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

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 (

Small-intestinal absorption of cadmium and the significance of mucosal metallothionein.

1 Although food intake is among the most important routes of Cd exposure, not many details are known about the intestinal absorption mechanisms of Cd. In this respect Cd is representative of most other nonessential, merely toxic metals. 2 Based on a concept of two distinguishable steps, intestinal absorption of Cd is characterized by high accumulation within the intestinal mucosa and a low rate of diffusive transfer into the organism. 3 After uptake into the mammalian organism, Cd is sequestered into hepatic metallothionein (MT). It is assumed that hepatic Cd-MT then gradually redistributes Cd to the kidney, which is the main target organ for chronic Cd toxicity. 4 When feeding low levels of dietary CdCl2, however, Cd accumulates preferentially in the kidney and to a lesser degree in the liver, a distribution pattern also found after intravenous and peroral administration of the Cd-MT complex itself. As dietary Cd induces intestinal MT, intestinal Cd-MT complexes could be at least partly responsible for the renal accumulation of dietary Cd. 5 For this mechanism, however, serosal release of mucosal Cd-MT is required. In fact, in vitro findings in rats reveal a concentration-dependent release of intestinal MT to the serosal side of the small intestine. These results indicate that endogenous intestinal MT may deliver Cd-MT to other inner organs, thus contributing to the preferential renal accumulation of ingested Cd.

Small-intestinal transfer mechanism of prunasin, the primary metabolite of the cyanogenic glycoside amygdalin.

1. The small-intestinal transfer of prunasin (D-mandelo-nitrile-beta-D-glucoside), the primary metabolite of amygdalin which is not absorbed in the small intestine as such, was studied in rat jejunum and ileum in vitro. 2. As shown by high pressure liquid chromatography, prunasin is transferred essentially intact across the intestinal wall, without cleavage of the glycosidic bond and thus no formation of benzaldehyde or cyanide during the mucosal passage. 3. Only the jejunal transfer of prunasin followed saturation kinetics (vmax = 1.6 mumol cm-1 min-1; KT = 460 mumol l-1) and exhibited a clearsodium-ion dependence. As indicated by the temperature dependence, only the jejunal mucosa-to-serosa transfer and the corresponding tissue uptake of prunasin required apparently high activation energies. Transfer in the terminal ileum showed diffusion characteristics. 4. Jejunal methyl alpha-D-glucoside transfer was inhibited by the presence of prunasin. Furthermore, the tissue uptake of methyl alpha-D-glucoside in rat jejunum was competitively inhibited by prunasin. 5. The results indicate that prunasin is absorbed unmetabolised in the jejunum of the rat via the transport system of glucose.

Intestinal first pass metabolism of amygdalin in the rat in vitro.

The intestinal first pass metabolism of amygdalin has been investigated in rat small intestine in vitro. The results show that amygdalin is hydrolyzed to prunasin, essentially in the wall of the proximal jejunum. This specific beta(1-6)hydrolytic cleavage of the terminal glucose residue is pH-dependent and can be inhibited by glucono-delta-lactone, a potent inhibitor of the lysosomal beta-glucosidase of the rat intestine. No substrate competition between phloridzin and lactose vs amygdalin was noted. None of the more common soluble beta- or alpha-enzymatic activities of mammalian intestine (alpha-glucosidase, alpha-amylase) or mammalian liver (beta-galactosidase, beta-glucuronidase) were capable of catalyzing the hydrolysis of the terminal glucose from amygdalin at pH's 5.0, 7.0 or 9.0. Furthermore, no metabolic activity of isolated rat livers toward amygdalin and prunasin was observed within two hours of recirculating perfusion. However, cecal contents of conventional rats, exhibited both amygdalin- and prunasin-hydrolyzing activities. The resulting mandelonitrile dissociates spontaneously into cyanide and benzaldehyde. Therefore, our findings indicate that metabolism of amygdalin to prunasin occurring in the proximal part of jejunum is apparently mediated by enzymatic beta(1-6)glucosidase activity of the gut wall. In contrast, the toxicity of amygdalin due to the release of cyanide obviously requires microbiological activities of the gut flora.

An all-glass perfusator for investigation of the intestinal transport and metabolism of foreign compounds in vitro.

The in vitro perfusion technique of surviving intestinal segments as described by Fisher and Parsons (1949) and modified by Rummel and Stupp (1960) was further improved by the introduction of an all-glass perfusator for studying the intestinal transport and metabolism of lipophilic xenobiotics.

Bidirectional transfer and tissue accumulation of folic acid by rat intestine in vitro.

The transfer and tissue content of 3H-pteroylmonoglutamate (PteGlu) from the mucosal to the serosal side (Jms, TCm) and in the reverse direction (Jsm, TCs) was studied using the everted sac technique. In the entire intestine, except for the colon, 3H-PteGlu was transferred preferentially into the serosal solution. When 3H-PteGlu was applied to the serosal side the final tissue concentration in either jejunal, duodenal, ileal or colonic segments was not significantly different from each other and about two-fold the serosal concentration. Apparently there exists a specific transfer process from the mucosal to the serosal side in the jejunum. The transfer of 3H-PteGlu shows saturation kinetics (S0.5 = 4.9 X 10(-5) mol/l). At low concentration (2 nmol/l) 3H-PteGlu was accumulated within the mucosal epithelium (tissue/mucosal fluid ratio = 3.8). Transfer and accumulation in the mucosal tissue of 3H-PteGlu apparently need high activation energy as indicated by the temperature dependency of these processes. Finally, transfer and accumulation in the tissue of 3H-PteGlu could be inhibited by salazosulfapyridine and phenobarbital.

Effect of lidocaine on in vitro absorption of glucose in jejunal and ileal segments of the rat.

The influence of lidocaine on the intestinal glucose absorption was investigated on Tyrode-perfused isolated intestinal segments of the rat. In ileal segments the transmural transfer of glucose decreased with increasing lidocaine concentration (5 X 10(-9) -2 X 10(-4) mol/l), whereas the water movement was unaffected. No inhibitory effect on glucose and water absorption was observed in the proximal jejunum.