1. l-Leucine was given to healthy, post-absorptive subjects as a continuous intravenous infusion (300 μmol/min) during 2 1/2 h. Arterial blood concentrations and regional exchange of amino acids were measured across the splanchnic region, the brain and a leg, by the catheter technique. Renal clearance of amino acids was also determined.

2. During the infusion of leucine its concentration rose four- to six-fold, while the concentrations of several other amino acids declined continually, the effect being most pronounced for isoleucine (−55% of initial value), methionine (−55%), valine (−40%), tyrosine (−35%) and phenylalanine (−35%).

3. The infused leucine was taken up by muscle tissue (55%), by the splanchnic region (25%) and by the brain (10%). Neither leg-muscle release nor splanchnic uptake of aromatic amino acids was affected. Renal clearance and tubular reabsorption of amino acids were uninfluenced by leucine infusion. The uptake of isoleucine and methionine by the brain, seen in the basal state, was inhibited during leucine infusion.

4. The marked reduction in the concentrations of the aromatic amino acids, the uptake of leucine by the brain and the inhibition of brain methionine uptake, which accompany leucine infusion in healthy subjects,-5-be of relevance for the treatment of patients with portal-systemic encephalopathy.

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