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PubMed1994

Osmotic flow in membrane pores of molecular size.

Water transfer by osmosis through pores occurs either by viscous flow or diffusion depending on whether the driving osmolyte is able to enter the pore. Analysis of osmotic permeabilities (Pos) measured in antibiotic and cellular pore systems supports this distinction, showing that Pos approaches either the viscous value (Pf) or the diffusive value (Pd) depending on the size of the osmolyte in relation to the pore radius. Macroscopic hydrodynamics and diffusion theory, when used with drag and steric coefficients within an appropriate osmotic model, apply with remarkable accuracy to channels of molecular dimensions where water molecules cannot pass each other, without the need to postulate any special flow regimens. It becomes apparent that the true viscous to diffusive flow ratio, Pf/Pd, can be separated from the effects of tracer filing by osmotic measurements alone. It does not monotonically decrease with the pore radius but rises steeply at the smaller radii which would apply to pores in cell membranes. Consequently, the application of the theory to osmotic and diffusive flow data for the red cell predicts a pore radius of 0.2 nm in agreement with other recent measurements on isolated components of the system, showing that the viscous-diffusive distinction applies even in molecular pores.

Anti-Bacterial Agents↗
PubMed2001

Isotonic transport by the Na+-glucose cotransporter SGLT1 from humans and rabbit.

1. In order to study its role in steady state water transport, the Na+-glucose cotransporter (SGLT1) was expressed in Xenopus laevis oocytes; both the human and the rabbit clones were tested. The transport activity was monitored as a clamp current and the flux of water followed optically as the change in oocyte volume. 2. SGLT1 has two modes of water transport. First, it acts as a molecular water pump: for each 2 Na+ and 1 sugar molecule 264 water molecules were cotransported in the human SGLT1 (hSGLT1), 424 for the rabbit SGLT1 (rSGLT1). Second, it acts as a water channel. 3. The cotransport of water was tightly coupled to the sugar-induced clamp current. Instantaneous changes in clamp current induced by changes in clamp voltage were accompanied by instantaneous changes in the rate of water transport. 4. The cotransported solution was predicted to be hypertonic, and an osmotic gradient built up across the oocyte membrane with continued transport; this resulted in an additional osmotic influx of water. After 5-10 min a steady state was achieved in which the total influx was predicted to be isotonic with the intracellular solution. 5. With the given expression levels, the steady state water transport was divided about equally between cotransport, osmosis across the SGLT1 and osmosis across the native oocyte membrane. 6. Coexpression of AQP1 with the SGLT1 increased the water permeability more than 10-fold and steady state isotonic transport was achieved after less than 2 s of sugar activation. One-third of the water was cotransported, and the remainder was osmotically driven through the AQP1. 7. The data suggest that SGLT1 has three roles in isotonic water transport: it cotransports water directly, it supplies a passive pathway for osmotic water transport, and it generates an osmotic driving force that can be employed by other pathways, for example aquaporins.

Animals↗
PubMed2005

Osmosis and solute-solvent drag: fluid transport and fluid exchange in animals and plants.

In 1903, George Hulett explained how solute alters water in an aqueous solution to lower the vapor pressure of its water. Hulett also explained how the same altered water causes osmosis and osmotic pressure when the solution is separated from liquid water by a membrane permeable to the water only. Hulett recognized that the solute molecules diffuse toward all boundaries of the solution containing the solute. Solute diffusion is stopped at all boundaries, at an open-unopposed surface of the solution, at a semipermeable membrane, at a container wall, or at the boundary of a solid or gaseous inclusion surrounded by solution but not dissolved in it. At each boundary of the solution, the solute molecules are reflected, they change momentum, and the change of momentum of all reflected molecules is a pressure, a solute pressure (i.e., a force on a unit area of reflecting boundary). When a boundary of the solution is open and unopposed, the solute pressure alters the internal tension in the force bonding the water in its liquid phase, namely, the hydrogen bond. All altered properties of the water in the solution are explained by the altered internal tension of the water in the solution. We acclaim Hulett's explanation of osmosis, osmotic pressure, and lowering of the vapor pressure of water in an aqueous solution. His explanation is self-evident. It is the necessary, sufficient, and inescapable explanation of all altered properties of the water in the solution relative to the same property of pure liquid water at the same externally applied pressure and the same temperature. We extend Hulett's explanation of osmosis to include the osmotic effects of solute diffusing through solvent and dragging on the solvent through which it diffuses. Therein lies the explanations of (1) the extravasation from and return of interstitial fluid to capillaries, (2) the return of luminal fluid in the proximal and distal convoluted tubules of a kidney nephron to their peritubular capillaries, (3) the return of interstitial fluid to the vasa recta, (4) return of aqueous humor to the episcleral veins, and (5) flow of phloem from source to sink in higher plants and many more examples of fluid transport and fluid exchange in animal and plant physiology. When a membrane is permeable to water only and when it separates differing aqueous solutions, the flow of water is from the solution with the lower osmotic pressure to the solution with the higher osmotic pressure.

Animals↗
PubMed1978

Distribution of osmotic flow in stomach and gallbladder.

Stomach and gallbladder actively transport fluids which are nearly isotonic to plasma. Consideration of the measured areas of the appropriate transporting surfaces gives a more realistic view of the osmotic gradient required to account for the observed net flow of water. Simple osmosis may be adequate if the transporting membrane has an osmotic permeability in the range observed for synthetic lecithin-cholesterol bilayer membranes.

Animals↗
PubMed1999

Hyperosmosis of cerebral injury.

Changes in tissue osmolarity or cerebrospinal fluid osmolarity after cerebral injury have received little attention in the literature, but osmosis may be an important cause of early cerebral edema. This paper reviews concepts and terms relating to osmosis, and reviews the few papers in the literature which have studied osmolarity after cerebral injury. In studies of both traumatic brain injury and ischemia, tissue osmolarity is elevated. Osmolarity of cerebrospinal fluid has also been shown to increase with injury. There have been no human studies examining osmolarity of tissue or cerebrospinal fluid after cerebral injury. Theoretical implications of the osmotic gradient are discussed.

Animals↗
PubMed1981

Removal of bound calcium from nematocyst contents causes discharge.

Nematocysts are the stinging organelles of jellyfish, sea anemones and other cnidarians. Each one consists of a closed capsule filled with fluid. In the resting state, part of the surface of the capsule is inverted, forming a tubular thread, which is everted explosively on excitation. The mechanism of explosion is not yet understood, it may be relevant to exocytosis in general, as the nematocyst is a specialized type of exocytotic secretion originating in the Golgi apparatus. Picken and Skaer observed that the capsular fluid showed a large depression of freezing point, suggesting that the osmotic pressure might be as high as 140 bar. They tentatively ascribed the explosion to a sudden increase in permeability of the capsule wall, allowing a rapid osmotic influx of water. However, there is evidence that the material of the thread itself may be capable of some degree of extension and it has been suggested that osmosis plays no part in discharge. We present here a new theory: that the capsule wall is permeable to water even in the undischarged state; discharge is initiated by an increase in the osmotic pressure of the capsular fluid which is brought about by removal of bound calcium ions.

Animals↗
PubMed1985

A central role for cell osmolarity in isotonic fluid transport across epithelia.

Previous theoretical models for solute-solvent coupling in epithelia that dealt only with the intercellular channel did not predict isotonic transport except when very high cell membrane permeabilities were assumed. To study this issue, we have developed the formalisms for osmotic equilibration at an alternative location, the apical cell membrane (including its adjacent unstirred layer), which are somewhat simpler than those for the channel. Much as in other models, we confirm that only rather unrealistically high values of the cell membrane permeability lead to isotonic transport. We have also found, however, that isotonic transport can occur at much lower values of the cell membrane permeability if the concentration within the cell differs slightly from that in the ambient medium. This emphasizes the importance of incorporating the intracellular concentration as an integral part to any transport model, such as in the present apical membrane version of local osmosis.

Animals↗
PubMed1991

Aspects of silicone rubber as an encapsulant for neurological prostheses. Part 1. Osmosis.

Silicone rubber is not, at first sight, a promising material with which to encapsulate a piece of implantable microelectronics expected to last ten or twenty years. It is well known to be very permeable to water. In fact, if correctly applied, it performs very well for ten years, possibly longer. The paper considers the part played by osmosis in achieving satisfactory performance combined with simple technology.

Biomedical Engineering↗
PubMed1998

Routes and mechanism of fluid transport by epithelia.

The mechanism of fluid transport by leaky epithelia and the route taken by the transported fluid are in dispute. A consideration of current mathematical models for coupling of solutes and water, as well as the methodologies for the study of fluid transport, shows that local osmosis best accounts for water movement. Although it seems virtually certain that the tight junctions are water permeable, the fraction of absorbed fluid that crosses the tight junction cannot yet be determined with confidence.

Animals↗
PubMed1986

Effect of inositol hexaphosphate on the transient behavior of red cells following a DMSO-induced osmotic pulse.

An osmotic pulse can be used to incorporate inositol hexaphosphate (IHP) into red cells. The pulse is induced by equilibrating a red cell suspension with DMSO and then rapidly diluting with an isotonic IHP solution. Since IHP binds to hemoglobin and lowers the affinity for oxygen, this method may find application in the preparation of low-affinity cells for experimental and clinical use. The experiments reported here examined the dynamic changes of several red cell variables immediately following the osmotic pulse. The effect of IHP, which has been shown to dissociate red cell cytoskeletons, was evaluated by comparison with a matched phosphate-buffered saline (PBS) diluent. Red cell morphology, volume, and hemoglobin permeability were studied by fixing the cells at times ranging from 0.06 to 300 sec after dilution. Mechanical fragility was measured by subjecting the cells to a short period of shear stress at the same times after dilution. With both diluents, the cells underwent a rapid increase in volume followed by a return towards normal volume with a maximum at less than 250 msec. With IHP diluent, the period of hemoglobin permeability immediately followed the size peak and was completed by about 1 sec after dilution. PBS also induced a second leakage at longer times (10-120 sec), which resulted in a morphological dichotomy with ghosts and intact cells. The choice of diluent also affected sensitivity to shear stress. The IHP-treated cells had a mechanical fragility maximum at about 1 sec. The PBS-treated cells exhibited no enhanced mechanical fragility. An unexpected result was the inhibition of the second phase of lysis in PBS-treated cells by a properly timed shear stress.

Cell Membrane Permeability↗
PubMed1988

Kinetics of osmotic water movement in chondrocytes isolated from articular cartilage and applications to cryopreservation.

The ability of chondrocytes to survive conditions encountered during cryopreservation depends on the responses of the cells to the physiochemical changes that result when water is removed from the environment of the cells in the form of ice. Cellular responses are therefore closely related to the osmotic permeability properties of the plasma membrane. In order to optimize the conditions for cryopreservation of chondrocytes, osmotic properties of the chondrocyte membrane were determined from the kinetics of volume change in hypertonic solutions at different temperatures. The hydraulic conductivity of the plasma membrane was 0.305 +/- 0.025 micron3/micron2/min/atmosphere at 24 degrees C, with an Arrhenius activation energy of 8.06 kcal/mol. These values are similar to those reported for other cell types, but the osmotically inactive volume of the chondrocytes (0.41 +/- 0.04) was significantly higher than for other cells, implying that chondrocytes have a higher dry weight ratio or that they contain a higher proportion of osmotically inactive or bound water. These results were used to calculate the osmotic responses of chondrocytes at low temperatures and to predict that the least damaging cooling rate for isolated chondrocytes in the absence of cryoprotective compounds is 10 degrees C/min. The ultimate goal of this study is the development of an analytical model applicable to the optimization of techniques for cryopreservation of intact cartilage and other tissues.

Animals↗
PubMed1977

The generation of the proton electrochemical potential and its role in energy transduction.

The evidence that all energy transducing membranes can generate a proton electrochemical potential difference, delta micronH, across the membrane and that this potential can be used to transfer energy among energy transducing units and to generate ATP, has increased the interest for the view that delta micronH plays an obligatory role in energy transduction and ATP synthesis. In the present article we shall concentrate on two experimental questions related with the generation and role of delta micronH: (a) the charge/site ratio; (b) the relation between the proton electrochemical potential on one side and the cation electrochemical potential, the phosphate potential and the redox potential on the other. We shall then discuss the view that energy transduction corresponds to a molecular energy machine rather than to a fuel cell.

Adenosine Triphosphate↗
PubMed1978

[The influence of capillary permeability on the extravascular protein quantities as measured in situ (author's transl)].

The extravascular protein contents in the perivascular connective tissue of a rat's mesenterial plate was measured ultramicrospectrophotometrically in situ and in vivo after changing the hydrostatic and colloidosmotic pressures of the blood. We analized the perivascular area of the different microcirculatory vessels, arterioles, capillaries and venules. The perivascular protein contents, which corresponds besides arterioles to 40% and besides venules to 59% in comparison to the intravascular blood plasma, decreased in a comparison group under the osmotic influence of the perfusion liquid on the mesenterial plate. An injection of an isotonic saline solution is followed only by small fluctuations of the perivascular protein contents during one hour. After blood loss a transitional increase of extravascular protein at the perivenular area was observed indicating the mobilisation of protein depots. Around arterioles the extravascular protein contents did not change significantly. An intravenous injection of albumin solution was followed by a short-termed increase of tissue protein around the arterioles. Around the venules after 50 minutes the extravascular protein contents increased significantly. Within one hour after the changes of the permeability conditions the maximal induceable protein movements in the perivascular space were calculated as +/- 1.35 g% plasma protein concentration corresponding to a maximun protein exchange of +/- 3.1 mg ml(-1) tissue.

Animals↗
PubMed2004

Mathematical modelling of corneal swelling.

This paper presents a differential model of the corneal transport system capable of modelling thickness changes in response to osmotic perturbations applied to either limiting membrane. The work is directed towards understanding corneal behaviour in vivo. The model considers the coupled viscous flows within the corneal stroma and across the epithelial and endothelial membranes. The flows within the stroma are established based on transport theory in porous media, while the flows across the membranes are described using the phenomenological equations of irreversible thermodynamics. The ability of the numerical model to reproduce corneal thickness changes in response to endothelial perturbations was tested against available experimental data. The sensitivity of the model to changes in stromal and membrane transport coefficients was examined.

Cell Membrane Permeability↗