THE INFLUENCE OF LUMINAL PH ON TRANSPORT OF NEUTRAL AND CHARGED DIPEPTIDES BY RAT SMALL-INTESTINE, IN-VITRO

Four hydrolysis-resistant dipeptides (D-phenylalanyl-L-alanine, D-phen ylalanyl-L-glutamine, D-phenylalanyl-L-glutamate and D-phenylalanyl-L- lysine) were synthesized to investigate the effects of net charge on t ransmural dipeptide transport by isolated jejunal loops of rat small i ntestine. At a luminal pH of 7.4 and a concentration of 1 mM the two d ipeptides with a net charge of -1 and +1 were transported at substanti ally slower rates (18 +/- 1.3 and 8.4 +/- 1.3 nmol min(-1) (g dry wt.) (-1), respectively) than neutral D-phenylalanyl-L-alanine and D-phenyl alanyl-L-glutamine (87 +/- 0.2 and 197 +/- 14 nmol min(-1) (g dry wt.) (-1), respectively). We investigated the effects of luminal pH on dipe ptide transport by varying the NaHCO3 content of Krebs Ringer perfusat e equilibrated with 95%0(2)/5% CO2. The pH changes did not affect wate r transport, but serosal glucose appearance increased significantly at pH 6.8. Transmural transport of D-phenylalanyl-L-alanine and D-phenyl alanyl-L-glutamine at pH 6.8 was stimulated (P < 0.01) by 61% and 49%, respectively, whereas the lower pH increased the rate for negatively charged D-phenylalanyl-L-glutamate by 306% (P < 0.01) and decreased th at for positively charged D-phenylalanyl-L-lysine by 46% (P < 0.05). I ncreasing luminal pH to 8.0 inhibited D-phenylalanyl-L-alanine transpo rt by 60%, whereas D-phenylalanyl-L-lysine transport was 60% faster.