Dependence of intestinal absorption in vivo on the unstirred layer

Utilizing the Combination of Binding Kinetics and Micro-Pharmacokinetics Link in Vitro α-Glucosidase Inhibition to in Vivo Target Occupancy

Many compounds with good inhibitory activity (i.e., high affinity) within in vitro experiments failed in vivo studies due to a lack of efficacy from limited target occupancy (TO) in the drug discovery process. Recently, it was found that rate constants of the formation and dissociation of the binary drug-target complex, rather than affinity, often govern in vivo efficacy. Therefore, the binding kinetics (BK) properties of compound-target interaction are emerging as a pivotal parameter. However, it is obvious that BK rate constants of the compound against target would not be directly linked to the in vivo TO unless the compound concentration in the target vicinity at any time point (TPK) can be evaluated. Here, we developed a novel simulation model to quantitate the dynamic change of target engagement over time in rat with a combined use of BK and TPK features of Epicatechin gallate (ECG) and epigallocatechin gallate (EGCG) on the basis of α-glucosidase (AGH). Analysis of the results displayed that the percent of maximum AGH occupancies by the ECG were varied significantly from 48.9 to 95.3% and by the EGCG slightly from 96 to 99.8%; that the time course of above 70% engagement by ECG spanned a range from 0 to 0.64 h and by EGCG a range of 1.5 to 8.9 h in four different intestinal segments of the rat. It was clearly analyzed how each parameter in the simulation model effected on the in vivo the AGH engagement by ECG and EGCG. Our results provide a novel approach for assessing the potential inhibitory activity of the compounds against AGH.

Deconstructing the physical processes of digestion: reductionist approaches may provide greater understanding

I provide a broad overview of the physical factors that govern intestinal digestion i.e. the admixture of food particles in digesta with secreted enzymes and the subsequent mass transfer of liberated nutrients from the surfaces of particles to the gut wall, with a view to outlining the quantitative work that is required to determine the relative importance of these factors in the digestion of particular foods. I first discuss what is known of the mechanical forces generated by contraction of the walls of the various segments of the gut and the level of diffusive, and advective mixing that it generates within the lumen. I then discuss the particular physical effects that may limit the digestion of solid, physically and/or chemically homogenous and heterogeneous food particles, notably capillarity, porosity, poro-elastic flow and compaction and their likely effects on diffusive and convective mass transfer at particulate surfaces. Similarly, I discuss mucins and morphology on mass transfer of nutrients to the gut wall i.e. the mucosa.

An exploration of the microrheological environment around the distal ileal villi and proximal colonic mucosa of the possum (Trichosurus vulpecula)

Multiple particle-tracking techniques were used to quantify the thermally driven motion of ensembles of naked polystyrene (0.5 µm diameter) microbeads in order to determine the microrheological characteristics around the gut mucosa. The microbeads were introduced into living ex vivo preparations of the wall of the terminal ileum and proximal colon of the brushtail possum (Trichosurus vulpecula). The fluid environment surrounding both the ileal villi and colonic mucosa was heterogeneous; probably comprising discrete viscoelastic regions suspended in a continuous Newtonian fluid of viscosity close to water. Neither the viscosity of the continuous phase, the elastic modulus (G') nor the sizes of viscoelastic regions varied significantly between areas within 20 µm and areas more than 20 µm from the villous mucosa nor from the tip to the sides of the villous mucosa. The viscosity of the continuous phase at distances further than 20 µm from the colonic mucosa was greater than that at the same distance from the ileal villous mucosa. Furthermore, the estimated sizes of viscoelastic regions were significantly greater in the colon than in the ileum. These findings validate the sensitivity of the method and call into question previous hypotheses that a contiguous layer of mucus envelops all intestinal mucosa and restricts diffusive mass transfer. Our findings suggest that, in the terminal ileum and colon at least, mixing and mass transfer are governed by more complex dynamics than were previously assumed, perhaps with gel filtration by viscoelastic regions that are suspended in a Newtonian fluid.

Intestinal permeability and its relevance for absorption and elimination

Human jejunal permeability (P(eff)) is determined in the intestinal region with the highest expression of carrier proteins and largest surface area. Intestinal P(eff) are often based on multiple parallel transport processes. Site-specific jejunal P(eff) cannot reflect the permeability along the intestinal tract, but they are useful for approximating the fraction oral dose absorbed. It seems like drugs with a jejunal P(eff) > 1.5 x 10(-4) cm s(-1) will be completely absorbed no matter which transport mechanism(s) are utilized. Many drugs that are significantly effluxed in vitro have a rapid and complete intestinal absorption (i.e. >85%) mediated by passive transcellular diffusion. The determined jejunal P(eff) for drugs transported mainly by absorptive carriers (such as peptide and amino acid transporters) will accurately predict the fraction of the dose absorbed as a consequence of the regional expression. The data also show that: (1) the human intestinal epithelium has a large resistance towards large and hydrophilic compounds; and (2) the paracellular route has a low contribution for compounds larger than approximately molecular weight 200. There is a need for more exploratory in vivo studies to clarify drug absorption and first-pass extraction along the intestine. One is encouraged to develop in vivo perfusion techniques for more distal parts of the gastrointestinal tract in humans. This would stimulate the development of more relevant and complex in vitro absorption models and form the basis for an accurate physiologically based pharmacokinetic modelling of oral drug absorption.

Bias in estimation of transporter kinetic parameters from overexpression systems: Interplay of transporter expression level and substrate affinity

The objective was to investigate the interplay between transporter expression levels and substrate affinity in controlling the influence of aqueous boundary layer (ABL) resistance on transporter kinetics in an over-expression system. Taurocholate flux was measured across human apical sodium-dependent bile acid transporter (hASBT)-Madin-Darby canine kidney monolayers on different occasions and kinetic parameters estimated with and without considering ABL. In error-free simulation/regression studies, flux values were generated across a range of J max, Kt, and substrate concentrations. Similar evaluation was performed for transport inhibition studies. Additionally, simulation/regression studies were performed, incorporating 15% random error to estimate the probability of successfully estimating Kt. Across different occasions, experimental J max and Kt estimates for taurocholate were strongly associated (p < 0.001; r2 = 0.82) when ABL was not considered. Simulation/regression results indicate that not considering ABL caused this association, such that Kt estimates were highly positively biased at high hASBT expression. In reanalyzing taurocholate flux data using the ABL-present model, Kt was relatively constant across occasions (approximately 5 microM) and not associated with J max (p = 0.24; r2 = 0.13). Simulations suggest that J max and Kt collectively determined ABL influence, which is most prominent under conditions of low monolayer resistance. Additionally, not considering ABL lead to negatively biased Ki estimates, especially at high J max. Error-inclusive simulation/regression studies indicated that the probability of successfully estimating Kt depended on the contribution of ABL resistance to flux; when flux became increasingly ABL-limited, probability of success decreased. Results indicate that ABL resistance can bias Kt and Ki estimates from overexpression systems, where the extent of bias is determined by transporter expression level and substrate affinity.

High-methoxyl pectin has greater enhancing effect on glucose uptake in intestinal perfused rats

OBJECTIVE

Pectins have been known to decrease blood glucose levels. However, the mechanism of this effect is unclear. The direct action of various pectins (high- or low-methoxyl pectins) on the intestinal absorption of glucose was investigated in gut-perfused rats.

METHODS

After equilibrium, jejunal and ileal segments were simultaneously perfused with an isotonic electrolyte solution (pH 7.4) containing glucose (10 mM/L) and high- or low-methoxyl pectins (10 g/L). Each test or control solution was perfused in a random sequence, with perfusion times of 30 min. Changes in glucose concentration of perfusate solution reservoir were determined over the experimental period.

RESULTS

High- and low-methoxyl pectins in the perfusate significantly inhibited jejunal uptake of glucose compared with the control (P < 0.05). High-methoxyl pectins had greater inhibitive effect on intestinal absorption of glucose than low-methoxyl pectins. The observed changes in glucose and water absorptions caused by high- or low-methoxyl pectins were reversible by switching to a pectin-free perfusate. In addition, net water absorption changed to secretion after addition of high- or low-methoxyl pectins.

CONCLUSIONS

These results suggest that the decrease in intestinal absorption of glucose observed after perfusion of high- or low-methoxyl pectins may be caused by viscosity-related increases in mucosal unstirred layer thickness.

The water-soluble extract of chicory reduces glucose uptake from the perfused jejunum in rats

Among the components of dietary fiber, the soluble fibers have been found to impair glucose absorption. Little is known, however, about the mechanism of this effect. The direct action of soluble fibers (chicory water-soluble extract and inulin) on the intestinal absorption of glucose was investigated in gutperfused rats. After equilibrium, both jejunal and ileal segments were simultaneously perfused with an isotonic electrolyte solution (pH 7.4) containing glucose (10 mmol/L) and chicory water-soluble extract (chicory extract) or inulin (10 g/L). Each test or control solution was perfused in random sequence, with perfusion times of 30 min. Chicory extract or inulin in the perfusate (10 g/L) inhibited the absorption of glucose from jejunum (P < 0.05). The observed changes in glucose and water absorption caused by chicory extract or inulin were reversible after switching to a fiberfree perfusate. Additionally, net water absorption changed to secretion upon addition of chicory extract or inulin. These results suggest that the reduction in intestinal absorption of glucose observed after perfusion of chicory extract or inulin may be caused by viscosity-related increases in mucosal unstirred layer thickness.

Adhesive mucous gel layer and mucus release as intestinal barrier in rats

BACKGROUND

Although it has been reported that total parenteral nutrition induces an increased intestinal permeability and a decreased mucous gel layer covering the intestinal epithelium, the role of mucous gel on intestinal permeability has not been well understood. We examined the in vivo effects of N-acetyl cysteine (NAC) as mucolytic agent and colchicine as suppressant of the mucus production on the intestinal transmission of fluorescein isothiocyanate dextran 70,000 (FITC-dextran).

METHODS

Rats were divided into four groups. In each group, FITC-dextran (750 mg/kg) with or without NAC (3000 mg/kg) was injected into the small intestinal lumen 3 hours after intraperitoneal injection of saline or colchicine (Col, 10 mg/kg). Thirty minutes after injection of FITC-dextran, blood samples were taken from portal vein to analyze plasma fluorescein concentration by fluorescence spectrometry. Samples of small intestine were sectioned in a cryostat for fluorescence microscopy, and the identical sections were stained by periodic acid-Schiff reaction.

RESULTS

Plasma FITC-dextran level in NAC group was higher than that in control group (p < .01), that in Col + NAC group was higher than that in Col group (p < .01) and that in Col + NAC group was higher than that in NAC group (p < .05). The spaces between villi were filled with mucous gel in the control and Col groups, whereas those were not entirely filled with mucous gel in NAC and Col + NAC groups. FITC-dextran and mucous gel showed complementary distribution in all rats. The villous interstitial edema was recognized in NAC group and the villi were disrupted in Col + NAC group.

CONCLUSIONS

These results suggest that intestinal permeability is possibly affected not only by the mucous gel covering the intestinal epithelium but also by mucus release from goblet cells of the small intestine.

Permeability characteristics of various intestinal regions of rabbit, dog, and monkey

The in vitro permeability of a series of both hydrophilic and lipophilic compounds, as defined by the octanol/water partition coefficient, was measured in four segments of rabbit, monkey, and dog intestine using a side-by-side diffusion cell. A linear relationship was established for tissue resistance to hydrophilic compound diffusion in jejunum and colon among rabbit, monkey, and dog. The results suggest that rabbit jejunum is twice as permeable as monkey and dog jejunum. The colonic tissues of monkey, rabbit, and dog demonstrate similar permeabilities. Measuring the permeabilities of different tissues with compounds of similar physicochemical properties allows comparison of tissue restriction to transport. Thus, in vitro permeability measurements may be used to investigate physiological differences of various intestinal tissue segments that influence tissue permeability. Investigating the permeability of different intestinal segments from various species could allow the identification of an appropriate in vitro intestinal permeability model that will lead to the prediction of intestinal absorption in humans, eliminating the need for extensive and often misleading in vivo animal testing.

Calculation of the aqueous diffusion layer resistance for absorption in a tube: application to intestinal membrane permeability determination

The single-pass intestinal perfusion technique has been used extensively to estimate the wall permeability in rats. The unbiased membrane parameters can be obtained only when the aqueous resistance is properly accounted for. This aqueous resistance was calculated numerically from a convective diffusive mass transfer model, including both passive and carrier-mediated transport at the intestinal wall. The aqueous diffusion layer resistance was shown to be best described by a function of the form, [formula: see text] where G zeta, P*m, P*c, Km, and Co are, respectively, Graetz number, passive permeability, carrier-mediated permeability, Michaelis constant, and the drug concentration entering the tube. Asterisked are dimensionless quantities obtained by multiplying the permeability constants with R/D, where R and D being radius and drug diffusivity, respectively. A, B, C, D and E were obtained by a least-squares nonlinear regression method, giving values of 1.05, 1.74, 1.27, 0.0659, and 0.377, respectively, over the range of 0.001 less than or equal to G zeta less than or equal to 0.5, 0.01 less than or equal to P*m less than or equal to 10, 0.01 less than or equal to P*c less than or equal to 10, and 0.01 less than or equal to Km/Co less than or equal to 100. This aqueous resistance was found to converge to those calculated from Levich's boundary layer solution in low Graetz range, indicating the correct theoretical limit. Using an iteration method, the equation was shown to be useful in extracting the intrinsic membrane permeability from the experimental data.

Intestinal perfusion in vivo for the study of absorptive processes

1. Intestinal absorption can be studied by in vitro and in vivo techniques. Among the in vivo ones, intestinal perfusion is the one more employed. 2. Intestinal perfusion could be performed by a simple perfusion of an intestinal segment or by a double perfusion of the intestine and the vascular bed simultaneously. 3. The double perfusion has the advantage of measuring the substrate appearance in the vascular circuit. 4. In this review we compare the different techniques described in the literature, paying attention to their advantages. 5. The best method is the one that maintains the animal alive throughout the experiment, because it provides information about intestinal absorption under conditions similar to the natural ones.

Diffusion coefficient in native mucus gel of rat small intestine

The diffusion coefficients of [3H] water, urea, benzoic acid, antipyrine, aminopyrine, alpha-methyl-glucoside, L-phenylalanine and of hydrogen ions were measured at 38 degrees C in native mucus gel from rat small intestine. The diffusion in the gel was reduced to 37-53% (for hydrogen ions to 7%) compared with buffer solution. In addition, the buffering capacity of the gel retarded the permeation of hydrogen ions before a steady state flux was attained. A model calculation revealed that in the preparation a gel layer of 80 microns thickness represents 23% of the total permeation resistance for substances with high epithelial permeability. The aqueous part of the pre-epithelial diffusion resistance amounts to 77% of the total resistance.

The influence of pH on rectal absorption of sodium benzoate studied in man by rectal lumen perfusion

The influence of pH on rectal absorption of sodium benzoate in man was studied by means of a rectal lumen perfusion method and compared with in vitro measurements on diffusional transport of sodium benzoate across an octanol/water interface. For nonbuffered solutions of benzoate in vitro, it was shown that mass flux across an octanol/water interface occurs in agreement with the pH-partition model. In vivo however, mass flux increases less with decreasing pH of unbuffered perfusate than is anticipated on the basis of the pH-partition model. Probably an alkaline flow across the rectal mucosa into the lumen is present as a physiological neutralization mechanism. In contrast, buffered solutions of benzoate show a linear relationship between mass flux and decreasing pH in vitro as well as in vivo. The effect of buffer on the concentration profile of benzoic acid is qualitatively explained. It is shown that an alkaline flow across the rectal mucosa only slightly influences absorption of benzoic acid from strongly buffered solutions in the rectal lumen. It is concluded that the use of strong buffers in rectal solutions induces a drastic effect on the pH of the boundary layer, an effect not seen for unbuffered solutions. This phenomenon does not invalidate the pH-partition hypothesis but can be explained by it.

Comparative assessment of the resistance of the unstirred water layer to solute transport between two different intestinal perfusion systems

The resistance of the unstirred water layer to solute transport was estimated in two different intestinal single-pass perfusion systems for a comparative study, using D-glucose as a model compound. One is a well established perfusion system in anesthetized rats as a standard (system A). The other is the one in unanesthetized rats for comparison (system B). It was demonstrated that in system B as well as in system A the resistance of the unstirred water layer to D-glucose transport should be taken into account and this resistance, accordingly, the effective thickness of the unstirred water layer (delta) which is assumed to be in proportion to its resistance, could be described as a function of the perfusion rate by using a film model. The delta decreased with increasing perfusion rate and was larger in system A than in system B at each perfusion rate; 785 microns in system A versus 319 microns in system B at the perfusion rate of 0.16 ml/min and 337 microns versus 184 micron at that of 2.95 ml/min. Thus in system B the effective thickness, accordingly, the resistance, of the unstirred water layer was reduced to about 50% of that in system A, but the resistance of the unstirred water layer could still account for 85% of the total resistance at the maximum as far as D-glucose absorption was concerned, while 93% in system A. These results suggest that, compared with perfusion experiments in anesthetized rats (system A), the resistance of the unstirred water layer is reduced but cannot be left out of consideration even if perfusion experiments are performed in unanesthetized rats (system B). And the lower resistance of the unstirred water layer in system B was attributed to a turbulent flow in contrary to a laminar flow in system A.

Closed rat jejunal segment in situ: role of pre-epithelial diffusion resistance (unstirred layer) in the absorption process and model analysis

After intraluminal injection of 0.5 ml buffer solution into closed jejunal segments (length, 3-5 cm) of anesthetized rats the appearance rates of a series of labeled substances in jejunal venous blood were measured for 30 min in situ (initial concentration, 0.02-10 mmol/l or 1 GBq/l tritiated water). The appearance rates quickly rose to a maximum and then declined almost exponentially. Model analysis of the descending of branch of the curves by two one-compartment models (perfect luminal mixing, radial diffusion without convection) revealed a relative pre-epithelial diffusion resistance of nearly 100% for benzoic acid, salicylic acid, L-lysine (0.02 and 1 mmol/l), alpha-methyl-D-glucoside, and L-phenylalanine; 80% to 95% for aniline and butanol; 50% to 80% for benzyl alcohol, theophylline, aminopyrine, antipyrine, dodecanol, and D-galactose; approximately 40% for tritiated water; approximately 30% for L-lysine (10 mmol/l); 10% to 20% for urea and benzylamine; and approximately 4% for erythritol. The shape of the curves was well described by a two-compartment model (intestinal lumen and "tissue", radial luminal diffusion without convection); the variability of the data, however, prevented closer analysis of the parameters of this model. Since pre-epithelial diffusion (unstirred layer) in the closed jejunal segment without peristalsis is the rate limiting step in the absorption process of highly permeant substances, information on intestinal epithelium can be obtained only with poorly permeant substances.

A comparison of the effects of hyperosmotic salicylate solutions in closed and perfused rectal in-situ loops of rats

The effects of presentation of hyperosmotic solutions of sodium salicylate to the closed and perfused rectal loop preparations commonly used in assessing drug absorption have been compared. Epithelial cell loss was quantified in control and treated loops. Tissue damage was significantly greater (P less than 0.01) in treated perfused loops. Fluid efflux, which was associated with a small but significant change in the haematocrit value, was noted in both systems.

Determination of kinetic parameters of a carrier-mediated transport in the perfused intestine by two-dimensional laminar flow model: effects of the unstirred water layer

The kinetic parameters of a carrier-mediated transport for D-glucose and for taurocholate were determined from rat in situ intestinal single perfusion experiments. The true parameters were obtained by the two-dimensional laminar flow model, in which the solute concentration at the aqueous-intestinal membrane interface can be calculated numerically without assuming the aqueous diffusion layer, discriminating the effects of the unstirred water layer. The true Michaelis constant was 4.5 mM for D-glucose and 1.5 mM for taurocholate. The true maximal transport velocity was 3.4 nmol/s per cm2 for D-glucose and 0.29 nmol/s per cm2 for taurocholate. The apparent Michaelis constant was raised by the factor of 6.6 for D-glucose and 3.6 for taurocholate due to the effects of the unstirred water layer. The maximal transport velocity was relatively unaffected by the unstirred water layer in both compounds. The values of the effective (operational) thickness of the unstirred water layer were compatible with those reported previously by employing various experimental methods. The kinetic parameters obtained in vitro everted sacs, for comparison, almost coincided with the true ones in situ. Therefore, the two-dimensional laminar flow model is shown to be valid not only for determining the kinetic parameters of a carrier-mediated transport in situ but also for predicting the absorption rate in situ from the uptake rate in vitro.

Comparison of four experimental techniques for studying drug absorption kinetics in the anesthetized rat in situ

Theophylline absorption kinetics were determined using in situ perfusion techniques. The objectives of this study were to obtain information on the effective permeability constant (ke) of theophylline and its variance during the course of an experiment, and to study the dependence of ke on the experimental technique used. Four in situ intestinal perfusion techniques were compared in the rat: single-pass perfusion, recirculating perfusion, oscillating perfusion, and the closed loop method. The absorption of theophylline appeared to be strongly dependent on the hydrodynamics in the lumen. Constant values and similar coefficients of variation for ke values were obtained for the single-pass perfusion, the recirculating perfusion, and the oscillating perfusion methods. The closed-loop method suffered both from a dropping value of ke with time and a relatively large coefficient of variation.

Determination of the membrane permeability coefficient and the reflection coefficient by the two-dimensional laminar flow model for intestinal perfusion experiments

We performed single perfusion experiments in the small intestine of rats in order to prove that the two-dimensional laminar flow model is suitable to determine the membrane permeability coefficient and the reflection coefficient. We used progesterone as an aqueous-diffusion-limited drug, urea as a membrane transport-limited drug and the tritiated water as an intermediate substance. The membrane permeability coefficient for progesterone was calculated to be 3.6 X 10(-4) cm/s. This value did not change when the thickness of the aqueous diffusion layer was altered by increasing the perfusion rate 10-fold. It was directly demonstrated that the two-dimensional laminar flow model was suitable to analyze the data of intestinal perfusion experiments. Membrane permeability coefficients for urea and tritiated water were determined to be 3.4 X 10(-5) cm/s and 8.9 X 10(-5) cm/s, respectively. In the presence of water absorption with the hypotonic perfusion solution, the reflection coefficient for urea was 0.84. This value is thought to be theoretically reasonable, suggesting the usefullness of the two-dimensional laminar flow model to obtain the reflection coefficient in the intestinal membrane.

Use of carbon monoxide to measure luminal stirring in the rat gut

We used carbon monoxide (CO) as a probe to quantitatively measure intestinal unstirred water layers in vivo. CO has several features that make it uniquely well suited to measure the unstirred layer in that its tight binding to hemoglobin makes uptake diffusion limited, and its relatively high lipid solubility renders membrane resistance negligible relative to the water barriers of the unstirred layer and epithelial cell. The unique application of CO was the measurement of the absorption rate of CO both from the gas phase as well as a solute dissolved in saline. Several lines of evidence showed that a gut stripped free of saline and then filled with gas contained a negligible unstirred layer. Thus, absorption of CO from the gas phase measured resistance of just the epithelial cell. Subtraction of this value from the resistance of CO absorption from saline provided a direct measure of unstirred layer resistance. Studies in the rat showed for a 3-min absorption period that the conventionally calculated apparent unstirred layer for the jejunum was 411 micron and for the colon was 240 micron. However, this conventionally calculated unstirred layer resistance did not truly depict the situation in the rat gut, since there was a continuing depletion of CO from outer surfaces of luminal contents throughout the experiment period. This produced a continually increasing diffusion barrier with time. Calculation of expected absorption rate from unstirred cylinders with the dimensions of the rat gut indicated that there was virtually no stirring in the small intestine and minimal stirring in the colon. The technique described in this paper appears to be simpler and to require fewer assumptions for validity than other techniques previously used to measure unstirred layers in vivo.

Concentration profile in the intestinal tract and drug absorption model: two-dimensional laminar flow in a circular porous tube

The calculation method of the concentration profile in the intestinal tract was developed by adapting the two-dimensional laminar flow in a circular porous tube to the fluid flow and considering a small water absorption or secretion in the intestinal perfusion experiment. The concentration profile was changed by the axial component of velocity, the radial component of velocity, the membrane permeability coefficient, the reflection coefficient and so on. According to the calculated values, the concentration decreased from the center of the intestinal tract to the intestinal membrane as well as from the inlet to the outlet of the intestinal tract. The concentration at the aqueous-intestinal membrane interface increased when water was absorbed and decreased when water was secreted. Consequently, the drug absorption (or secretion), not only because of the effect of the solvent drag but also because of the increase (or decrease) of the concentration gradient at the aqueous-intestinal membrane interface. Using the proposed model in the present study, the concentration at the aqueous-intestinal membrane interface can be calculated directly under the various conditions. Therefore, the true membrane permeability coefficient can be obtained by a graphic method, without correcting the apparent membrane permeability coefficient by the effective thickness of the aqueous diffusion layer (the unstirred water layer).

Drug absorption by the rat jejunum perfused in situ. Dissociation from the pH-partition theory and role of microclimate-pH and unstirred layer

In anaesthetized rats the rate of appearance of benzoic acid and aminopyrine in jejunal venous blood was measured; the pH of the luminal perfusion solution was varied between 4 and 10.5. The pH-absorption curves were less steep than predicted by the unmodified pH-partition theory. A reduction of the mucosal unstirred layer thickness by means of the segmented-flow technique considerably increased the absorption rate without essentially changing the shape of the pH-absorption curves. The pH at the surface of the jejunal mucosa was 6.0, 6.5, 6.6, and 8.0 for luminal solutions of pH 4.0, 6.0, 8.0, and 10.8, respectively. From the absorption data the microclimate-pH was calculated which would explain best the observed pH-absorption curves. These calculated pH-values correspond well to the values measured at the mucosal surface. Therefore, it was concluded that a microclimate-pH caused the deviation of the intestinal pH-absorption curves of benzoic acid and aminopyrine from the prediction of the unmodified pH-partition theory. The mucosal unstirred layer represented only a considerable permeation resistance and was not responsible for the deviating shape of the pH-absorption curves.

Influence of the enteric surface coat on the unidirectional flux of acetamide across the wall of rat small intestine

In vivo treatment of the jejunal mucosa with glycosidic enzymes seems to remove the enteric surface coat of the enterocyte. As a consequence, the mucosa-to-serosa unidirectional flux of acetamide increases remarkably. The glycocalyx probably represents a barrier to the diffusion of small hydrosoluble solutes.

Comparison of jejunal and ileal absorptive functions for glucose and valine in vivo--a technique for estimating real Km and Jmax in the domestic fowl

1. A technique is described that enables the kinetic characterisation of the saturable absorption mechanisms in the chicken jejunum and ileum for glucose and valine in vivo (after correction for non-saturating components and unstirred layers) by estimation of real Km and Jmax. 2. In the ileum both nutrients have lower real Km and higher Jmax values than in the jejunum indicating, at least for hexose and amino acids, that the ileal enterocytes are functionally equipped and anatomically well-sited to fulfil the role of scavengers of the small intestine.

Amino acid influx across the mucosal border of the rat intestine in vivo

The lack of an in vivo method for measuring influx of amino acid into the mucosa has prevented a systematic comparison of characteristics of amino acid influx in vivo with prior in vitro studies. We developed and validated a technique for measuring amino acid influx in vivo. The mucosa is exposed briefly to labelled amino acid perfused luminally at a rapid rate and tissue uptake is measured. The brief exposure period insures that amino acid is confined to the segment. The rapid perfusion rate minimizes concentration of endogenous Na+ in the lumen and permits Na-dependency for alpha-aminoisobutyric acid influx to be demonstrated in vivo for the first time. We also demonstrated the inhibitory effect of K+ and competition by glycine on alpha-aminoisobutyric acid influx in vivo. The saturation kinetics for L-leucine in vivo and in vitro were compared under varying perfusion rates and with and without stirring with air. Under optimal conditions of agitation (rapid perfusion and bubbling with air), the apparent Michaelis constant (Kt) is decreased to be almost equal to that determined under comparable influx conditions in vitro. These studies demonstrate no major difference between characteristics of amino acid transport under more physiologic in vivo conditions as compared with prior in vitro studies.

Analysis of models for determining intestinal wall permeabilities

In determining intestinal wall permeabilities, several mass transport models may be applied to analyze the results from external perfusion experiments. The appropriateness of any given model depends on the applicability of the model assumptions to the experimental system. This report compares several mass transport models with respect to their assumptions and applicability to a particular experimental design. The models are shown to differ in their assumptions regarding convection and diffusion in the perfusing fluid. However, since the wall permeability is an unknown parameter in each model and is estimated from the data, all of the models fit the mass transfer results reasonably well, despite fundamentally different assumptions. However, the determined permeabilities differ. Residence time distribution analysis of the experimental system is more sensitive to the model assumptions. It is shown that, in a particular experimental system, laminar flow in a cylindrical tube is the most appropriate model. The model also has the advantage of implicitly accounting for the convection-diffusion problem in the perfusing fluid. Hence, the diffusion layer thickness is not estimated from the data. With the hydrodynamics defined, the relative permeabilities resulting from the application of the several models to the data can be interpreted. The wall permeability determined in the suggested manner provides an estimate of the limiting assistance under perfect mixing conditions.

Rate-limiting barriers to intestinal drug absorption: a review

The intestinal epithelium is composed of several structures that could serve as barriers to the transfer of drugs from the GI lumen to the systemic circulation. An aqueous stagnant layer that overlies the apical membrane and the subepithelial blood flow are potential barriers to the absorption of drugs that readily penetrate the absorbing cell of the epithelium. The apical, basal, and basement membranes are potential barriers to the absorption of less permeable drugs. The cytoplasm of the absorbing cell is a relatively, thick barrier that must also be traversed. While the location and structure of these potential barriers are well known, those barriers that are operative and the kinds of molecules for which they are operative are not known. The structure and permeability properties of the potential barriers are considered, along with the roles of the paracellular pathway and countercurrent exchange in the villus circulation.

Intestinal structure and function related to toxicology

The study of toxic effects on small intestinal function is complicated by the integration of the activity of the small intestine with the activities of other regions of the GI tract. Also, the barrier and portal functions of the intestine are not as clearly defined as sometimes assumed. The intestinal surface functions as a barrier to the ingress of large quantities of large water soluble molecules. Lipidic substances enter the body quite readily as do small water-soluble molecules. The small intestinal surface is more a portal than a barrier, with its portal functions divided between nonspecific diffusional entry, which depends on physical properties and electric charge, and entry by specific membrane transport, which depends upon chemical structure. The implications of these properties of the small intestine for toxicological studies are stressed.

The longitudinal intraluminal concentration gradient in the perfused rat jejunum and the appropriate mean concentration for calculation of the absorption rate

In anaesthetized rats, jejunal segments 30 cm in length were perfused (rates:0.1, 0.2,0.5 ml/min) with solutions containing antipyrine, salicylic acid, urea, L-lysine, L- and D-phenylalanine. The intraluminal concentration was determined approx. 10, 20, and 30 cm from the inflow cannula. The intraluminal concentration gradients were almost exponential and corresponded to the gradient predicted for a laminar flow through a solute-permeable circular tube. This gradient has a slightly greater curvature than the exponential one. Theoretical considerations showed that the logarithmic mean (Co-Cz)/ln(Co/Cz) of the in- and outflow concentration is the best average value for application in non-integrated absorption equations. This is also valid for the temporal concentration gradient after single administration of a substance into an intestinal segment.

Rat jejunum perfused in situ: effect of perfusion rate and intraluminal radius on absorption rate and effective unstirred layer thickness

In anaesthetized rats a jejunal segment was perfused in situ varying the perfusion rate (0.1, 0.2, 0.5 ml/min) in a randomized order. The intraluminal radius of the segments was small (1.7 mm) or enlarged (3.1 mm) by increasing the intraluminal pressure. The appearance rate of butanol, antipyrine, salicylic acid, D- and L-phenylalanine but not of urea in the venous blood of the jejunal segments was increased up to 35%, when the intraluminal perfusion rate was raised from 0.1 to 0.5 ml/min. Two factors contribute to this effect: the flattening of the concentration gradient down the segment and the reduction of the effective unstirred layer thickness. The length and the intraluminal radius of the perfused segments was not altered, when the perfusion rate was varied. Therefore, a change of the absorbing area did not contribute to the increase of the absorption rate induced by the increase of the perfusion rate. In the series with small intraluminal radius the experimental data corresponded to the theoretical predictions obtained for a laminar intraluminal flow. In the segments with enlarged intraluminal radius the increase of the absorption rate by raising the perfusion rate was less than expected for a laminar flow indicating that the flow might have been turbulent. The enlargement of the intraluminal radius from 1.7 to 3.1 mm increased the absorption rate up to 100%.

Role of unstirred layer in intestinal absorption of phenylalanine in vivo

The appearance rate of L- and D-phenylalanine in the venous blood of rat jejunal loops in vivo is increased up to 60% if the intraluminal solution is mixed more efficiently by the simultaneous perfusion of air. The effect decreases as the luminal concentration is increased to 100 mmol/l. Thus, the apparent Michaelis constants are by 50% lower in the case of the reduced unstirred layer (26 to 17 for L- and 9 to 6 mmol/l for D-phenylalanine). The enhancement of the absorption and the reduction of the Michaelis constants can be attributed to the reduction of the effective unstirred layer thickness by about 400--500 micrometer.