MUCOSAL AND HEPATIC PROTEIN SYNTHESIS MEASURED SIMULTANEOUSLY WITH INTRAGASTRIC AND INTRAVENOUS STABLE ISOTOPIC TRACERS IN FED PIGLETS

Abstract

The pool of amino acids used for intestinal mucosal and hepatic protein synthesis can derive from the diet, from intracellular proteolysis and from arterial amino acids. The relative contributions of these sources to mucosal and hepatic protein were quantified in piglets (N=4; 7.4 kg body weight), previously implanted with gastric (IG), portal (PORT), jugular (IV) and arterial (ART) catheters. After an overnight fast, the animals were fed hourly meals of a milk-based diet and were infused, for 6 h, with U13C-algal protein (IG) and 2H5-phenylalanine(PHE; IV). The tracer:tracee ratios of [U13C] and [2H5]PHE in ART and PORT blood, as well as in VLDL apolipoproteinB-100 (APOB), fibrinogen, hepatic and mucosal protein were measured by methane negative chemical ionization selected ion monitoring mass spectrometry. The table shows the mean ratios (± SD) of the isotopic enrichments of IV tracer to that of the IG tracer in ART, PORT and APOB-100 over the last h, and in plasma fibrinogen and hepatic protein at the end of the infusion. The ratio of the IV:IG tracers was (P<0.01) lower in PORT than in ART PHE. The tracer:tracee ratio of the IG tracer in APOB (4.98 ± 1.69 mol%) was higher (P<0.025) than in hepatic free PHE (1.92 ± 0.85). The IV:IG tracer ratio of APOB-100, fibrinogen and hepatic protein were all lower (P<0.05) than that of ART PHE, but the IV:IG ratio was higher in APOB than in fibrinogen and liver constitutive protein. The ratio of the two tracers in mucosal protein (0.27 ± 0.02) was 30% (P=0.05 by paired t-test) higher than their ratio in mucosal free PHE (0.21 ± 0.06). We conclude (i) that PHE for mucosal and hepatic protein synthesis derives from the extracellular input rather than from amino acids released by intracellular proteolysis and (ii) that ART PHE may be a preferential source of mucosal and PORT PHE a preferential source of hepatic protein synthesis.