A multitracer stable isotope quantification of the effects of arginine intake on whole body arginine metabolism in neonatal piglets

We have previously shown that deficient arginine intake increased the rate of endogenous arginine synthesis from proline. In this paper, we report in vivo quantification of the effects of arginine intake on total endogenous arginine synthesis, on the rates of conversion between arginine, citrulline, ornithine, and proline, and on nitric oxide synthesis. Male piglets, with gastric catheters for diet and isotope infusion and femoral vein catheters for blood sampling, received a complete diet for 2 days and then either a generous (+Arg; 1.80 g·kg^-1·day^-1; n = 5) or deficient (-Arg; 0.20 g·kg^-1·day^-1; n = 5) arginine diet for 5 days. On day 7, piglets received a primed, constant infusion of [guanido- ¹⁵N₂]arginine, [ureido-¹³C;5,5- ²H₂]citrulline, [U-¹³C₅]ornithine, and [¹⁵N;U-¹³C₅]proline in an integrated study of the metabolism of arginine and its precursors. Arginine synthesis (μmol·kg^-1·h^-1) from both proline (+Arg: 42, -Arg: 74, pooled SE: 5) and citrulline (+Arg: 67, -Arg: 120; pooled SE: 15) were higher in piglets receiving the -Arg diet (P < 0.05); and for both diets proline accounted for ∼60% of total endogenous arginine synthesis. The conversion of proline to citrulline (+Arg: 39, -Arg: 67, pooled SE: 6) was similar to the proline-to-arginine conversion, confirming that citrulline formation limits arginine synthesis from proline in piglets. Nitric oxide synthesis (μmol·kg^-1·h^-1), measured by the rate conversion of [guanido-¹⁵N₂]arginine to [ureido-¹⁵N]citrulline, was greater in piglets receiving the +Arg diet (105) than in those receiving the -Arg diet (46, pooled SE: 10; P < 0.05). This multi-isotope method successfully allowed many aspects of arginine metabolism to be quantified simultaneously in vivo.