Competition for transport between methyldopa and other amino acids in rat gut loops

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Abstract
Transport of L- and D-methyldihydroxy- phenylalanine-methyldopa) and their inhibition of natural amino acid transport was studied in situ, with rat jejunal loops. L-Methyldopa was absorbed very slowly although significantly more than the D isomer. In a latin-square experiment, the effects of L- and D-methyldopa and mannitol (as a poorly absorbed control) on histidine transport were compared. Mannitol significantly reduced histidine absorption, indicating sensitivity of histidine transport to adverse osmotic gradients. D-Methyldopa reduced histidine absorption but not significantly more than mannitol. L-Methyldopa reduced histidine absorption significantly more than mannitol and D-methyldopa. It was thought that L-methyldopa inhibited amino acid transport mechanism(s) having specificity for L configurations. Mannitol and L- and D-methyldopa were all without effect on glucose absorption, suggesting that methyldopa had no non-specific effects on gut metabolism. Further studies demonstrated inhibition of transport of glutamic acid, glycine, lysine, taurine, and phenylalanine, but not methionine or proline by L-methyldopa, suggesting that it had a more general inhibitory-action on amino acid transport than competitive inhibition of one transport mechanism. Incidental evidence suggested that the amino compound, taurine, was absorbed by a process other than simple diffusion.