by 5 d of 1 of 4 test diets: the arginine-deficient diet (basal), or the basal diet with either α-ketoglutarate [ + α - KG; 4.6 mmol/(kg d)], ornithine [ + Orn; 9.2 mmol/(kg d)] or both [ + α - KG/ + Orn; 4.6 mmol/(kg d) α - ketoglutarate + 9.2 mmol/(kg d) ornithine]. Piglets received primed, constant infusions of [1-¹⁴C]ornithine infused intragastrically (either d 5 and d 7) to determine ornithine kinetics, and [guanido-¹⁴C]arginine intragastrically to measure arginine flux (d 6). Piglets receiving the ornithine-containing diets had a higher intragastric ornithine flux (P < 0.0001) and ornithine oxidation (P < 0.05). Ornithine supplementation did not increase arginine synthesis, although the ornithine supplemented piglets had a greater conversion of ornithine to proline (P < 0.0001). The fates of supplemental ornithine in piglets fed an arginine-deficient diet appear to be oxidation and proline synthesis; this was not affected by the presence of α-ketoglutarate.