Active transport inhibition in rat small intestine by amphiphilic amines: an in vitro study with various local anaesthetics.

ABSTRACT

In the present investigation with rings of everted rat small intestine, amphiphilic amines such as local anaesthetics (e.g. lidocaine, procaine, tolycaine) were employed to study their effects on intestinal absorption of methyl alpha-D-glucoside, L-leucine, D-fructose, and 2-deoxy-D-glucose. All the amphiphilic amines tested, except for benzocaine, significantly inhibited Na(+)-dependent active uptake of methyl alpha-D-glucoside and L-leucine while leaving uptake of D-fructose (facilitated diffusion) and 2-deoxy-D-glucose (passive diffusion) unaffected. Increasing concentrations of lidocaine in the incubation medium inhibited the uptake of methyl alpha-D-glucoside (IC(50) approximately 3.5 mmol/L) and L-leucine (IC(50) approximately 6 mmol/L) in a dose-dependent manner. Complete reversibility of the inhibitory effect could only be achieved at short-term incubations (lidocaine concentrations (kinetics of methyl alpha-D-glucoside and L-leucine in the presence of lidocaine revealed a significant increase in the apparent Michaelis constant, leaving the maximal transport capacity essentially unaltered. Reducing the Na(+) concentration in the incubation medium aggravated inhibition by lidocaine of the uptake of methyl alpha-D-glucoside. Analysis of the inhibition kinetics by Dixon plots revealed a competitive interaction between Na(+) and the amphiphiles. However, phlorizin binding was not affected by lidocaine. Changing the pH of the incubation medium from 5.6 to 8.0 increased the inhibitory effect of the amphiphiles, which indicated that the non-ionised and, thus, more lipophilic form participates in the mechanism of inhibition. However, benzocaine, a rather lipophilic local anaesthetic with no aliphatic amino group, did not impair active uptake of methyl alpha-D-glucoside. Whether the amphiphilic amines act by their partition into the membrane matrix or directly interact with sodium binding sites remains to be elucidated, however.