Effects of essential and nonessential amino acids on gastric emptying in the dog

Five dogs with gastric fistulas were studied to assess the effect on gastric emptying of two potent cholecystokinin (CCK) releasers (tryptophan and phenylalanine) and six other essential amino acids; the nonessential amino acids alanine, beta-alanine, arginine, asparagine, aspartic acid, citrulline, cysteine, cystine, glutamic acid, glutamine, D-glutamine, histidine, hydroxyproline, ornithine, proline, serine, tyrosine, and D-tryptophan; four peptide and amino acid preparations; and solutions of glucose, glycine, and mannitol. Of the essential amino acids, only tryptophan significantly slowed emptying and it was above 4 mM that there was a difference between control and test meal. The delay in response to tryptophan was dose-related and approached maximum at 40 mM; D-tryptophan had no effect at these concentrations. In concentrations up to 80 mM, none of the nonessential amino acids slowed emptying significantly. The four peptide and amino acid preparations in concentrations ranging from 80 to 700 milliosmoles had dose responses identical to those of D-glucose, glycine, and mannitol at similar osmolalities. It is concluded that L-tryptophan is a uniquely potent delayer of gastric emptying in the dog which is dose-dependent and stereospecific. Phenylalanine, a potent CCK releaser, did not slow emptying, which suggests that CCK release may not be the only mechanism by which tryptophan acts. The peptide and amino acid preparations (casein hydrolysate, Bacto-peptone, Amigen, FreAmine) seem to delay emptying by stimulation of osmoreceptors which is distinct from the mechanism of action of tryptophan.