Role of unstirred water layer in the exsorption of quinidine

Effect of Theophylline on the Intestinal Clearance of Drugs in Rats

Intestinal eχsorption of salicylic acid, urea and quinidine was measured during the perfusion of the rat intestinal lumen with Tyrode solution. The intestinal clearance (CLi) of the three compounds was measured by dividing the rate of appearance in the intestinal luminal perfusate by the plasma concentration of the compound. Co-administration of theophylline (0 −2 mg h−1) with the test agents increased the CLi of salicylic acid, did not alter the CLi of urea, but decreased the CLi of quinidine. The effect of theophylline on the CLi of quinidine was enhanced with increasing dose. Theophylline was found to increase microclimate-pH at the intestinal surface, but the magnitude of ΔpH alone could not explain the effect of theophylline on the CLi of quinidine. The data, together with previous observations, suggest that the intestinal eχsorption of drugs was affected by the microclimate pH and by the unstirred water layer. Theophylline affects CLi of salicylic acid and quinidine partly by increasing the microclimate pH of the intestine. Theophylline may also affect quinidine CLi by inhibiting the carrier-mediated pathway.

Concentration-dependent Exsorption of Quinidine in the Rat Intestine

During intravenous infusion, the luminal concentration of quinidine was higher than the plasma concentration. The intestinal clearance (CLi) of the drug was measured by dividing the rate of appearance of the drug in the intestinal luminal perfusate by the plasma concentration. The CLi of quinidine was therefore much higher than the rate of luminal perfusion. Over the infusion dose range of 0·1–2 mg h−1, the CLi of quinidine decreased with increasing plasma concentration of quinidine. Adding quinidine into the luminal perfusate had little effect on the CLi of quinidine. Co-administration of quinidine with other agents intravenously did not alter the CLi of salicylic acid and urea, while the same treatment decreased the CLi of theophylline and 5-disopyramide. In-vitro experiments on brush-border membrane vesicles showed that quinidine decreased the rate of Na+ uptake and H+ efflux. The inhibition was significant at quinidine concentrations above 20 μm. Quinidine was a more potent inhibitor than amiloride. At quinidine infusion rates less than 2 mg h−1, quinidine concentration in plasma or in the luminal perfusate was at the lower limit of the inhibitory concentration. Microclimate pH at the intestinal surface was also measured. At mid-jejunum, the microclimate pH increased 0·3 pH units by infusing 2 mg h−1 of quinidine, while the microclimate pH at most other measuring sites was not significantly altered by quinidine infusion. It was concluded that quinidine is exsorbed from blood into the intestinal lumen by a carrier-mediated pathway in addition to the passive diffusion. At high plasma concentration, quinidine exsorption becomes saturated. Quinidine inhibited the intestinal exsorption of theophylline and S-disopyramide possibly by competition on the carrier.

The Effect of Dinoprost on Transport of Water and Imipramine Through Rat Small Intestinal Membranes

The relation between transmucosal fluid movement and its effect on absorption and exsorption of imipramine was studied with the in-situ single-pass perfusion technique in rats. Dinoprost (prostaglandin F2α, PGF2α) caused a dose-related inhibition of both absorption and secretion of water across the intestinal membrane. When PGF2α was infused at a rate of 5 μmol kg−1 h−1, the absorption rate of water decreased from 51·7 to 21·5 mL h−1 and the secretion rate decreased from 48·9 to 26·8 mL h−1. Net water flux changed from net water absorption (0·9 mL h−1)tonet water secretion (5·33 mL h−1) by infusion of PGF2α. However, absorption and exsorption of imipramine were little affected by infusion of PGF2α. The absorption rates of imipramine were 3·03 and 2·36 mg h−1 in the absence and presence of PGF2α, respectively. Furthermore, the average amounts of imipramine exsorbed into the intestinal lumen in 2 h were 7·82 and 8·10% in the absence and presence of PGF2α, respectively. Infusion of PGF2α, also enhanced motility of the small intestine compared with the control. From these results, it appears that PGF2α has no effect on the absorption and exsorption of imipramine across the intestinal membrane although it is reasonable to use PGF2α in the case of patients with overdoses of drugs which decrease gastrointestinal motility.

Experimental demonstration of the unstirred water layer effect on drug transport in Caco-2 cells

We previously demonstrated that P-glycoprotein and MRP2 contribute to the secretory transport of grepafloxacin in the small intestine. Although inhibitors of these secretory transporters increased absorptive transport of grepafloxacin, secretory transport was not altered in Caco-2 cells, as determined by a conventional Transwell method. Because the value of the permeability coefficient of grepafloxacin is high, permeation through the unstirred water layer (UL) might be the rate-limiting step. To examine the possibility that the UL effect may mask the involvement of membrane transporters in the transport of drug with high permeability in Caco-2 cells, transport experiments were performed by agitating the experimental solution to decrease the thickness of the UL, and by lowering the temperature to decrease permeation via active transporters. Under these conditions, the UL effect was not rate limiting, and the inhibitory effects of transporter modulators were reflected in the apparent permeability as a decrease in secretory transport as well as an increase in absorptive transport. In conclusion, it was demonstrated that the UL can be the rate-limiting factor for transport of drugs with high membrane permeability in Caco-2 cells. When the UL affects the apparent permeability in an experimental apparatus in vitro, careful analysis is required to evaluate the contributions of transporters from the apparent permeability of drugs.

Drug exsorption from blood into the gastrointestinal tract

Drugs are exsorbed from the blood across the gastrointestinal membranes by passive or active processes. In the case of a passive transport mechanism, the exsorption of drugs depends on the concentration gradients between the serosal and mucosal sides. The extent of secretion (exsorption) is determined by numerous factors such as extent of binding to serum proteins, distribution volume, lipophilicity, pKa and molecular size of drugs, and the blood flow rate in the gut. Specific transport systems such as P-glycoprotein (P-gp), organic cation and organic anion transporters are found to be involved in active intestinal secretion of drugs. Intestinal secretory transport systems reduce the extent of drug absorption sometimes resulting in low oral bioavailability. It is, therefore, important to know whether poor drug absorption is due to the involvement of specialized secretory transport systems. Modulation of intestinal secretory transport can be a means to enhance absorption of drugs with low oral bioavailability if exsorption of drugs is based on active secretion pathways that are open for control from the "outside".

Pectin-supplemented enteral diet reduces the severity of methotrexate induced enterocolitis in rats

BACKGROUND

Administration of methotrexate (MTX) to rats fed an elemental diet results in a high mortality from severe enterocolitis. Previous studies have shown that pectin is an important precursor of substrates for intestinal structure and function and may facilitate intestinal recovery after enterocolitis. The aim of this study is to evaluate the effect of pectin on MTX-induced enterocolitis in rats.

METHODS

Rats received intragastric infusion of either 1% pectin-supplemented or pectin-free elemental diet from the beginning of the study via a gastrostomy. On the 4th day animals received either MTX, 20 mg/kg intraperitoneally, or saline injection and were killed on the 7th day for sampling.

RESULTS

Pectin supplementation significantly decreased body weight loss, organ water content, and intestinal myeloperoxidase levels and increased mucosal protein, DNA, and RNA content in enterocolitis rats. The intestinal permeability was increased by administration of MTX, and pectin supplementation significantly reversed the increased permeability in the distal small bowel and colon. Pectin supplementation also lowered the magnitude of bacterial translocation, decreased plasma endotoxin levels, and restored bowel microecology.

CONCLUSIONS

Pectin significantly decreased MTX-induced intestinal injury and improved bowel integrity.

Effect of activated charcoal and atropine on absorption and/or exsorption of organophosphorus compounds in rats

Effects of activated charcoal and atropine for the removal of organophosphorus compounds, which remain in the gastrointestinal tract or have already been absorbed into the systemic circulation, were investigated in rats. Activated charcoal extensively adsorbed the organophosphates fenitrothion, tolelofos methyl, piperophos and salithion, and its immediate administration after oral ingestion of fenitrothion remarkably reduced serum fenitrothion levels, but had no effect on the serum levels of the compound which had been absorbed from the gastrointestinal tract. Thus, all of the organophosphorus compounds were poorly exsorbed (0.002-0.39% of the dose in 120 min) from the blood into the intestinal lumen probably due to their extensive protein binding and large distribution volumes. Atropine inhibited absorption of fenitrothion in the perfusion in-situ and also delayed the absorption of the compound in-vivo, but had no significant effect on exsorption of fenitrothion. The serum fenitrothion levels on treatment with both atropine and charcoal significantly decreased compared with those of the control. We conclude that, oral activated charcoal will not be able to enhance the elimination of organophosphorus compounds which have already been absorbed into the systemic circulation, but constituted useful method for the removal of the compounds remaining in the gastrointestinal tract because of its excellent adsorptive capacity.

Inhibition of intestinal P-glycoprotein and effects on etoposide absorption

P-glycoprotein (Pgp) actively pumps a number of antineoplastic drugs, such as etoposide, out of cancer cells and causes multidrug resistance. Pgp is also expressed at the brush-border membrane of the small intestine under normal physiological conditions. We hypothesized that inhibition of intestinal Pgp might decrease the efflux of etoposide from the blood into the intestinal lumen, thereby, increasing the bioavailability of etoposide. The absorption of etoposide was studied using everted gut sacs prepared from rat jejunum and ileum. The addition of C219, a monoclonal antibody of Pgp, at 100 ng/ml or of 0.2 M 5'-adenylylimidodiphosphate, a nonhydrolyzable adenosine triphosphate (ATP) analog, increased the absorption of etoposide. Quinidine, an antiarrythmic agent, has been demonstrated to circumvent multidrug resistance in cell lines, possibly by interfering with Pgp function. Adding quinidine at 1 mg/ml to the everted gut sac increased the absorption of etoposide. In vivo absorption of etoposide was also studied by intraluminal perfusion of the drug in the small intestine of anesthetized rats. Intravenous infusion of quinidine at either 1 or 2 mg/h increased the serum level of etoposide in a dose-dependent manner. Intravenous infusion of etoposide at 0.2 mg/h resulted in luminal exsorption of the drug in the small intestine. The intestinal clearance of etoposide was 41.7 +/- 7.2 ml kg-1, which decreased to 18.4 +/- 3.9 ml kg-1 with the infusion of quinidine at 1 mg/h. The present data confirm that intestinal Pgp mediates the efflux of etoposide and that the use of Pgp-inhibiting agents such as quinidine may increase the bioavailability of etoposide.