Luminal threonine concentration acutely affects intestinal mucosal protein and mucin synthesis in piglets

Intestinal mucin synthesis is sensitive to dietary threonine supply, which suggests that the gut's requirement for threonine may comprise a significant proportion of the whole body requirement. We used a continuously perfused gut loop model and intraluminal flooding dose technique in 6 young pigs to study the acute effects of varying luminal availability of threonine on intestinal protein and mucin syntheses. A complete amino acid mixture containing 0, 21, or 56 mg threonine/g total amino acids (whole body requirement ratio is approximately 28 mg/g) was continuously perfused in isolated loops for 120 min, including a 30-min (3)H-phenylalanine flooding dose. We measured fractional synthesis rates of total mucosal protein and mucin by analyzing (3)H-phenylalanine incorporation. Fractional rates of total mucosal protein synthesis were higher in loops perfused with solutions containing threonine at 56 mg/g (66 +/- 4%/d) compared with 0 mg/g (42 +/- 9%/d) and 21 mg/g (53 +/- 6%/d) (P < 0.05). For mucin, fractional rates of synthesis differed between 0 mg/g (323 +/- 72%/d), 21 mg/g (347 +/- 49%/d), and 56 mg/g (414 +/- 31%/d) (P < 0.05). In addition, total proline and threonine concentrations in the protein hydrolysates increased with luminal threonine concentration (P < 0.05), indicating an increase in threonine- and proline-rich proteins. De novo synthesis of mucosal and mucin proteins is acutely sensitive to luminal threonine concentration, which demonstrates the importance of dietary amino acid supply to gut protein metabolism.