Gut mucosal protein synthesis measured using intravenous and intragastric delivery of stable tracer amino acids.

We measured the rates of mucosal protein synthesis during the simultaneous delivery of [1-13C]leucine and [1-13C]valine delivered either intragastrically or intravenously to investigate any influence of the route of supply of the tracers. Dependent on the route, there were marked differences in the gradient of labeling between the plasma and intramucosal leucine and valine; i.e., for intravenous tracers the ratio was 1.73 +/- 0.16, but for intragastric tracers it was 0.65 +/- 0.12 (P < 0.05). Incorporation of intravenous tracer into mucosal protein was linear with time, and irrespective of tracer route, the calculated fractional rates of protein synthesis were identical when based on the intracellular labeling of the leucine or valine tracer, i.e., with intravenous 2.58 +/- 0.32%/h and with intragastric 2.45 +/- 0.36%/h. The results demonstrate that a robust and reproducible method of measurement of gastrointestinal mucosal protein synthesis has been developed and that use of either intragastric or intravenous routes of tracer administration gives comparable results. The high rates measured suggest that the gastrointestinal mucosa contributes substantially to whole body protein synthesis in normal healthy subjects.