Endogenous Gut Nitrogen Losses in Growing Pigs Are Not Caused by Increased Protein Synthesis Rates in the Small Intestine

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Abstract
The purpose of this study was to establish, using a flooding dose of l-[ring 2, 6-3H] phenylalanine, whether feeding pigs diets that induce high endogenous gut nitrogen losses (ENL) also increases protein synthesis rates in (PSR) the visceral organs. Twelve 18-kg Yorkshire barrows with catheters in the right and left jugular veins were fed for 3 wk either casein-cornstarch- (CC) or barley-canola meal- (BCM) based diets formulated to a similar digestible energy /crude protein ratio and designed to induce either low or high ENL, respectively. Pigs were infused with 10 mL/kg body weight of a 150 mmol · L−1 phenylalanine solution containing 230 MBq · L−1 labeled phenylalanine for 12 min and killed 20 min later. Plasma phenylalanine specific radioactivity (SRA) rose to a plateau value within 3 min of starting the infusion and did not change (P > 0.10) thereafter. Fractional rates of protein synthesis (Ks, %/d) based on SRA in plasma- or intracellular-free phenylalanine did not differ (P > 0.10) in all tissues except pancreas (P < 0.05). Diet affected Ks in liver (P < 0.01) and colon (P < 0.05) but not in pancreas, duodenum, jejunum and cecum. Based on plasma-free phenylalanine SRA, liver Kswere 85.4 ± 11.0 vs. 60.5 ± 5.2 (mean ± sem) in CC- and BCM-fed pigs, respectively; these values were 82.3 ± 4.7 vs. 98.2 ± 5.8 in the colon. The absolute amount of protein synthesis (g · d−1) was higher in the liver (P < 0.05) and pancreas (P < 0.05) of the CC pigs compared to BCM pigs. No dietary effects were observed in all other organs (P > 0.10). The present results suggest that feeding growing pigs a BCM diet that induces high ENL does not affect PSR in the small intestine of growing pigs from which >50% of ENL originates.