Abstract
The kinetics of three different tracers of phenylalanine were studied when continuously infused together either by an intravenous (IV) or intragastric (IG) route in six young healthy men during a fasting state. During IV infusion, mean flux values were 39.2 ± 1.8, 40 ± 3, 41.8 ± 3.6 μmol · kg −1 · h −1 for L-[ring- 2H 5], [ 15N], and L-[1- 13C]phenylalanine, respectively (differences not significant). Fluxes were higher ( P < .001) during IG than IV infusion, indicating a disappearance of tracer during the first pass through the splanchnic area. Also, higher fluxes were found for [ring- 2H 5]phenylalanine (74 ± 6.23 μmol · kg −1 · h −1) compared with [ 15N] and L-[1- 13C]phenylalanine (54.24 ± 4.7 and 61.15 ± 5.3 μmol · kg −1 · h −1) during IG infusion. Proton exchange might explain this difference, possibly limiting in vivo use of this label when the tracer is to be administered by the IG route.
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