Absorption of cyclosporine from rabbit small intestine in situ

The absorption of the immunosuppressive drug, cyclosporine, from intestinal segments in the anesthetized rabbit in situ is reported. The experimental technique allows serial blood sampling with simultaneous sampling of the intestinal perfusate. The method of Loo and Riegelman for estimating the systemically absorbed fraction of the drug was used in conjunction with the in situ method. Taking the length of the rabbit small intestine as 300 cm and using the apparent permeability of cyclosporine determined in polyethylene glycol 400 (PEG 400) at a bulk fluid flow rate of 0.27 mL/min, the anatomical reserve length for cyclosporine was calculated to be -302 cm. The studies demonstrate that a significant portion of the loss of cyclosporine from rabbit small intestine cannot be accounted for as systemically absorbed drug. Furthermore, the apparent permeability coefficient of cyclosporine in these studies is so low that the length of small intestine in the rabbit is not sufficient for complete absorption of the drug from the lumen under the conditions used.     

Drug absorption from the irradiated rat small intestine in situ.

The absorption of acidic drugs phenobarbitone and sulphafurazole, basic drugs mecamylamine and quinidine, and a neutral drug isoniazid was studied in situ. Rats were irradiated 750 rad whole-body with 60Co and the absorption experiment was done three and six days thereafter using the cannulated small intestine of urethane-anaesthetized rats. Drug disappearance from the intestinal lumen and drug levels in the whole blood and intestinal wall were measured. In control rats phenobarbitone showed the most rapid absorption and mecamylamine the slowest. Irradiation retarded the disappearance of all drugs from the intestinal lumen on the third postirradiation day. Fluid absorption was also diminished. On the sixth postirradiation day the absorption of phenobarbitone, sulphafurazole and mecamylamine had returned to the control level, but the absorption of quinidine and isoniazid was still retarded. After i.v. administration of drugs they were not significantly excreted into the intestinal contents and irradiation did not modify excretion. The distribution of drugs between the intestinal fluid and the intestinal wall was complete in the first 10 min of experiment. Mecamylamine and quinidine were lowered in the whole blood by irradiation. Blood levels of drugs did not correlate well to the rate of disappearance of drugs from the intestinal lumen. The reversible changes in absorption induced by irradiation are probably secondary effects of irradiation on intestinal morphology, permeability and transport capacity, composition, and possibly blood flow.     

Absorption and metabolism of acetaminophen by the in situ perfused rat small intestine preparation

The in situ perfused rat small intestine preparation was used to examine the extents of segmental absorption and metabolism of acetaminophen (A). Additionally, the preparation was employed to investigate any intestinal excretion of A and its conjugates from the circulation to the intestinal lumen. In this preparation, blood perfusate (300 ml) recirculated the intestinal preparation at 7.5 ml/min, entering via the superior mesenteric artery and returned to the reservoir via the portal vein. To demonstrate the extent of segmental absorption, metabolism, and excretion by different segments of the intestine, tracer doses of 3H-A (0.41 to 0.55 mumol in 0.3 ml of saline) were administered into the (a) entire intestine; (b) segments (first, second, and third) of one-third the length of the intestine, by instillation of the dose into the lumens of the segments; and (c) reservoir of perfusate. Exudates of luminal fluid from the injected segment and segments not exposed to drug were monitored for A and its conjugates during the experiment and at the end of 2 hr. Absorption of A was usually complete by 60 min; the extent of absorption of A at the end of 2 hr by the entire length of the intestine and by its three (first, second, and third) individual segments were 71.7 +/- 2.6, 50.5 +/- 4.0, 73.9 +/- 2.1, and 58.8 +/- 6.1% of dose (mean +/- SE), respectively. At the end of 2 hr, the total amount of acetaminophen glucuronide in perfusate and luminal fluid accounted for 3.1-5.5% and 0.14-0.1% of dose, respectively, among these preparations; acetaminophen sulfate was present only as a small percentage of dose in the lumen. Glucuronidation activity, when expressed as a percentage of the absorbed dose, was fairly constant for the entire intestine and first and second segments (8%) but decreased slightly for the third segment (7%). When A was present in blood perfusing the intestine, no metabolite was detected in perfusate or luminal fluid. Instead, unchanged A was excreted (5.6% dose) into the lumen. The effect of dose and vehicle on the extents of absorption and metabolism of A in the preparation was investigated by the instillation of different doses of A (0.16, 99.2, and 396.9 mumol in 0.3 ml of polyethene glycol 400) into the entire intestine at the duodenum.(ABSTRACT TRUNCATED AT 400 WORDS)     

The absorption of warfarin from the rat small intestine in situ.

The in situ absorption from the rat small intestine of the weakly acidic drug, warfarin (pKa 5-05), at 200 mug ml-1 in the instilled fluid with initial pH levels of 3, 5, 7 or 8 has been examined. These initial pH's in the buffer changed rapidly towards neutrality. The buffers at pH's 3 and 5 probably caused different amounts of warfarin precipitation, which resulted in different rates of warfarin disappearance from the instilled fluid which paralleled the initial rates of accumulation of warfarin in (or on) the intestinal wall. Where greater drug precipitation had probably occurred the initial rates of absorption into the plasma were slower. At the initial pH of 3 and by solubilization of warfarin with propylene glycol, the rate of absorption was similar to that from a fluid of pH 7. Propylene glycol in 15% solution did not affect the system significantly. The relatively high transfer of warfarin into octanol from buffer solution at pH 7 might indicate that the small fraction of unionized drug (1 : 100) at pH 7 is enough for remarkable transfer of this highly lipid-soluble drug.     

Absorption of folic acid from the small intestine of the rat

The absorption of folic acid from segments of the small intestine of the rat has been measured in situ with tritium-labelled folic acid. The fraction absorbed was independent of concentration below 10^-6 M but was depressed to half at 4×10^-5 M. Direct measurements of fluxes showed that the mucosal → serosal flux was about 14 times the serosal → mucosal flux, and therefore that uptake of folic acid is an active process. In the ileum but not in the duodeno-jejunum, absorption was depressed by the presence of electrolyte. There was little difference in absorptive capacity between jejunum and ileum nor was there any significant change in animals suffering from a dietary deficiency of folic acid.     

Drug absorption from in situ rat small intestine during metoclopramide administration.

The effect of metoclopramide on the absorption of drugs in solution in the small intestinal lumen of rats in situ was studied. Metoclopramide in doses up to 50 mg/kg sc did not significantly modify the disappearance of isoniazid and quinidine from the small intestinal lumen. At the end of the absorption experiments, quinidine in the whole blood of the experimental animals was increased after metoclopramide. The blood level did not correlate to the drug disappearance from the intestinal lumen. The results probably differ from those obtained when drugs are given orally to subjects treated with metoclopramide.     

Absorption and metabolic extraction of diltiazem from the perfused rat small intestine

The metabolic extraction of diltiazem was examined in conjunction with its absorption, using rat small intestine perfused in situ by the single-pass method, to clarify its intestinal metabolism. This is a topic of increasing interest which has not been fully clarified, particularly as far as the extent of metabolic extraction and the enzymes involved (cytochrome P450 (CYP) 3A and/or others) are concerned. The intestinal availability (Fi) of diltiazem was evaluated at steady-state by dividing the fraction absorbed into the mesenteric venous blood (Fa,b) by the fraction that disappeared from the intestinal lumen (Fa). The Fi of diltiazem (0.05 mM) was 0.126 and, hence, the extraction ratio (Ei=1-Fi) was 0.874, indicating that diltiazem undergoes extensive first-pass metabolism during its passage through the intestinal mucosa. The Ei was unchanged when the concentration was increased to 0.5 mM, suggesting that metabolism is linear over this concentration range. Thereafter, Ei decreased with concentration, demonstrating saturable metabolism, and reached an insignificant level at the highest concentrations of 30 and 50 mM. The decrease in Ei, or increase in Fi, was brought about by an increase in Fa,b (from about 0.02 to about 0.05) in the concentration range up to 10 mM and by a decrease in Fa (from about 0.15 to about 0.05) at concentrations higher than that. These results suggest that the extraction observed at the lower concentrations is almost solely attributable to metabolic extraction of a saturable nature. However, ketoconazole and cyclosporin A, which are specific CYP3A inhibitors, inhibited the metabolic extraction of diltiazem (0.05 mM) by only about 20% at the concentration (40 microM) at which they inhibited CYP3A almost completely, suggesting that the contribution of CYP3A to intestinal diltiazem metabolism is not marked. Thus, the present study demonstrates that diltiazem undergoes extensive first-pass metabolism in the rat small intestine, although the contribution of CYP3A seems to be relatively minor.     

T2毒素在原位灌流大鼠小肠中的吸收和代谢动力学

应用原位肠灌流标本研究了T2毒素在大鼠小肠中的吸收和代谢功力学,T2毒素对小肠标本进行血管灌流时消除半衰期为46.9 min.主要代谢产物HT2毒素的生成半衰期为12.9 min.当T2毒素由十二指肠注射后再进行灌流时,可同时发生吸收前和吸收后的小肠代谢转化,并主要以HT2,3’-OHHT2等代谢产物的形式吸收,此时毒素在小肠中的吸收和消除半衰期分别为33.0和25.6 min,实验结果表明肠道也是代谢消除T2毒素的主要器官,尤其在消化道中毒时,毒素将经历显著的肠道首过代谢.     

Absorption and metabolism of procaine by the rat small intestine

Summary The aim of this study was to obtain information about the absorption of procaine in the rat small intestine (Fisher-Parsons preparation). In the range from 0.25–10 mmol · l^–1 procaine in the luminal perfusate, much more of the unchanged drug was absorbed in segments of the ileum than of the duodenum and jejunum. Besides procaine, two metabolites, p-aminobenzoic acid (PABA) and acetylated p-aminobenzoic acid (AABA), formed in the intestinal mucosa, appeared in the absorbate. With increasing substrate concentration in the perfusate the PABA in the absorbate increased considerably in all three segments; from 0.75 mmol · l^–1 procaine upwards the PABA produced was highest in the jejunum. AABA formed in the mucosa and measured in the absorbate did not increase in the same manner with increasing substrate concentration; in the absorbate of jejunal segments the amount of AABA was significantly higher than in duodenal and ileal segments. Taking into account that in rats the microclimate of the ileum differs considerably from that of the upper part of the small intestine, the marked difference observed in the absorption of procaine between ileal segments on the one side, and duodenal and jejunal segments on the other, can be explained on the basis of the non-ionic diffusion theory.Send offprint requests to H. P. Büch at the above address     

Absorption and metabolic characteristics of p-aminobenzoic acid and its isomer, m-aminobenzoic acid, from the rat small intestine.

Absorption and metabolic characteristics of p-aminobenzoic acid (PABA) and m-aminobenzoic acid (MABA) from the rat small intestine were examined by means of in situ recirculation and in vitro everted sac experiments. p-Aminobenzoic acid was extremely rapidly absorbed from the rat small intestine, whereas the absorption of MABA, the m-isomer of PABA, was comparably slower. This finding was partly explained by the result that PABA is more lipophilic than MABA. The metabolite percentage of PABA was considerably greater than that of MABA in mucosal fluid, tissue, and serosal fluid. On the other hand, a concentration-dependent and a directional difference in the transfer rate of these drugs were observed in everted and noneverted sacs of rat small intestine. Furthermore, mucosal uptake of PABA or MABA was inhibited by 1 mM 2,4-dinitrophenol, 10 mM sodium azide, and pretreatment with HgCl2 (10 mM). These results indicate that MABA, as well as PABA, is transported through the intestine by a carrier-mediated transport system, and that the molecular structure of these drugs is important for their absorption and metabolic characteristics.     

Absorption of selenite in the rat small intestine: interactions with glutathione

Incubation of isolated rat intestinal epithelial cells with selenite resulted in a rapid decrease of intracellular glutathione (GSH) and about 5 nmol of Se (added as 75Se labelled selenite, 50 microM) was accumulated per 10(6) cells during 20 min. Addition of exogenous GSH enhanced cellular accumulation of Se, while the gamma-glutamyl transferase inhibitor, serine-borate, decreased uptake. The added exogenous GSH was rapidly consumed (oxidized) by selenite. Depletion of GSH by diethylmaleate (DEM) inhibited selenite uptake both in in situ isolated intestinal loop, everted gut sacs and in isolated intestinal cells. The findings indicate that both intracellular and extracellular GSH, and gamma-glutamyltransferase, play a role in the absorption of selenite in the intestine.     

First pass biotransformation of ethinylestradiol in rat small intestine in situ

Summary The intestinal absorption and biotransformation of ethinylestradiol and of ethinylestradiol glucuronide in rats were studied using a jejunal loop preparation in situ. Radioactivity associated with ethinylestradiol amost completely appeared in the venous outflow within 80 min. Forty two percent was present as unchanged compond, 56% in the glucuronide fraction and 2% as sulfate ester. Absorption of ethinylestradiol glucuronide was about 20 times slower than absorption of the nonconjugated molecule. The data indicate, that intestinal absorption of ethinylestradiol involves several superimposed kinetics and that glucuronidation in the gut may result in a decreased bioavailability of the compound.     

齐墩果酸磷酸酯二钠盐的小肠吸收动力学

目的研究齐墩果酸磷酸酯二钠盐(OANa2)大鼠小肠的吸收情况。方法采用大鼠在体、离体小肠回流实验装置,利用高效液相色谱法测定肠循环液中OANa2的含量,采用紫外分光光度法测定肠循环液中酚红的含量。结果在体小肠胆管结扎与不结扎的药物吸收百分率(PA)分别为43.4%、43.7%。实验浓度下,离体小肠上、下段的吸收速率常数(Ka)分别为0.318 h、0.300 h。此外,离体小肠实验在3种不同浓度下的吸收速率常数分别为0.270 h、0.322 h、0.284 h。胆管结扎与不结扎的药物吸收百分率无显著性差异(P>0.05);OANa2在小肠不同肠段的吸收速率常数无显著差异(P>0.05)。结论在实验浓度范围内,OANa2的小肠吸收机制属于被动扩散。     

Absorption of methylmercury compounds from rat intestine

Intestinal absorption of methylmercury complexed with non-protein sulfhydryl compounds (NPSHs) as occurs in bile was studied by means of direct injection of mercury compounds into ligated intestinal segments of rats. The extent of absorption of methylmercury-cysteinylglycine (MM-CysGly) was similar to that of methylmercury-cysteine (MM-Cys) and 1.5 times larger than that of methylmercury-glutathione (MM-GSH). This result suggested that MM-CysGly, which is recognized as a major component of methylmercury in rat bile, can be easily reabsorbed from the intestine. These results indicate that not only MM-GSH and MM-Cys but also MM-CysGly may play important roles in the intestinal reabsorption of methylmercury during its enterohepatic circulation.

When the ligated intestine was pretreated with probenecid and acivicin, the intestinal absorption of MM-GSH was depressed much more than in the case of treatment with acivicin alone. This indicates the possibility that there are at least two systems for intestinal transport of MM-GSH i.e. γ-glutamyltrans-peptidase (GGT)-dependent and -independent systems.     

Absorption of benzoic acid in segmental regions of the vascularly perfused rat small intestine preparation.

Oral bioavailability is a consequence of intestinal absorption, exsorption, and metabolism and is further modulated by the difference in activities among segmental regions. The influence of these factors on the net absorption of benzoic acid (BA), a substrate that is metabolized to hippurate and is transported by the monocarboxylic acid transporter 1, was studied in the recirculating, vascularly perfused, rat small intestine preparation. Metabolism of BA was not observed for both systemic and intraluminal injections into segments of varying lengths. But, secretion of BA into lumen was noted. Absorption of BA (0.166-3.68 micromol) introduced at the duodenal end for absorption by the entire intestine was complete (>95% dose at 2 h) and dose-independent, yielding similar absorption rate constants (k(a) of 0.0464 min(-1)). The extent of absorption remained high (92-96% dose) when BA was injected into closed segments of shorter lengths (12 or 20 cm), suggesting a large reserve length of the rat intestine. However, k(a) was higher for the jejunum (0.0519 and 0.0564 min(-1), respectively, for the 12- and 20-cm segments) and exceeded that for the duodenum (12-cm segment, 0.0442 min(-1)) and ileum (20-cm segment, 0.0380 min(-1)) at closed injection sites. The finding paralleled the distribution of monocarboxylic acid transporter isoform 1 detected by Western blotting along the length of the small intestine. Fits of the systemic and oral data (based on duodenal injection for absorption by the whole intestine) to the traditional, physiological model and to the segregated flow model (SFM) that describes partial intestinal flow to the enterocyte region showed a better fit with the SFM even though metabolite data were absent.     

运用单向灌流模型研究抗糖尿病创新药物西格列松大鼠在体肠的吸收

目的研究西格列松大鼠在体各肠段的吸收情况。方法运用单向灌流模型、采用高效液相色谱法对药物的质量浓度进行检测来研究药物在不同质量浓度、不同pH值下的吸收。结果西格列松在各肠段的表观吸收系数有显著性差异(P<0.05);不同质量浓度药物的表观吸收系数无显著性差异(P>0.05);不同pH值条件下药物的表观吸收系数有显著性差异(P<0.05)。结论西格列松的吸收不受药物质量浓度的影响,药物在较大质量浓度下(100 mg.L-1)也无饱和现象,在肠黏膜的转运为被动扩散过程;药物吸收有pH值依赖性,pH值越低吸收越好。     

Absorption of iron from iron succinyl-protein complexes by mouse small intestine.

The absorption of iron from iron succinyl-protein complexes was investigated in mice. 59Fe-labelled succinyl-casein and -albumin complexes, [59Fe]ferritin and 59FeSO4, at doses of 20 or 200 micrograms of iron, were administered orally to normal mice or mice with absorption enhanced by chronic hypoxia. 59Fe from iron succinyl-protein was well absorbed in normal mice (greater than 10% of dose) and showed enhanced absorption in hypoxic mice (greater than 40% of dose). Intestinal uptake was predominantly by the duodenum for all compounds studied. In-vivo absorption of 59Fe from an iron succinyl-protein complex was studied using tied-off segments of mouse duodenum, jejunum or ileum of normal or hypoxic mice. Incubation for up to 15 min in duodenum or 60 min in ileum showed very little absorption of 59Fe. No enhancement of absorption was seen in hypoxic mice. It was concluded that absorption of the intact iron succinyl-protein complex cannot explain absorption seen after oral dosing.     

Absorption and presystemic glucuronidation of 1-naphthol in the vascularly fluorocarbon emulsion perfused rat small intestine

Summary Using the isolated vascularly fluorocarbon emulsion perfused rat small intestine some factors which determine the extent of the intestinal glucuronidation of 1-naphthol to 1-naphthol--d-glucuronide were studied. Increasing the luminal 1-naphthol concentration resulted in a concomitant increase in the 1-naphthol appearance in the vascular perfusate. In contrast, the total appearance of 1-naphthol--d-glucuronide increased less than proportional to the increase in the luminal 1-naphthol concentration. About 88% of the total amount of 1-naphthol--d-glucuronide excreted was released into the vascular perfusate. The capacity-limited intestinal glucuronide efflux is most likely due to saturation of the excretory mechanism for 1-naphthol--d-glucuronide. Decreasing the vascular flow rate influenced both the appearance of 1-naphthol and 1-naphtol--d-glucuronide in the vascular perfusate, whereas the appearance of 1-naphthol--d-glucuronide in the luminal perfusate was essentially flow-independent. A noradrenaline-induced change in the haemodynamic state of the vascular bed (with the total flow kept constant) resulted in a marked decrease in the 1-naphthol vascular concentration. The vascular 1-naphthol--d-glucuronide concentration was only slightly affected. These results indicate that changes in blood flow and blood flow distribution within the intestinal wall can affect the extent of presystemic intestinal metabolism by interfering with the absorption of the parent compound and the efflux of formed conjugates. These parameters can be of paramount importance for causing variable intestinal first-pass effects of drugs in vivo. Send offprint requests to M. H. de Vries at the above address