Routes of nonelectrolyte permeation across epithelial membranes

Characteristics of Passive Solute Transport across Primary Rat Alveolar Epithelial Cell Monolayers

Primary rat alveolar epithelial cell monolayers (RAECM) were grown without (type I cell-like phenotype, RAECM-I) or with (type II cell-like phenotype, RAECM-II) keratinocyte growth factor to assess passive transport of 11 hydrophilic solutes. We estimated apparent permeability (Papp) in the absence/presence of calcium chelator EGTA to determine the effects of perturbing tight junctions on "equivalent" pores. Papp across RAECM-I and -II in the absence of EGTA are similar and decrease as solute size increases. We modeled Papp of the hydrophilic solutes across RAECM-I/-II as taking place via heterogeneous populations of equivalent pores comprised of small (0.41/0.32 nm radius) and large (9.88/11.56 nm radius) pores, respectively. Total equivalent pore area is dominated by small equivalent pores (99.92-99.97%). The number of small and large equivalent pores in RAECM-I was 8.55 and 1.29 times greater, respectively, than those in RAECM-II. With EGTA, the large pore radius in RAECM-I/-II increased by 1.58/4.34 times and the small equivalent pore radius increased by 1.84/1.90 times, respectively. These results indicate that passive diffusion of hydrophilic solutes across an alveolar epithelium occurs via small and large equivalent pores, reflecting interactions of transmembrane proteins expressed in intercellular tight junctions of alveolar epithelial cells.

Nanomaterial interactions with and trafficking across the lung alveolar epithelial barrier: implications for health effects of air-pollution particles

Studies on the health effects of air-pollution particles suggest that injury may result from inhalation of airborne ultrafine particles (<100 nm in diameter). Engineered nanomaterials (<100 nm in at least one dimension) may also be harmful if inhaled. Nanomaterials deposited on the respiratory epithelial tract are thought to cross the air-blood barrier, especially via the expansive alveolar region, into the systemic circulation to reach end organs (e.g., myocardium, liver, pancreas, kidney, and spleen). Since ambient ultrafine particles are difficult to track, studies of defined engineered nanomaterials have been used to obtain valuable information on how nanomaterials interact with and traffic across the air-blood barrier of mammalian lungs. Since specific mechanistic information on how nanomaterials interact with the lung is difficult to obtain using in vivo or ex vivo lungs due to their complex anatomy, in vitro alveolar epithelial models have been of considerable value in determining nanomaterial-lung interactions. In this review, we provide information on mechanisms underlying lung alveolar epithelial injury caused by various nanomaterials and on nanomaterial trafficking across alveolar epithelium that may lead to end-organ injury.

Kinetics of manganese absorption in ligated small intestinal segments of broilers

Two experiments were conducted with 28-day-old male commercial broilers to study mechanisms of Mn absorption and the effect of Mn treatment on divalent metal transporter 1 (DMT1) mRNA levels in ligated segments from different intestinal regions of broilers. The results from experiment 1 showed that the amount of Mn absorption was asymptotic with respect to time within 40 min after perfusion of the duodenal, jejunal, and ileal segments of broilers with 2.18 mmol/L of Mn as MnSO(4). In experiment 2, a kinetic study of Mn absorption was performed with duodenal, jejunal, and ileal loops perfused with solutions containing 0, 0.13, 0.27, 0.54, 1.09, 2.18, 4.37, or 8.74 mmol/L of Mn as MnSO(4). Manganese concentrations in perfusates were determined at 5 min after perfusion. In the control group and in the group treated with 2.18 mmol/L Mn as MnSO(4), DMT1 mRNA levels of ligated intestinal regions at 30 min after perfusion were analyzed by real-time reverse transcription PCR. The kinetic curves of Mn absorption showed that Mn absorption was a carrier-mediated process in the duodenum and jejunum. The maximum absorption rate (J(max)) in duodenal segments was greater (P < 0.05) than that in the jejunum (94.08 vs. 81.17 nmol/cm per min). There was no significant difference (P = 0.85) in the Michaelis-Menten constant (K(m)) values between the duodenum and jejunum (3.41 vs. 3.60 mmol/L). In the ileum of Mn-deficient broilers, the most probable mechanism of Mn absorption was a nonsaturable diffusion process, and the diffusive constant (P; means +/- SE) was 2.42 x 10(-2) +/- 5.22 x 10(-4) cm(2)/min. The DMT1 mRNA levels in the duodenum and jejunum of broilers were greater (P < 0.001) than the level in the ileum. The DMT1 mRNA level in the small intestine of broilers in the Mn treatment group decreased significantly (P < 0.001) compared with that of the control. The different mechanisms of Mn absorption found in different intestinal segments suggest that the ileum is the main site of Mn absorption in the small intestine of broilers.

Ultrastructural basis for alveolar-capillary permeability to protein

The intravenous injection into mice of small volumes (less than 0.1 ml) of peroxidatic enzymes of molecular weight of 40 000 daltons or greater results in little if any penetration of these probe molecules into endothelial junctions. The injection of cytochrome c (12 000 daltons), on the other hand, results in the localization of this tracer in some but not all endothelial junctions. When horseradish peroxidase (EC 1.11.1.7) is injected in a large volume of saline (0.5 ml), reaction product is present in endothelial junctions and basement membrane, but is prevented from entering the alveolar space by zonulae occludentes between epithelial cells. These experiments indicate that although endothelial junctions, under physiological conditions, are largely impermeable to molecules the size of horseradish peroxidase, and presumably most serum proteins, they are labile and susceptible to stretching if intravascular pressure is increased. Freeze-fracture studies show that pulmonary capillary endothelial junctions are composed of one or at the most two strands which show areas of discontinuity. Epithelial junctions, by contrast, are composed of a continuous, complex network of anastomosing fibres. These observations confirm physiological experiments which indicate that it is the pulmonary epithelium rather than the endothelium which determines the permeability properties of the alveolar-capillary membrane to lipid-insoluble molecules. Bidirectional pinocytic transport is an additional mechanism whereby lipid-insoluble molecules are transported across both endothelial and epithelial layers. The relative contribution of this transport mechanism to the total amount transported remains to be established.

Equivalent radius of paracellular "pores" of the mesothelium

Diffusional permeability (P) to water (P(w)), Cl(-) (P(Cl(-))), and mannitol (P(man)) was determined in specimens of rabbit parietal pericardium without and with phospholipids added on the luminal side, as previously done with sucrose and Na(+). P to the above-mentioned molecules and to Na(+) (P(Na(+))) was also determined after mesothelium was scraped away from specimens. P(w), P(Cl(-)), P(Na(+)), and P(man) of connective tissue were the following (x10(-5) cm/s): 73.1 +/- 7.3 (SE), 59.5 +/- 4.5, 41.7 +/- 3.4, and 23.4 +/- 2.4, respectively. From these and corresponding data on integer pericardium, P(w), P(Cl(-)), P(Na(+)), and P(man) of mesothelium were computed. They were the following: 206, 17.9, 9.52, and 3.93, and 90.2, 14.4, 4.34, and 1.75 x 10(-5) cm/s without and with phospholipids, respectively. As previously found for P to sucrose, P to solutes is smaller in mesothelium than in connective tissue, although the latter is approximately 35-fold thicker; instead, P(w) is higher in mesothelium, suggesting marked water diffusion through cell membrane. Equivalent radius of paracellular "pores" of mesothelium was computed with two approaches, disregarding P(w). The former, a graphical analysis on a P-molecular radius diagram, yielded 6.0 and 1.7 nm without and with phospholipids, respectively. The latter, on the basis of P(man), P to sucrose, and function for restricted diffusion, yielded 7.8 and 1. 1 nm, respectively.

Comparison of four markers of intestinal permeability in control subjects and patients with coeliac disease

BACKGROUND

Controversy surrounds the issue of intestinal permeability in patients with coeliac disease, polyethylene glycol 400 indicating reduced and di-/mono-saccharide urine excretion ratios and 51Cr-labeled ethylenediaminetetraacetic acid (EDTA) indicating increased permeability.

METHODS

We assessed the suitability of polyethylene glycol 400, L-rhamnose, lactulose, and 51Cr-EDTA as markers of intestinal permeability by assessing urine excretions after simultaneous intravenous instillation of these markers and after oral administration in normals and patients with coeliac disease.

RESULTS

After intravenous administration the 24-h urine excretion of polyethylene glycol 400, L-rhamnose, lactulose, and 51Cr-EDTA was 40%, 72%, 93%, and 97%, respectively. There was no significant difference between controls and patients with coeliac disease. Oral administration of the markers in an iso- and hyper-osmolar test solution demonstrates reduced permeation due to an osmotic retention effect of lactulose. In contrast, hyperosmolar glycerol increases permeation of all markers except L-rhamnose. Timing of urines and altering osmolarity is important for the behavior of individual markers but does not enhance the discrimination between controls and patients when the differential urine excretion of lactulose/L-rhamnose is used. The sensitivity of the urine excretion ratio of lactulose/L-rhamnose was comparable to that of 51Cr-EDTA used by itself. Whereas lactulose/L-rhamnose and 51Cr-EDTA showed increased intestinal permeability in coeliac disease, the permeation of polyethylene glycol was reduced. Permeation of the markers did not correlate significantly with jejunal histology.

CONCLUSIONS

Correlations of marker permeation rates with test dose osmolarity in controls and patients with coeliac disease shows a variable lack of conformity, suggesting that the markers may permeate the intestine by different routes, which are affected to a different extent in coeliac disease.

Permeability properties of rat renal lysosomes

Although lysosomes maintain large pH gradients and may be subjected to significant osmotic gradients in vivo, little is known about their passive permeability properties. In recent studies, vacuolar H(+)-adenosine-triphosphatases (ATPases), such as those found in lysosomes, have been suggested to act as water channels. In addition, the erythrocyte and proximal tubule water channel CHIP28 is present on the plasma membrane of proximal tubule cells and may undergo endocytosis so that it is incorporated in lysosomes. We therefore examined water, proton, and small nonelectrolyte permeabilities in freshly purified lysosomes from rat renal proximal tubule. Lysosomes were purified by differential and Percoll gradient centrifugation. The preparation contained only lysosomes when examined by electron microscopy. Moreover, analysis by flow cytometry showed virtually all particles to be positive for acid phosphatase and cathepsin B activities. Permeabilities were measured on a stopped-flow fluorimeter by monitoring the self-quenching or pH-sensitive quenching of entrapped fluorescein derivatives. Osmotic water permeability (Pf) averaged 0.011 +/- 0.003 cm/s (n = 6), a value similar to that of biological membranes containing water channels. However, Pf was insensitive to the organic mercurial reagent p-chloromercuribenzene-sulfonate and to HgCl2 and exhibited an activation energy of 10.8 +/- 0.8 kcal/mol. These results indicate that water flux in lysosomes occurred via the lipid bilayer, and not via water channels. Addition of ATP led to lysosomal acidification (proton flux = 4.6 +/- 0.8 x 10(-11) mmol H+.s-1.cm-2), which was completely inhibited by 0.1 microM bafilomycin. Pf was insensitive to this agent as was the passive proton permeability (0.36 +/- 0.18 cm/s, n = 4). Permeabilities to small nonelectrolytes varied in proportion to the oil-water partition coefficient, confirming the applicability of Overton's rule to lysosomes. We conclude that proximal tubular lysosomes exhibit high Pf, which occurs via the lipid bilayer and not via vacuolar H(+)-ATPase.

The structural barrier of absorptive mucosae: site difference of the permeability of fluorescein isothiocyanate-labelled dextran in rabbits

The permeability of fluorescein isothiocyanate-labelled dextran (FD, M.W. 4400-71,200) across nasal, buccal, duodenal, jejunal, ileal, colonic, and rectal mucosae excised from rabbits has been measured to estimate the structural barrier of absorptive mucosae using Ussing-type diffusion chambers. The permeability coefficient of FD in all these mucosae decreased with increasing molecular weight. The rank order of FD permeability did not always correlate with the electrical resistance of the mucosae. Among components of the small intestine (duodenum, jejunum, ileum) and the large intestine (colon, rectum), however, the rank order of FD permeability corresponded to the magnitude of the electrical resistance in each instance; the upper colonic mucosa showed the highest permeability, especially in FD of low molecular weight, and permeability of the rectal mucosa was lowest except for the duodenal mucosa. The nasal mucosa showed the lowest electrical resistance and the highest permeability of those studied, suggesting that it has a leaky structural barrier.

Water and nonelectrolyte permeabilities of apical membranes of toad urinary bladder granular cells

Certain types of epithelial cells such as those lining the toad urinary bladder have been classified as "tight" because their apical membranes exhibit low permeabilities to water, ions, and small nonelectrolytes. However, the permeability properties and structural features of these specialized apical membranes remain unclear because these membranes have never been purified. To isolate toad bladder granular cell apical membranes, we derivatized the bladder apical surface with the membrane-impermeant bifunctional reagent N-hydroxysulfosuccinimydyl-S,S-biotin (NHS-SS-biotin). After cell disruption, these derivatized apical membranes were purified using streptavidin-coated magnetic beads in a magnetic field. With the use of lactoperoxidase-mediated radioiodination as a marker for apical membrane, this preparative procedure purified apical membrane 48- or 72-fold as compared with homogenate. Thin section electron microscopy revealed unilamellar vesicles with some nonvesiculated membranes, while fragments of organelles such as mitochondria were absent. Water and nonelectrolyte permeabilities of purified apical membrane vesicles were similar to those obtained in intact bladders in the absence of antidiuretic hormone stimulation. The results demonstrate that isolated apical vesicles do not contain water channels and confirm the applicability of Overton's rule to the apical membrane of the toad urinary bladder. The technique has general applicability to isolation of other plasma membranes, and the apical membranes obtained are suitable for structural analysis.

Drug delivery via the mucous membranes of the oral cavity

The delivery of drugs via the mucous membranes lining the oral cavity (i.e., sublingual and buccal), with consideration of both systemic delivery and local therapy, is reviewed in this paper. The structure and composition of the mucosae at different sites in the oral cavity, factors affecting mucosal permeability, penetration enhancement, selection of appropriate experimental systems for studying mucosal permeability, and formulation factors relevant to the design of systems for oral mucosal delivery are discussed. Sublingual delivery gives rapid absorption and good bioavailability for some small permeants, although this site is not well suited to sustained-delivery systems. The buccal mucosa, by comparison, is considerably less permeable, but is probably better suited to the development of sustained-delivery systems. For these reasons, the buccal mucosa may have potential for delivering some of the growing number of peptide drugs, particularly those of low molecular weight, high potency, and/or long biological half-life. Development of safe and effective penetration enhancers will further expand the utility of this route. Local delivery is a relatively poorly studied area; in general, it is governed by many of the same considerations that apply to systemic delivery.

Role of paracellular pathway in nonelectrolyte permeation across rat colon epithelium enhanced by sodium caprate and sodium caprylate

The enhancing effects of 0.25% sodium caprate (C10) and sodium caprylate (C8) on the paracellular permeation of seven water-soluble nonelectrolytes (inulin, polyethylene glycol 900, mannitol, erythritol, glycerol, thiourea, and urea) across the isolated rat colonic epithelium were examined using the Ussing-type chamber technique. The paracellular changes were also measured by impedance analysis. In both the presence and the absence of enhancers, the permeation clearances (Pm) for inulin (12-15 A in molecular radius) to erythritol (3.2 A) increased linearly with the increase in their free diffusion coefficients (Dfr), showing the existence of a paracellular shunt pathway unrestricted to any molecular size. Glycerol (2.9 A), thiourea (2.6 A), and urea (2.3 A) had higher clearances than the expected linear values, showing the existence of a restricted paracellular or transcellular pathway. Both C10 and C8 increased the permeabilities in the two pathways, but C10 was more effective than C8. The increase in the permeabilities via the shunt pathway caused by the enhancers was greater than that via the restricted pathway, and thus, the two-phase pattern in the relationship of Pm and Dfr was similar to that in the absence of enhancers. The transcellular permeabilities for urea and thiourea, which were obtained from the efflux experiments, were increased by the enhancers. However, the relative increase caused by C10 was smaller than that of the paracellular-restricted permeabilities. The paracellular changes probably were due to the increase in pore area per unit diffusive path length.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of [Ca2+]i and [Ca2+] sensitization in the caffeine contracture of rat myocytes: measurement of [Ca2+]i and [caffeine]i

1. Fluorescence measurements have been made in single, isolated rat ventricular myocytes using the Ca2(+)-sensitive indicators Fura-2 and Indo-1. In Fura-2-loaded cells, the application of caffeine (2-20 mM) produced a change of fluorescence indicating an increase of [Ca2+]i which then spontaneously decayed to control levels. These changes of [Ca2+]i were accompanied by a contracture. 2. In contrast, in Indo-1-loaded cells, in addition to the changes of fluorescence expected for the transient increase of [Ca2+]i produced by caffeine, there was a maintained decrease of fluorescence. 3. Measurements in vitro showed that caffeine quenches the fluorescence of Indo-1 (but not of Fura-2) in a [Ca2+]-and wavelength-independent manner. Caffeine therefore had no effect on the ratio of Indo-1 fluorescence measured at two wavelengths. This inhibition by caffeine could be described by an apparent Ki of 4 mM. In the cell the Ki was considerably larger (18 mM). 4. We have separated the Indo-1 fluorescence changes into caffeine- and [Ca2+]i-dependent components. The time course of change of intracellular caffeine was calculated. When [caffeine]o was rapidly increased, [caffeine]i changed with a rate constant of 8 s-1 giving an apparent permeability to caffeine of 2 x 10(-3) cm s-1. 5. This method was used to measure [caffeine]i and [Ca2+]i simultaneously during caffeine-induced contractures. The shape of the caffeine contracture was found to depend on both the speed of application of caffeine and the concentration applied. If caffeine was applied quickly then the contracture developed within 1 s to a maximum level and then relaxed to a lower maintained level. With slower application, there was a more complete relaxation of the initial contraction followed by a slower redevelopment of contraction. 6. Despite the difference in contraction time course, irrespective of the flow rate, [Ca2+]i decayed monotonically. The slow secondary development of contraction has the same time course as the increase of [caffeine]i. The caffeine contracture can be reproduced by a model in which both [Ca2+]i and [caffeine]i affect contraction. 7. The increase of [Ca2+]i is not greatly affected by altering the caffeine concentration from 2.5 to 50 mM. In contrast the maintained level of contraction increases over this range showing that the Ca2(+)-independent effects of caffeine on the myofilaments have a low affinity for caffeine.

The acute effects of smokeless tobacco on transport and barrier function of buccal mucosa

The acute effects of smokeless tobacco (ST) on buccal mucosal transport and barrier function were studied by means of in vivo and in vitro techniques. In humans, in vivo exposure to 0.5 g ST transiently increased the transmural electrical potential difference (PD). However, despite continued exposure, PD returned to baseline within 20 min. The mechanisms for these changes were explored by use of dog buccal mucosa mounted in Ussing chambers. Luminal exposure to a Ringer-extract of ST (EOST) increased PD and short-circuit current (Isc) and decreased electrical resistance (R), with changes reversible upon removal of EOST from the bath. Further, radioisotopic fluxes showed that the increase in Isc in EOST-exposed tissues was accompanied by increased absorption of electrolytes (Na, Cl, and other ions), and the decrease in R was accompanied by increased permeability to mannitol. Light microscopy of tissues exposed to EOST showed no morphological changes after exposure to 0.5 g of ST, but after exposure to 1.5-2.5 g of ST, dilated intercellular spaces were identified. Contact of aqueous media with ST led to the release of electrolytes and other soluble compounds into solution. To determine the effect of electrolyte release on buccal function, we exposed mucosae luminally to a solution with ion composition and/or osmolality similar to EOST or to one with an EOST previously dialyzed against Ringer. Solutions with similar ion composition and/or osmolarity changed PD, Isc, and R in a manner similar to EOST, while dialyzed-EOST had no effect. In addition, luminal nicotine produced effects different from EOST, decreasing PD and Isc and increasing R.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium transfer at the blood-nerve barrier of the frog sciatic nerve

Calcium transfer across the blood-nerve barrier of the frog sciatic nerve was studied using an in situ perfusion technique and an in vivo i.v. bolus injection technique. The permeability-surface area product of 45Ca at the blood-nerve barrier, (PA)BNB, calculated from radioactivity in the desheathed nerve segment after 5 min of circulation of tracer, and corrected for the residual radioactivity in the blood space, equaled 4.4 +/- 0.4 (S.E.M.) X 10(-5) ml.s-1.g-1 wet wt. The (PA)BNB of 45Ca was independent of [Ca2+] in the perfusion medium between 0.18 and 18 mM. The permeability-surface area products of 45Ca across the perineurium [(PA)per] also was measured by an in situ incubation technique, and equaled 1.45 +/- 0.41 X 10(-5) ml.s-1.g-1 wet wt. (n = 8). The half time (t 1/2) for nerve calcium to equilibrate with plasma calcium was calculated to be 60 min. The low, passive permeability to calcium of the blood-nerve barrier probably limits marked calcium concentration changes in nerve endoneurium following transient changes of plasma calcium, but should not alter steady-state responses.

Gastrointestinal permeability to polyethylene glycol: an evaluation of urinary recovery of an oral load of polyethylene glycol as a parameter of intestinal permeability in man

The permeability of the gastrointestinal tract was investigated by means of polyethylene glycol (PEG), given orally. The PEG solutions contained oligomers with molecular weights from 414 to 1206. The urinary recovery of ingested PEG was determined by high performance liquid chromatography (HPLC). Considerable inter- and intra-individual variations in recovery were found in a reference group of 33 healthy subjects. Intake of food did not change the recovery of PEG, 6.11% (SEM 0.52, n = 13) compared with that for the reference group, 6.45% (SEM 0.39, n = 33). The PEG recovery in a group of ileostomy patients was 5.86% (SEM 0.62, n = 13). No correlation was found between PEG recovery and small bowel transit time. The lipophillic properties of PEG were determined by measuring the partition coefficients in 1-octanol/water and in methylhexanoate/water systems. From these results, correlated with the values for the hydrodynamic volumes of PEG, it was concluded that the intestinal permeation of PEG observed was probably determined by the hydrophillic/hydrophobic properties of the mucosal membrane, rather than by the presence of intercellular junctions or paracellular pores. The PEG molecules with molecular weights from 414 to 1206 were, therefore, not suitable as solitary probes for permeability studies in man.

Water and nonelectrolyte permeability of isolated rat hepatocytes

We have measured the diffusive permeability coefficients of isolated rat hepatocytes to 3H2O, [14C]urea, [14C]erythritol, [14C]mannitol, [3H]sucrose, and [3H]inulin, employing a technique previously developed for erythrocytes (Redwood et al., J. Gen. Physiol 64:706-729, 1974). Diffusion coefficients for the tracer molecules were measured in packed hepatocytes, supernatant fluid, and intracellular medium (lysed hepatocytes) and were calculated assuming one-dimensional semi-infinite diffusion through a homogeneous medium. By applying the series-parallel pathway model, the following permeability coefficients (10(-5) cm/sec) for the hepatocyte plasma membrane were obtained. 3H2O, 98.6 +/- 18.4; [14C]urea, 18.2 +/- 5.3; [14C]erythritol, 4.8 +/- 1.6; [14C]mannitol, 3.1 +/- 1.4; [3H]sucrose, 0; [3H]inulin, 0. These results indicate that isolated rat hepatocytes are highly permeable to water and polar nonelectrolytes, when compared with other transporting epithelia. This relatively high cellular permeability is consistent with a model in which nonelectrolyte permeation is via an aqueous pathway of equivalent pore diameter of 8-12 A. The finding that [14C]erythritol and [14C]mannitol cross the hepatocyte plasma membrane indicates that these molecules enter the bile canaliculus through the transcellular route. Conversely, the failure of [3H]sucrose and [3H]inulin to permeate the hepatocyte in the isolated condition supports the concept that biliary entry of these large carbohydrates, at least that fraction which cannot be accounted for by a vesicular mechanism, must occur via the transjunctional shunt pathway.

Permeability characteristics of the guinea pig biliary apparatus

To determine the mechanism and pathway of entry of polar nonelectrolytes into bile, we studied first the permeation of [3H]H2O, [14C]urea, [14C]erythritol, [14C]mannitol, [3H]sucrose, [3H]inulin and [3H]dextran across an isolated, in situ perfused segment of the guinea pig's extrahepatic bile duct. All of these molecules, except [3H]dextran, permeated the bile duct. The diffusive permeability coefficients (cm per sec per 10(6) ranged from 248 for [3H]H2O to 1.2 for [3H]inulin. On the basis of these results, we formulated two models of solute biliary excretion. In Model I, permeation across both the canaliculus and ductule/duct was assumed to be by simple diffusion. In Model II, solute was assumed to enter the canaliculus by convection and diffusion, and the ductule/duct by diffusion. Reflection coefficients and/or permeability coefficients for the canalicular membrane were then determined by fitting the equations describing these processes to the bile-to-plasma ratios of [14C] erythritol, [14C]mannitol, [3H]sucrose and [3H]inulin observed at different rates of bile flow produced by partially clamping the bile duct cannula and subsequently infusing taurodehydrocholate. Only when convection was included as a mechanism of canalicular permeation (Model II) was excellent fitting observed. In such a case, the reflection coefficients and permeability coefficients for the carbohydrates in question were similar to those reported for other transporting epithelia. Cholestasis produced by taurolithocholate (10 to 40 mumoles per kg, i.v.) was associated with an irreversible increase in both the sieving coefficient and permeability coefficient for [3H]sucrose and [3H]inulin, even when the inhibition of bile flow was fully reversible. The permeability to [14C] erythritol and [14C]mannitol was either not affected or minimally increased. These findings suggest that, in the guinea pig: solutes as large as [3H]inulin enter the biliary tree both at the canaliculus and bile ductule/duct; [14C]erythritol and [14C]mannitol gain access into the canalicular lumen primarily by convection, whereas [3H]sucrose and [3H]inulin permeate mainly by diffusion; distal permeation of these carbohydrates is small, and accounts for 1 to 11% of their total biliary entry; the canalicular membrane permeability to [3H]sucrose and [3H]inulin is not functionally important in bile secretion, and a fraction of canalicular bile flows through the transjunctional shunt pathway.

Factors affecting the absorption of phenolics and carboxylic acids in the guppy (Poecilia reticulata)

The rate of absorption of 17 phenols, anisoles, and carboxylic acids in the guppy (Poecilia reticulata Peters) was measured at pH levels from 3 to 9. The rate was directly proportional to the concentration, and the compounds did not interfere with each other's absorption. The primary route of uptake was across the gill epithelium, but 25-40% of the total amount penetrated across the skin. When the fish were exposed at a pH low enough to prevent ionization of acidic compounds, the initial rate of uptake increased with lipophilicity up to a partition coefficient of about 10(4). Above this point, the absorption rate did not correlate with lipophilicity indicating that the uptake rate of the most lipophilic compounds was limited by the unstirred water layers. The effect of lipophilicity on absorption accounts for about half of the effect found in toxicity and bioconcentration. The absorption rate of acids declined with pH rising, but did not follow changes in the concentration of nonionized acid. The slopes of the absorption rate vs pH curves were distinctly less steep than the slope of the dissociation curve. In addition, the curves of the most lipophilic acids showed a shift to the right, which can be ascribed to the unstirred layers on the surface of the epithelia.

Transepithelial transport of nonelectrolytes in the rabbit mandibular salivary gland

The characteristics of nonelectrolyte secretion by the rabbit mandibular salivary gland have been investigated in an in vitro, perfused preparation. The concentrations of 14C-labeled nonelectrolytes were measured in saliva samples collected over a range of flow rates during the secretory response of the gland to continuous acetylcholine infusion. Of the nine nonelectrolytes studied, the two particularly lipid-soluble molecules, ethanol and antipyrine, appeared in the saliva at approximately the same concentration as in the perfusate, regardless of the secretory flow rate. The more polar molecules (urea, ethanediol, thiourea, glycerol, erythritol, mannitol and sucrose) appeared at saliva/perfusate concentration ratios (phi) which showed a strong dependence on flow. With the exception of thiourea, this could be attributed to the combined contributions of diffusion and solvent drag. For the polar nonelectrolytes, estimates have been obtained of both the permeability coefficients of the gland (P) and the solvent-drag filtration coefficients (1 - sigma). The relation between 1 - sigma and molecular radius suggests that small polar nonelectrolytes and the bulk of the secreted water cross the epithelium via aqueous channels that are approximately 0.8 nm in width. The location of the channels remains uncertain because tissue space measurements indicate that the nonelectrolytes most affected by solvent drag have access to both transcellular and paracellular pathways.

Small intestinal permeability to mannitol, lactulose, and polyethylene glycol 400 in celiac disease

Mannitol (molecular weight 182), lactulose (342), and polyethylene glycol 400 (range 242-550) absorption was studied in 25 controls, 22 untreated celiacs, and 13 treated celiacs. Untreated celiacs absorbed less mannitol and more lactulose than controls. Absorption of higher as well as lower molecular-weight polyethylene glycols was reduced in untreated celiac disease. Absorption returned towards normal on treatment. Polyethylene glycol and lactulose absorption was enhanced by administering them in a hypertonic solution. Polyethylene glycol 400 but not lactulose or mannitol was lipid soluble in vitro. It was concluded that the mucosa in untreated celiac disease was more "leaky" than normal. Polyethylene glycol 400 absorption data suggested that its absorption may largely be determined by its lipid solubility and was decreased in celiac disease because of the reduced surface area of the small intestine. Polyethylene glycol 400 cannot be recommended as a suitable marker for permeability studies of the small intestine.

Permeability of the rat oral mucosa to organic solutes measured in vivo

The permeability of the oral mucosa to organic solutes of different molecular volumes, oil-to-water distribution coefficients, and pKa was studied in an in-vivo system. The calculated permeability coefficients were independent of time of application and the concentration of the solutes, a finding consistent with transport by simple diffusion. The permeability of organic acids was pH dependent whereas permeability to compounds with a degree of ionization that did not change over the pH range was not affected by pH. Increasing the chain length from two to eight carbons in an organic acid series resulted in an increase of both the oil-to-water distribution coefficient and the permeability coefficient; the permeability coefficient was decreased by the addition of oxygen in the form of hydroxyl groups. These results indicate that there is similarity in the way that lipid-soluble substances diffuse across oral mucosa and other lipid membranes. Results using compounds with oil-to-water distribution coefficients less than that of water suggest that these compounds traverse the oral mucosa by two additional routes. One, for compounds with molar volumes less than 80 cm3/ml, is via pores; whereas, the second, utilized by larger compounds, is probably by an intercellular route.

Dynamics of clomethiazole edisylate interaction with plastic infusion systems

The dynamics of the interaction of clomethiazole edisylate (1) with polyvinyl chloride and cellulose propionate, the main plastics used in the manufacture of infusion bags and sets, was examined. An experimental system in which the plastic was either open or closed to the environment was used to determine the relative contribution of the sorption and permeation processes to loss from solutions of clomethiazole edisylate (I) in contact with the plastic infusion systems. Sorption by the plastic infusion materials accounted for most of the drug loss, while permeation into the external environment accounted for the remainder. The sorption and permeation into and through polyvinyl chloride was temperature dependent. The diffusion coefficient and permeation rate constant both increased with temperature, while the polyvinyl chloride-water partition coefficients were independent of temperature. The activation energy for the diffusion in polyvinyl chloride was 13.5 kcal/mol. The permeability of the infusion bag plastic and the evaporation across an unstirred air boundary layer adjacent to the external surface of the plastic both appeared to contribute to the overall diffusional resistance encountered in the permeation process. The plastic-water partition coefficients are independent of initial concentration, suggesting that the concentration-dependent loss of the drug from solutions stored in plastic infusion bags and burets is a result of the greater diffusivity of the drug in the plastic at the higher initial concentrations. Plasticization of the polymers by the drug is indicated by the increase in the diffusivity of the drug in polyvinyl chloride and cellulose propionate, the increase in the rate and extent of sorption of a radiolabeled marker (diazepam) by the plastic, and the decreased stiffness of polyvinyl chloride exposed to higher concentrations of the drug.

Secretion and re-absorption of glucose in rat submandibular and sublingual saliva

Glucose permeation from blood to saliva appeared to follow the paracellular pathway in rat submandibular and sublingual glands, and the permeability was much higher in the sublingual than in the submandibular gland. The duct system re-absorbed glucose in the submandibular but not the sublingual gland. The glucose concentration in sublingual saliva was inversely related to the flow rate.

Effects of environmental salinity on branchial permeability of rainbow trout, Salmo gairdneri

1. The gill fluxes of various non-electrolytes were measured in fresh-water- and sea-water-adapted trout (Salmo gairdneri). The studies were performed in vitro with a 'perfused-head' preparation. 2. The results allow one to specify different transepithelial pathways according to the physico-chemical characteristics of the permeant molecules: (1) for hydrophilic the physico-chemical characteristics of the permeant molecules: (1) for hydrophilic and lipophilic molecules of small molecular radius, a transcellular pathway in the respiratory cells of the secondary lamellae, (2) for hexose, all paracellular pathways in the gill epithelium, and (3) for hexose polymers (inulin, dextran), a transcellular pathway in the chloride cells of the primary lamellae. 3. The selectivity of the respiratory cells as a function of the liposolubility of the molecules tested is low. The decrease of this selectivity in the course of salt-water adaptation taken together with the modification of lipid composition of membranes and the effect of adrenaline on the branchial permeability suggests that non-electrolytes diffuse through a lipid phase able to form hydrogen bonds. 4. The high permeability of gills to hexose polymers of high molecular weight suggests a vesicular transport, especially in fish adapted to fresh water.

Prolactin effects on ion transport across cultured mouse mammary epithelium

Prolactin is a known osmoregulatory hormone in lower vertebrates, and recent evidence indicates that this hormone modulates ionic concentrations in milk. In an ultrastructurally and biochemically differentiated primary cell culture system in which mouse mammary epithelium is maintained on floating collagen gels, prolactin causes an increase in short-circuit current (Isc) of monolayers of cells derived from midpregnant (24.6 to 48.0 microA . cm-2) and lactating (10.4 to 16.1 microA . cm-2) glands. Transepithelial potential differences (basal side ground) average about -12 mV and are similar to those seen in vivo. Prelactating mammary epithelial cell cultures have transepithelial resistances ranging from 374 omega . cm2 (prolactin present) to 507 omega . cm2 (prolactin absent), and lactating cell cultures have resistances averaging almost 1,000 omega . cm2. Prolactin effects require at most one day of culture maintenance in prolactin-containing medium, and the effects are not due to known contamination of prolactin preparations with arginine vasopressin or growth hormone. Medium concentrations of prolactin as low as 1 ng/ml can elicit these effects. In prelactating cell cultures not treated with prolactin, the Isc is equal to the rate of sodium absorption. Prolactin increases sodium absorption fourfold but increases Isc only twofold. Clearly, prolactin induces other active transport; neither potassium nor chloride movements can account for this additional transport. Resistance values, current-voltage plots, and permeability coefficients indicate that in vitro mammary epithelium is a moderately "tight" tissue. Comparisons with intact glands indicate that in vitro mammary epithelium closely resembles its in vivo counterpart. Floating collagen gel cultures appear suitable for elucidating transport properties in cellularly heterogeneous and structurally complex mammalian tissues.

D-Glucose transport across the apical membrane of the surface epithelium in Nereis diversicolor

Epidermal D-glucose transport was investigated in vivo in the brackish-water polychaete worm Nereis diversicolor. Transfer across the apical membrane is rate-limiting to D-glucose uptake, but the cuticle and/or mucus presents some resistance to D-glucose diffusion between bulk solution and transporting membrane. Maximal D-glucose influx is about 10(-12) mol sec-1 per cm2 of apical plasmalemma. Under natural conditions (approximately 1 microM D-glucose in the medium), backflux from the epidermal transport pool is negligible, but a significant paracellular outflux may occur. D-glucose influx across the apical membrane is Na+-dependent and completely inhibitable by phlorizin and harmaline; phloretin is less effective, and cytochalasin B has no effect. Influx is moderately depressed by KCN and iodoacetate, alpha-methyl-D-glucopyranoside is an effective substitute of D-glucose in transport. Animals acclimated to a low salinity, in which epidermal salt transport takes place, show a marked decrease of D-glucose transport capacity. On transfer of animals from a high to a low salinity, or vice versa, the corresponding change of influx occurs after a time-lag of at least an hour. Permeability of the epidermis to simple diffusion of D-glucose is 8 X 10(-8) cm sec-1 (on basis of gross epidermal area).

Effect of chemical structure on nonelectrolyte penetration of oral mucosa

The in vitro permeability of oral mucosa from New Zealand rabbits to 17 nonelectrolytes was studied using radioactive tracer techniques. Calculated permeability coefficients were independent of solute concentration and were similar when measured from outer to inner surface or inner to outer surface. Generally, an increase in chain length and increased lipid solubility within a series of alcohols or diols resulted in increased permeability. However, the first member of each series displayed anomalous behavior in that it penetrated more rapidly than the second member of the series. Addition of one or more hydroxyl groups to a compound brings about a decrease in permeability. The addition of a second or third hydroxyl group has less of an effect in decreasing permeability if it is adjacent to an existing hydroxyl as compared to when it is further separated. Addition of a hydroxyl group to a compound decreases permeability to a greater extent than addition of a ketone. Also, replacement of a hydroxyl group with an amide results in decreased permeability. These results point out the importance of lipid solubility and ability to form hydrogen bonds to permeability.

Permeability of gingival sulcular epithelium in the development of scorbutic gingivitis

Young adult nonhuman primates were initially fed an ascorbic-acid free diet and, subsequently, a diet with a suboptimal level of the vitamin. Following approximately 12 weeks of this dietary regime, five of seven experimental animals of the gingival sulcular epithelium to 3H-inulin, but not to 14C-butyric acid or 14C-urea, had increased significantly (+86%, P less than 0.01) in the experimental group as compared to ad libitum or pair-fed controls. This defect in the intercellular permeability barrier of the sulcular epithelium could have contributed to the pathogenesis of scorbutic gingivitis.

Permeability characteristics of muscle membrane

Unidirectional flux rates of saturated fatty acids, saturated alcohols, and bile acids were measured in an intact rat diaphragm preparation. The logarithm of the permeability coefficients for fatty acids containing from five to ten carbon atoms was a linear function of the number of carbon atoms in the fatty acid chain. Incremental free energies of solution were +336 cal x mol-1 for the addition of a hydroxyl group and -258 cal x mol(-1) for the addition of a methylene group. These incremental free energies were similar to those obtained by other investigators in other animal tissues, and our data suggest a structural similarity between membranes in different tissues and in different species. The muscle membrane exhibited anomalously high permeabilities for fatty acids containing less than five carbon atoms. Since muscle lacks tight junctions, this result suggest that small non-electrolytes traverse polar regions or aqueous pores within the cellular membrane.

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.

Effect of cycloheximide on urea facilitated transport through toad gallbladder epithelium

Transepithelial urea outfluxes across toad gallbladder were determined before and after the addition of cycloheximide. The drug inhibits the movement of urea but has no effect on thiourea and antipyrine outfluxes. The inhibition of amide transport is time dependent as also shown in counterflow experiments. These results are consistent with the hypothesis that cycloheximide inhibits the synthesis of membrane proteic sites involved in urea mediated transport.