The effect of surgical bowel manipulation and anesthesia on intestinal glucose absorption in rats

Changes in the gut microbiota of patients with sarcopenia based on 16S rRNA gene sequencing: a systematic review and meta-analysis

Introduction

Sarcopenia, an age-related disease, has become a major public health concern, threatening muscle health and daily functioning in older adults around the world. Changes in the gut microbiota can affect skeletal muscle metabolism, but the exact association is unclear. The richness of gut microbiota refers to the number of different species in a sample, while diversity not only considers the number of species but also the evenness of their abundances. Alpha diversity is a comprehensive metric that measures both the number of different species (richness) and the evenness of their abundances, thereby providing a thorough understanding of the species composition and structure of a community.

Methods

This meta-analysis explored the differences in intestinal microbiota diversity and richness between populations with sarcopenia and non-sarcopenia based on 16 s rRNA gene sequencing and identified new targets for the prevention and treatment of sarcopenia. PubMed, Embase, Web of Science, and Google Scholar databases were searched for cross-sectional studies on the differences in gut microbiota between sarcopenia and non-sarcopenia published from 1995 to September 2023 scale and funnel plot analysis assessed the risk of bias, and performed a meta-analysis with State v.15. 1.

Results

A total of 17 randomized controlled studies were included, involving 4,307 participants aged 43 to 87 years. The alpha diversity of intestinal flora in the sarcopenia group was significantly reduced compared to the non-sarcopenia group: At the richness level, the proportion of Actinobacteria and Fusobacteria decreased, although there was no significant change in other phyla. At the genus level, the abundance of f-Ruminococcaceae; g-Faecalibacterium, g-Prevotella, Lachnoclostridium, and other genera decreased, whereas the abundance of g-Bacteroides, Parabacteroides, and Shigella increased.

Discussion

This study showed that the richness of the gut microbiota decreased with age in patients with sarcopenia. Furthermore, the relative abundance of different microbiota changed related to age, comorbidity, participation in protein metabolism, and other factors. This study provides new ideas for targeting the gut microbiota for the prevention and treatment of sarcopenia.

Systematic Review Registration

https://www.crd.york.ac.uk/PROSPERO/display_record.php?RecordID=475887, CRD475887.

Paracellular permeability and tight junction regulation in gut health and disease

Epithelial tight junctions define the paracellular permeability of the intestinal barrier. Molecules can cross the tight junctions via two distinct size-selective and charge-selective paracellular pathways: the pore pathway and the leak pathway. These can be distinguished by their selectivities and differential regulation by immune cells. However, permeability increases measured in most studies are secondary to epithelial damage, which allows non-selective flux via the unrestricted pathway. Restoration of increased unrestricted pathway permeability requires mucosal healing. By contrast, tight junction barrier loss can be reversed by targeted interventions. Specific approaches are needed to restore pore pathway or leak pathway permeability increases. Recent studies have used preclinical disease models to demonstrate the potential of pore pathway or leak pathway barrier restoration in disease. In this Review, we focus on the two paracellular flux pathways that are dependent on the tight junction. We discuss the latest evidence that highlights tight junction components, structures and regulatory mechanisms, their impact on gut health and disease, and opportunities for therapeutic intervention.

General anesthesia bullies the gut: a toxic relationship with dysbiosis and cognitive dysfunction

Perioperative neurocognitive disorder (PND) is a common surgery outcome affecting up to a third of the elderly patients, and it is associated with high morbidity and increased risk for Alzheimer's disease development. PND is characterized by cognitive impairment that can manifest acutely in the form of postoperative delirium (POD) or after hospital discharge as postoperative cognitive dysfunction (POCD). Although POD and POCD are clinically distinct, their development seems to be mediated by a systemic inflammatory reaction triggered by surgical trauma that leads to dysfunction of the blood-brain barrier and facilitates the occurrence of neuroinflammation. Recent studies have suggested that the gut microbiota composition may play a pivotal role in the PND development by modulating the risk of neuroinflammation establishment. In fact, modulation of gut microbiome composition with pre- and probiotics seems to be effective for the prevention and treatment of PND in animals. Interestingly, general anesthetics seem to have major responsibility on the gut microbiota composition changes following surgery and, consequently, can be an important element in the process of PND initiation. This concept represents an important milestone for the understanding of PND pathogenesis and may unveil new opportunities for the development of preventive or mitigatory strategies against the development of these conditions. The aim of this review is to discuss how anesthetics used in general anesthesia can interact and alter the gut microbiome composition and contribute to PND development by favoring the emergence of neuroinflammation.

Effect of Type 2 Diabetes and Impaired Glucose Tolerance on Digestive Enzymes and Glucose Absorption in the Small Intestine of Young Rats

The reactions of intestinal functional parameters to type 2 diabetes at a young age remain unclear. The study aimed to assess changes in the activity of intestinal enzymes, glucose absorption, transporter content (SGLT1, GLUT2) and intestinal structure in young Wistar rats with type 2 diabetes (T2D) and impaired glucose tolerance (IGT). To induce these conditions in the T2D (n = 4) and IGT (n = 6) rats, we used a high-fat diet and a low dose of streptozotocin. Rats fed a high-fat diet (HFD) (n = 6) or a standard diet (SCD) (n = 6) were used as controls. The results showed that in T2D rats, the ability of the small intestine to absorb glucose was higher in comparison to HFD rats (p < 0.05). This was accompanied by a tendency towards an increase in the number of enterocytes on the villi of the small intestine in the absence of changes in the content of SGLT1 and GLUT2 in the brush border membrane of the enterocytes. T2D rats also showed lower maltase and alkaline phosphatase (AP) activity in the jejunal mucosa compared to the IGT rats (p < 0.05) and lower AP activity in the colon contents compared to the HFD (p < 0.05) and IGT (p < 0.05) rats. Thus, this study provides insights into the adaptation of the functional and structural parameters of the small intestine in the development of type 2 diabetes and impaired glucose tolerance in young representatives.

Mechanisms of Glucose Absorption in the Small Intestine in Health and Metabolic Diseases and Their Role in Appetite Regulation

The worldwide prevalence of metabolic diseases such as obesity, metabolic syndrome and type 2 diabetes shows an upward trend in recent decades. A characteristic feature of these diseases is hyperglycemia which can be associated with hyperphagia. Absorption of glucose in the small intestine physiologically contributes to the regulation of blood glucose levels, and hence, appears as a putative target for treatment of hyperglycemia. In fact, recent progress in understanding the molecular and cellular mechanisms of glucose absorption in the gut and its reabsorption in the kidney helped to develop a new strategy of diabetes treatment. Changes in blood glucose levels are also involved in regulation of appetite, suggesting that glucose absorption may be relevant to hyperphagia in metabolic diseases. In this review we discuss the mechanisms of glucose absorption in the small intestine in physiological conditions and their alterations in metabolic diseases as well as their relevance to the regulation of appetite. The key role of SGLT1 transporter in intestinal glucose absorption in both physiological conditions and in diabetes was clearly established. We conclude that although inhibition of small intestinal glucose absorption represents a valuable target for the treatment of hyperglycemia, it is not always suitable for the treatment of hyperphagia. In fact, independent regulation of glucose absorption and appetite requires a more complex approach for the treatment of metabolic diseases.

The Effect of Anesthetic Regimens on Intestinal Absorption of Passively Absorbed Drugs in Rats

PURPOSE

Different anesthetic regimens are used during single pass intestinal perfusion (SPIP) experiments for the study of intestinal drug absorption in rats. We examined the ketamine/xylazine anesthetic combination to evaluate its influence on drug absorption compared to older regimens. Additionally, we examined whether supplementary analgesia has any effect on drug absorption and the effect of the different anesthetic regimens on induction time and stress response.

METHODS

Rats were anesthetized using four different anesthetic regimens; ketamine/midazolam, pentobarbital, ketamine/xylazine and ketamine/xylazine/butorphanol. Three model drugs were administered to rat intestines and P_eff was calculated. Stress response was evaluated by quantifying blood corticosterone levels and induction time was recorded.

RESULTS

We found absorption under pentobarbital to be higher or similar to absorption under ketamine/midazolam. These results partly correlate with past literature data. Ketamine/xylazine was found to give similar or higher P_eff compared to pentobarbital and ketamine/midazolam. Addition of butorphanol did not affect absorption and reduced induction time and stress.

CONCLUSIONS

In studies of intestinal drug absorption, the ketamine/xylazine combination is superior to other anesthetic regimens as it is more convenient and seems to affect absorption to a lesser extent. Addition of butorphanol is highly recommended as it did not affect absorption but led to a more effective and less stress inducing experiment.

Low intake of digestible carbohydrates ameliorates duodenal absorption of carbohydrates in mice with glucose metabolism disorders induced by artificial sweeteners

BACKGROUND

Long-term artificial sweetener consumption has been reported to induce glucose intolerance, and the intestinal microbiota seems as an important target. While the impacts of artificial sweeteners on energy balance remain controversial, this work aimed to evaluate the protective effects in mice of a low digestible carbohydrate (LDC) diet on plasma glucose, plasma fasting insulin, sweet taste receptors, glucose transporters, and absorption of carbohydrates, together with consumption of acesulfame potassium (AK) or saccharin (SAC).

RESULTS

Artificial sweetener was administered to mice for 12 weeks to induce glucose metabolism disorders; mice were treated with an LDC diet for the final 6 weeks. The experimental groups were treated with an LDC diet that had the same energy as the normal-diet group. Prolonged administration of artificial sweeteners led to metabolic dysfunction, characterized by significantly increased plasma glucose, insulin resistance, sweet taste receptors, glucose transporters, and absorption of carbohydrates. Treatment with an LDC diet positively modulated these altered parameters, suggesting overall beneficial effects of an LDC diet on detrimental changes associated with artificial sweeteners.

CONCLUSIONS

Reducing digestible carbohydrates in the diet can significantly reduce the absorption of carbohydrates and improve glucose metabolism disorders caused by dietary factors. These effects may be due to the fact that reducing the amount of digestible carbohydrates in the feed can reduce the number of intestinal sweet receptors induced by exposure to artificial sweeteners. © 2019 Society of Chemical Industry.

Pitfalls in short-chain fatty acid research: A methodological review

This methodological review suggests what to do and what not to do in short‐chain fatty acid (SCFA) research for researchers, supervisors, scientific reviewers, and regulatory officers. High viscosity of gut contents, existence of bacterial biofilm and of mucus layer at the mucosal surface, and rapid absorption of SCFAs make it difficult to know their concentrations at the very surface of the mucosa. As lumen or fecal concentration of SCFAs does not reflect their rate of production, these parameters should not be used as measures of SCFA production or absorption. Effects of SCFAs can vary and even become opposite at different dose, time of/after exposure or time of the day. Thus, results without dose–response, time‐course, and diurnal variance experiments can be seriously misleading. It is also to note that too much emphasis on n‐butyrate should be avoided.

Integrative physiology of transcellular and paracellular intestinal absorption

Glucose absorption by the small intestine has been studied for nearly a century. Despite extensive knowledge about the identity, functioning and regulation of the relevant transporters, there has been and there remains controversy about how these transporters work in concert to determine the overall epithelial absorption of key nutrients (e.g. sugars, amino acids) over a wide range of dietary and/or luminal concentrations. Our broader, integrative understanding of intestinal absorption requires more than the reductionist dissection of all the components and their elaboration at molecular and genetic levels. This Commentary emphasizes the integration of discrete molecular players and processes (including paracellular absorption) that, in combination, determine the overall epithelial absorption of key nutrients (e.g. sugars, amino acids) and putative anti-nutrients (water-soluble toxins), and the integration of that absorption with other downstream processes related to metabolic demands. It identifies historic key advances, controversies and future research ideas, as well as important perspectives that arise through comparative as well as biomedical physiological research.

Small intestinal epithelial permeability to water-soluble nutrients higher in passerine birds than in rodents

In the small intestine transcellular and paracellular pathways are implicated in water-soluble nutrient absorption. In small birds the paracellular pathway is quantitatively important while transcellular pathway is much more important in terrestrial mammals. However, there is not a clear understanding of the mechanistic underpinnings of the differences among taxa. This study was aimed to test the hypothesis that paracellular permeability in perfused intestinal segments is higher in passerine birds than rodents. We performed in situ intestinal perfusions on individuals of three species of passerine birds (Passer domesticus, Taeniopygia guttata and Furnarius rufus) and two species of rodents (Mus musculus and Meriones ungiculatus). Using radio-labelled molecules, we measured the uptake of two nutrients absorbed by paracellular and transcellular pathways (L-proline and 3-O-methyl-D-glucose) and one carbohydrate that has no mediated transport (L-arabinose). Birds exhibited ~2 to ~3 times higher L-arabinose clearance per cm² epithelium than rodents. Moreover, paracellular absorption accounted for proportionally more of 3-O-methyl-D-glucose and L-proline absorption in birds than in rodents. These differences could be explained by differences in intestinal permeability and not by other factors such as increased retention time or higher intestinal nominal surface area. Furthermore, analysis of our results and all other existing data on birds, bats and rodents shows that insectivorous species (one bird, two bats and a rodent) had only 30% of the clearance of L-arabinose of non-insectivorous species. This result may be explained by weaker natural selection for high paracellular permeability in animal- than in plant-consumers. Animal-consumers absorb less sugar and more amino acids, whose smaller molecular size allow them to traverse the paracellular pathway more extensively and faster than glucose.

Low intake of digestible carbohydrates ameliorates the duodenal absorption of carbohydrates in mice with glucose metabolic disorders induced by sucralose

In the current study, the protective effects of diets with low digestible carbohydrates (LDCs) on plasma glucose, plasma fasting insulin, sweet taste receptors, glucose transporters and absorption of carbohydrates in mice that consume sucralose were evaluated.

Predicting Drug Extraction in the Human Gut Wall: Assessing Contributions from Drug Metabolizing Enzymes and Transporter Proteins using Preclinical Models

Intestinal metabolism can limit oral bioavailability of drugs and increase the risk of drug interactions. It is therefore important to be able to predict and quantify it in drug discovery and early development. In recent years, a plethora of models-in vivo, in situ and in vitro-have been discussed in the literature. The primary objective of this review is to summarize the current knowledge in the quantitative prediction of gut-wall metabolism. As well as discussing the successes of current models for intestinal metabolism, the challenges in the establishment of good preclinical models are highlighted, including species differences in the isoforms; regional abundances and activities of drug metabolizing enzymes; the interplay of enzyme-transporter proteins; and lack of knowledge on enzyme abundances and availability of empirical scaling factors. Due to its broad specificity and high abundance in the intestine, CYP3A is the enzyme that is frequently implicated in human gut metabolism and is therefore the major focus of this review. A strategy to assess the impact of gut wall metabolism on oral bioavailability during drug discovery and early development phases is presented. Current gaps in the mechanistic understanding and the prediction of gut metabolism are highlighted, with suggestions on how they can be overcome in the future.

Beyond Ussing's chambers: contemporary thoughts on integration of transepithelial transport

In the mid-20th century, Hans Ussing developed a chamber that allowed for the simultaneous measurement of current and labeled probe flux across epithelia. Using frog skin as a model, Ussing used his results to propose mechanisms of transcellular Na(+) and K(+) transport across apical (exterior/luminal) and basolateral (interior) membranes that is essentially unchanged today. Others took advantage of Ussing's chambers to study mucosal tissues, including bladder and intestines. It quickly became clear that, in some tissues, passive paracellular flux, i.e., across the tight junction, was an important component of overall transepithelial transport. Subsequent work demonstrated that activation of the apical Na(+)-glucose cotransporter SGLT1 regulated paracellular permeability such that intestinal paracellular transport could coordinate with and amplify transcellular transport. Intermediates in this process include activation of p38 MAPK, the apical Na(+)/H(+) exchanger NHE3, and myosin light chain kinase (MLCK). Investigators then focused on these processes in disease. They found that TNF induces barrier dysfunction via MLCK activation and downstream caveolin-1-dependent endocytosis of the tight junction protein occludin. TNF also inhibited NHE3, and both barrier loss and PKCα-dependent NHE3 inhibition were required for TNF-induced acute diarrhea, emphasizing the interplay between transcellular and paracellular transport. Finally, studies using immune-mediated inflammatory bowel disease models showed that mice lacking epithelial MLCK were initially protected, but became ill as epithelial damage progressed and provided a tight junction-independent means of barrier loss. None of these advances would have been possible without the insights provided by Ussing and others using Ussing's ingenious, and still useful, chambers.

Intestinal and hepatic first-pass extraction of the 11β-HSD1 inhibitor AMG 221 in rats with chronic vascular catheters

1. A catheterized rat model was used to define the intestinal and hepatic components of oral bioavailability for an 11β-HSD1 inhibitor, AMG 221. These data were integrated with standard in vivo metabolism studies to elucidate the components contributing to the oral disposition of a novel drug candidate. 2. Intestinal and hepatic extraction ratios of AMG 221 obtained using a five-catheter rat model were 0.56 and 0.32, respectively. Therefore, both intestinal and hepatic extraction contributed to the first-pass component of oral bioavailability. There was no evidence for significant gut extraction of systemically administered drug. 3. Mass balance data and in vivo metabolite characterization obtained after administration of [(14)C] AMG 221 to rat showed that AMG 221 was completely absorbed from the gut lumen following an oral dose, primarily excreted in urine and was almost completely metabolized prior to excretion. 4. Hepatic bioavailability (FH), measured in two animals at various time points after oral dose administration was somewhat variable but generally characterized by an initial reduction during the absorption phase followed by an increase during the elimination phase, consistent with hepatic distribution of AMG 221. 5. The five-catheter rat model afforded estimates of hepatic and intestinal contribution to oral bioavailability that were used with other data to define the preclinical ADME characteristics of a drug candidate.

Effects of exogenous glucagon-like peptide-1 on gastric emptying and glucose absorption in the critically ill: relationship to glycemia

OBJECTIVE

To determine the acute effects of exogenous glucagon-like peptide-1 on gastric emptying, glucose absorption, glycemia, plasma insulin, and glucagon in critically ill patients.

DESIGN

Randomized, double-blind, crossover study.

SETTING

Intensive care unit.

SUBJECTS

Twenty-five mechanically ventilated patients, without known diabetes, studied on consecutive days.

INTERVENTIONS

Intravenous glucagon-like peptide-1 (1.2 pmol/kg/min) or placebo was infused between -30 and 330 mins. At 0 min, 100 mL liquid nutrient (1 kcal/mL) including 100 microg of 13C-octanoic acid and 3 grams of 3-O-methyl-glucose was administered.

MEASUREMENTS AND MAIN RESULTS

Blood glucose, serum 3-O-methyl-glucose (as an index of glucose absorption), insulin and glucagon concentrations, as well as exhaled 13CO2 were measured. The gastric emptying coefficient was calculated to quantify gastric emptying. Data are presented as mean (sd). There was a nonsignificant trend for glucagon-like peptide-1 to slow gastric emptying (gastric emptying coefficient) (glucagon-like peptide-1, 2.45 [0.93] vs. placebo, 2.75 [0.83]; p = .09). In 11 of the 25 patients, gastric emptying was delayed during placebo infusion and glucagon-like peptide-1 had no detectable effect on gastric emptying in this group (1.92 [0.82] vs. 1.90 [0.68]; p = .96). In contrast, in patients who had normal gastric emptying during placebo, glucagon-like peptide-1 slowed gastric emptying substantially (2.86 [0.58] vs. 3.41 [0.37]; p = .006). Glucagon-like peptide-1 markedly reduced the rate of glucose absorption (3-O-methyl-glucose area under the curve(0-330), 37 [35] vs. 76 [51] mmol/L/min; p < .001), decreased preprandial glucagon (at 0 min change in glucagon, -15 [15] vs. -3 [14] pmol/L; p < .001), increased the insulin/glucose ratio throughout the infusion (area under the curve(-30-330), 1374 [814] vs. 1172 [649] mU/mmol/min; p = .041), and attenuated the glycemic response to the meal (glucose area under the curve(0-330), 2071 [353] vs. 2419 [594] mmol/L/min; p = .001).

CONCLUSIONS

Exogenous glucagon-like peptide-1 lowers postprandial glycemia in the critically ill. This may occur, at least in part, by slowing gastric emptying when the latter is normal but not when it is delayed.

Effect of abdominal surgery on the intestinal absorption of lipophilic drugs: possible role of the lymphatic transport

Although abdominal surgery is a routine procedure in clinical practice and in preclinical investigation, little is known regarding its effect on the intestinal absorption of drugs. The aim of this study was to investigate the effect of abdominal surgery on the intestinal absorption of highly lipophilic compounds with different absorption mechanisms following oral administration. The 2 compounds that were tested were biopharmaceutical classification system (BCS) class 2 model lipophilic cannabinoid derivatives, dexanabinol and PRS-211,220. Although dexanabinol is mostly absorbed via passive diffusion to the portal blood, PRS-211,220 is absorbed mostly via lymphatic transport. In this work, we compared the absorption of these compounds after abdominal surgery in rat with the absorption data obtained from naïve animals. The outcomes of this investigation showed that the abdominal surgery mostly affected the absorption process on the preenterocyte level, as indicated by the 2-fold increase in the extent of intestinal absorption of dexanabinol, which is a compound with a low degree of intestinal lymphatic transport. However, the lymphatic transport was not affected by the surgical procedure as evident by the absence of change in the extent of absorption of PRS-211,220, which is transported to the systemic circulation mainly by intestinal lymphatics. In conclusion, abdominal surgery can significantly affect the intestinal absorption of lipophilic drugs; however, intestinal lymphatic transport seems to be less affected by the abdominal surgery.

Hydroxyitraconazole, formed during intestinal first-pass metabolism of itraconazole, controls the time course of hepatic CYP3A inhibition and the bioavailability of itraconazole in rats

Itraconazole (ITZ) is a substrate of CYP3A and both ITZ and hydroxyitraconazole (OH-ITZ), a major metabolite formed by CYP3A, are potent inhibitors of CYP3A. The concentration- and time-dependent changes in the hepatic availability (F(H)) of ITZ were evaluated in rats after oral doses of 5 and 40 mg/kg. Simultaneous blood samples were obtained from the aorta, portal vein, and hepatic vein for 24 h following duodenal ITZ administration, and concentrations of ITZ and OH-ITZ determined by LC/MS. During the absorption phase, the F(H) of ITZ increased from 0.2 to 1.0, reflecting the time course of hepatic CYP3A inhibition. A counterclockwise hysteresis was observed between ITZ concentrations entering the liver (C(IN,ITZ)) and F(H), whereas there was no time delay observed between the change in F(H) and the OH-ITZ concentrations entering the liver (C(IN,OH-ITZ)). The direct relationship between C(IN,OH-ITZ) and F(H) suggested that OH-ITZ was mainly responsible for the inhibition of CYP3A. A positive portal venous-aortic gradient for OH-ITZ was measured after duodenal administration of ITZ, indicating intestinal formation of OH-ITZ. The in vivo Ki for OH-ITZ (38 +/- 3 nM) was estimated from C(IN,OH-ITZ) versus F(H) of ITZ, and is similar to values obtained from inhibition of midazolam hydroxylation in CYP3A4 supersomes (Drug Metab Dispos 32:1121-1131, 2004). The data suggest that OH-ITZ, formed by intestinal CYP3A, controls the time course of hepatic CYP3A inhibition and is mainly responsible for the observed increase in F(H) of ITZ.

Hydrotropic polymeric micelles for enhanced paclitaxel solubility: in vitro and in vivo characterization

The purpose of this investigation was to characterize the in vitro stability and in vivo disposition of paclitaxel in rats after solubilization of paclitaxel into hydrotropic polymeric micelles. The amphiphilic block copolymers consisted of a micellar shell-forming poly(ethylene glycol) (PEG) block and a core-forming poly(2-(4-vinylbenzyloxy)-N,N-diethylnicotinamide) (P(VBODENA)) block. N,N-Diethylnicotinamide (DENA) in the micellar inner core resulted in effective paclitaxel solubilization and stabilization. Solubilization of paclitaxel using polymeric micelles of poly(ethylene glycol)-b-P(D,L-lactide) (PEG-b-PLA) served as a control for the stability study. Up to 37.4 wt % paclitaxel could be loaded in PEG-b-P(VBODENA) micelles, whereas the maximum loading amount for PEG-b-PLA micelles was 27.6 wt %. Thermal analysis showed that paclitaxel in the polymeric micelles existed in the molecularly dispersed amorphous state even at loadings over 30 wt %. Paclitaxel-loaded hydrotropic polymeric micelles retained their stability in water for weeks, whereas paclitaxel-loaded PEG-b-PLA micelles precipitated in a few days. Hydrotropic polymer micelles were more effective than PEG-PLA micelle formulations in inhibiting the proliferation of human cancer cells. Paclitaxel in hydrotropic polymer micelles was administered orally (3.8 mg/kg), intravenously (2.5 mg/kg), or via the portal vein (2.5 mg/kg) to rats. The oral bioavailability was 12.4% of the intravenous administration. Our data suggest that polymeric micelles with a hydrotropic structure are superior as a carrier of paclitaxel due to a high solubilizing capacity combined with long-term stability, which has not been accomplished by other existing polymeric micelle systems.

Calcium absorption by Cav1.3 induces terminal web myosin II phosphorylation and apical GLUT2 insertion in rat intestine

Glucose absorption in rat jejunum involves Ca(2+)- and PKC betaII-dependent insertion of GLUT2 into the apical membrane. Ca(2+)-induced rearrangement of the enterocyte cytoskeleton is thought to enhance paracellular flow. We have therefore investigated the relationships between myosin II regulatory light chain phosphorylation (RLC(20)), absorption of glucose, water and calcium, and mannitol clearance. ML-7, an inhibitor of myosin light chain kinase, diminished the phloretin-sensitive apical GLUT2 but not the phloretin-insensitive SGLT1 component of glucose absorption in rat jejunum perfused with 75 mM glucose. Western blotting and immunocytochemistry revealed marked decreases in RLC(20) phosphorylation in the terminal web and in the levels of apical GLUT2 and PKC betaII, but not SGLT1. Perfusion with phloridzin or 75 mM mannitol, removal of luminal Ca(2+), or inhibition of unidirectional (45)Ca(2+) absorption by nifedipine exerted similar effects. ML-7 had no effect on the absorption of 10 mM Ca(2+), nor clearance of [(14)C]-mannitol, which was less than 0.7% of the rate of glucose absorption. Water absorption did not correlate with (45)Ca(2+) absorption or mannitol clearance. We conclude that the Ca(2+) necessary for contraction of myosin II in the terminal web enters via an L-type channel, most likely Ca(v)1.3, and is dependent on SGLT1. Moreover, terminal web RLC(20) phosphorylation is necessary for apical GLUT2 insertion. The data confirm that glucose absorption by paracellular flow is negligible, and show further that paracellular flow makes no more than a minimal contribution to jejunal Ca(2+) absorption at luminal concentrations prevailing after a meal.

Portal venous hyperinsulinemia does not stimulate gut glucose absorption in the conscious dog

The purpose of the present study was to assess whether physiological portal vein hyperinsulinemia stimulates gut glucose absorption in vivo. Chronically catheterized (femoral artery, portal vein, inferior vena cava, and proximal and distal duodenum) and instrumented (Doppler flow probe on portal vein) insulin (INS, 2 mU.kg(-1).min(-1), n = 6) or saline (SAL, n = 5) infused dogs were studied during basal (30 minutes) and experimental (90 minutes) periods. Arterial and portal vein plasma insulin were 3.3- and 3.2-fold higher, respectively, throughout the study in INS compared to SAL. An intraduodenal glucose infusion of 8 mg.kg(-1).min(-1) was initiated at t = 0 minutes. At t = 20 and 80 minutes, a bolus of 3-O-[3H]methylglucose (MG) and L-[14C]glucose (L-GLC) was injected intraduodenally. Phloridzin, an inhibitor of the Na+ -dependent glucose transporter (SGLT1), was infused from t = 60 to 90 minutes in the presence of a peripheral isoglycemic clamp. Net gut glucose output (NGGO) was 5.2 +/- 0.6 and 4.6 +/- 0.8 mg.kg(-1).min(-1) in INS and SAL, respectively, from t = 20 to 60 minutes. Transporter-mediated absorption was 87% +/- 2% of NGGO in both INS and SAL. Passive gut glucose absorption was 13% +/- 2% of NGGO in both INS and SAL. Phloridzin-induced inhibition of transporter-mediated absorption completely abolished passive absorption of L-GLC in both groups. This study shows that under physiological conditions, a portal vein insulin infusion that results in circulating hyperinsulinemia does not increase either transporter-mediated or passive absorption of an intraduodenal glucose load.

THE EFFECT OF ANESTHESIA AND SURGERY ON CYP3A ACTIVITY IN RATS

The purpose of this investigation was to examine the effects of surgery and anesthesia on in vivo CYP3A activity and portal venous blood flow. Midazolam, a CYP3A probe for both rats and humans, was administered orally (2.7 mg), intravenously (0.57 mg), or via the portal vein (0.57 mg) to rats 4 h after anesthesia with ketamine/xylazine and surgery for placement of indwelling vascular and duodenal catheters and 3 days after surgery (chronic). The systemic clearance of midazolam was 51 +/- 4 ml/min/kg in the chronic animals, and this was significantly decreased (29 +/- 1 ml/min/kg, P = 0.024) in acute rats studied 4 to 6 h after anesthesia and surgery. The hepatic availability (FH), directly determined from the aortic and hepatic venous concentration gradient, was significantly higher in the acute animals (0.57 +/- 0.05) compared with the chronic animals (0.33 +/- 0.07, P = 0.001). Hepatic availability was determined using a classical approach in which FH was calculated from the area under the plasma concentration versus time curve ratio after portal venous or intravenous administration. FH was higher in the acute rats (0.48) compared with the chronic animals (0.27 +/- 0.03). Portal venous blood flow was significantly lower in the acute animals (5.0 +/- 0.4 ml/min/100 g body weight) compared with the chronic animals (9.1 +/- 0.9 ml/min/100 g body weight, P = 0.015). The effect of surgery and anesthesia was confirmed using the indicator dye dilution method after infusion of [14C]polyethylene glycol 4000 into the superior mesenteric artery. Our data suggest that anesthesia and surgery decreases both hepatic CYP3A activity and hepatic blood flow in rats. Studies performed in rats within 3 days of surgery and anesthesia are conducted under nonphysiologic conditions and therefore provide inaccurate assessment of drug disposition, in particular, clearance and bioavailability.

Prior exercise enhances passive absorption of intraduodenal glucose

The purpose of this study was to assess whether a prior bout of exercise enhances passive gut glucose absorption. Mongrel dogs had sampling catheters, infusion catheters, and a portal vein flow probe implanted 17 days before an experiment. Protocols consisted of either 150 min of exercise (n = 8) or rest (n = 7) followed by basal (-30 to 0 min) and a primed (150 mg/kg) intraduodenal glucose infusion [8.0 mg x kg-1x min-1, time (t) = 0-90 min] periods. 3-O-[3H]methylglucose (absorbed actively, facilitatively, and passively) and l-[14C]glucose (absorbed passively) were injected into the duodenum at t = 20 and 80 min. Phloridzin, an inhibitor of the active sodium glucose cotransporter-1 (SGLT-1), was infused (0.1 mg x kg-1 x min-1) into the duodenum from t = 60-90 min with a peripheral venous isoglycemic clamp. Duodenal, arterial, and portal vein samples were taken every 10 min during the glucose infusion, as well as every minute after each tracer bolus injection. Net gut glucose output in exercised dogs increased compared with that in the sedentary group (5.34 +/- 0.47 and 4.02 +/- 0.53 mg x kg-1x min-1). Passive gut glucose absorption increased approximately 100% after exercise (0.93 +/- 0.06 and 0.45 +/- 0.07 mg x kg-1 x min-1). Transport-mediated glucose absorption increased by approximately 20%, but the change was not significant. The infusion of phloridzin eliminated the appearance of both glucose tracers in sedentary and exercised dogs, suggesting that passive transport required SGLT-1-mediated glucose uptake. This study shows 1). that prior exercise enhances passive absorption of intraduodenal glucose into the portal vein and 2). that basal and the added passive gut glucose absorption after exercise is dependent on initial transport of glucose via SGLT-1.

Intestinal passive absorption of water-soluble compounds by sparrows: effect of molecular size and luminal nutrients

We tested predictions that: (1) absorption of water-soluble probes decreases with increasing molecular size, consistent with movement through effective pores in epithelia, and (2) absorption of probes is enhanced when measured in the presence of luminal nutrients, as predicted for paracellular solvent drag. Probes (L-arabinose, L-rhamnose, perseitol, lactulose; MW 150.1-342.3 Da) were gavaged in nonanesthetized House sparrows ( Passer domesticus), or injected into the pectoralis, and serially measured in plasma. Bioavailability was calculated as F=AUC by gavage/AUC by injection, where AUC is the area under the curve of plasma probe concentration vs. time. Consistent with predictions, F declined with probe size by 75% from the smallest to the largest probe, and absorption of probes increased by 40% in the presence of luminal glucose or food compared to a mannitol control. Absorption of water-soluble probes by sparrows is much higher than in humans, which is much higher than in rats. These differences seem mainly attributable to differences in paracellular solvent flux and less to differences in effective paracellular pore size.

Chronic Staphylococcal enterotoxin B and lipopolysaccharide induce a bimodal pattern of hepatic dysfunction and injury

OBJECTIVE

To determine the effect of chronic exposure to endotoxin (lipopolysaccharide) and Staphylococcal enterotoxin B on hepatic injury and function.

DESIGN

Prospective, controlled trial.

SETTING

Research laboratory in a university hospital.

SUBJECTS

Male Sprague-Dawley rats weighing 325-350 g with chronic vascular and bile catheters.

INTERVENTIONS

Chronically catheterized rats were treated daily with saline, 50 microg/kg Staphylococcal enterotoxin B alone, 1000 microg/kg lipopolysaccharide alone, 1000 microg/kg lipopolysaccharide with 50 microg/kg Staphylococcal enterotoxin B, or 100 microg/kg lipopolysaccharide with 50 microg/kg Staphylococcal enterotoxin B for 10 days. Serum and biliary measures of hepatic injury and dysfunction were measured before and then 6 hrs and 1, 2, 3, 7, and 10 days after the start of treatment. The animals were killed at 10 days and the livers examined histologically.

MEASUREMENTS AND MAIN RESULTS

Mean rates of bile flow, biliary indocyanine green excretion, and bile acid flux were significantly decreased immediately after treatment (6 hr, 1 and 2 days) and then at 10 days. Increases in biliary and serum gamma-glutamyltransferase and serum bile acids also occurred in a similar bimodal pattern. Animals treated with lipopolysaccharide or Staphylococcal enterotoxin B alone became tolerant and did not develop the bimodal pattern of hepatic dysfunction. Histologic examination of the liver at 10 days revealed periportal inflammation and fibrosis.

CONCLUSIONS

The combination of lipopolysaccharide and Staphylococcal enterotoxin B leads to late liver injury, whereas either toxin alone does not. These data may explain the frequent development of liver dysfunction in patients exposed to multiple bacterial toxins such as in sepsis, multiple-system organ failure, and other diseases with altered intestinal permeability.

Endotoxin-induced hyperlactatemia results from decreased lactate clearance in hemodynamically stable rats

OBJECTIVE

To determine whether endotoxin-induced hyperlactatemia in hemodynamically stable animals is due to increased lactate production or decreased lactate clearance by measuring lactate turnover rate in the vascular compartment (LTRvc).

DESIGN

Prospective, controlled trial.

SETTING

Research laboratory in a university hospital.

SUBJECTS

Male Sprague-Dawley rats weighing 275-425 g with chronic vascular catheters.

INTERVENTIONS

Chronically catheterized rats were treated with 6 microg/kg endotoxin or saline. LTRvc was determined from the specific activity of carbon-14 [14C]lactate in aortic blood during a constant infusion of [14C]lactate into the inferior vena cava. The role of the splanchnic organs in lipopolysaccharide-induced alterations in LTRvc was determined from the splanchnic first-pass clearance of [14C]lactate infused into the superior mesenteric artery and direct measurements of blood lactate concentration gradients across the splanchnic organs.

MEASUREMENTS AND MAIN RESULTS

Despite a 260% increase in lactate concentrations after lipopolysaccharide treatment, the specific activity of [14C]lactate and the LTRvc did not change, indicating that lipopolysaccharide-induced hyperlactatemia is caused by decreased lactate clearance from the vascular compartment rather than increased lactate flux into the vascular compartment. In contrast, lactate clearance by the splanchnic system was increased. The specific activity of [14C]lactate in aortic blood decreased 33% after lipopolysaccharide treatment when the [14C]lactate was infused into the superior mesenteric artery, indicating increased first-pass clearance of [14C]lactate by the splanchnic organs. Furthermore, the hepatic venous-aortic concentration gradient of lactate became increasingly negative after lipopolysaccharide treatment, indicating increased vascular extraction of lactate by the splanchnic system (0.07 +/- 0.07 micromol/mL vs. -0.34 +/- 0.14 micromol/mL).

CONCLUSIONS

Lipopolysaccharide-induced hyperlactatemia in hemodynamically stable rats is caused by a net decrease in lactate clearance from the vascular compartment despite the fact that the clearance of lactate by the splanchnic system remains intact.

The active and passive components of glucose absorption in rat jejunum under low and high perfusion stress

In order to determine how perfusion design affects the relationship of the apparent "active" and "passive" components of glucose absorption, rat jejunum was perfused with 50 mM glucose under conditions of low and high mechanical stress. Phloretin or cytochalasin B was used to inhibit GLUT2 and phloridzin to inhibit SGLT1. In low stress perfusions, the ratios of the "passive" to the "active" components determined using phloretin and phloridzin were 2.2 and 0.43, respectively. This discrepancy was explained by the fact that phloridzin inhibits not only SGLT1 but also indirectly that part of the GLUT2-mediated component controlled by SGLT1 through the glucose-induced activation and recruitment of GLUT2 to the brush-border membrane. In high stress perfusions, the ratios of the "passive" to the "active" components determined using phloretin and phloridzin were 0.94 and 0.95, respectively; cytochalasin B gave 0.95. The identity of these results was explained by the observation that the passive component is not dependent on the active component, because glucose-induced activation and recruitment of GLUT2 does not occur in high stress perfusions. Simultaneous inhibition of SGLT1 and GLUT2 in high stress perfusions with phloridzin and cytochalasin B inhibited absorption by 92 +/- 7 %; non-carrier-mediated transport is therefore minimal. Our data provide support for the view that the term "facilitated" should be used to replace the term "passive" in describing the component now known to be mediated by GLUT2. The study of the mechanism and regulation of this facilitated component depends crucially on the design of the perfusion system.

Loads, capacities and safety factors of maltase and the glucose transporter SGLT1 in mouse intestinal brush border

Safety factors are defined as ratios of biological capacities to prevailing natural loads. We measured the safety factor of the mouse intestinal brush-border hydrolase maltase in series with the glucose transporter SGLT1, for comparison with previous studies of sucrase and lactase. Dietary maltose loads increased 4-fold from virgin to lactating mice. As in previous studies of intestinal adaptive regulation, that increase in load without change in dietary composition resulted in an increase in maltase and SGLT1 capacities mediated non-specifically by an increase in intestinal mass, without change in maltase or SGLT1 activities per milligram of tissue. Maltase and SGLT1 capacities increased only sublinearly with load during lactation, such that safety factors decreased with load: from 6.5 to 2.4 for maltase, and from 1.1 to 0.5 for SGLT1. The apparently high safety factor for maltase may be related to the multiple natural substrates hydrolysed by the multiple sites of maltase activity. The apparently low safety factor for SGLT1 is made possible by the contribution of hindgut fermentation to carbohydrate digestion. SGLT1 activity is paradoxically higher for mice consuming sucrose than for mice consuming maltose, despite maltose hydrolysis yielding double the glucose load yielded by sucrose hydrolysis, and despite glucose constituting the load upon SGLT1.

Transporter-mediated absorption is the primary route of entry and is required for passive absorption of intestinal glucose into the blood of conscious dogs

To determine the contributions of transporter-mediated and passive absorption during an intraduodenal glucose infusion in a large animal model, six mongrel dogs had sampling catheters (portal vein, femoral artery, duodenum), infusion catheters (vena cava, duodenum) and a portal vein flow probe implanted 17 d before an experiment. Protocols consisted of a basal (-30 to 0 min) and an experimental (0-90 min) period. An intraduodenal glucose infusion of 44 micromol/(kg. min) was initiated at t = 0 min. At t = 20 and 80 min, 3-O-[3H]methylglucose and L-[14C]glucose (L-Glc) were injected intraduodenally. Phloridzin, an inhibitor of the Na+/K+ ATP-dependent transporter (SGLT1), was infused from t = 60 to 90 min in the presence of a peripheral isoglycemic clamp. Net gut glucose output was 21.1 +/- 3.0 micromol/(kg. min) from t = 0 to 60 min. Transporter-mediated glucose absorption was calculated using three approaches, which involved either direct measurements or indirect estimates of duodenal glucose analog radioactivities, to account for the assumptions and difficulties inherent to duodenal sampling. Values were essentially the same regardless of calculations used because transporter-mediated absorption was 89 +/- 1%, 90 +/- 2% and 91 +/- 2% of net gut glucose output. Phloridzin-induced inhibition of transporter-mediated absorption completely abolished passive absorption of L-Glc. We conclude that in dogs, transporter-mediated glucose absorption constitutes the vast majority of glucose absorbed from the gut and is required for passive glucose absorption. The method described here is applicable to investigation of the mechanisms of gut glucose absorption under a variety of nutritional, physiologic and pathophysiologic conditions.

The antisecretory factor: synthesis, anatomical and cellular distribution, and biological action in experimental and clinical studies

The antisecretory factor (AF) is a 41-kDa protein that provides protection against diarrheal diseases and intestinal inflammation. Its cDNA has been cloned and sequenced. AF is highly potent, with 10(-12) mol of recombinant AF being sufficient to counteract experimentally induced diarrhea in rat. The antisecretory activity is exerted by a peptide located between positions 35 and 50 of the AF sequence. Synthetic peptides based on this sequence are promising candidates for drugs to counteract intestinal hypersecretion, as well as imbalances of fluid transport in other body compartments. AF probably exerts its effects via nerves; AF immediately and potently inhibits ion transport across isolated nerve membranes from Deiters' cells. Immunocytochemistry has shown that AF is present in most tissues in the body, and in situ nucleic acid hybridization has shown that cells that store AF are also capable of AF synthesis. The endogenous plasma level of AF is increased by enterotoxins and by certain food constituents such as hydrothermally processed cereals. These cereals significantly improve clinical performance in patients suffering from inflammatory bowel diseases. AF-enhancing food also protects domestic animals against diarrheal diseases, and such feed has been used successfully in Swedish swine farming for the past 10 years. Increased understanding of AF action might result in expanded clinical applications and confirm that AF is an important regulator of homeostasis.

Enhanced intestinal motility influences absorption in anaesthetized rat

The subject of this investigation was to study influence of the intestinal motility on absorption of 3-o-methyl-D-glucose (3-OMG), mannitol and polyethylene glycol (PEG 4000), used as absorption route markers, while monitoring cardiovascular parameters in an intestinal in situ model in rats. Rats were anaesthetized with Inactin(R) and Rapinovet(R). A segment of duodenum, approximately 10 cm, was perfused single-pass with saline containing unlabelled and radioactive 3-OMG, PEG 4000 or mannitol. The PEG 4000 was recovered almost completely in the intestinal perfusate suggesting an intact mucosal integrity. Most animals exhibited an intestinal contractile activity resembling fed motility except seven out of 19 given Rapinovet, which showed a 'burst-type' pattern resembling migrating motor complex (MMC). Absorption of 3-OMG in rats with MMC-like motility appears to be lower than in rats with fed-like motility, while no such difference was seen for mannitol. Moreover, there was a positive correlation (r 2=0.75) between intestinal activity (fed) and absorption of 3-OMG, but not with absorption of mannitol. The carrier-mediated absorption of 3-OMG was not only influenced by intestinal motility, but also by its pattern. This was not observed with mannitol, which is passively absorbed.

Endotoxin-induced reduction in biliary indocyanine green excretion rate in a chronically catheterized rat model

Using a nonstressed chronically catheterized rat model in which the common bile duct was cannulated, we studied endotoxin-induced alterations in hepatic function by measuring changes in the maximal steady-state biliary excretion rate of the anionic dye indocyanine green (ICG). Biliary excretion of ICG was calculated from direct measurements of biliary ICG concentrations and the bile flow rate during a continuous vascular infusion of ICG. Despite significant elevations in mean peak serum tumor necrosis factor-alpha (TNF-alpha) concentrations (90.9 +/- 16.2 ng/ml), there was no effect on mean rates of bile flow or biliary ICG clearance after administration of 100 microg/kg endotoxin at 6 or 24 h. Significant differences from mean baseline rates of bile flow and biliary ICG excretion did occur after administration of 1,000 microg/kg endotoxin (mean peak TNF-alpha 129.6 +/- 24.4 ng/ml). Furthermore, when rats were treated with up to 16 microg/kg of recombinant TNF-alpha, there was no change in mean rates of bile flow or ICG biliary clearance compared with baseline values. These data suggest that the complex regulation of biliary excretion is not mediated solely by TNF-alpha.

Staphylococcal enterotoxin B potentiates LPS-induced hepatic dysfunction in chronically catheterized rats

Most models of liver dysfunction in sepsis use endotoxin (lipopolysaccharide; LPS) to induce a pathophysiological response. In our study published in this issue (Beno DWA, Uhing MR, Goto M, Chen Y, Jiyamapa-Serna VA, and Kimura RE. Am J Physiol Gastrointest Liver Physiol 280: G858-G865, 2001), the adverse effect of LPS on hepatic function in vivo was only significant at relatively high LPS doses despite high tumor necrosis factor-alpha concentrations. However, many patients with sepsis are exposed to multiple bacterial toxins that may augment the immune response, resulting in increased hepatic dysfunction. We have developed a model of polymicrobial sepsis by parentally administering a combination of staphylococcal enterotoxin B (SEB) and LPS. Using this model, we demonstrate that SEB (50 microg/kg) potentiates the effect of LPS-induced hepatic dysfunction as measured by decreased rates of biliary indocyanine green clearance and bile flow. These increases were most pronounced with doses of 10 and 100 microg/kg LPS, doses that by themselves do not induce hepatic dysfunction. This may explain the seemingly increased incidence and severity of liver dysfunction in sepsis, and it suggests that the exclusive use of LPS for replicating septic shock may not be relevant for studies of hepatic dysfunction.

Passive absorption of hydrophilic carbohydrate probes by the house sparrow Passer domesticus

To evaluate the permeability of the intestine of the house sparrow Passer domesticus to hydrophilic compounds, we applied a pharmacokinetic technique to measure in vivo absorption of two carbohydrate probes, l-arabinose and d-mannitol. Probes were fed or injected, and blood and excreta were subsequently collected and analyzed by gas chromatography/mass spectrometry. Following injection, plasma probe concentration decreased in a log-linear fashion, implying single-compartment, first-order kinetics. Following oral administration, plasma probe concentrations increased, reached a maximum at 10 min and then decreased in log-linear fashion. Mannitol and arabinose absorption were calculated from the areas under the post-absorption plasma curve and the respective distribution spaces and elimination constants. The amounts absorbed increased linearly with the concentration administered (range 1–1000 mmol × l(−1)), implying a passive process. The mouth-to-cloaca retention time of digesta, measured using the non-absorbable compound potassium ferrocyanide, was independent of probe concentration. On average, 69% of the oral dose of probe was absorbed and this was independent of the concentration of probe administered. This paper supports an earlier report of substantial passive glucose absorption in house sparrows and offers a method to study the extent of hydrophilic solute absorption, which has importance for future research in areas as diverse as biomedical, ecological and evolutionary physiology.

Noninvasive in vivo analysis of human small intestinal paracellular absorption: regulation by Na+-glucose cotransport

Activation of intestinal Na+-glucose cotransport increases paracellular movement of inert tracers in cultured monolayers, isolated rodent intestinal mucosae, and in rodents in vivo. However, not all studies have demonstrated comparable effects on human intestinal paracellular absorption. We sought to assess the effects of Na+-glucose cotransport on paracellular absorption in human beings using a simple noninvasive assay. Study subjects drank six 200-ml doses of test solution, composed of 0.8% w/v creatinine (sufficient to overwhelm endogenous creatinine) in 277 mM glucose or mannitol and urine was collected. Intestinal creatinine absorption is paracellular. Once absorbed, creatinine is cleared into the urine. Therefore, urinary creatinine recovery reflects intestinal paracellular creatinine absorption. Total urinary creatinine recovery was 55% +/- 4% of creatinine ingested with glucose and 38% +/- 9% of creatinine ingested with mannitol (p < 0.001). Thus, intestinal paracellular absorption of creatinine is increased by the presence of luminal glucose. Our results are consistent with in vivo human regulation of mucosal permeability by Na+-glucose cotransport.

Permeability of the rat small intestinal epithelium along the villus-crypt axis: effects of glucose transport

BACKGROUND & AIMS

The aim of this study was to elucidate the permeability characteristics of the epithelium along the villus-crypt axis and investigate the effect of glucose transport on these characteristics along this axis.

METHODS

The disappearance rates of (14)C-mannitol and (51)Cr-EDTA or (3)H-inulin were determined as clearance (Cl(x)) from a recirculating perfusion system of the jejunal lumen in anesthetized rats. Net fluid transport was varied over a large range by exchanging mannitol with glucose in the perfusate solution and by inhibition of nervously mediated secretory processes with hexamethonium. The perfusion rate was 0.5 or 0.2 mL/min.

RESULTS

Cl(Man) enhanced significantly with increasing net fluid transport (secretion 8.50+/-1.88, to absorption 16.72+/-1.75 microL x min(-1) x g(-1)) and with glucose perfusates. Cl(Cr-EDTA) was constant irrespective of net fluid transport and was reduced to insignificant values at a perfusion rate of 0.2 mL/min. Cl(In) was not different from zero.

CONCLUSIONS

The absorbing apical part of the villus contains small pores (radius, <6 A) allowing passive transport via solvent drag of, e.g., monosaccharides, whereas the pores in the crypts are large (50-60 A) and inaccessible to the luminal content. The basal part of the villus contains medium-sized pores (10-15 A) through which no solvent drag occurs. Active glucose transport in the rat mainly increases the number of small pores accessible for passive transport, whereas the size of these pores seems to stay constant.

Current methodologies used for evaluation of intestinal permeability and absorption

This review article will focus on the various techniques that are currently employed by drug discovery scientists in evaluating permeability/absorption of drug candidates during the drug candidate selection process. Various preclinical methodologies are available; each having advantages and disadvantages, but it is the judicious use of these techniques that can help identify drug candidates that will be well absorbed in humans. It is well recognized that the human intestinal permeability cannot be accurately predicted based on a single methodology (in vitro: tissue/cell culture, in situ, or in vivo).

Show me the pathway! Regulation of paracellular permeability by Na(+)-glucose cotransport

The physiological impact of Na(+)-nutrient cotransport-dependent regulation of intestinal tight junction permeability has been controversial. Nonetheless, increased permeability of small intestinal mucosae and enterocyte tight junctions as a consequence of Na(+)-nutrient cotransport has been documented by a significant number of in vivo and in vitro studies. Some details of the intracellular signaling events that regulate this process have been described recently. The aims of this article are to: (i) review studies of tight junction regulation and paracellular nutrient absorption in mammalian intestine, (ii) identify potential applications of tight junction regulation, and (iii) summarize recent progress in defining molecular mechanisms that lead to altered tight junction permeability.

Discrepancies between effects of recombinant human growth hormone on absorption and secretion of water and electrolytes on the human jejunum compared to results reported on rat jejunum

Previous studies in rats showed that the administration of recombinant human growth hormone markedly increased intestinal absorption of electrolytes and water and suggested that growth hormone would be a useful antidiarrheal agent. We therefore examined the effect of recombinant human growth hormone on the human jejunum in vivo, using a triple lumen nonabsorbable marker technique. Healthy subjects were studied on two different test days, one as a control and a second where recombinant human growth hormone was injected subcutaneously in a dose of 100 microg/kg. With this dose we achieved equal or higher growth hormone serum levels than in previous rat studies. However the administration of recombinant human growth hormone did not stimulate absorption or inhibit secretion of water and electrolytes in the human jejunum in vivo. We believe that the discrepancy between humans and rats is most likely due to the species difference rather than to differences in methods that were used. Therefore recombinant human growth hormone cannot be considered a useful proabsorptive antidiarrheal agent in humans.

Detection and analysis of gastrointestinal sounds in normal and small bowel obstructed rats

This study is aimed at detecting gastrointestinal sounds (GIS) and correlating their characteristics with gastrointestinal (GI) conditions. The central hypotheses are that GIS generation depends on the motility patterns and the mechanical properties of the gut, and that changes in those result in measurable differences in GIS. An animal model which included both healthy rats and those with small bowel obstruction (SBO) was developed. The acoustic bursts of GIS were detected by amplitude thresholding the signal envelope. Three methods of envelope estimation were proposed and evaluated. Envelope estimation using a Hilbert transform was found to produce the best results in the current application. The duration and dominant frequency of each detected GIS event was estimated and clear differences between healthy and diseased rats were discovered. In the control state, GIS events were found to consistently be of relatively short duration (3-65 ms). Although the majority of events in the SBO state had similar short duration, infrequent longer events were also detected and appeared to be pathognomonic. Long duration events (> 100 ms) occurred in each of seven obstructed, but in none of 14 non-obstructed, cases (p < 0.001). It is concluded that GIS analysis may prove useful in the non-invasive, rapid, and accurate diagnosis of SBO.

Importance of colonic bacterial fermentation in short bowel patients: small intestinal malabsorption of easily digestible carbohydrate

The small intestine's large capacity for glucose absorption and for adaptation seems to contradict the reported importance of carbohydrate malabsorption in short bowel (SB) patients. The aim of the present study was to investigate the occurrence of malabsorption in these patients ingesting realistic amounts of carbohydrates. We performed a dose-response study [ingestion of increasing amounts of glucose and complex carbohydrates (boiled rice and wheat bread), and the nonabsorbable disaccharide lactulose] in SB patients with an intact colon. The hydrogen (H2) -breath test and changes in serum acetate were used to evaluate colonic fermentation and, thus, indirectly, the lack of small intestinal carbohydrate assimilation. Blood glucose and plasma insulin were measured to evaluate absorption. Plasma concentrations of the ileal brake hormones--glucagon-like peptide-1 (GLP-1) and peptide tyrosine tyrosine (PYY)--were measured to test whether release of these hormones was related to colonic fermentation. Significant amounts of 25 g and 50 g glucose, and of the bread and rice meals were fermented rather than absorbed, as judged by the increases in end-expiratory H2. Serum acetate concentrations were significantly higher in SB patients than in healthy controls. The orocecal transit times of all test meals ranged from 15 to 120 min. GLP-1 and PYY releases in SB patients were significantly higher than in healthy volunteers. They were mutually parallel and paralleled the increase in insulin. They were not related to ongoing fermentation or to intraluminal carbohydrate content per se, but most probably to absorption of glucose in the distal bowel. In conclusion, well-adapted SB patients had pronounced small intestinal malabsorption of carbohydrate, even after ingestion of small amounts of easily absorbable carbohydrates. A fast small intestinal spreading of carbohydrates, once in the small intestine, and a spill-over to the colon seem to explain the data best.

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.