Isolated perfusion of the small intestine using perfluorotributylamine as artificial oxygen carrier

A model of the isolated perfused rat small intestine

Intestinal edema remains a serious clinical problem, and novel approaches to study its pathophysiology are needed. It was our aim to develop a long-term stable isolated perfused rat small bowel preparation permitting analysis of vascular, luminal, interstitial, and lymphatic compartments and to demonstrate the utility of this model by studying the effects of the proinflammatory mediator platelet-activating factor (PAF). A temperature-controlled chamber with an integrated balance was designed to perfuse isolated intestines through the mesenteric artery and the gut lumen. Steroids or oxygen carriers were not needed. Functional and morphological integrity of the tissue was preserved for several hours as confirmed by oxygen consumption, venous lactate-to-pyruvate ratio, arterial and venous pH, lactose digestion and galactose uptake, intravascular and luminal pressures, maintained fluid homeostasis, gut motility, and quantitative light microscopic analysis. Administration of PAF caused typical effects such as vasoconstriction, gut atony, and loss of galactose uptake. PAF also elicited a transient loss of 20% of the perfusate liquid from the mesenteric vascular bed, two-thirds of which were transferred to the lumen. All these responses were entirely reversible. This new model provides detailed insights into the physiology of the small intestine and will allow to study fundamental processes such as fluid homeostasis, barrier functions, transport mechanisms, and immune responses in this organ. Using this model, here we show a dramatic and yet reversible response of the rat small bowel to PAF, suggesting luminal water clearance as a novel safety factor in the intestine that may be of clinical relevance.

Intestinal absorption of rutin in free and conjugated forms

Quercetin is one of the most common flavonoids in nature, occurring mainly in glycosidic forms such as rutin. Rutin has been reported to exert numerous biochemical and pharmacological activities, though information about its absorption and metabolism is scarce. The aim of this study was to investigate intestinal handling of luminally administered rutin in an isolated preparation of luminally and vascularly perfused rat small intestine. A synthetic perfusate free from blood components was used as vascular medium, with a perfluorocarbon as oxygen carrier. Luminal media consisted of a bicarbonate-buffered sodium chloride solution spiked with rutin (40.5 +/- 1.8 micromol/L). Viability was maintained during the entire perfusion; no differences between rutin and control perfusions for perfusion pressure, lactate-pyruvate ratio, oxygen uptake, and acid-base homeostasis were observed. About 10% of the administered rutin appeared at the vascular side, chiefly as free rutin (5.6%), but some rutin sulfate (2.5%) and glucuronide (2.0%) were also detected. The conjugates were preferentially absorbed to the vascular side, while only traces of the glucuronide (0.2%) were found in the luminal perfusate. Minute amounts of the rutin administered were located in the intestinal tissue (1.1%) in the form of unchanged rutin and its glucuronide and sulfate conjugates. The model used serves as a valuable tool for understanding intestinal handling of the bioactive flavonol glycoside rutin, and the obtained results confirm uptake of rutin in the rat small intestine.

Isoflavones from tofu are absorbed and metabolized in the isolated rat small intestine

Studies suggest a variety of biological effects of soybean isoflavones, but there is little information regarding small intestinal absorption and metabolism. The aim of this study was to investigate intestinal handling of luminally administered soybean-based tofu in an isolated preparation of the luminally and vascularly perfused rat small intestine (male Sprague-Dawley, approximately 45 d old). A synthetic emulsion free from blood components was used as vascular medium, with a perfluorocarbon as oxygen carrier. Luminal media consisted of tofu, predigested with pepsin and pancreatin and emulsified with bile acids, containing 39. 5 micromol/L genistein compounds and 19.1 micromol/L daidzein compounds. Viability of the organ preparation was maintained during the entire perfusion, confirmed by lack of significant differences between tofu and control perfusion experiments for arterial pressure, glucose consumption, oxygen uptake, lactate-pyruvate ratio and acid-base homeostasis. Daidzein (8.9%) and genistein (8.0%) compounds from tofu exhibited almost the same (P: > 0.05) absorption rate during small intestinal passage. The majority of the absorbed genistin appeared vascularly as genistein (4.4%), in addition to minor amounts of unchanged genistin (2.1%) and genistein glucuronide (1.5%). In the luminal effluent, a considerable increase of genistein (338%) as well as daidzein (190%) as cleavage products of the glucosides and malonyl-glucosides was observed. The distribution of daidzein compounds in the small intestine was not different from that of genistein compounds (P: > 0.05), except for the blood vessels, which had extremely low total amounts. Sulfate derivatives of genistein and daidzein compounds were not detectable. An effect of tofu ingredients was observed on absorption rate of genistin, on glucuronidation and on distribution of genistein glucuronide in the intestine.

Absorption and metabolism of genistin in the isolated rat small intestine

Uptake and intestinal metabolism of physiologically active genistin were studied in an ex vivo intestinal perfusion model; luminally applied concentrations were 5.9, 12.0, and 23.8 micromol/l. The intestinal absorption of genistin was 14.9% (+/-2.3, n=9), irrespective of the amounts applied. The majority of the absorbed genistin appeared as genistein glucuronide (11.6%), also recovered as the main metabolite on the luminal side (19.5%). Minor amounts of genistin (1.3%) and genistein (1.9%) were found on the vascular side, whereas 15.4% of applied genistin was luminally cleaved to yield genistein. Sulfate derivatives of genistein or genistin were not observed.

Absorption and metabolism of genistein in isolated rat small intestine

Studies suggest a variety of biological effects for the isoflavonoid genistein, but there is little information regarding small intestinal absorption and metabolism. The aim of this study was to investigate the absorption and metabolism of luminally administered genistein in an isolated preparation of luminally and vascularly perfused rat small intestine. A synthetic perfusate free from blood components was used as vascular medium, with a perfluorocarbon as oxygen carrier. Luminal media consisted of a bicarbonate buffered sodium chloride solution spiked with genistein (12 micromol/L). Viability was maintained during the entire perfusion as indicated by no significant differences between genistein and control perfusions for perfusion pressure, lactate-pyruvate ratio, oxygen uptake and acid-base homeostasis. Luminal disappearance rate of genistein did not change throughout the entire perfusion time. After a significant increase until about 30 min, vascular appearance rate of total genistein reached an equilibrium. The intestinal absorption of luminally administered genistein was 40.6%, corresponding to an average uptake of 2.9 nmol. min(-1). g dry intestine(-1). The majority (31.3%) of the absorbed genistein appeared as genistein glucuronide, also recovered as the main metabolite on the luminal side (13.3%). Only small amounts of genistein (2.6%) and genistein glucuronide (2.9%) were found in the intestinal tissue. The results demonstrate a favorable uptake of genistein, a pertinent addition to the ongoing discussion about health benefits of isoflavones.

Estimation of oral drug absorption in man based on intestine permeability in rats

Predicting the fraction of an oral dose absorbed in humans is of considerable interest at an early stage of a research program in the pharmaceutical industry. Models described in the literature to predict the oral absorption in man include: the permeability in Caco-2 cells, absorption from a perfused segment of rat intestinal lumen and uptake into everted rings. The present study used an isolated and vascularly perfused rat small intestine to determine the permeability values of eleven compounds across the intestinal epithelium. A good correlation was obtained between the permeability values determined in this model and the proportion of an oral dose absorbed in humans. Compared to the other models, the present one could allow the appearance in the artificial bloodstream and the intestinal metabolism of a compound to be studied simultaneously.

Origins of motility patterns in isolated arterially perfused rat intestine

BACKGROUND/AIMS

Assessment of the neuromuscular control of small intestinal motility and movement of luminal contents is hampered in vivo by measurement techniques and in vitro by tissue viability. The aim of this study was to establish the structural and functional integrity of an isolated segment of rat ileum and characterize its motility.

METHODS

Segments of rat ileum were perfused arterially with oxygenated fluorocarbon and luminally with saline. Oral and aboral pressures were correlated with conformational changes detected by concurrent video imaging.

RESULTS

Light and electron microscopy showed no neuromuscular abnormalities after experiments, and acetylcholine-induced pressure amplitudes were unchanged during experiments. Under basal conditions, low-frequency contractions showing constant frequency (0.27/min) and amplitudes (oral, 17 hPa; aboral, 15 hPa) corresponded to luminally occlusive aborally propagated contractions, which were eliminated by tetrodotoxin. High-frequency contractions with a constant frequency (27/min) were also seen; their basal amplitude (0.3 hPa) increased immediately before and after low-frequency contractions and after tetrodotoxin. Tetrodotoxin also increased basal intestinal tone.

CONCLUSIONS

An isolated, arterially perfused segment of rat ileum retains structural and functional integrity. It shows low-frequency propulsive contractions, controlled by the enteric nervous system, and myogenic high-frequency contractions, probably subject to tonic neural inhibition.

Studies of human airways in vitro: a review of the methodology

The pathophysiology of human airway narrowing is only partly understood. In order to gain more insight in the mechanisms of human lung diseases and potential beneficial therapeutic agents, adequate models are needed. Animal airway models are of limited value since lung diseases such as asthma and chronic obstructive pulmonary disease (COPD) are unique to humans and because the mechanisms of airway narrowing differ between species. Therefore, it is important to perform studies on human isolated airways. We describe the models that have been developed to study airway function in vitro, emphasizing human airway preparations. The easily prepared airway strip and ring preparations are described first. The potential damage during preparation and the interference with airway structure are important drawbacks in these preparations. Lung parenchymal strips, described next, were designed in order to study responsiveness of small airways. However, parenchymal strips are anatomically complex, and responsiveness is determined by the relative amounts of airway and vascular smooth muscle. The lack of reproducibility between species and even within one animal limits their usefulness. Airway tube preparations, in which luminal and serosal stimulation can be separated, enable us to study the modulatory role of the airways epithelium in vitro. Furthermore, airway compliance can be measured. In the isolated perfused lung preparation, relationships between the airways and the vascular system are preserved and the interaction between these two systems can be studied. Weight gain due to fluid extravasation is a problem in this model which has not been used yet to study human lungs in vitro. Next, methodological aspects such as tissue handling and storage, recording of responses, removal of the epithelium, and electrical field stimulation are discussed in some detail. Although animal airways tissue can be studied immediately after removal, human tissue is often obtained with some delay. However, this seems tenable since electron microscopy of lung tissue obtained at autopsy showed that recovery of the preparation occurs during incubation of carbogenated Krebs-Henseleit (K-H) buffer. Dissected airways can be stored overnight in cooled K-H buffer until up to 55 hr after resection without losing viability. Commonly used physiological salt solutions which bath the tissue contain osmotic molecules, ions important for contractility, glucose as a substrate, and a bicarbonate-carbon dioxide buffer.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of dexamethasone on glutamine metabolism in the isolated vascularly perfused rat small intestine

Post-stress metabolism is associated with a large glutamine (Gln) efflux from muscle and an increased Gln utilization by the small intestine. Both appear to be modulated by corticosteroids. The present investigation was performed to better characterize the mechanism of corticoid action on Gln metabolism in an isolated preparation of vascularly perfused rat small intestine. In all perfusions, a synthetic perfusate free from blood components was used with only 0.6 mM Gln and 10 mM glucose as substrates. Irrespective of dexamethasone concentrations in the vascular perfusate (none, 0.25 mg l-1, or 2.5 mg l-1, isolated intestines from normal rats exhibited unchanged extraction rates of Gln (-85 +/- 8, -89 +/- 10, and -87 +/- 16 nmol min-1 g-1) and unchanged production rates of alanine (43 +/- 9, 40 +/- 7, and 51 +/- 5 nmol min-1 g-1) and ammonia (49 +/- 15, 45 +/- 13, and 54 +/- 13 nmol min-1 g-1). Similarly, when intestines were vascularly perfused 2 or 9 days after dexamethasone injection (0.45 mg kg-1 BW), net Gln uptake also remained unchanged (-88 +/- 16 and -84 +/- 11 nmol min-1 g-1). There was, however, a shift in nitrogenous products of Gln metabolism from ammonia (-31% and -38%) to alanine (+16% and +64%). Thus, the failure of dexamethasone to increase Gln uptake in the isolated rat intestine may indicate that rather than acting directly on the mucosa, dexamethasone could regulate intestinal Gln consumption in vivo by indirect mechanisms possibly involving extramucosal tissues. Dexamethasone pretreatment may modulate the pattern of nitrogenous products in portal venous blood presented to the liver and thus support enhanced nitrogen loss through ureagenesis by metabolic cooperation between gut and liver.

Use of an artificial oxygen carrier in isolated rat liver perfusion: first demonstration of net glucose uptake at physiological portal glucose concentrations using a hemoglobin-free perfusate

A defect in isolated perfused rat-liver (IPRL) preparations has been proposed to explain discrepancies between in vivo and in vitro findings regarding hepatic glucose metabolism. The aim of the present study was to investigate whether a preparation of IPRL using a synthetic hemoglobin-free perfusate was capable of net glucose uptake and glycogen deposition at physiological portal substrate concentrations. Livers from fed anaesthetized rats were perfused in a recirculating system using a fluorocarbon emulsion as artificial oxygen carrier. Depending on the prevailing glucose concentration, livers exhibited net glucose uptake or release with a threshold value of 5.5-6.0 mM glucose. Net glucose uptake was associated with net glycogen deposition (+0.23 to +0.59 mumol C6 min-1 g-1). From 5.8 mM (n = 3) and 10.0 mM (n = 8), initial concentration glucose levels fell to 5.3 +/- 0.2 mM after 210 min (n = 3) and 6.3 +/- 0.9 mM after 120 min (n = 8), respectively. This was equivalent to a net glucose uptake of -0.16 and -0.45 mumol min-1 g-1. Anoxia reversibly switched hepatic glucose balance from net uptake (-0.42 mumol min-1 g-1) to release (+0.69 mumol min-1 g-1) followed by net uptake (-0.50 mumol min-1 g-1) after reinstitution of aerobic conditions. We conclude that the composition of perfusion media might play a pivotal role for studies of glucose metabolism in the isolated perfused rat liver. In our experimental model, using a hemoglobin-free synthetic medium, net glucose uptake was readily demonstrated at physiological portal substrate concentrations similar to the in vivo situation.

Dipeptide metabolism in the isolated perfused rat small intestine

The purpose of the present study was to investigate the utilisation of vascularly administered leucyl-leucine (Leu-Leu), alanyl-glutamine (Ala-Gln) and glycyl-glutamine (Gly-Gln) by the isolated vascularly perfused rat small intestine. Fractional extraction rates were 49%, 35.5%, and 12% for Leu-Leu, Ala-Gln, and Gly-Gln (0.15mM) corresponding to a net uptake of -63.5, -31.5, and -17 nmol/min/g wet weight. Nitrogen metabolism in terms of glutamine uptake and release of alanine and ammonia was not different when perfusion with dipeptides or with free amino-acids were compared. No soluble dipeptidase activity was released into the plasma- and cell-free synthetic vascular perfusate. No dipeptide could be recovered from the luminal perfusate. Considering the high fractional extraction rate for Leu-Leu, it is conceivable that dipeptide assimilation may occur also in extramucosal gut tissue. Although dipeptide transport cannot be excluded, dipeptide assimilation in small intestine may involve membrane bound peptidase(s), as in liver.

Pharmacokinetic modelling of the effect of activated charcoal on the intestinal secretion of theophylline, using the isolated vascularly perfused rat small intestine

The effect of activated charcoal administration on the secretion of theophylline from the blood into the intestinal lumen has been examined by use of the rat isolated vascularly perfused small intestine. A closed two compartment model was used to analyse the vascular and luminal concentration-time curves obtained. An equation was derived to calculate the time-dependent intestinal clearance. From control experiments it was concluded that theophylline is secreted by a diffusional transport system through the intestinal wall. The intestinal clearance declined rapidly with time as a result of the concomitant increase in luminal theophylline concentration. After 120 min a steady state between the vascular and luminal perfusate was established. Administration of activated charcoal in the lumen had a profound effect on the kinetics of the drug. The vascular steady state concentration was depressed dramatically. The theophylline clearance remained nearly constant with time, because the blood to lumen concentration gradient was maximized. The maximal value for the intestinal theophylline clearance was estimated to be 0.88 mL min-1 and it equalled the value for the intestinal blood flow at the absorptive site. By use of the concept of absorptive site blood flow, the maximal effect of charcoal on systemic theophylline clearance could be adequately predicted for rats, dogs and man. Activated charcoal administration is only useful to enhance the systemic clearance of drugs or toxicants if that clearance is of the same order of magnitude as the absorptive site blood flow or lower.

A ready-to-use activated charcoal mixture. Adsorption studies in vitro and in dogs: its influence on the intestinal secretion of theophylline in a rat model

A practical, ready-to-use preparation of activated charcoal (AZU mixture) for application in toxicology has been formulated. To establish its efficacy, the formulation was tested in vitro and in dogs. The in vitro adsorption capacity was compared to that of freshly prepared charcoal suspension in water (CW) and to Carbomix. Langmuir adsorption coefficients demonstrated small but clinically insignificant differences in adsorption capacity between the preparations. The laxative sodium sulfate did not reduce the adsorption capacity of charcoal in vitro. Dogs were given 60 mg of paracetamol per kg as an oral solution followed by 5 g of activated charcoal preparation. The area under the plasma concentration versus time curve (control 2955 +/- 353 mg.min-1.l-1) was significantly reduced following CW (921 +/- 453) and AZU (786 +/- 270). The premixed AZU charcoal formulation is efficacious, inexpensive and overcomes the problems of bed-side preparation. An isolated vascularly perfused rat small intestine can be used to describe the effect of activated charcoal on the intestinal secretion of theophylline.