Studies on amino acid inhibition of monosaccharide exit from anuran small intestinal epithelium

1. The effect of the addition of amino acids to the intestinal lumen upon the movement of the monosaccharide alpha-methyl-D-glucopyranoside (alpha MG) from the preloaded epithelium into the blood and into the lumen of the vascularly perfused frog small intestine has been studied. 2. The neutral hydrophobic amino acids tryptophan, leucine, phenylalanine, tyrosine, isoleucine, valine, norleucine and cycloleucine all rapidly inhibit the exit of alpha MG out of the epithelium into the vascular bed. They stimulate backflux of the sugar from the epithelium into the lumen to a very much smaller extent. 3. L-Leucine is a more effective inhibitor of alpha MG exit into the blood than is D-leucine. Near-maximal inhibition of alpha MG exit is seen with 10 mM-L-leucine in the intestinal lumen. 4. The addition of leucine (10 mM) to the lumen of the intestine preloaded with alpha MG approximately halves the rate constant for alpha MG washout into the blood from 12.6 +/- 1.7 (4) x 10(-3) to 5.5 +/- 1.4 (4) x 10(-3) min-1, without appreciably altering the pool of monosaccharide in the tissue. The inhibitory effect of L-leucine upon alpha MG exit into the blood is not abolished by the presence of phlorizin (5 x 10(-5) M) in the intestinal lumen. 5. The complex pattern of inhibition of alpha MG transfer from the lumen to the blood observed upon the addition of L-leucine to the lumen is consistent with the finding that the amino acid inhibits the exit of the monosaccharide out of the epithelium into the blood in addition to any inhibitory effect upon sugar entry across the brush border. 6. It is suggested that alpha MG may be a substrate for a proposed transport system for neutral hydrophobic amino acids which, it is suggested, is present in the basolateral membrane of the epithelial cell.