Effect of portal versus jugular methionine infusion on circulating amino acids and nitrogen metabolism in sheep.

A double cross-over experiment with 2 rumen-, bilateral jugular vein-, and portal vein-cannulated sheep was performed to determine the influence of route of infusion of methionine on nitrogen balance and blood amino acids. Nutrients were provided solely by intravenous (glucose, amino acids, water soluble vitamins) and intraruminal (VFA, minerals) infusion. Treatments consisted of the infusion of methionine plus serine via either the jugular vein (J) or portal vein (P). The respective order of treatments for the sheep were J, P, J, P and P, J, P, J. Urinary nitrogen was lower and nitrogen balance was greater when methionine was infused intrajugularly. No differences in plasma or blood urea nitrogen were noted. Plasma amino acid (PAA) concentrations showed no significant differences due to treatment. However, whole blood amino acid (BAA) concentrations revealed significantly greater levels of methionine when methionine was administered via the jugular vein. Expression of PAA as percentages of the total resulted in no treatment effects, while percentage expression of BAA showed that methionine was greater and phenylalanine was lower during intrajugular methionine infusion. The central and peripheral methionine infusion routes appeared to influence slightly the subsequent utilization of methionine as determined by effects on BAA levels of methionine. It was demonstrated that results from analysis of circulating amino acids may differ according to whether plasma or whole blood samples are used, and whether the data are expressed as concentrations or proportions.