Characteristics of threonine, valine and methionine absorption in the jejunum and ileum of sheep

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Virginia Polytechnic Institute and State University


Wethers fitted with double re-entrant cannulae located in either the jejunal or ileal section of the small intestine were used to study the absorption characteristics of threonine, valine and methionine. Perfusions of the cannulated intestine were conducted 4 hr after feeding for a 1 hr period. Absorption of amino acids was measured as the total amount of amino acid removed from the perfusion solution during the 1 hr period. Concentrations of each amino acid ranged from 0.6 to 7.4 millimolar. The experiment design, a three dimensional central composite design, required only 15 different concentration combinations of the three amino acids used to determine their absorption characteristics.

Removal of amino acid from the perfusion solution was constant over the 1 hr perfusion period, but absorption did vary with time after feeding at which the wethers were perfused. Four or 8 hr after feeding tended to have higher absorption rates than 2 or 6 hours. Absorption of each amino acid increased with each replication indicating that the cannulae remained healthy and functional. Wethers with cannulae located in the ileum absorbed more (P < .05) threonine, valine and methionine than those with jejunal cannulae.

Equations were derived for each amino acid to predict the amount which would be absorbed when the initial concentrations of all three amino acids were known. Site of absorption and effluent volume were also part of the absorption prediction equations, Absorption of threonine and valine was affected by the quadratic component (P < .10) of their own concentrations. The linear and quadratic components of methionine concentration exerted a positive influence (P < .05) on methionine absorption. Site of absorption was not an important (P < .10) component of the prediction equation with the exception of methionine. Effluent volume was a significant (P < .01) component of all three absorption prediction equations. The amounts of each amino acid absorbed was dependent upon the initial concentration of that amino acid. As the initial concentration was increased, the amount absorbed increased, but no saturation points were noted.

Absorption, as observed in this experiment, was subject to inhibition and stimulation by the other amino acids present in the perfusion solution. Valine was inhibitory of threonine absorption when present in low concentrations, but increasing the concentration of valine resulted in a stimulatory effect on threonine absorption. Methionine inhibited the absorption of threonine when valine was present at concentrations below 3 mM, but the stimulation of threonine absorption by high concentrations of valine was enough to override the inhibitory effects of methionine. The amount of valine absorbed was decreased by increasing the methionine concentration. Low concentrations of threonine decreased the amount of valine absorbed while high concentrations had a stimulatory effect at high concentrations of valine and low concentrations of methionine. Valine exerted inhibitory effects on methionine absorption when threonine was present in low concentrations and threonine was also inhibitory if valine was present at low concentrations. When the concentration of threonine was increased to 7.4 mM, each increase in valine concentration resulted in more methionine being absorbed.