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

The effect of pH and concentration on alpha-methyldopa absorption in man

An open crossover study of the absorption of alpha-methyldopa has been conducted in normal healthy adult male volunteers in whom a triple lumen perfusion tube had been placed. Three volunteers were perfused on separate occasions with 0.1 mM alpha-methyldopa at pH 4.5, 6.0 and 7.4. Three other volunteers were perfused on separate occasions with 0.1, 1.0 and 10 mM alpha-methyldopa at pH 6.0. Two additional subjects were perfused with 0.1 mM alpha-methyldopa at pH 6.0. Absorption was not a linear function of concentration above 1 mM alpha-methyldopa. There was also a weak trend toward greater absorption near pH 6.0. At higher concentrations of drug in the perfusion solution (10 vs 1 mM), the free fraction of alpha-methyldopa in plasma samples was increased significantly. Thus, although absorption of alpha-methyldopa is more efficient at lower concentrations, bioavailability may not be substantially enhanced due to increased sulphation in the gut wall. Comparison of permeabilities with previous results from our laboratories suggests the rat is a good model for predicting the behaviour of alpha-methyldopa after its oral administration to man.

Concentration and pH dependency of alpha-methyldopa absorption in rat intestine

The intestinal absorption of alpha-methyldopa from perfused segments of rat intestine was determined. Jejunal and ileal segments were studied at pH 4.5, 6.0 and 7.4 at an alpha-methyldopa concentration of 0.01 mM, and at pH 6.0 at concentrations of 0.01, 0.10 and 10 mM. Intestinal absorption was found to depend on both concentration and pH but not on segment. A weak positive correlation (r = 0.5) was observed between net water absorption and alpha-methyldopa absorption, similar to that observed for amino acids. The low intestinal wall permeability of alpha-methyldopa under normal intestinal pH conditions (pH 7.4) is consistent with the incomplete oral bioavailability of this drug.

A third dipeptide carrier system typified by l-prolyl-lhydroxyproline and independent of l-leucyl and beta-alanyl dipeptides in rat gut loops

1. The intestinal transport of L-prolyl-L-hydroxyproline (10 mmol/l) was investigated in rat small gut loops in vivo under pentobarbitone anaesthesia. Osmolality of test solutions was adjusted to eliminate any positive effect of solvent drag on disappearance of solutes from the lumen. 2. L-Leucylglycine and beta-alanyl-L-histidine (carnosine), representative members of two distinctly different dipeptide transport groups previously delineated, were tested for competitive action on L-prolyl-L-hydroxyproline uptake at ten times equimolar concentration (100 mmol/l), but were found to have no effect on the carrier system. 3. L-Prolyl-L-hydroxyproline uptake was markedly blocked by other L-prolyl dipeptides, indicating that they shared a common carrier system. Disappearance of L-prolyl-L-hydroxyproline from the gut lumen was reduced from 48% 15 min-1 10 cm-1 (control, containing 70 mmol/l mannitol) to 11% or 20% in the presence of L-prolylglycine (100 mmol/l) or L-prolyl-L-leucine (25 mmol/l), respectively. 4. It was concluded that at least three separate dipeptide carrier protein systems exist in the rat small gut, the disappearance of L-prolyl-L-hydroxyproline from the gut lumen being inhibited by two other L-prolyl dipeptides but not by L-leucyl or beta-alanyl dipeptides.