Determination of human lens capsule permeability and its feasibility as a replacement for Bruch's membrane

We have investigated human anterior lens capsule as a potential replacement for Bruch's membrane as a treatment for age-related macular degeneration. Any substrate to replace Bruch's membrane should possess certain characteristics to maintain proper function of the overlying retina. One of the important properties of Bruch's membrane is allowing the flow of nutrients and waste between the retinal pigment epithelium and the choriocapillaris. Here, we measured the permeability of the lens capsule by studying the diffusion of various molecular weight FITC-dextran molecules. Expressions for extraction of diffusion coefficients from concentration vs. time data from a blind-well chamber apparatus were derived for both a single and double membrane experiments. The diffusion coefficients in the lens capsule were found to be in the range of 10(-6) to 10(-10)cm2/s. We demonstrated a power law relationship, with the diffusion coefficient possessing a -0.6 order dependence on molecular weight. The molecular weight exclusion limit was determined to be 150+/-40 kDa. We have compared this value with reported values of Bruch's membrane molecular weight exclusion limit and find that the lens capsule has the potential to act as a substitute Bruch's membrane.     

Totally implantable artificial hearts and left ventricular assist devices: selecting impermeable polycarbonate urethane to manufacture ventricles

In the development of a new generation of totally implantable artificial hearts and left ventricular assist devices (VADs) for long-term use, the selection of an acceptable material for the fabrication of the ventricles probably represents one of the greatest challenges. Segmented polyether urethanes used to be the material of choice due to their superior flexural performance, acceptable blood compatibility, and ease of processing. However, because they are known to degrade and to be readily permeable to water, they cannot meet the rigorous requirements needed for a new generation of implantable artificial hearts and VADs. Therefore, the objective of the present study was to identify alternative polymeric materials that would be satisfactory for fabricating the ventricles, and in particular, to determine the water permeability through membranes made from four commercial polycarbonate urethanes (Carbothane PC3570A, Chronoflex AR, Corethane 80A, and Corethane 55D) in comparison to those made from two traditional polyether urethanes (Tecoflex EG80A and Tecothane TT-1074A). In addition to determining the rate of water transmission through the six membranes by exposing them to deionized water, saline, and albumin-Krebs solution under pressure and measuring the displacement of liquid by means of a recently developed capillary method, the inherent surface and chemical properties of the six membranes were characterized by SEM, contact angle measurements, FTIR, DSC, and GPC techniques. The results of the study demonstrated that the rate of water transmission through the four polycarbonate urethane membranes was significantly lower than through the two polyether urethanes. In fact the lowest values were recorded with the two Corethane membranes, and the harder type 55D polymer had a lower value (2.7 x 10(-7) g/s cm2) than the softer 80A version (3.3 x 10(-7) g/s cm2). This level of water vapor permeability, which appears to be controlled primarily by a Fickian diffusion mechanism, is between 2 and 4 times lower than that obtained with traditional polyether urethane membranes of equivalent thickness. The superior performance of the polycarbonate urethanes is likely due to the inherently lower chain mobility of the carbonate structure in the soft segment phase. In addition, the study shows that additional impermeability to water vapor can be achieved by selecting a polyurethane polymer with a high hard segment content, an aromatic rather than aliphatic diisocyanate comonomer, and a more hydrophobic surface. The use of a higher molecular weight polyurethane is not necessarily efficacious if the above requirements are not met. As expected by Raoult's Law, the study found that the use of physiological media instead of deionized water further decreases the rate of water vapor transmission. Because none of today's commercial polyurethanes are totally impervious to water vapor transmission, additional work is needed to develop permeable polymers or to apply additional treatments to existing candidates to achieve an acceptable impermeable ventricle material.     

Permeability of subcutaneous tissues surrounding long-term implants to oxygen

Certain types of implanted medical devices depend on oxygen supplied from surrounding tissues for their function. However, there is a concern that the tissue associated with the foreign body response to implants may become impermeable to oxygen over the long term and render the implant nonfunctional. We report oxygen flux recordings from electrochemical oxygen sensor devices with wireless telemetry implanted in subcutaneous porcine tissues. The devices remained implanted for up to 13 weeks and were removed with adjacent tissues at specified times for histologic examination. There are four main observations: (1) In the first few weeks after implantation, the oxygen flux to the sensors, or current density, declined to a sustained mean value, having unsynchronized cyclic variations around the mean; (2) The oxygen mass transfer resistance of the sensor membrane was negligible compared to that of the tissue, allowing for a sensitive estimate of the tissue permeability; (3) The effective diffusion coefficient of oxygen in tissues was found to be approximately one order of magnitude lower than in water; and (4) Quantitative histologic analysis of the tissues showed a mild foreign body response to the PDMS sensor membrane material, with capillaries positioned close to the implant surface. Continuous recordings of oxygen flux indicate that the tissue permeability changes predictably with time, and suggest that oxygen delivery can be sustained over the long term.     

Separative pathways for urea and water, and for chloride in chicken erythrocytes.

1. Urea and water permeabilities of chicken erythrocytes are considerably lower than those of mammalian red cells. 2. The permeabilities to urea, thiourea and to N-methylurea (about 10(-6) cm/sec at 25 degrees C) were independent of concentration within a very broad range, and we found no evidence of interaction between transport of analogue molecules. The activation energies were between 17 and 19 kcal/mole, and urea transport was not inhibited by phloretin, which inhibits urea transport in mammalian red cells. 3. The water permeability of chicken red cells (as measured by the diffusion of tritiated water) was 1-35 X 10(-3) cm/sec at 25 degrees C. The activation energy was 10 kcal/mole, and the water permeability was not affected by phloretin or parachloromercuribenzoate. 4. It is concluded that the urea and water permeabilities of the chicken erythrocyte membrane are similar to those of a non-porous bimolecular phospholipid membrane. 5. Like the red cells of other animal species the chicken red cell membrane contains an anion transport system, mediating a rapid exchange of chloride across the cell membranes. The pH dependence, temperature dependence, and sensitivity to inhibitors were similar to the properties of the anion transport system found in mammalian red cells. Our study shows, therefore, that the transport system offers a highly specific pathway to the exchange of anions, without presenting an inspecific leak to the permeation of water and urea.     

Sorption kinetics and equilibrium uptake for water vapor in soft-contact-lens hydrogels

A gravimetric-sorption technique was used to obtain kinetic and equilibrium adsorption/desorption data for water vapor in four different soft-contact-lens (SCL) polymers at 35 degrees C. The SCL materials are a conventional hydrogel (polymacon) with a low water content at saturation (<50 wt %); two conventional hydrogels (hilafilcon A and alphafilcon A) with a high water content at saturation (>50 wt %); and a siloxane hydrogel (balafilcon A). Absorption and desorption equilibrium isotherms (water activity versus water weight fraction) overlap at high water contents, whereas significant hysteresis is observed at low water contents. The hysteresis loop is likely due to trapping of water in the polymer during the desorption process because of a rubber-to-glass transition of the SCL-film surfaces. Sorption data were interpreted using Flory-Rehner theory. The positive Zimm and Lundberg cluster function suggests that water tends to cluster in these SCL materials, except at very low water content. For polymacon and hilafilcon A, Fickian diffusion is observed for all activities for both water sorption and desorption. However, for alphafilcon A and balafilcon A, non-Fickian features appear at intermediate/low activities, in particular during water desorption, suggesting coupling of the diffusion process with polymer-matrix relaxation. The diffusion coefficient increases significantly with water concentration for polymacon and hilafilcon A (from approximately 0.3 x 10(-8) to 4.0 x 10(-8) cm2/s) because of augmented mixture free volume induced by water sorption, whereas a more complex composition dependence is observed for alphafilcon A and balafilcon A probably as consequence of a combined effect of polymer relaxation, plasticization, and water clustering.     

Measurements of dog blood-brain transfer constants by ventriculocisternal perfusion.

Ventriculocisternal perfusions of mongrel dogs were performed for 1-6 h with solutions containing isotopically labeled compounds. At the conclusion of the perfusion period, serial brain samples were taken from the caudate nucleus and analyzed for radioactivity. Tissue concentration profiles were constructed from the data, and apparent tissue diffusion and capillary exchange coefficients were determined. The tissue diffusion constant of sucrose was 3 X 10(-6) cm2/s, which is approximately 45% of its free-water value. The permeability of the brain capillary complex to creatinine, sodium, and mannitol was so low that it could not be accurately measured by this technique. Capillary transfer coefficients, expressed as half-times, were determined for water, urea, and ethylene glycol; the t1/2 values were 1.5, 15, and 17 min, respectively. These numbers were converted to PS products and compared to other published values. This work suggests that the exchange of these compounds between blood and brain is partially (water) or nearly completely (urea and ethylene glycol) limited to membrane permeability.     

Permeability of individual human erythrocytes to thiourea.

The osmotic swelling to haemolysis of individual red blood cells by isosmotic thiourea has been studied using microcine photography. 2. Crenation occurs immediately upon addition of isosmotic thiourea. The cell becomes a crenated sphere without volume decrease. 3. Subsequently, the cell volume increases linearly with time with maximum swelling occurring at about 102 sec which is 81% of the total haemolysis time. 4. At maximum swelling, the cell volume is 92% greater than the initial cell volume. This volume increase is about double that measured with other permeating substances. 5. The much larger maximum volume implies that thiourea increases the area of the cell membrane. This increase varies from 0 to 75% for individual cells, with a mean of 22%. 6. Membrane expansion varies inversely as the initial cell membrane area and cell volume (r=0-790). 7. Using the increased surface area, increased maximum volume and the swelling time, the mean permeability is calculated to be 5-52 X 10(-7) cm/sec (S.D. of mean=+/-1-19 X 10(-7) cm/sec). The distribution of permeabilities represents a normal distribution. 8. The pre-lytic potassium loss ranged from 0 to 36% with a mean value of 16-5%. This is consistent with values reported in the literature for slow haemolysis. As with other permeants the distribution is skewed towards lower values. 9. Membrane permeability of individual cells varies with the amount of membrane expansion observed. Coefficient of correlation between permeability and expansion index is 0-674. 10. There is no correlation between permeability and the reciprocal of the haemolysis time (r=-0-035). The correlation between permeability and the reciprocal of the swelling time is also poor (r=0-303), probably owing to the variability in membrane expansion by thiourea in individual cells. 11. As has been shown previously for faster permeants, the permeability coefficient cannot be calculated from the haemolysis time. Because thiourea alters the membrane area and the haemolytic volume, the coefficient cannot be calculated from the swelling time unless the changes in the membrane area are also taken into account.     

The effect of hormones and of an osmotic gradient on the structure and properties of mammalian foetal urinary bladder in vitro.

1. Water and isotope fluxes were measured by incubating urinary bladders of foetal pigs and sheep in vitro in the presence and absence of a concentration and osmotic gradient. The structure of the urinary bladder of foetal pigs under various conditions was studied by electron microscopy. Its ultrastructure was found to be closely similar to that of foetal sheep. 2. Antidiuretic hormone (ADH) (0-2 U. ml-1) enhanced the enlargement of intercellular spaces caused by dilute mucosal medium in pig bladders; prolactin (1 u. ml-1) prevented osmotic dilatation of the intercellular spaces. 3. The hydraulic conductivity, Lp, was estimated to be 0-5 X 10 (-7) cm.s-1atm-1 in sheep and pigs at about 100 days gestation; the ratio of osomotic to diffusional permeability, (LpRT/VW)/PD, in the presence and absence of ADH, was 2-1 and 1-6 respectively. These are similar to the values found in fish gills. 4. Prolactin reduced bulk flow of water to zero in seven out of eight bladders investigated. Incubation with ADH or vasotocin (55 mu. ml-1) in the presence of prolactin restored water flux to 22% and 45% of control values respectively. 5. There was no significant net flux of sodium from mucosa to serosa in pig bladder except in the presence of prolactin. No net flux of sodium occurred from mucosal to serosal side of pig or sheep bladders in the presence of an adverse electrochemical gradient, although in sheep the permeability ratio was significantly greater than one. 6. The diffusional flux ratio for water remained unity under all conditions; vasotocin increased unidirectional fluxes and prolactin reduced them. The flux ratios were unaffected by the direction of bulk fluid flow, probably because diffusion was rapid compared to flow: the ratio of diffusional flux to volume flow was between 11 and 18.     

Chloride transport by self-exchange and by KCl salt diffusion in gramicidin-treated red blood cells.

The permeability of gramicidin-treated human red blood cell membranes to K+ and Cl- has been measured at normal ionic strength (1) by tracer exchange at steady-state distribution of salt, and (2) by net transport of salt in the presence of a salt concentration gradient. Under both conditions KCl was the only inorganic salt in cells and medium. In the studies of self-exchanges the electrical driving force on the ions was zero. Calculaton of permeability coefficients from net salt transport was simplified because the experiment was designed as a special case of the Nerst-Planck diffusion regime, i.e. the single salt case. Gramicidin altered the cell membranes from being anion to become cation selective. Gramicidin increased the potassium exchange without affecting the chloride exchange measurably. The chloride exchange showed saturation kinetics as does chloride exchange in normal cells. The net transport of KCl in the presence of a constant concentration gradient increased to a constant value with increasing gramicidin concentration. At high gramicidin concentrations (0 degree C, pH 7.2) the "chloride permeability coefficient" calculated from tracer exchange (1.9 x 10(-6) cm/s) was 290 times the chloride permeability coefficient calculated from net salt transport (0.65 x 10(-8) cm/s). The latter value corresponds to a chloride conductance of 4.2 x 10(-6) ohm-1 cm-2. The chloride permeability coefficient was 2.1 x 10(-6) cm/s at 25 degrees C (pH 6.8) indicating a value of 3 for the Q25. It appears that normal red cells are anion selective in the sense that anion permeability exceeds cation permeability with a factor of more than a hundred between 0 degrees C and body temperature. The anion exchange, i.e. the Hamburger shift, is a tightly coupled transport process which is several orders of magnitude faster than anion transport by salt diffusion.     

Influence of changes in external potassium and chloride ions on membrane potential and intracellular potassium ion activity in rabbit ventricular muscle

1. The membrane responses of rabbit papillary muscles to rapid changes in [K](o) and [Cl](o) were measured with open-tipped micropipettes and with closed micropipettes made from K-selective glass.2. The muscle cells behaved primarily as a K electrode, and responses to changes in [K](o) with constant [Cl](o) or with constant [K](o) x [Cl](o) were substantially the same.3. When [Cl](o) was changed at a constant [K](o) the membrane potentials changed rapidly and symmetrically by a small value and remained constant for 30 min.4. Measurement of potential with K(+)-selective micro-electrodes in these experiments showed no change in intracellular K activity. In addition to permitting calculation of K permeability, these measurements reassured us that the K(+)-selective electrodes were well insulated and not influenced by electrical shunts at the impalement site.5. Although the membrane response to changes in [Cl](o) was small, it was possible to calculate that the permeability ratio (P(Cl)/P(K)), was 0.11. The Cl and K conductances were about 0.015 mmho/cm(2) and 0.09 mmho/cm(2) respectively, resulting in a conductance ratio (g(Cl)/g(K)) of about 0.17.6. The time course of depolarization by increase in [K](o) was rapid (half-time 5 sec), but repolarization on return to lower [K](o) was much slower (half-time 50 sec). The depolarization time course was easily fitted by the potential change calculated by assuming the need for K diffusion into the extracellular spaces and taking account of the logarithmic relation between membrane potential and [K](o). These calculations did not fit the time course of repolarization, which was slowed in the fashion expected from an inward-rectifying membrane.7. The influence of [K](i) on membrane potential was investigated by changes in tonicity of the external solution. Hypotonic solution produced a change in intracellular K activity close to that produced by ideal water movement. However, in hypertonic solution, intracellular K activity did not rise as much as predicted, suggesting a change in intracellular activity coefficient.     

Comparative nuclear magnetic resonance studies of diffusional water permeability of red blood cells from different species. IX. Australian feral chicken and domestic chicken (Gallus domesticus)

The diffusional water permeability (P _d) of Australian feral chicken and Australian and European domestic chicken red blood cells (RBCs) was measured by a doping nuclear magnetic resonance (NMR) technique. The values of P _d were around 1.7 × 10^?3 cm/s at 15°C, 2.0 × 10^?3 cm/s at 20°C, 2.5 × 10^?3 cm/s at 25°C, 3.7 × 10^?3 cm/s at 30°C, 4.3 × 10^?3 cm/s at 37°C, and 6.1 × 10^?3 cm/s at 42°C, with no significant differences between the three strains of chicken. There was no effect of p-chloromercuribenzene sulphonate on water diffusion. The activation energy of water diffusion was around 37 kJ/mol for all strains of chicken. These results suggest that no changes in the RBC water permeability are correlated with marked alterations in the habitat of chicken introduced to Australia (and that membrane proteins play little role in the diffusion of water across chicken RBC membrane).     

Glycerol transport in human red cells.

The kinetics of 14C-glycerol exchange was studied in human red cells. Glycerol appeared to be transported by two mechanisms: (i) by facilitated diffusion with permeability depending on glycerol concentration, and (ii) by an unspecific pathway, presumably representing the diffusion of individual glycerol molecules through the membrane with permeability independent of glycerol concentration. The latter permeability was 8 X 10(-8) cm/s at 20 degrees C, it was independent of pH, and had an activation energy of 25 kcal/mol. The facilitated transport of glycerol was completely inhibited by Cu++, and the activation energy was low, about 10 kcal/mol. The transport system was competitively inhibited by H+, reacting with at least three hydrogen analogue, as well as dimethylsulfoxide (a hydrogen bonding molecule with no structural resemblance to glycerol), inhibited glycerol transport competitively. Steins "dimerizer hypothesis" was revised according to our findings. A kinetic scheme describing the reactions of a transport controlling site with glycerol is presented in the Appendix. It is demonstrated in the article that the scheme accounts for out experimental results.     

Membrane permeability to the molecular and ionic forms of DMO in barnacle muscle

The time course of the intracellular pH (pHi) of barnacle muscle fibers was followed using microelectrodes while the fibers were exposed to 15 mM of the weak acid 5,5-dimethyloxazolidine-2,4-dione (DMO) at pH 6.5. The rapid initial fall in pHi was used to determine the membrane permeability to the DMO molecule, while the much slower fall during the subsequent plateau phase yielded the permeability to the DMO ion. This experimental approach and the mathematical treatment of the data can be used to obtain the membrane permeabilities to other weak acids or bases. We found values of 1.9 X 10(-4) cm/s for the permeability to the DMO molecule and 1.5 X 10(-7) cm/s for the permeability to the ion, assuming the fiber to be a cylinder. Thus the permeability of the neutral form is about 10(3) times that of the ionized form. At commonly encountered conditions of pHi = 7.3, outside pH = 7.5, and membrane potential = -52 mV, this permeability ratio introduces an error of only -0.01 to -0.02 into the determination of pHi based on the distribution of DMO.     

The motile response of lung macrophages: theoretical and experimental approaches using the linear under-agarose assay

The alveolar macrophage plays an important role in the lung's defense against inhaled particles, but few studies have addressed the motile behavior of these cells. In this study, we measured alveolar macrophage random motility using a modification of the under-agarose assay. The cells were exposed to a uniform concentration of F-norleucyl-leucyl-phenylalanine (FNLLP) in an agarose system, establishing conditions for stimulated random motility (or chemokinesis). Experimental results were compared with a theoretical model of cell migration. In this model, chemokinesis was expressed as a random motility coefficient, mu, which is the cellular equivalent of a diffusion coefficient for a molecule. The experimental data that described the migration of alveolar macrophages (density profiles) agreed well with the theoretical model. The random motility coefficient ranged from 1 X 10(-9) cm2/sec (no FNLLP) to a maximum of 1 X 10(-8) cm2/sec at 10(-9) M FNLLP. For concentrations of FNLLP greater than 10(-9) M, the random motility decreased to a constant value of 3 X 10(-9) cm2/sec. The maximum random motility response was observed at an FNLLP concentration one order of magnitude below the measured FNLLP equilibrium dissociation constant, Kd, of 6 X 10(-8) M for alveolar macrophages and was essentially constant over a large range of FNLLP concentrations on either side of the Kd value. These results suggest that such a combined experimental and theoretical approach reduces the limitation of previous techniques that depended largely on physical characteristics of the assay and more closely identifies and measures intrinsic properties of cell motility.     

溶剂在玻璃状高分子材料中的非Fick扩散机理的研究

本文对高分子材料中的非Ｆｉｃｋ 扩散的机理进行了研究，采用了扩散与粘弹性模型相结合的方法来刻画这一现象，同时考虑了溶剂的移动的影响，建立了带有可动边界的数学模型，运用奇异摄动方法对此模型进行了解析，所得结果与实验结果是一致的     

Allan Hoffman: A 70th Birthday Celebration

Water sorption which is not classically Fickian has been observed in a variety of polymers. Deviation from Fickian kinetics is widely assumed to be caused by rate-limiting polymer relaxation, despite minimal proof of this. To the contrary, the evidence accumulated in this work indicates that water transport in initially glassy poly(2-hydroxyethyl methacrylate) (PHEMA), an important water-swellable biomedical polymer, is controlled by Fickian diffusion. First of all, the fractional water uptake is initially linear and independent of sample thickness when plotted against the square root of time over initial thickness, as expected for a Fickian process. Furthermore, the moving solvent front also advanced with the square root of time. Temperature, polymer thermal history and initial solvent concentration all affected the sorption kinetics of PHEMA in manners consistent with a Fickian process. The invariably Fickian sorption mechanism is believed to be the consequence of the water molecule's small size and affinity for hydrophilic, swellable polymers.     

Outer membrane and porin characteristics of Serratia marcescens grown in vitro and in rat intraperitoneal diffusion chambers.

The composition and antibiotic permeability barrier of the outer membrane of Serratia marcescens were assessed in cells grown in vivo and in vitro. Intraperitoneal diffusion chambers implanted in rats were used for the in vivo cultivation of bacteria. Outer membranes isolated from log-phase bacterial cells recovered from these chambers were compared with membranes isolated from cells grown in vitro. Analysis revealed that the suspected 41-kilodalton porin and the OmpA protein were recovered on sodium dodecyl sulfate-polyacrylamide gels in equal quantities. Several high-molecular-weight proteins, thought to be iron starvation induced, appeared in the diffusion chamber-grown cells. The outer membrane permeability barriers to cephaloridine were similar in in vivo- and in vitro-grown cells based on permeability coefficient calculations. The permeability coefficient of cephaloridine in S. marcescens cells (30.3 x 10(-5) to 38.9 x 10(-5) cm s-1) was greater than that obtained for an Escherichia coli strain expressing only porin OmpC but smaller than those obtained for the E. coli wild type and a strain expressing only porin OmpF. Functional characterization of the suspected porin was performed by using the planar lipid bilayer technology. The sodium dodecyl sulfate-0.4 M NaCl-soluble porin from both in vitro- and in vivo-grown cells showed an average single-channel conductance in 1 M KCl of 1.6. A partial amino acid sequence (19 residues) was obtained for the S. marcescens porin. The sequence showed a very high homology to the E. coli OmpC porin. These data identified the S. marcescens outer membrane 41-kilodalton protein as a porin by both functional and amino acid analyses. Also, the methodology used allowed for efficient growth and recovery of diffusion chamber-grown bacterial cells and permitted identification of specific in vivo-induced changes in bacterial cell membrane composition.     

Linear diffusion of lead in the intestinal wall: a theoretical study

In this paper, the authors present a diffusion model of the intestinal wall to describe the transient overall uptake of lead ions across the epithelium of gastrointestinal mucosa and their subsequent diffusion in the underlying blood capillaries. The gastrointestinal mucosa is treated as a heterogeneous two-phase medium, consisting of a continuous extracellular phase and a dispersed cellular phase. The main mode of uptake is considered to be bulk diffusion, since transport of lead across the mucosal membrane occurs mainly by a passive diffusion process. The equations give the variation with time of concentrations of lead in both the phases across the intestinal wall. The mean concentration of lead available for absorption by the blood capillaries has been studied as a function of time. It is found that a steady state is reached in 19.45 min. It has been possible to estimate the intestinal membrane permeability for the lead molecule from this analysis: it is calculated to be 3.34 X 10(-3) s-1. Finally, the rate of lead uptake by the blood for different gut concentrations has been calculated.     

Potassium permeability of the mesothelium of the frog mesentery.

The mesothelium of the mesentery is a single cell layer described to offer no or every little hindrance to diffusion of small water soluble solutes. We have measured the potassium permeability of the mesothelium of the frog mesentery both in vivo and in vitro. The permeability measured in vivo--using K+--sensitive microelectrodes is 2.4 X 10(-5) cm s-1. In vitro measurements using conventional tracer flux technique on isolated mesentery yields a K+-permeability of 5.7 X 10(-5) cm s-1. These values are 15--30 times smaller than values previously reported from in vitro experiments on rat and rabbit mesentery. Also, the permeability is 12--15 times lower than the K+-permeability of the capillary wall determined on single capillaries in the frog mesentery. In the frog mesentery the mesothelium thus represents an important diffusion barrier compared to the capillary wall. This may be critical in experiments where filtration and reflexion coefficients of the capillaries are determined from measurements of fluid exchange across the capillary wall in response to application of hypertonic solutions on the surface of the mesentery.     

Activation energy for water transport in toad bladder

Activation energies (Ea) for water movement across vasopressin-(ADH) sensitive epithelia have been reported to be about 10 kcal/mol (1, 12). The present study shows that measurements of Ea for osmotic water flow across toad bladders are unreliable, because a temperature change induces marked alterations in membrane permeability to water within a 2.5-min interval. Thus bladders equilibrated with ADH either at room temperature or at 33 degrees C and then suddenly subjected to a lower temperature were found to exhibit a marked increase in membrane permeability to water. This observation suggests that there is a rapid turn-over of water permeability sites and that sudden exposure to cold inhibits the removal more than the induction of sites by ADH. To stabilize ADH-induced water channels for Ea measurements, bladders were exposed to ADH at room temperature, fixed with glutaraldehyde, and subjected to osmotic gradients at different temperatures. The Ea values for osmotic water flow across these ADH-permeabilized, glutaraldehyde-fixed bladders were 5.1 (4-12 degrees C), 4.3 (12-21 degrees C), 3.6 (21-36 degrees C), and 3.6 kcal/mol (30-38 degrees C). Ea values for shear viscosity of water in these temperature ranges were calculated to be 4.7, 4.2, 4.1, and 3.6 kcal/mol, respectively. The close correlation between Ea values for bulk water viscosity and osmotic water flow across the bladder wall suggests that an equivalent number of hydrogen bonds must be broken to achieve an increase in water flow through ADH-induced channels and an increase in fluidity of water in bulk solution.