Histological reevaluation of everted gut technique for studying intestinal absorption

Hyaluronic Acid in the Intestinal Tract: Influence of Structure, Rheology, and Mucoadhesion on the Intestinal Uptake in Rats

Oral hyaluronic acid (HA) is a ubiquitous biopolymer that has gained attention as a treatment for local or systemic diseases. Here, we prepared and characterized structures of free HA (f-HA) with a high (>10⁵ Da), intermediate (≤10⁵ Da), and low (≤10⁴ Da) average molar mass (MM); nanoparticles crosslinked with adipic dihydrazide (n-HA); and mixed formulations (mixed-HA) containing f-HA and n-HA. MM distribution determined the structure, hydrodynamic diameter, and zeta potential of the f-HAs. Crosslinking changed the physicochemical properties in n-HA. In vitro tack adhesion assays, using mucin tablets or a viable rat intestinal mucosa, showed better mucoadhesion with f-HA (intermediate MM) and mixed-HA (25% n-HA), especially in the jejunum segment. High MM f-HA presented negligible mucoadhesion. n-HA showed the deepest diffusion into the porous of the membranes. In vivo results showed that, except for high MM f-HA, there is an inverse relationship between rheological changes in the intestinal membrane macerates resulting from mucoadhesion and the effective intestinal permeability that led to blood clearance of the structures. We conclude that the n-HA formulations are promising for targeting other tissues, while formulations of f-HA (intermediate MM) and mixed-HA are better for treating dysbiosis.

Comparison of conventional and advanced in vitro models in the toxicity testing of nanoparticles

Humans are exposed to a wide variety of nanoparticles (NPs) present in the environment, in consumer, health and medical products, and in food. Conventional cytotoxicity testing compared to animal testing is less expensive, faster and avoids ethical problems at the expense of a lower predictive value. New cellular models and exposure conditions have been developed to overcome the limitations of conventional cell culture and obtain more predictive data. The use of three-dimensional culture, co-culture and inclusion of mechanical stimulation can provide physiologically more relevant culture conditions. These systems are particularly relevant for oral, respiratory and intravenous exposure to NPs and it may be assumed that physiologically relevant application of the NPs can improve the predictive value of in vitro testing. Various groups have used advanced culture and exposure systems, but few direct comparisons between data from conventional cultures and from advanced systems exist. In silico models may present another option to predict human health risk by NPs without using animal studies. In the absence of validation, the question whether these alternative models provide more predictive data than conventional testing remains elusive.

In vitro drug absorption models. I. Brush border membrane vesicles, isolated mucosal cells and everted intestinal rings: characterization and salicylate accumulation

Brush border membrane vesicles, isolated mucosal cells and everted rings from rat intestine were compared for their suitability for drug uptake studies. Vesicles from brush border membranes were judged to be metabolically and morphologically functional on the basis of biochemical and microscopic criteria. With the use of a collagenase-vascular-perfusion method, populations of villus, mid villus and crypt cells were separated. An alternative approach that is based on an EDTA-dissociation procedure afforded fractions enriched in villus and crypt cells. Although several enzymatic and metabolic activities of these two cell preparations were comparable, cell viability based on the Trypan Blue dye exclusion test, ultrastructural appearance and glucose uptake more closely conformed to in vivo values for cells isolated according to the EDTA-dissociation method. These cells were chosen as a model for drug transport investigation. The morphological and functional integrity of everted rings was verified by histological examination, extracellular space estimation and assessment of glucose transport ability. Sodium salicylate uptake studies using brush border membrane vesicles and isolated mucosal cells were highly variable, whereas everted segments exhibited good reproducibility in uptake experiments. Time dependence of salicylate uptake was demonstrated with membrane vesicles and everted rings. Time dependence was not observed in mucosal cell uptake studies, probably because of the time required to separate the cells from the incubation solution. Based on ease of preparation, technical aspects of in vitro incubation and reproducibility of results, everted intestinal rings were considered to be a good potential model for in vivo drug absorption. Brush border membrane vesicles were generally regarded as unacceptable because of variations after storage and between experiments. Isolated cells offered certain advantages, but the utility of cells as an in vitro model remains equivocal.

In vitro and ex vivo intestinal tissue models to measure mucoadhesion of poly (methacrylate) and N-trimethylated chitosan polymers

PURPOSE

The adhesion of a range of polymers based on poly(2-(dimethylamino-ethyl) methacrylate (pDMAEMA) was assessed using human mucus-secreting and non mucus-secreting intestinal cell monolayers, HT29-MTX-E12 (E12) and HT29 monolayers, as well as excised non-everted intestinal sacs from rats. Differentiation of mucoadhesion from bioadhesion was achieved by pre-treatment with the mucolytic agent, N-acetyl cysteine (NAC). Adherence of pDMAEMA polymers was compared to that obtained with the mucoadhesive, N-trimethylated chitosan (TMC).

METHODS

The quantity of adherent coumarin 343-conjugated polymers to HT29, E12, and intestinal sacs was measured by fluorescence. Confocal laser scanning microscopy (CLSM), light microscopy, and fluorescent microscopy were used to provide direct evidence. Measurements of transepithelial electrical resistance (TEER), permeability to FITC-dextran 4000 (FD-4), and the release of lactate dehydrogenase (LDH) were used to assess potential cytotoxicity of polymers.

RESULTS

Adherence of unquaternized and of 10%, 24%, and 32% methyl iodide-quaternized pDMAEMA polymers was measured in E12, HT29, and sacs. All pDMAEMA polymers showed significantly higher levels of adhesion to mucus (mucoadhesion) than to epithelium (bioadhesion). Colocalization of pDMAEMA with mucus was confirmed in E12 by microscopy. TMC showed equally high levels of mucoadhesion as unquaternized and 24% quaternized pDMAEMA, but displayed higher levels of bioadhesion. pDMAEMA-based polymers demonstrated lower levels of adherence to E12 and rat sacs in the presence of NAC, whereas adherence of TMC was unchanged. pDMAEMA significantly decreased the permeability of FD-4 across E12 monolayers and sacs and was less cytotoxic in E12 than in HT29. In contrast, TMC increased the permeability of FD-4 across E12 and sacs and was less cytotoxic in E12 than in HT29.

CONCLUSIONS

Human mucus-producing E12 monolayers can be used to assess polymer mucoadhesion and give similar data to isolated rat intestinal sacs. pDMAEMA displayed similar levels of mucoadhesion and lower levels of bioadhesion than a chitosan derivative and it was not cytotoxic. pDMAEMA decreased FD-4 flux in the presence of mucus, whereas TMC increased it. The combination of mucus and methacrylate polymers appears to increase barrier function of the apical membrane.

Kinetic analysis of hexose transport to determine the mechanism of amygdalin and prunasin absorption in the intestine

Evidence is accumulating that glucose-conjugated compounds may be carried across the gut mucosa via the epithelial sodium-dependent monosaccharide transporter SGLT1. A modification of the everted intestinal sac technique was utilized to study the transport of the cyanogenic glycoside amygdalin (D-mandelonitrile beta-D-gentiobioside) and its metabolite D-mandelontrile beta-D-glucoside (prunasin). Everted sacs of rat jejunum and ileum were bathed in isotonic oxygenated sodium chloride-potassium phosphate buffer containing 2.8 microCi D-[(3)H]-mannose and 0.187 microCi D-[(14)C]-glucose. For treatment groups, buffers contained phloridzin, galactose, amygdalin or prunasin. The rate constant (k) for the transport process was calculated. Compared with the control (n = 33), phloridzin (n = 25) significantly reduced the rate constants of both D-[(14)C]-glucose and D-[(3)H]-mannose. Substitution of sodium with choline and incremental galactose treatments similarly reduced D-[(14)C]-glucose influx, indicating that a fraction of the transport is carrier-mediated. Treatment with amygdalin did not significantly affect the rate constants of D-[(14)C]-glucose or D-[(3)H]-mannose transport. However, treatment with 1 mM prunasin (n = 16) did reduce the influx of D-[(14)C]-glucose without affecting D-[(3)H]-mannose values. This is consistent with the reports finding that glycoside absorption may be mediated by SGLT1.

The roles of P-glycoprotein and intracellular metabolism in the intestinal absorption of methadone: in vitro studies using the rat everted intestinal sac

Methadone is used as a treatment for opiate detoxification in methadone maintenance programs. Intra- and inter-patient variations in methadone bioavailability have been observed after oral methadone treatment and this makes it difficult to predict a dosing regimen. Intestinal absorption and metabolism could explain these variations. The in vitro gut sac model was used to study the intestinal absorption of methadone, and it confirmed that methadone is a substrate for P-glycoprotein. The transport of methadone was increased in presence of P-gp inhibitors verapamil and quinidine. The appearance of a major metabolite of methadone, 2-ethylidene-1, 5-dimethyl-3, 3-diphenyl pyrrolidine (EDDP) in the gut sac contents also demonstrated the existence of intestinal metabolism of methadone.

Gastrointestinal absorption of drugs: methods and studies

Physico-chemical descriptors of drug molecules are often not adequate in predicting their oral bioavailability. In vitro methods can be useful in evaluating some of the different factors contributing to bioavailability. While physical parameters such as drug solubility may effect oral bioavailability, in most cases, the major determining factors are likely to be metabolism, and absorption at the intestinal level. Metabolism may be preabsorptive, as with peptides, or during absorption, particularly as a result of the activity of the intracellular enzyme CYP3A4. Absorption may be transcellular (membrane diffusion, carrier-mediated, endocytosis) or paracellular, while p-glycoprotein activity in the apical cell membrane may limit bioavailability by expelling drugs from the mucosal cells. Knowledge of the absorption mechanism is important in determining formulation strategies. The different in vitro techniques used to study absorption have advantages and disadvantages. Ussing chambers can be useful to measure bidirectional transport, but most studies use simple salt media, and full tissue viability is doubtful. Caco-2 cell monolayers are human cells, but the system is static, and gives very low rates of transport, and exagerated enhancement of the paracellular route compared with small intestine. The rat everted gut sac incubated in tissue culture medium maintains tissue viability and gives reliable data, although it is a closed system. In situ perfusion gives no information on events at the cellular level, and absorption may be reduced by anaesthesia and surgical manipulation. In vivo perfusion in man, with multichannel tubes, gives valuable data, but is not practical for screening. Pharmacokinetic modelling can also give useful data such as the existence of different absorption sites. Permeability values from the literature show that for small hydrophilic molecules, which pass by the paracellular route, the improved everted sac gives values close to those for humans, while values with Caco-2 cells are orders of magnitude lower.

An improved everted gut sac as a simple and accurate technique to measure paracellular transport across the small intestine

An improved everted gut sac system has been developed in which the sacs were carefully prepared from rat small intestine and incubated in tissue culture medium. Under these conditions, the tissue showed good morphology at the electron microscope level, and was metabolically active for up to 2 h at 37 degrees C. Mannitol, an established probe of paracellular transport, was transported from the mucosal to the serosal side of the sac tissue. Excellent kinetic data showed that transport was linear up to 75 min and over a wide range of concentrations (0.025 - (10 mM). Mannitol was not detected in the tissue and transport was enhanced by EGTA, confirming the paracellular route of passage. Sacs prepared from colon also showed mannitol transport, but at a slower rate. Comparisons with Caco-2 cell monolayers showed that the everted sacs exhibited higher levels of paracellular transport than the cultured cell line. The improved everted gut sac system is an inexpensive and relatively simple technique with considerable potential as an in vitro tool to study the mechanisms, kinetics and enhancement of drug absorption across the small intestine at different sites and in the colon.

Reduced intestinal absorption of arginine during sepsis

OBJECTIVE

To investigate the effect of sepsis on the intestinal absorption of arginine.

DESIGN

Controlled, nonintervention study.

SETTING

Surgical research laboratories of Sinai Hospital of Baltimore.

SUBJECTS

Male Sprague-Dawley rats.

INTERVENTIONS

Experimental sepsis induced by cecal ligation and puncture or intraperitoneal injection of lipopolysaccharide.

MEASUREMENTS AND MAIN RESULTS

Sepsis assessed by peritoneal and blood cultures. Intestinal absorption estimated by measuring the transfer of 3H-arginine by everted jejunal sacs prepared from septic and control animals (n = 6 per group) at multiple time points after the induction of sepsis (6, 12, 24, 48, and 72 hrs after cecal ligation and puncture; 6 and 12 hrs after intraperitoneal injection of lipopolysaccharide). Induction of peritonitis in the rat by cecal ligation and puncture significantly reduced the in vitro uptake of arginine by everted jejunal sacs at 12, 24, and 48 hrs after laparotomy. Arginine transfer by everted jejunal sacs was also significantly reduced in rats as early as 6 hrs after intraperitoneal injection of endotoxin (endotoxin 273 +/- 14; saline 377 +/- 14 nmol/sac/hr). Data are expressed as mean +/- SEM. Recovery from sepsis was associated with normalization of arginine transfer by intestinal sacs.

CONCLUSIONS

Experimental sepsis, induced by either cecal ligation and puncture or intraperitoneal injection of lipopolysaccharide, resulted in impaired intestinal amino acid uptake. Impaired intestinal arginine absorption may explain the lack of benefit of enteral, compared with parenteral, arginine therapy on survival from a septic insult.

Radiolabeling of methylphosphonate and phosphorothioate oligonucleotides and evaluation of their transport in everted rat jejunum sacs

PURPOSE

The therapeutic use of antisense oligonucleotides will likely involve their administration over protracted periods of time. The oral route of drug dosing offers many advantages over other possible routes when chronic drug administration is necessary. However, little is known about the potential for oligonucleotide uptake from the gastrointestinal tract. This issue is addressed in the current work.

METHODS

We have developed a simple procedure for radiolabeling oligonucleotides by reductive alkylation with 14C-formaldehyde. We have utilized this approach, as well as 5' addition of fluorophores, to prepare labeled methylphosphonate and phosphorothioate oligonucleotides for use in intestinal transport studies. An everted rat gut sac model was employed to compare the transport of oligonucleotides to that of model compounds whose permeation properties are better understood.

RESULTS

We demonstrate that both methylphosphonate and phosphorothioate oligonucleotides are passively transported across the intestinal epithelium, probably by a paracellular route. The rates of transport for both types of oligonucleotides were similar, and were significantly greater than that of the very high MW polymer blue dextran, but were lower than the transport rate of valproic acid, a low MW compound known to have high oral availability.

CONCLUSIONS

A significant degree of permeation of oligonucleotides across the gastrointestinal epithelium does occur, but it is still unclear whether this is sufficient to permit effective oral administration of oligonucleotides as drugs.

Transmucosal impedance of small intestine: correlation with transport of sugars and amino acids

Transmucosal impedances of isolated perfused segments of jejunum from mice and hamsters were measured at frequencies from 10-100,000 Hz in the presence and absence of sugars and amino acids. Na-coupled transport of organic substrates caused large decreases of transmucosal impedance, reflecting contraction of cytoskeletal proteins controlling permeability of tight junctions, functional surface of basolateral membranes, and width of extracellular pathways. The observed changes of impedance were closely correlated with molar rates of Na-coupled active transport rather than with molecular species. Thus amino acids and sugars having the same molar rates of active transport also have the same effects on transmucosal impedance. It is proposed that a nonspecific increase of intracellular osmotic pressure during active transport is the first step initiating cytoskeletal contraction. Cell volume regulatory responses, including increased basolateral K+ conductance and Ca2+ influx, may be subsequent steps leading to contraction of perijunctional actomyosin, formation of junctional dilatations, and exposure of lateral membranes. Enhancement of oxygen capacity of perfusion fluids (e.g., with fluorocarbon emulsion) is required to maintain viability of isolated intestinal epithelium; in plain oxygenated Ringer-HCO3 solution, the transmucosal impedance is abnormally low and cytoskeletal contractile responses to Na-coupled transport are attenuated. An electrical circuit analog is presented that simulates almost exactly the observed transmucosal impedances and provides quantitative evaluation of the effects of Na-coupled transport of sugars and amino acids on resistances of tight junctions, capacitance of basolateral membranes, and postjunctional resistances of lateral intercellular spaces and villus cores.

Explant organ culture: a review

Organ explant culture models offer several significant advantages for studies of patho-physiologic mechanisms like cell injury, secretion, differentiation and structure development. Organs or small explants/slices can be removed in vivo and maintained in vitro for extended periods of time if careful attention is paid to the media composition, substrate selection, and atmosphere. In the case of human tissues obtained from autopsy or surgery, additional attention must be paid to the postmortem interval, temperature, hydration, and cause of death. Explant organ culture has been effectively utilized to establish outgrowth cell cultures and characterize the histiotypic relationships between the various cell types within an organ or tissue.

Reevaluation of the absorption of carbenoxolone using an in situ rat intestinal technique

The absorption of the model drug carbenoxolone was reevaluated using an in situ rat intestinal perfusion technique in which disappearance from the intestinal lumen, binding to the perfused jejunal segment, and appearance in the mesenteric (jejunal) vein were measured. The effect of the degree of ionization on these processes was examined by employing perfusion solutions of pH 4.0, 4.4, 5.0, and 6.5. Tissue binding was observed to be independent of pH. There was a rank-order correlation of the transfer rate of carbenoxolone with the degree of ionization which indicated that carbenoxolone was absorbed faster in its ionized form. This observation is in direct opposition to the pH-partition hypothesis, a finding which appears to support the previous work of Bridges et al. Ion-pairing of carbenoxolone with sodium ion present in the pH 6.5 buffer is one possible explanation for the unusually high transfer rate seen at this pH. A more likely explanation is that at the low pH values, some carbenoxolone precipitated out of solution during the perfusion experiments, thereby reducing the driving force for diffusion across the intestinal wall.

Intestinal absorption of aluminum in rats: stimulation by citric acid and inhibition by dinitrophenol

The effects of citric acid and dinitrophenol on the mucosal uptake of aluminum (Al) and its appearance in the portal and systemic blood were investigated to establish the energy dependence of these processes. Therefore, the rat small intestine was perfused in situ with media containing 20 mmol Al/liter, with or without 5 mmol citric acid/liter, and with or without 0.1 mmol/liter of the metabolic inhibitor DNP. It is concluded that (1) the appearance of Al in systemic blood depends on the mucosal Al uptake after perfusion and the systemic blood level before perfusion and (2) citric acid stimulates and DNP inhibits both mucosal uptake and Al absorption in situ.

Time-dependent deterioration of active transport in duodenal segments of rat intestine

In this study, a time-dependent deterioration of the active transport in duodenal segments of rat intestine was observed. The everted gut-sac technique was used to quantitate both calcium and glucose transport in intestinal segments. The results indicate that both calcium and glucose transport decreased significantly in intestinal segments of animals killed by cervical dislocation 10-20 min prior to tissue removal. It was further determined that animal anesthesia permitted excision of intestinal segments prior to death and thus avoided transport deterioration.

Physiological regulation of transepithelial impedance in the intestinal mucosa of rats and hamsters

Isolated intestinal segments from rats or hamsters were recirculated with balanced salt solutions containing fluorocarbon emulsion to provide 6 vpc oxygen. The lumen contained an axial Ag-AgCl electrode, and the serosal surface was surrounded by a cylindrical shell of Ag-AgCl. Transmural impedances were measured at frequencies from 0.01-30 kHz before and after removal of the mucosal epithelium. The resistance of intercellular junctions, RJ, the distributed resistance of the lateral spaces, RL, and the distributed membrane capacitance, CM, were computed from the relations between frequency and impedance. Activation of Na-coupled solute transport by addition of glucose, 3-0-methyl glucose, alanine or leucine caused two- to threefold decreases of transepithelial impedance. Typical changes induced by glucose in hamster small intestine were RJ 30----13 omega, RL 23----10 omega, and CM 8----20 microF (per cm length of segment). Half maximal response occurred at a glucose concentration of 2-3 mM. The area per unit path length of the junctions (Ap/delta chi = specific resistance divided by RJ) in glucose activated epithelium was 3.7 cm in hamster midgut and 6.8 cm in rat. These values are close to the 4.3 cm estimated independently from coefficients of solvent drag and hydrodynamic conductance in glucose-activated rat intestine in vivo. The transepithelial impedance response to Na-coupled solute transport was reversibly dependent upon oxygen tension. It is proposed that activation of Na-coupled solute transport triggers contraction of circumferential actomyosin fibers in the terminal web of the microvillar cytoskeletal system, thereby pulling apart junctions and allowing paracellular absorption of nutrients by solvent drag as described in the previous accompanying paper. Anatomical evidence in support of this hypothesis is presented in the following second accompanying paper.

Intestinal absorption of biotin and biocytin in the rat

The uptake of biotin and biocytin was investigated in rat intestine using the everted sac technique. It has been shown that at biotin and biocytin concentrations less than 40 and 50 nM respectively, absorption proceeds by a saturable process, whereas at higher concentrations uptake by passive diffusion predominates. Fractionation of solubilized brush border preparations indicates that biotinidase is the only protein which binds biotin in this preparation.

Assessment of an in situ rat intestine preparation with perfused vascular bed for studying the absorption and first-pass metabolism of drugs

The perfused in situ rat jejunum preparation originally described by Hanson and Parsons (1976) was adapted for use in absorption and metabolism studies with drugs. The preparation allows simultaneous perfusion of the gut lumen and associated vasculature and is viable for one hour. The viability of the preparation was assessed, and the application of the method is illustrated by experiments with the opiate analgesic, buprenorphine.

Intestinal absorption studies with glycyl-proline, glycine and ethanol in rats infected with Eimeria nieschulzi

The absorption of glycyl-proline, glycine, and ethanol through the intestinal wall was studied in vitro by an everted sac technique in rats infected with the coccidial parasite Eimeria nieschulzi which causes damage and atrophy to the intestinal villi. The absorption of the dipeptide and of the amino-acid was reduced through tissue from infected animals but the transport of ethanol was similar in both infected and uninfected rats. The replacement of sodium by potassium in the mucosal bathing fluid as well as the separate addition of the metabolic inhibitors, potassium cyanide and dinitrophenol, reduced the amount of amino-acids transferred in both the infected and uninfected tissue in a similar proportion. The results support the conclusion that infection by the parasite affects several different elements of transport across the ileal wall but does not selectively reduce any single one.

Structural and functional assembly of rat intestinal cytochrome P-450 isozymes. Effects of dietary iron and selenium

We have reported previously that both dietary iron and selenium regulate intestinal cytochrome P-450 content by modulating the synthesis of its prosthetic heme moiety. Whether these elements are required for synthesis and/or viability of its apocytochrome moiety is unknown. We have examined the effects of intraluminal deprivation of these elements on the apocytochrome moieties of the constitutive (P-450) and the beta-naphthoflavone inducible (P-448) intestinal isozymes. The relative content of intestinal apocytochrome P-450 moieties generated by dietary deprivation of iron and/or selenium was assessed indirectly by complexing with exogenous heme in vitro, to reassemble the holocytochromes which could be monitored spectrally and catalytically. We now report that, whereas both intraluminal iron and selenium are required for maintenance of the prosthetic apocytochrome moiety of the constitutive intestinal isozyme, only intraluminal selenium is required for the viability of apocytochrome P-448. The latter apparently survives in the absence of intraluminal iron and can be assembled to the holocytochrome, with exogenously added heme. The mechanistic basis of the critical requirement of intestinal apocytochromes for intraluminal selenium is unclear. It is intriguing, however, that the deleterious effects of selenium deprivation are principally exerted in cell systems actively synthesizing protein and inexorably dependent on their extracellular milieu for their nutriment.

A method of studying the intestinal absorption of aluminium in the rat

Aluminium (Al) intoxication in dialysis patients is held to be caused not only by Al in the dialysis fluid but also by Al from orally administered phosphate binders. Studies on Al absorption in patients and healthy individuals as well as in animals are still scarce, and do not provide sufficient data to characterize the absorption process. This paper presents a method of studying the process of Al absorption in a perfusion system of rat small intestine in vivo, in combination with a cannulation system of the portal vein for serial blood sampling. Determination of concentrations of an absorbed substance in samples of both the perfusion medium and the portal blood, collected during the perfusion medium and the portal blood, collected during the perfusion period, may clarify the nature of the absorption process. Although this method appears to be useful for the study of the intestinal absorption of any substance, it was adapted for the study of the intestinal absorption of Al compounds. The usefulness of this method for studying Al absorption was demonstrated in an experiment in which Al chloride (0.5 g/l) in buffered media of pH 7.0, 7.5, and 8.0 was perfused through the rat small intestine over a period of at least 30 min. The results of this experiment indicate that a decrease in pH of the perfusion medium leads to an increase in absorption of Al in the portal blood.

Gastrointestinal absorption and metabolism of capsaicin and dihydrocapsaicin in rats

Gastrointestinal absorption of capsaicin and dihydrocapsaicin was studied in rats in vivo and in situ. Rapid absorption of capsaicin or dihydrocapsaicin from stomach and small intestine occurred in vivo. About 85% of the dose was absorbed in the gastrointestinal tract within 3 hr. In situ, within 60 min after the administration of capsaicin and dihydrocapsaicin into stomach, jejunum, and ileum, about 50, 80, and 70% of the respective dose had disappeared from the lumen. When 2,4-dinitrophenol or NaCN was added, no significant reduction in uptake of [3H]dihydrocapsaicin was observed in the jejunum. These results suggested that capsaicin and its analogs were absorbed by a nonactive process in jejunum. [3H]Dihydrocapsaicin was mainly absorbed via the portal system but not a mesenteric lymphangial one. The radioactivity in the portal blood was composed of 85% of [3H]dihydrocapsaicin and 15% of its metabolite (8-methyl nonanoic acid) bound to the albumin fraction. Dihydrocapsaicin-hydrolyzing enzyme activity was found in jejunal tissue. These results suggest that capsaicin and its analogs partly received a first-pass effect, i.e., metabolism of a compound following first absorption in the gastrointestinal tract. It is concluded that capsaicin and its analogs are readily transported to the portal vein through the gastrointestinal tract by a nonactive process and partly metabolized during absorption.

Effect of pH on theophylline transfer across the everted rat jejunum

The effect of pH on the cumulative transfer of theophylline across the everted rat jejunum in vitro was investigated. Intestinal integrity was assessed by light and scanning electron microscopy, while the biochemical viability of the intestine was evaluated using glucose transfer measurements. The initial (0-30 min) clearance of theophylline was directly proportional to the fraction un-ionized at pH 5.5, 7.4, 8.0, and 10.0. Plots of cumulative theophylline transfer versus time over 60 min were nonlinear, but could be subdivided into two linear segments of 30-min duration. Due to this nonlinearity, differences in theophylline transfer with pH were significant only over the first 30 min of the experiment. Intestinal tissue integrity and viability correlated with the time at which the clearance (slope) increased, while the magnitude of the increase in clearance was proportional to the degree of ionization of theophylline.

Kinetics of transport and metabolism of 1-beta-D-arabinofuranosylcytosine and structural analogs by everted perfused rat jejunum

Few reports have dealt with the kinetics and metabolism of AraC and analogs by rat intestine. Using everted rat jejunum with continuous perfusion, it was possible to demonstrate that AraC and Cyd cross the intestinal barrier(s) by a carrier mediated process which was saturable and exhibited fairly good fitting of the flux rate by Michaelis-Menten equation. The transport rate of different analogs was not consistent with the pH-partition theory of membrane transport of drugs being rather dependent on the chemical structure of the nucleoside. A free amino group of cytosine increased the rate of transport within the present series of AraC analogs. There was a detectable deaminase as well as esterase activity towards AraC and its analogs in rat jejunum.

Intestinal absorption and metabolism of 6-mercaptopurine in the rat small intestine

The intestinal absorption of 6-mercaptopurine was examined in the rat by in vitro and in situ techniques for the purpose of establishing absorption characteristics which might explain the poor systemic oral availability of this drug. Experiments were designed to evaluate the significance of intestinal metabolism and active secretion processes. Since mucosal/serosal drug concentration gradients across in vitro segments were not significantly different (P greater than 0.05) for everted and noneverted preparations, with values of 1.21 +/- 0.27 and 0.91 +/- 0.12, respectively, it was concluded that active secretion or absorption mechanisms were absent. Varying the concentration of 6-mercaptopurine from 0.24 to 5.88 mM demonstrated saturability of the biotransformation of 6-mercaptopurine to 6-thiouric acid with a maximum rate of 1.6 X 10(-5) mmoles per min g for jejunal portions. Distal segments displayed 85% higher rates of biotransformation at concentrations of 1.47 mM. Inclusion of allpurinol (2 mM) completely inhibited biotransformation. With in situ loops perfused with 1.47 mM drug, collection of mesentery blood showed that absorption rates of 6-thiouric acid were 0.67 that of the parent drug.

Further Investigations on the Ileum Model as a Possible Alternative to In Vivo Eye Irritancy Testing

Summary

The concentration of six surfactants required to block spontaneous contractions of rabbit isolated ileum by 50% (EC50) correlated well with in vivo eye irritancy (correlation coefficient -0.996) only when fresh ileum was tested at 35°C. Storage of ileum for 2 hr or 4 hr before testing did not significantly alter the potency of five of the surfactants, but the potency of triethanolamine decyl sulphate (TDS), the only C-10 surfactant tested, was significantly reduced as compared to fresh controls. When tested on fresh ileum at 29°C all the surfactants, except TDS, were more potent at blocking contractions while TDS was significantly less potent.

It is suggested that the penetration barrier to surfactants is considerably more complex than previously envisaged, possibly involving a transport system capable of concentrating TDS, but not the other agents tested.

It would appear that, provided experimental conditions are carefully controlled, the ileum model can yield results which correlate well with in vivo ocular irritancy and as such may provide the basis for the development of a viable screen for irritancy of surfactants and surfactant-based products.

1,25-Dihydroxyvitamin C3-stimulated active uptake of phosphate by rat jejunum

An assay system for measuring an active phosphate uptake in rat jejunal disks has been developed and used to study the rat's response to 1,25-dihydroxyvitamin D3. The active component of phosphate uptake is stimulated by added KCl, fructose, sodium, and CaCl2, but not MgCl2. Half-saturation values obtained with this assay are 0.08 mM (1,25-dihydroxyvitamin D3-treated) and 0.11 mM (D-deficient). The maximal velocities are 9.6 nmol . cm-2 . min-1 (D-deficient). Using a 5-min preincubation and a 15-min incubation, a dose-response curve for 1,25-dihydroxyvitamin D3 shows 6 pmol to be the lowest dose to give a significant response and 60 pmol to be the level at which the maximal response is reached. Measurement of phosphate uptake versus time after dosing with 1,25-dihydroxyvitamin D3 (300 pmol) revealed a biphasic response with peaks at 8 and 36 h and a trough at 20 h.

Testosterone metabolism by the rat gastrointestinal tract, in vitro and in vivo

We have shown previously that the capacity of the jejunal mucosa to oxidise testosterone to the weaker androgen, androstenedione, by the enzyme 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD), is considerable. The present study extends these earlier observations by measuring 17 beta-HSD activity in different regions of the gastrointestinal tract, by investigating the potential for testosterone metabolism by slices and everted sacs of rat jejunum, and estimating the contribution of intestinal testosterone metabolites to circulating levels of plasma androgens, by portal vein sampling in the rat, in vivo. 17 beta-HSD activity in homogenates of gastric and duodenal mucosa was significantly higher than that in jejunum, and was also present in ileum and colon. In addition to androstenedione, slices and everted sacs of rat jejunum produced various metabolites, one of which was probably dihydrotestosterone. It was not, however, a major metabolite in vivo. It is suggested that 5 alpha-reduction may be favoured in vitro by a lower oxidation-reduction potential resulting from tissue anoxia. The major portal vein metabolite was androstenedione, the same major metabolite produced by mucosal homogenates. We conclude that oxidation of testosterone is the major metabolic pathway in intestinal mucosa and the capacity of the gastrointestinal tract to reduce the potency of testosterone is considerable. Our findings suggest that the gut, rather than the liver, is responsible for the failure of oral testosterone to provide effective androgen replacement therapy. The qualitative difference in testosterone metabolism between in vitro and in vivo preparations emphasises the need for caution in the interpretation of similar in vitro experiments.

Hyperglycemia and net transintestinal glucose and sodium transport in the rat

Hyperglycemia produced by intravenous infusion of glucose cause, in the everted jejunum of rat, an increase of net transintestinal transport of glucose, Na and water without modifying their cell concentration. The concentration of glucose in the fluid pouring out at the serosal side is higher than the calculated intracellular concentration, especially in the experiments in which the serosal compartment is initially empty. This fact could be explained by the possible existence of an active extrusion mechanism in the basolateral membrane of the enterocyte.

Non-ionic surfactants and gastric mucosal transport of paraquat

The influence of a range of polyoxyethylated non-ionic surfactants upon the transport of [14C]-paraquat dichloride across rabbit isolated gastric mucosa was investigated. Paraquat was shown to cross the mucosal epithelium by passive diffusion. Certain members of the surfactant series enhanced transmucosal paraquat transfer at low surfactant concentrations (e.g. 0.001%) but the occurrence and magnitude of this effect was not dependent in a simple manner upon surfactant structure of physiochemical properties. At micellar concentrations the increase in paraquat transport was greatest with those surfactants possessing both ethylene oxide chains of 10-20 units and alkyl chains longer than C7-C9. The most effective absorption promoter was found to be Brij 36T (C12E10). At micellar surfactant concentrations the enhancement of paraquat transfer appeared, from histological evidence, to be related to the ability of the surfactants to solubilize membrane components and disrupt epithelial cells.

Oxygen-dependent 1,25-dihydroxycholecalciferol-induced calcium ion transport in rat intestine

O2-dependent CA2+ uptake by rat duodenal discs has been characterized and used in a revised assay for 1,25-dihydroxycholecalciferol-induced intestinal Ca2+ transport. Although both muscle and mucosal surfaces are exposed in this free-floating-disc assay, the Ca2+ influx across the muscle surface is small, not O2- or vitamin D-dependent, and can be subtracted out. Depriving the animals of food for 9-14 h before assay increases the O2-dependent uptake by about 75%. Half-saturation values for O2-dependent Ca2+ uptake as determined with this assay are: 0.8mM-Ca2+ (fed) and 0.5mM-Ca2+ (food-deprived) for vitamin D-deficient rats, and 0.9mM-Ca2+ (fed) and 1.5mM-Ca2+ (food-deprived) for rats dosed with 1,25-dihydroxycholecalciferol. The maximum velocity of uptake varies from 6.7nmol of Ca2+ per cm2/min (fed) to 7.0nmol of Ca2+ per cm2/min (food-deprived) for vitamin D-deficient rats and 16.7nmol of Ca2+ per cm2/min (fed) to 29 nmol of Ca2+ per cm2/min (food-deprived) for 1,25-dihydroxycholecalciferol-treated rats. By using a 5 min preincubation and 15 min incubation with 1.0mM-Ca2+, duodenal tissue taken from vitamin D-treated rats shows about a 3-fold increase in O2-dependent Ca2+ uptake when compared with tissue taken from vitamin D-deficient animals. The calcium ionophore A23187, depending on concentration, either has no significant effect on or inhibits the O2-dependent uptake, rather than increasing it. Actinomycin D, at a dose of 2 micrograms/g, inhibits the O2-dependent uptake in intestinal discs from both vitamin D-deficient and vitamin D-treated rats by 58 and 80% respectively, when administered in vivo 3 1/2 h before assay.

Permeability of everted rat small intestine to lidocaine and derivatives

The transfer rates of lidocaine and five derivatives were measured across the everted rat intestine. No obvious relationship was observed between the first-order rate constants for transfer and the lipophilicity of the compounds as measured by their apparent partition coefficients. The intestinal mucosal epithelium also did not appear to be the rate-limiting barrier for the passage of these agents across the intestinal membrane regardless of the respective clearance of the compound. It is suggested that the ionized form of these agents may be capable of crossing the intestinal barrier at substantial rates, although it is not known if a similar situation exists in vivo. The transfer rate is believed to be due to the passage of intact drug and not the metabolite.

Extracellular space determination in rat small intestine by using markers of different molecular weights

The apparent extracellular space (ECS) of rat jejunum, everted and cannulated "in vitro", has been measured by using extracellular markers of different molecular weights. The markers used were two polyethyleneglycols, 14C and 3H labelled (14C-PEG MW 4000 and 3H-PEG MW 900) and 3H-sucrose. The ECSs for the mucosal and serosal sides have been separately determined throughout the time course, and it has been found that the two spaces are identical when PEG 4000 was used but the serosal ECS is almost the double when using PEG 900. The serosal ECS determined with sucrose is four times as big as the mucosal ECS. It seems reasonable to conclude that the best marker for the measure of total apparent ECS is sucrose, placed in the serosal compartment, taking into account that the mucosal ECS is four times smaller than the serosal one. All the markers used reach equilibrium with ECS, more rapidly in the mucosal than in the serosal ECS. Finally, by comparing cell water and cell Na concentrations, one observes that there is a statistical difference between the results obtained by using PEG 4000 as an extracellular marker and those obtained with sucrose.

Organ culture of adult mouse intestine. I. Morphological results after 24 and 48 hours of culture

Explants of adult mouse intestine have been maintained in organ culture for 24 to 48 hr. The best results have been obtained with a mixture of DMEM-HEPES medium and NCTC-135 enriched with 10% fetal bovine serum. The morphology of the mucosa is well preserved at the light and electron microscopic level: absorbing cells exhibit an increase in secondary lysosomes; goblet cells and Paneth cells remain active; numerous mitoses are observed in the crypts; and vigorous re-epithelization takes place on the margin of the explants.