Cimetidine transport in brush-border membrane vesicles from rat small intestine

In previous studies, sulfoxide metabolite was observed in animal and human intestinal perfusions of cimetidine and other H-2-antagonists. A sequence o f follow-up studies is ongoing to assess the intestinal contributions of dr ug metabolism and drug and metabolite transport to variable drug absorption . An evaluation of these contributions to absorption variability is carried out in isolated fractions of the absorptive cells to uncouple the processe s involved. In this report, data is presented on the drug entry step from a study on [H-3]cimetidine uptake into isolated brush-border membrane vesicl es from rat small intestine. A saturable component for cimetidine uptake wa s characterized with a V-max and K-m (mean +/- S.E.M.) of 6.1 +/- 1.5 nmol/ 30s/mg protein and 8.4 +/- 2.0 mM, respectively. Initial binding, and possi bly intravesicular uptake, was inhibited by other cationic compounds includ ing ranitidine, procainamide, imipramine, erythromycin, and cysteamine but not by TEA or by the organic anion, probenecid. Initial uptake was not inhi bited by amino acids methionine, cysteine, or histidine, by the metabolite cimetidine sulfoxide, or by inhibitors of cimetidine sulfoxidation, methima zole, and diisothiocyanostilbene-2,2'-disulfonic acid. Equilibrium uptake w as inhibited by ranitidine, procainamide, and cysteamine but not by erythro mycin or imipramine. Initial cimetidine uptake was stimulated by an outward ly directed H+ gradient, and efflux was enhanced by an inwardly directed H gradient. Collapse of the H+ gradient as well as voltage-clamping potentia l difference to zero significantly reduced initial cimetidine uptake. The d ata is supportive of both a cimetidine/H+ exchange mechanism and a driving- force contribution from an inside negative proton or cation diffusion poten tial.