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A brief description is given of the site at which an extensive programme of in situ constant head permeability tests has been carried out. The effect of hydraulic fracture on the results of such permeability tests is discussed, from which a method of estimating K0 is evolved. Comparison is made with a test method given by Bjerrum and Andersen (1972) and with independent measurements carried out by Wroth and Hughes (1973). On donne une brève description d'un site où l'on a effectué un large programme de mesures de perméabilité à charge constante in situ. On présente une discussion de l'effet de la fracture hydraulique sur les résultats obtenus lors des mesures de perméabilité dans ces conditions, et on en ddduit une méthode d'estimation de Ko. On compare cette méthode à celle proposée par Bjerrum et Anderson (1972) et aux mesures indépendantes effectuées par Wroth et Hughes (1973).

The distribution of a random variable is determined by the probability density functions (PDF) of all other random variables with which the variable in question is jointly distributed. If the PDF of the random variable of interest is normal, or skewed normal, then the distributions with which it is jointly distributed determine its mean and standard deviation. In the case described here (where hemolysis time of the red blood cell is a function of the permeability coefficient and geometric variables of the cell) the mean and standard deviation of the permeability coefficient and the known distributions of the geometric variables on which the hemolysis time depends determine a predicted distribution of hemolysis time. An observed distribution of the hemolysis time is obtained spectrophotometrically. By choosing the mean and standard deviation of the permeability coefficient so that the predicted PDF of the hemolysis time matches the observed PDF best by least-squares criterion, the complete distribution of the permeability coefficient is determined.

The unidirectional movement of glycerol across rabbit brush border was studied in vitro. The permeability coefficient for glycerol across the ileal brush border decreased from 3.41 x 10-4 cm · sec-1 at 0.05 mm to 0.65 at 5 mm, and remained constant at this level from 5 to 50 mm. Glucose and phlorizin did not inhibit glycerol movement. N-ethyl-maleimide inhibited glycerol by more than 55% from a 0.05 mm solution, but did not affect glycerol uptake from a 50 mm solution. Glycerol movement across the ileal brush border measured from a 0.05 mm solution was 2 times greater than the movement across proximal intestine. By contrast, glycerol movement across proximal and distal intestine was similar when measured from a 50 mm solution. It is concluded that glycerol movement occurs both by diffusion via the aqueous regions of the membrane and by a saturable carrier-mediated mechanism.

The two parameters usually invoked when discussing transport across membranes are the "diffusion permeability coefficient" and the "hydrodynamic permeability coefficient." In this study the magnitude of these two coefficients is established experimentally for collodion membranes of differing porosities. The hydrodynamic permeability is predominant while convergence of the two permeabilities tends to obtain as the membranes become less coarse. The flux data obtained are used to calculate "average pore diameter" and the meaningfulness of these calculations is interpreted. The relationship between the two coefficients and transport across membranes as treated by the system of irreversible thermodynamics is discussed.

The ratio of unsaturated to saturated hydraulic conductivity was determined for mixtures of glass beads and clay, and for a clay soil. In shrinking soils, the relative hydraulic conductivity remained fairly high over a wide range of moisture contents, while in nonshrinking soils the relative hydraulic conductivity rapidly approached zero with decreasing moisture content.

The problem of exchange of substances between circulating blood and tissue is discussed in terms of a model with continuously distributed tissue characteristics. Consideration is given to the distribution of perfusion rates, the distribution of metabolism coefficients for a substance consumed at a rate proportional to its concentration, and the distribution of permeability coefficients for a slowly penetrating inert substance. Equations are derived for obtaining crude estimates of the moments of these distributions from exchange data.

The permeation of He, N2, CH4, C2H6, C2H4, CO2, and N2O through polyethylene membranes has been studied at pressures up to 60 atm and at temperatures between −10° and 60° C. The experimental temperature range encompassed the critical temperature of the last four penetrants. The mean permeability coefficient, P, for C2H6, C2H4, CO2, and N2O exhibited an exponential dependence on pressure at reduced penetrant temperatures below and near unity. This dependence decreased with increasing reduced temperature. The values of P for CH4, which were obtained at reduced temperatures above unity, were essentially constant over the entire pressure range. Finally, the P's for N2 and He, which were measured at the highest reduced temperatures, appeared to decrease slightly with increasing pressure. Possible causes for the observed behavior are discussed, and a new correlation for the pressure dependence of the permeability coefficients is presented. The separation of two different N2O-CO2 mixtures by selective p...

An investigation of the permeability to nitrogen of various mixtures of natural rubber and synthetic elastomers has brought to light one highly important fact. The lower permeability obtained by the addition of a synthetic elastomer does not depend solely on the permeability coefficient itself of the particular synthetic elastomer. Rather, it depends in addition on another factor, which, in all probability, is a function of the internal structure of the mixture of natural rubber and synthetic elastomer. A study of this structure is not a simple problem, and mention is made only of the fact that, after having attempted unsuccessfully to analyze the structure by classic methods of microscopy, very encouraging results have been obtained with mixtures of natural rubber and Paracril-26 by measuring, at radio frequencies, the loss factor. The results indicate that, at least with natural rubber-Paracril-26 mixtures, milling brings about a certain degree of mutual diffusion of the two elastomers on a molecular scale. However, the vulcanizates give indications of a two-phase colloidal structure, one phase relatively rich in Paracril, the other phase relatively rich in natural rubber. Such a structure would be in harmony, at least qualitatively, with the conclusions drawn in the present investigation on the permeability to gases of mixtures of elastomers.

The validity of Darcy's law in unsaturated media and the assumptions made in its use are discussed. The relationship of the permeability coefficient in Darcy's law to various soil parameters is briefly considered, the total soil moisture potential is defined, and its components discussed. By use of the equation of continuity and Darcy's law, an equation of flow is derived, and the general problem of obtaining solutions of this equation is outlined. The special mathematical difficulties encountered when the permeability is considered as a function of the moisture content or the capillary potential are also indicated. A numerical solution of the flow equation for a semi-infinite system is given, using a functional relationship between the permeability and the moisture content, and the phenomenon of a wetting front is shown to be indicated.

A simple model is presented for one-dimensional diffusion in an ensemble of semi-infinite and finite pores or capillaries in which the boundary at one end of each capillary is allowed to fluctuate randomly between a perfectly reflecting barrier and a perfectly absorbing barrier. The model is independent of the spatial distribution of the capillaries; it is only assumed that there are a large number of them and that they are noninteracting. Exact solutions are possible and results are obtained, in terms of the fluctuation parameters, for the total amount per unit area of solute passed through the capillary system in the semi-infinite case, and for a permeability coefficient and time lag to steady state in the finite system. Applications of the model to diffusion in biological membranes are discussed.

The effects of relative humidity on the survival of desiccation by the tardigrade, Macrobiotus areolatus, were investigated. The most survivals were obtained when the animals were dried at relative humidities greater than 70% at 20° C. At these high humidities the animals form tuns, while at lower humidities they become flattened or crumpled. Anesthetized animals do not form tuns at any humidity.The rate of evaporative water loss from tuns in air was investigated by recording weight loss from animals kept at known relative humidities. Tuns formed by active animals lose water during the early stages of dehydration at a rate approximately 0.3 times the rate of anesthetized animals. Anesthetized animals equilibrate with the surrounding air within one hour, while tuns require more than 100 hours to equilibrate. At the end of 100 hours, the water content of tuns at 80% RH is 10-25%. During dehydration the permeability coefficient of tuns decreases a hundredfold (from 2.0-5.0 x 10-4 cm hr-11 atm-1 to 1.6-6.0 x ...

1. 1. The transmural Na+, K+, Cl− and water flow in the total intestine of Aphanius dispar was measured by an in vitro in situ perfusion technique. 2. 2. A neutral NaCl absorption without a net K+ flow was observed; this occurred in the absence of a transmural osmotic difference followed by an isosmotic water absorption. 3. 3. The osmotic permeability coefficient and the NaCl mobility of the intestinal wall seemed high.

Studies in electro-osmosis and electro-osmotic pressure difference have been carried out with methanol using a sintered glass disk as the membrane. The measurements are reported at voltages varying from 0 to 300 V and temperatures varying from 30 to 50°C. The cross-phenomenolgical coefficient and permeability coefficient have been evaluated graphically. The dependence of the coefficients on temperature and voltage is discussed.

1. 1. A modification of the osmotic lysis method, taking into account differences in osmotic resistance, is introduced. This method allows a more fundamental physicochemical interpretation of the permeability process and the calculation of permeability coefficients. 2. 2. Evidence is presented that the reflection coefficient for glycerol is near to one for human red blood cells as well as for pig red blood cells. 3. 3. The permeability coefficient proved to be independent of differences in lysis behaviour of erythrocytes, which were treated with isotonic solutions of different non-permeants. 4. 4. The osmotic resistance in NaCl solutions is strongly time dependent, whereas in sucrose solutions this parameter is time independent and gives the best approximation of the original osmotic resistance of the red blood cell.

Uptake rate of a gas from a rat's subcutaneous gas pocket was divided by the surface area and by the apparent pocket-to-tissue tension difference to yield an exchange coefficient, K'. Values in (ml x 10–4)/(min cm2 atm) were O2, 6.6; CO2, 150; and N2, 2. Blood flow in adjacent tissue appeared to have little influence on uptakes of O2 and CO2, since the K'co2:K'o2 ratio indicated that the uptakes were governed by diffusion alone, and drastic alteration of blood flow (death of the animal) decreased K'o2 by only 10%. In contrast, blood flow apparently affected N2 uptake. Because O2 and CO2 uptakes were not blood flow limited, K'o2 and K'co2 are estimates of true permeability coefficients; the calculated permeability coefficient for N2 is 3.3 (ml x 10–4)/(min cm2 atm). Comparison shows the pocket surface to be 1/50–1/150 as effective for O2 transfer as the lung. Finally, corrections are calculated for pocket-to-tissue pO2 and pCO2 differences in gas pockets used for tissue tonometry.

By employing the technique of immersion refractometry to measure volume changes, a study has been made of the rate of entry of water into chick heart fibroblasts cultured in a fluid medium. The time required for the cytoplasm of the fibroblast to reach a given refractive index, when the cell is exposed to a low external osmotic pressure, is a measure of its permeability. It is pointed out that all calculations applied to such data depend on various more or less arbitrary assumptions. A number of calculations made on different assumptions give estimates of the permeability coefficient for the cell membrane varying from 1.41 to 2.82 μ min -1 atm -1 at 38°C, or on the other hand, estimates of the diffusion coefficient of water in the protoplasm varying from 1.7 x 10 -10 to 6.6 x 10 -10 cm 2 /s.

Moisture permeability of most double-layer films has a directional property. This “two-sidedness” may be brought about mainly by a change in the permeability coefficient as a result of the change in vapor pressure. To utilize this characteristic, it should be clarified as to how the permeability coefficient varies. For this purpose the differential permeability coefficient was calculated, making it easy to estimate the permeability of moisture under various conditions and making it possible to obtain the distribution of both vapor pressure and the water concentration in double-layer films. When the permeability on single films under various moisture conditions is given, the “two-sidedness” feature of double-layer films made from them will be grasped.

A study has been made of water entry into human red blood cells under an osmotic pressure gradient. The measurements were made using a rapid reaction stop flow apparatus, whose construction, calibration, and performance are described in detail. Red cell volume changes were determined from 90 degrees scattered light. The permeability coefficient for water entry under a relative isosmolar concentration of 1 to 1.5 was found to be 0.22 +/- 0.01 cm(4)/sec osmol, which agrees well with our previously published value. The experiments were also designed to measure the thickness of the unstirred layer around the6 red cells. This was found to be 5.5 +/- 0.8 micro under the present experimental conditions. It is concluded that our previously measured permeability coefficient for water entrance under a diffusion gradient does not require correction on account of the unstirred layer.

Antimony pH microelectrodes were used to measure the pH of blastocoele fluid (pHb) of the six-day intact rabbit blastocyst. The average pHb of blastocysts measured a short time after collection in a medium containing 25 mM HCO3− at 5% CO2 in air was 7.49 ± 0.03 (S.E.). The pHb of blastocysts incubated in vitro was affected by changes in P and external bicarbonate ion concentration. In general, variations of pH of the medium bring about changes in pHb in the same direction. On the assumption that the changes in pHb were due to H+ movement across the blastocoele wall, a H+ flux of 1.8 × 10−8 mole/cm2/sec was calculated. The corresponding H+ permeability coefficient was 8 × 10−2 cm/sec. It is concluded that H+ is passively distributed across the blastocoele wall.

The case of a vessel, which supplies a region through which it passes with some substance, is considered for the situation in which the permeability is the limiting factor. Diffusion parallel to the vessel is neglected. The substance may, however, be consumed proportional to its concentration in the inner or outer region. A solution is given for the case in which the input is an arbitrary function of time. It is suggested that the results may be applied in some cases to data on the injection of substances into blood vessels, or they may be applied to the transient effects in the case of vapors or gases passing through the respiratory passages.

The rate of water exchange in the salmon egg was determined by means of the electromagnetic diver-balance. Salmon eggs treated with alcohol to remove the plasma membrane were used to determine the diffusion coefficient, D, for water in the cytoplasm, and untreated eggs to determine the permeability coefficient, E, for water through the plasma membrane. It was found that D is of the same order as that observed in other eggs, whereas the value of E is somewhat lower, but still within the same order of magnitude. The results do not confirm the classical notion that the salmon egg is impermeable to water, but they show that the exchange is extremely slow, the half time being about 24 hr. In order to explain this result the contribution of egg size and environmental temperature must be taken into account. It is concluded that the actually observed rate of swelling corresponds to that anticipated from the recorded permeability coefficient, and that there is therefore no reason to presume that any kind of osmoregulation takes place.

Muscle and hemolymph from the moth Hyalphora cecropia were analyzed for Na + , K + , Ca 2+ and Mg 2+ content using spectrophotometric and atomic absorption techniques. The direction and relative magnitude of the transmembrane gradients are summarized. 1. (1) Na + and Ca 2+ are about evenly distributed; the magnitude of the transmembrane gradient is not statistically significant. 2. (2) The transmembrane K + concentration gradient is directed outward while the Mg 2+ gradient is directed inward. Substitution of these numerical values into the Nernst equation (assuming a permeability coefficient of 1) gives no clue as to the origin of the electrical potential recorded, i.e. E K + = −11 mV; E Na + = +10 mV; E Ca 2+ = −2 mV; E Mg 2+ = +12 m mV.

Rates and amounts of water translocation across the mitochondrial membrane have been studied with a photometric technique. The process of water translocation can be described in terms of the diffusion equations, and the mitochondria behave as spherical bodies between 15 and 110 mosm. A permeability coefficient to water of 5.3×10(-3)cm sec(-1) has been calculated. The mitochondrial surface is about 1m(2)/g protein during incubation in 0.10M KCl, and increases to 30 m(2)/g protein during incubation in 0.005M KCl.The osmotic shrinkage of hypotonically swollen mitochondria has also been studied. Complete reversibility of hypotonic swelling occurs only after incubation of mitochondria in media below 60 to 90 mosm. The appearance of the reversibility is phenomenologically correlated with the rupture of the outer mitochondrial membrane., Below 30 mosm there is a change of the absorbance properties of the membrane. The change correlates with the complete unfolding of the cristae and is attributed to ultrastructural reorganization of the membrane following mechanical stretching.

The permeability coefficient for the transport of a gas, vapor, or liquid through a polymer film is the product of the penetrant solubility and a diffusion coefficient. A transient permeation experiment known as the time-lag technique can be used to separate this product, provided the diffusion coefficient is independent of penetrant concentration. In this well-known experiment the polymer is initially free of penetrant. A new transient permeation experiment where the polymer is initially saturated with penetrant is suggested here. A general mathematical proof is given to show that by using the results form these two transient experiments which have different initial conditions one can determine the penetrant solubility no matter how the diffusion coefficient depends on penetrant concentration. Also one can determine two different concentration averaged diffusion coefficients from the results.

1. The rate of osmotic influx of water into flexible and hardened erythrocytes has been studied using a stopped-flow rapid-reaction method, and diluting the cells 1:20 in hypotonic media.2. Neither the initial rate of swelling nor the time to the onset of haemolysis were changed after making the erythrocytes inflexible by adding formaldehyde or by heating at 48 degrees C for 30 min.3. From a plot of the initial rate of uptake with hypotonicity, the permeability coefficient k can be calculated as 0.098 mu(3)/mu(2). sec atm (+/- 10%) at 20.5 degrees C.4. The value of k calculated from the time to the onset of haemolysis is 0.28 mu(3)/mu(2) sec atm. This suggests individual cells may differ over a wide range of permeability.5. The results are consistent with the pores being at fixed sites in the membrane and unaffected by flexing of the cell.

A new technique to determine the rate of water passage through the membrane of the human erythrocyte under an osmotic gradient has been developed. It utilizes a rapid mixing apparatus of the Hartridge-Roughton type which permits measurements at short intervals after the reaction has begun. This is coupled with a light-scattering device of new design which permits the determination of very small changes in volume of the cells without disturbing them. With this technique it was possible to measure the change in volume of freshly drawn human erythrocytes after about 50, 100, 155, and 215 msec. of exposure to anisotonic media. The experimental curves were compared with theoretical curves derived from accepted equations for the process and a permeability coefficient of 0.23 ± 0.03 (cm.4/osm., sec.) was obtained.

Recently, quantitative methods were developed for determining the permeability coefficient of solutes in lecithin spherules. The technique involved following (a) the direct release of solutes from the dispersions, (b) the release after prior dilution, and (c) the release from dispersions partly equilibrated with the solutes for a predetermined period. A quantitative evaluation of several physical models indicated that the models that assume that the spherules are equally spaced, multiconcentric bilayers of lecithin were in satisfactory agreement with the experimental release data. In the present study, this technique was applied to the transport of 3-O-methyl-D-glucose in liposome dispersions prepared from lecithin-dicetyl phosphate (10:1) and lecithin-dicetyl phosphate-cholesterol (10:1:1). The transport results for 3-O-methyl-D-glucose yielded a permeability coefficient that was 50 times larger than that for D-glucose. The dispersions prepared from lecithin-dicetyl phosphate containing 10% cholesterol yielded a permeability coefficient that was 2.4 times smaller than the dispersions prepared without cholesterol. The analysis of the results indicated that, for relatively large permeability coefficients as obtained in these studies, the dilution-release experiments show greater sensitivity in the determination of this parameter compared to the direct-release experiments.

1. 1. For the purpose of quantifying the transport of drugs across phospholipid membranes in complex aqueous liposome dispersions, solute release experiments were conducted with different initial boundary conditions so that sensitive tests of appropriate physical models could be made. 2. 2. Simple physical models which assume monosize or multisize single membrane controlled solute transport failed to provide a reasonable agreement between the experimental data and the theory. 3. 3. A systematic evaluation of all the parameters which could introduce uncertainties then revealed that the monosize-multiconcentric models are generally in satisfactory agreement with the experimental transport data. These findings suggest that these models may be used in the reliable determinations of effective bulk permeability coefficients. Calculations using the multisize-multiconcentric models and comparisons between the monosize and multisize-multiconcentric models showed that the assumption in which the particle size distribution is neglected is a good one.

Fourth-instar larvae of Opifex fuscus were found to have a mean body weight of 7·61 mg, a water content of 85·2%, and a mean cuticular surface area of 29·3 mm2. Larvae ligatured at neck and anus were found to lose 1 % of the body weight per hour when immersed in sea water. The osmotic permeability coefficient of the cuticle was calculated from these data to be 2·4 × 10− 2 cm. h− 1. Measurement of the diffusional permeability coefficient (Pd) by means of tritiated water and Deuterium oxide gave a value for this parameter of 4·8 × 10− 3 cm.h− 1. The diffusional permeability of the cuticle of Opifex fuscus is thus less than that of typical freshwater insects, as predicted by Shaw & Stobbart.

ConclusionThe permeability of proximal convolutions and loops of Henle to 14C-urea has been studied using the microperfusion technique in the intact rat kidney. The permeability coefficient of both structures is about the same, 12.5 × 10-4 and 10.4 × 10-4 mm/sec, respectively, and these values are very close to those obtained previously by other authors.

The removal of carbon dioxide and other noxious vapors from the life-support system of a spacecraft is a formidable problem in manned space exploration. This is particularly true for flights of long duration, and in those instances when a significant number of astronauts are involved in the flight. Of the several different systems proposed, the gaseous diffusion, or permeability, process incorporates three attractive features.

An attempt was made to determine the sodium flux between the fast compartment of brain sodium - assumed to represent the pericapillary sodium - and plasma. Concentration of Na22 in plasma and brain of nephrectomized mice was measured from 40 sec. to 15 min. after intravenous isotope injection. Using a mathematical model of brain sodium compartments an expected course of Na22 concentration in brain corresponding to the course of plasma Na22 concentration was computed. The computed course was fitted to the actual values of brain Na22 concentration by changing the properties of the fast compartment in the model. An approximate value of sodium flux between the fast compartment and plasma determined in this way is 10.48 μEq/min. per 1 g of wet tissue. From this value the permeability coefficient of brain capillaries PNa = 0.50 × 10−5 cm/sec. was estimated.

Using methods described in Part I differences of pressure were applied between the external medium and the xylem elements of tomato root systems. The pressure permeability coefficient k p (the water flux per unit pressure difference), and the osmotic permeability coefficient k 0 (the water flux per unit difference of osmotic potential between the external medium and the xylem elements) were compared under similar conditions of applied pressure. Also k 0 was measured in the presence and absence of pressure gradients. The most striking effect of applying a pressure gradient was an increase in k 0 , 2 atm commonly causing a five-fold increase. Thus diffusion of water across the cortex was greatly facilitated by a pressure gradient. In addition it was found that under similar pressure conditions k p was consistently greater than k 0 , k p / k 0 being about 1·3 with an applied pressure difference of 2 atm. This indicated a mass flow of about a quarter of the total flux under these conditions. This dual nature of the water movement was confirmed by the action of cyanide and anaerobic conditions which caused a greater reduction in k 0 than k p showing that the diffusional pathway was more sensitive to decreased metabolism than the mass flow pathway. The effect of pressure gradients on diffusional water movement and the possible location of the mass flow pathway are briefly discussed as are their implications with reference to transpirational tensions and the uptake of solutes from the soil.

SummarySorbose alone and with glucose have been infused intravenously into pregnant rabbits near term, and permeability coefficients for sorbose under these 2 conditions have been calculated. It has been shown that there is a significant depression of permeability coefficient for sorbose when the maternal blood is overloaded with glucose. The results have been interpreted as indicating a competitive inhibition by glucose of sorbose transfer across the placenta.

A theoretical analysis is presented for the calculation of tritiated water (3HHO) permeability coefficient in red blood cells during osmotic water flow. This method can also be used to calculate the permeability coefficients of small hydrophilic solutes under similar experimental conditions. With the aid of this analysis it becomes feasible experimentally to test the pore model hypothesis for the red blood cell membrane. Previously determined experimental values were substituted for parameters in the final equation to show how this hypothesis can be tested.

The diffusion of 22Na+ through a composite system consisting of a layer of phospholipid sol and a layer of bovine plasma albumin have been made using a modified form of the continuous monitoring method. When 0.001 M CaCl2 and cholesterol is added to each layer a membrane is formed at the interface and affords a resistance to diffusional flow. Equations have been developed using Laplace transform theory from which the permeability coefficient of this membrane can be calculated.

A differential equation is developed describing volume changes as a function of time for a system such as a cell which is undergoing osmotically induced volume changes due to a solute which penetrates the cell membrane or boundary of the system. The parameters used to describe the membrane properties are the Staverman reflection coefficient, the water permeability coefficient and the solute permeability coefficient. The fluids bathing both sides of the membrane barrier are assumed to be spatially homogeneous with respect to water and solute concentrations. A first-order perturbation technique is used to solve the equations and obtain cell volume as a function of time. This solution is compared with data available in the literature and values for the cell membrane parameters are obtained for these cases.

The present experiments, in contrast to the conclusions of Harrison et al. [3] indicate that endotoxin in LD90 dose causes direct damage to the canine alveolocapillary membrane with an increase of permeability. The effect is readily apparent in both plasma-perfused hypoxic lungs and whole-blood-perfused oxygenated lungs. For plasma-perfused oxygenated lungs a greater dose of endotoxin is required to observe this effect.

It is suggested that the water permeability properties of cells are not as diverse as they appear to be when judged by means of their widely different conventional permeability coefficients. These coefficients ignore the possible rate-limiting effect of water diffusion in the cell interior. If it is assumed that the water permeability coefficients of cell membranes are similar in the wide variety of cells for which data are available, and that the diffusion coefficients of water in the cell interior are also similar, then both these quantities may be calculated. The permeability coefficient was thus estimated to lie within the range from 3 × 10−4 to 7 × 10−3 cm/sec, and the diffusion coefficient in the range from 8 × 10−10 to 2 × 10−8 cm2/sec. The significance of these values has been discussed.

A method has been devised to measure the permeability characteristics of the intestinal mucosal cells in the rat. The method makes use of an electrical recording balance to register changes in weight when the mucosal face of a small strip of intestine is exposed to anisotonic solutions. The permeability coefficient of the luminal surface of intestinal mucosal cells to water is measured as 0.15 cm4/OSM, sec. and reasons are adduced to suggest that the true value might be higher than this. The equivalent pore radius of the luminal face of the tissue, measured in experiments in which lipid-insoluble non-electrolytes have been used according to the method of Goldstein and Solomon, appears to be 4.0 Å.

Pinocytosis in amoeba was described, and its brief history reviewed at an earlier ACS meeting (4). It was pointed out that pinocytosis is the descriptive term for a behavioral pattern. In amoeba this pattern is initiated by a step increase in the cation concentration at a suitable pH and pCa. This report will consider in detail the initial phase of pinocytosis, the reaction of the ameoba plasma membrane to the cationic stimulus.

Freshly isolated frog rod outer segments are sensitive osmometers which retain their photosensitivity; their osmotic behavior reveals essentially the same light-sensitive Na+ influx observed electrophysiologically in the intact receptor cell. Using appropriate osmotic conditions we have examined freeze-etch replicas of freshly isolated outer segments to identify the membrane which regulates the flow of water and ions. Under isosmotic conditions we find that the disc to disc repeat distance is almost exactly twice the thickness of a disc. This ratio appears to be the same in a variety of vertebrate rod outer segments and can be reliably measured in freeze-etch images. Under all our osmotic conditions the discs appear nearly collapsed. However, when the length of the outer segment is reduced by hyperosmotic shocks the discs move closer together. This markedly reduces the ratio of repeat distance to disc thickness since disc thickness remains essentially constant. Thus, the length reduction of isolated outer segments after hyperosmotic shocks primarily results from reduction of the extradisc volume. Since the discs are free floating and since they undergo negligibly small changes in volume, the plasma membrane alone must be primarily responsible for regulating the water flux and the light-sensitive Na+ influx in freshly isolated outer segments. On this basis we calculate, from the osmotic behavior, that the plasma membrane of frog rod outer segment has a Na+ permeability constant of about 2.8 x 10-6 cm/s and an osmotic permeability coefficient of greater than 2 x 10-3 cm/s.