Salt-water coupling in leaky epithelia

Are standing osmotic gradients the main driver of cerebrospinal fluid production? A computational analysis

BACKGROUND

The mechanisms of cerebrospinal fluid (CSF) production by the ventricular choroid plexus (ChP) have not been fully deciphered. One prominent hypothesized mechanism is trans-epithelial water transport mediated by accumulation of solutes at the luminal ChP membrane that produces local osmotic gradients. However, this standing osmotic gradient hypothesis has not been systematically tested.

METHODS

To assess the plausibility of the standing gradient mechanism serving as the main driver of CSF production by the ChP, we developed a three-dimensional (3D) and a one-dimensional (1D) computational model to quantitatively describe the associated processes in the rat ChP inter-microvillar spaces and in CSF pools between macroscopic ChP folds (1D only). The computationally expensive 3D model was used to examine the applicability of the 1D model for hypothesis testing. The 1D model was employed to predict the rate of CSF produced by the standing gradient mechanism for 200,000 parameter permutations. Model parameter values for each permutation were chosen by random sampling from distributions derived from published experimental data.

RESULTS

Both models predict that the CSF production rate by the standing osmotic gradient mechanism is below 10% of experimentally measured values that reflect the contribution of all actual production mechanisms. The 1D model indicates that increasing the size of CSF pools between ChP folds, where diffusion dominates solute transport, would increase the contribution of the standing gradient mechanism to CSF production.

CONCLUSIONS

The models suggest that the effect of standing osmotic gradients is too small to contribute substantially to CSF production. ChP motion and movement of CSF in the ventricles, which are not accounted for in the models, would further reduce this effect, making it unlikely that standing osmotic gradients are the main drivers of CSF production.

Membrane transporters control cerebrospinal fluid formation independently of conventional osmosis to modulate intracranial pressure

Background

Disturbances in the brain fluid balance can lead to life-threatening elevation in the intracranial pressure (ICP), which represents a vast clinical challenge. Nevertheless, the details underlying the molecular mechanisms governing cerebrospinal fluid (CSF) secretion are largely unresolved, thus preventing targeted and efficient pharmaceutical therapy of cerebral pathologies involving elevated ICP.

Methods

Experimental rats were employed for in vivo determinations of CSF secretion rates, ICP, blood pressure and ex vivo excised choroid plexus for morphological analysis and quantification of expression and activity of various transport proteins. CSF and blood extractions from rats, pigs, and humans were employed for osmolality determinations and a mathematical model employed to determine a contribution from potential local gradients at the surface of choroid plexus.

Results

We demonstrate that CSF secretion can occur independently of conventional osmosis and that local osmotic gradients do not suffice to support CSF secretion. Instead, the CSF secretion across the luminal membrane of choroid plexus relies approximately equally on the Na⁺/K⁺/2Cl⁻ cotransporter NKCC1, the Na⁺/HCO₃⁻ cotransporter NBCe2, and the Na⁺/K⁺-ATPase, but not on the Na⁺/H⁺ exchanger NHE1. We demonstrate that pharmacological modulation of CSF secretion directly affects the ICP.

Conclusions

CSF secretion appears to not rely on conventional osmosis, but rather occur by a concerted effort of different choroidal transporters, possibly via a molecular mode of water transport inherent in the proteins themselves. Therapeutic modulation of the rate of CSF secretion may be employed as a strategy to modulate ICP. These insights identify new promising therapeutic targets against brain pathologies associated with elevated ICP.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-022-00358-4.

Cerebrospinal fluid production by the choroid plexus: a century of barrier research revisited

Cerebrospinal fluid (CSF) envelops the brain and fills the central ventricles. This fluid is continuously replenished by net fluid extraction from the vasculature by the secretory action of the choroid plexus epithelium residing in each of the four ventricles. We have known about these processes for more than a century, and yet the molecular mechanisms supporting this fluid secretion remain unresolved. The choroid plexus epithelium secretes its fluid in the absence of a trans-epithelial osmotic gradient, and, in addition, has an inherent ability to secrete CSF against an osmotic gradient. This paradoxical feature is shared with other 'leaky' epithelia. The assumptions underlying the classical standing gradient hypothesis await experimental support and appear to not suffice as an explanation of CSF secretion. Here, we suggest that the elusive local hyperosmotic compartment resides within the membrane transport proteins themselves. In this manner, the battery of plasma membrane transporters expressed in choroid plexus are proposed to sustain the choroidal CSF secretion independently of the prevailing bulk osmotic gradient.

Water permeability of rat liver mitochondria: A biophysical study

The movement of water accompanying solutes between the cytoplasm and the mitochondrial spaces is central for mitochondrial volume homeostasis, an important function for mitochondrial activities and for preventing the deleterious effects of excess matrix swelling or contraction. While the discovery of aquaporin water channels in the inner mitochondrial membrane provided valuable insights into the basis of mitochondrial plasticity, questions regarding the identity of mitochondrial water permeability and its regulatory mechanism remain open. Here, we use a stopped flow light scattering approach to define the water permeability and Arrhenius activation energy of the rat liver whole intact mitochondrion and its membrane subcompartments. The water permeabilities of whole brain and testis mitochondria as well as liposome models of the lipid bilayer composing the liver inner mitochondrial membrane are also characterized. Besides finding remarkably high water permeabilities for both mitochondria and their membrane subcompartments, the existence of additional pathways of water movement other than aquaporins are suggested.

The Role of Aquaporin Water Channels in Fluid Secretion by the Exocrine Pancreas

The mammalian exocrine pancreas secretes a near-isosmotic fluid over a wide osmolarity range. The role of aquaporin (AQP) water channels in this process is now becoming clearer. AQP8 water channels, which were initially cloned from rat pancreas, are expressed at the apical membrane of pancreatic acinar cells and contribute to their osmotic permeability. However, the acinar cells secrete relatively little fluid and there is no obvious defect in pancreatic function in AQP8 knockout mice. Most of the fluid secreted by the pancreas is generated by ductal epithelial cells, which comprise only a small fraction of the gland mass. In the human pancreas, secretion occurs mainly in the intercalated ducts, where the epithelial cells express abundant AQP1 and AQP5 at the apical membrane and AQP1 alone at the basolateral membrane. In the rat and mouse, fluid secretion occurs mainly in the interlobular ducts where AQP1 and AQP5 are again co-localized at the apical membrane but appear to be expressed at relatively low levels. Nonetheless, the transepithelial osmotic permeability of rat interlobular ducts is sufficient to support near-isosmotic fluid secretion at observed rates. Furthermore, apical, but not basolateral, application of Hg(2+) significantly reduces the transepithelial osmotic permeability, suggesting that apical AQP1 and AQP5 may contribute significantly to fluid secretion. The apparently normal fluid output of the pancreas in AQP1 knockout mice may reflect the presence of AQP5 at the apical membrane.

General models for water transport across leaky epithelia

The group of leaky epithelia, such as proximal tubule and small intestine, have several common properties in regard to salt and water transport. The fluid transport is isotonic, the transport rate increases in dilute solutions, and water can be transported uphill. Yet, it is difficult to find common features that could form the basis for a general transport model. The direction of transepithelial water transport does not correlate with the direction of the primary active Na+ transport, or with the ultrastucture as defined by the location of apical and basolateral membranes, of the junctional complex and the lateral intercellular spaces. The presence of specific water channels, aquaporins, increases the water permeability of the epithelial cell membranes, i.e., the kidney proximal tubule. Yet other leaky epithelia, for example, the retinal pigment epithelium, have no known aquaporins. There is, however, a general correlation between the direction of transepithelial transport and the direction of transport via cotransporters of the symport type. A simple epithelial model based on water permeabilities, a hyperosmolar compartment and restricted salt diffusion, is unable to explain epithelial transport phenomena, in particular the ability for uphill water transport. The inclusion of cotransporters as molecular water pumps in these models alleviates this problem.

Molecular water pumps

There is good evidence that cotransporters of the symport type behave as molecular water pumps, in which a water flux is coupled to the substrate fluxes. The free energy stored in the substrate gradients is utilized, by a mechanism within the protein, for the transport of water. Accordingly, the water flux is secondary active and can proceed uphill against the water chemical potential difference. The effect has been recognized in all symports studied so far (Table 1). It has been studied in details for the K+/Cl- cotransporter in the choroid plexus epithelium, the H+/lactate cotransporter in the retinal pigment epithelium, the intestinal Na+/glucose cotransporter (SGLT1) and the renal Na+/dicarboxylate cotransporter both expressed in Xenopus oocytes. The generality of the phenomenon among symports with widely different primary structures suggests that the property of molecular water pumps derives from a pattern of conformational changes common for this type of membrane proteins. Most of the data on molecular water pumps are derived from fluxes initiated by rapid changes in the composition of the external solution. There was no experimental evidence for unstirred layers in such experiments, in accordance with theoretical evaluations. Even the experimental introduction of unstirred layers did not lead to any measurable water fluxes. The majority of the experimental data supports a molecular model where water is cotransported: A well defined number of water molecules act as a substrate on equal footing with the non-aqueous substrates. The ratio of any two of the fluxes is constant, given by the properties of the protein, and is independent of the driving forces or other external parameters. The detailed mechanism behind the molecular water pumps is as yet unknown. It is, however, possible to combine well established phenomena for enzymes into a working model. For example, uptake and release of water is associated with conformational changes during enzymatic action; a specific sequence of allosteric conformations in a membrane bound enzyme would give rise to vectorial transport of water across the membrane. In addition to their recognized functions, cotransporters have the additional property of water channels. Compared to aquaporins, the unitary water permeability is about two orders of magnitude lower. It is suggested that the water permeability is determined from chemical associations between the water molecule and sites within the pore, probably in the form of hydrogen-bonds. The existence of a passive water permeability suggests an alternative model for the molecular water pump: The water flux couples to the flux of non-aqueous substrates in a hyperosmolar compartment within the protein. Molecular water pumps allow cellular water homeostasis to be viewed as a balance between pumps and leaks. This enables cells to maintain their intracellular osmolarity despite external variations. Molecular water pumps could be relevant for a wide range of physiological functions, from volume regulation in contractile vacuoles in amoeba to phloem transport in plants (Zeuthen 1992, 1996). They could be important building blocks in a general model for vectorial water transport across epithelia. A simplified model of a leaky epithelium incorporating K+/Cl-/H2O and Na+/glucose/H2O cotransport in combination with channels and primary active transport gives good quantitative predictions of several properties. In particular of how epithelial cell layers can transport water uphill.

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.

Molecular mechanisms for passive and active transport of water

Water crosses cell membranes by passive transport and by secondary active cotransport along with ions. While the first concept is well established, the second is new. The two modes of transport allow cellular H2O homeostasis to be viewed as a balance between H2O leaks and H2O pumps. Consequently, cells can be hyperosmolar relative to their surroundings during steady states. Under physiological conditions, cells from leaky epithelia may be hyperosmolar by roughly 5 mosm liter-1, under dilute conditions, hyperosmolarities up to 40 mosm liter-1 have been recorded. Most intracellular H2O is free to serve as solvent for small inorganic ions. The mechanism of transport across the membrane depends on how H2O interacts with the proteinaceous or lipoid pathways. Osmotic transport of H2O through specific H2O channels such as CHIP 28 is hydraulic if the pore is impermeable to the solute and diffusive if the pore is permeable. Cotransport of ions and H2O can be a result of conformational changes in proteins, which in addition to ion transport also translocate H2O bound to or occlude in the protein. A cellular model of a leaky epithelium based on H2O leaks and H2O pumps quantitatively predicts a number of so-far unexplained observations of H2O transport.

Cotransport of K+, Cl- and H2O by membrane proteins from choroid plexus epithelium of Necturus maculosus

1. The interaction between K+, Cl- and H2O fluxes was studied in the ventricular membrane of the choroid plexus epithelium from Necturus maculosus by means of ion-selective microelectrodes. 2. Three experimental strategies were adopted: the osmolarity of the ventricular solution was increased abruptly by addition of (i) mannitol or (ii) KCl; (iii) Na+ in the ventricular solution was replaced isosmotically by K+. 3. The mannitol experiments showed that H2O had two pathways across the ventricular membrane. One was purely passive, with a water permeability, L'p, of 0.64 x 10(-4) cm s-1 (osmol l-1)-1. This operated in parallel with an ion-dependent pathway of similar magnitude which was abolished in Cl(-)-free solutions. 4. When KCl was added there was a flow of H2O into the cell. Surprisingly, this took place despite the osmotic gradient which favoured an efflux of H2O. The effect was blocked by frusemide (furosemide), in which case KCl had the same effects as applications of NaCl or mannitol. 5. Replacement of Na+ with K+ caused an influx of H2O. This flux could proceed against osmotic gradients implemented by mannitol. 6. The present data and those of earlier publications show that the interdependence of the fluxes of K+, Cl- and H2O in the exit membrane can be described as cotransport. The fluxes have a fixed stoichiometry of 1:1:500, the flux of one species is able to energize the flux of the two others, and the transport exhibits saturation and is specific for K+ and Cl-. 7. A molecular model based upon a mobile barrier in a membrane spanning protein gives an accurate quantitative description of the data.

A kinetic model of rat proximal tubule transport--load-dependent bicarbonate reabsorption along the tubule

A model is presented of solute and water reabsorption along the proximal tubule of the rat kidney based on kinetic descriptions of the main membrane transport systems, in order to assess the extent to which these kinetics suffice to explain certain aspects of the global transport behaviour in this segment, especially with respect to bicarbonate reabsorption. The model includes in the apical membrane, an active proton pump, Na+/H+ antiport, Na-coupled transport of organic solutes, Cl-/formate exchange with formic acid recycling, and membrane conductances to protons and K+. In the baso-lateral membrane, besides the Na+/K+ pump, the model includes Na(+)-3HCO3- and electroneutral K(+)-Cl- cotransporters, and membrane conductances for K+, H+, and, optionally, for Cl-. Appropriate passive diffusional pathways were included in both cell membranes and in the paracellular pathway. Using mass balance and electroneutrality constraints, these transport systems were built into an epithelial model which was then integrated (by finite difference approximation) into a model of a longitudinal tubule. Simulated cellular solute concentrations and luminal concentration profiles were in good agreement with reported experimental observations. We show that, given the reported transport kinetics for the Na+/H+ antiporter, a hitherto unexplained observation concerning load-dependent bicarbonate reabsorption can be shown mainly to result from the nonlinear longitudinal concentration profile for bicarbonate and pH. We also discuss problems of transcellular Cl- transport in the light of recent reports of basolateral Cl- conductance and observations relevant to apical Cl-/formate (or other base) exchange.

Pseudo-streaming potentials in Necturus gallbladder epithelium. I. Paracellular origin of the transepithelial voltage changes

Apparent streaming potentials were elicited across Necturus gallbladder epithelium by addition or removal of sucrose from the apical bathing solution. In NaCl Ringer's solution, the transepithelial voltage (Vms) change (reference, basolateral solution) was positive with sucrose addition and negative with sucrose removal. Bilateral Cl- removal (cyclamate replacement) had no effect on the polarity or magnitude of the Vms change elicited by addition of 100 mM sucrose. In contrast, bilateral Na+ removal (tetramethylammonium [TMA+] replacement) inverted the Vms change (from 2.7 +/- 0.3 to -3.2 +/- 0.2 mV). Replacement of Na+ and Cl- with TMA+ and cyclamate, respectively, abolished the change in Vms. Measurements of cell membrane voltages and relative resistances during osmotic challenges indicate that changes in cell membrane parameters do not explain the transepithelial voltage changes. The initial changes in Vms were slower than expected from concomitant estimates of the time course of sucrose concentration (and hence osmolality) at the membrane surface. Paired recordings of the time courses of paracellular bi-ionic potentials (partial substitution of apical Na+ with tetrabutylammonium [TBA+]) revealed much faster time courses than those produced by sucrose addition, although the diffusion coefficients of sucrose and TBACl are similar. Hyperosmotic and hypoosmotic challenges yielded initial Vms changes at the same rate; thereafter, the voltage increased with hypoosmotic solution and decreased with hyperosmotic solution. These late voltage changes appear to result from changes in width of the lateral intercellular spaces. The early time courses of the Vms changes produced by osmotic challenge are inconsistent with the expectations for water-ion flux coupling in the junctions. We propose that they are pseudo-streaming potentials, i.e., junctional diffusion potentials caused by salt concentration changes in the lateral intercellular spaces secondary to osmotic water flow.

Water permeability of ventricular cell membrane in choroid plexus epithelium from Necturus maculosus

1. The osmotic water permeability Lp and the relations between the flows of H2O, K+ and Cl- were studied in the ventricular membrane of the epithelium from the choroid plexus of Necturus maculosus. 2. The flows were induced by abrupt changes in external osmolarity of the ventricular solution. Solution changes were convective and no effects of unstirred layers could be detected on measured parameters. 3. The initial rate of change in intracellular concentrations of K+ and Cl- was monitored by double-barrelled ion-selective microelectrodes. 4. The initial rate of flux of H2O could be monitored as the changes in the concentration of intracellular choline ions (Ch+i). When 0.5 mmol l-1 of choline chloride was added to the external solutions, Ch+i attained values of 1-5 mmol l-1. The dilution or concentration of Ch+i could be recorded by K+ electrodes since the sensitivity of these to Ch+ is more than 50 times greater than to K+. 5. The Lp of the ventricular membrane of the epithelium was 1.4-2.1 x 10(-4) cm s-1 (osmol l-1)-1 and independent of the direction of the induced water flux. Lp was unchanged in tissues adapted to osmolarities of half the physiological value. 6. The efflux of H2O induced by mannitol was associated with an instantaneous efflux of K+ which was inhibited by furosemide. The fluxes had a ratio of 40 mmol l-1. The influx of H2O induced by the removal of NaCl from the ventricular solution was associated with an instantaneous influx of K+. The H2O influx had a ratio to the flux of K+ of 70 mmol l-1. 7. The efflux of H2O induced by mannitol was associated with an efflux of Cl- which was inhibited by furosemide. The ratio of the two fluxes was in the range 15-44 mmol l-1. 8. The conclusion is that the Ch+ method gives a reliable measure of the movement of H2O across the ventricular membrane. The magnitude of the Lp and its relevance to transepithelial transport are discussed. The osmotically induced H2O movement is accompanied by furosemide-sensitive fluxes of K+ and Cl- of the same magnitude. This suggests that co-transport between H2O and KCl can take place in the membrane.

Osmotic water permeability of Necturus gallbladder epithelium

An electrophysiological technique that is sensitive to small changes in cell water content and has good temporal resolution was used to determine the hydraulic permeability (Lp) of Necturus gallbladder epithelium. The epithelial cells were loaded with the impermeant cation tetramethylammonium (TMA+) by transient exposure to the pore-forming ionophore nystatin in the presence of bathing solution TMA+. Upon removal of the nystatin a small amount of TMA+ is trapped within the cell. Changes in cell water content result in changes in intracellular TMA+ activity which are measured with intracellular ion-sensitive microelectrodes. We describe a method that allows us to determine the time course for the increase or decrease in the concentration of osmotic solute at the membrane surface, which allows for continuous monitoring of the difference in osmolality across the apical membrane. We also describe a new method for the determination of transepithelial hydraulic permeability (Ltp). Apical and basolateral membrane Lp's were assessed from the initial rates of change in cell water volume in response to anisosmotic mucosal or serosal bathing solutions, respectively. The corresponding values for apical and basolateral membrane Lp's were 0.66 x 10(-3) and 0.38 x 10(-3) cm/s.osmol/kg, respectively. This method underestimates the true Lp values because the nominal osmotic differences (delta II) cannot be imposed instantaneously, and because it is not possible to measure the true initial rate of volume change. A model was developed that allows for the simultaneous determination of both apical and basal membrane Lp's from a unilateral exposure to an anisosmotic bathing solution (mucosal). The estimates of apical and basal Lp with this method were 1.16 x 10(-3) and 0.84 x 10(-3) cm/s.osmol/kg, respectively. The values of Lp for the apical and basal cell membranes are sufficiently large that only a small (less than 3 mosmol/kg) transepithelial difference in osmolality is required to drive the observed rate of spontaneous fluid absorption by the gallbladder. Furthermore, comparison of membrane and transepithelial Lp's suggests that a large fraction of the transepithelial water flow is across the cells rather than across the tight junctions.

Pathways for water absorption and physiological role of the lateral interspaces in the kidney tubule

Possible routes for water and salt flow and the most likely theories that describe coupling between water and salt flow across leaky epithelia are presented. The osmotic theories seem the most likely ones. However, several of the theories have weaknesses that render them unsatisfactory, in particular because of the possibility of paracellular water flow in these epithelia. Puzzling are the findings that measurements of the cellular water osmotic permeability give figures that are too low for some of the exclusively transcellular theories to work. If these observations hold in the future, it may be shown that part of the water moves through paracellular pathways in these leaky epithelia. This view is supported by the observation that large extracellular markers are dragged by volume flow. Finally, experimental evidence is reviewed indicating that changes in the luminal area concentration may modulate the functional state of the nephron junctional complexes.

Osmotic water permeabilities of brush border and basolateral membrane vesicles from rat renal cortex and small intestine

The osmotic water permeability Pf of brush border (BBM) and basolateral (BLM) membrane vesicles from rat small intestine and renal cortex was studied by means of stopped-flow spectrophotometry. Scattered light intensity was used to follow vesicular volume changes upon osmotic perturbation with hypertonic mannitol solutions. A theoretical analysis of the relationship of scattered light intensity and vesicular volume justified a simple exponential approximation of the change in scattered light intensity. The rate constants extracted from fits to an exponential function were proportional to the final medium osmolarity as predicted by theory. For intestinal membranes, computer analysis of optical responses fitted well with a single-exponential treatment. For renal membranes a double-exponential treatment was needed, implying two distinct vesicle populations. Pf values for BBM and BLM preparations of small intestine were equal and amount to 60 microns/sec. For renal preparations, Pf values amount to 600 microns/sec for the fast component, BBM as well as BLM, and to 50 (BBM) and 99 (BLM) microns/sec for the slow component. The apparent activation energy for water permeation in intestinal membranes was 13.3 +/- 0.6 and in renal membranes 1.0 +/- 0.3 kCal/mole, between 25 and 35 degrees C. The mercurial sulfhydryl reagent pCMBS inhibited completely and reversibly the high Pf value in renal brush border preparations. These observations suggest that in intestinal membranes water moves through the lipid matrix but that in renal plasma membranes water channels may be involved. From the high Pf values of renal membrane vesicles a transcellular water permeability for proximal tubules can be calculated which amounts to approximately 1 cm/sec. This value allows for an entirely transcellular route for water flow during volume reabsorption.

The roles of paracellular and transcellular pathways and submucosal space in isotonic water absorption by rabbit ileum

Water movements have been studied in sheets of isolated rabbit ileum using a method which measures net volume flows across the mucosal and serosal surfaces of the tissue continuously with high resolution. At 35 degrees C, with the tissues incubated in isotonic Ringer solution containing D-glucose (25 mM) on both sides, there is a steady net inflow of fluid at the rate of 24 +/- 2 microliter cm-2 h-1 across the mucosal surface (Jm) and an outflow of 8 +/- 1 microliter cm-2 h-1 across the serosal surface (Js) (n = 16). The stable transepithelial p.d. across these tissues is 2.7 +/- 0.2 mV, serosa positive. Jm can be reversibly inhibited by anoxia. Ouabain (0.1 mM) added to the serosal solution inhibits inflow across the mucosal and serosal surfaces by 75% (n = 7) within 30 min. If phlorizin (0.1 mM) is added to the mucosal Ringer solution containing glucose (20 mM) within 30 min of the commencement of in vitro absorption, Jm is reduced from 37 +/- 3 to 28 +/- 2 microliter cm-2 h-1 (n = 3). Dilution of the mucosal Ringer solution by 50 mosmol kg-1 (with the serosal solution kept isosmolar) results in a rapid transient increase in mucosal inflow. An increase of 50 mosmol kg-1 in the mucosal Ringer solution with NaCl, sucrose or mannitol causes a transient reversal of mucosal flow, followed by a return of inflow at a reduced level. Rabbit ileum can transport water against gradients of approximately 75 mosmol kg-1 of sucrose, NaCl, or mannitol. Addition of polyethylene glycol (mol. wt. 20000; 3 mosmol kg-1) causes a sustained reversal of mucosal inflow; inflow can be restored only by removing polyethylene glycol from the mucosal Ringer solution. The tissue can absorb water against an osmotic gradient of 200 mM-glycerol. The above data have been incorporated into a new model to explain isotonic flow of fluid by this epithelium. The main features are that the hydraulic conductivity (Lp) of the mucosal boundary of the lateral intercellular space is approximately 1 X 10(-8) cm s-1 cmH2O-1. This Lp is too low to sustain isotonicity of the flow emerging from the lateral intercellular space at the observed rates. Hypertonic fluid emerging from the lateral intercellular space is diluted by transcellular water flow generated by the hypertonicity of the submucosa and back-diffusion of solute via mucosal shunt channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Water permeability of Necturus gallbladder epithelial cell membranes measured by nuclear magnetic resonance

In order to assess the contribution of transcellular water flow to isosmotic fluid transport across Necturus gallbladder epithelium, we have measured the water permeability of the epithelial cell membranes using a nuclear magnetic resonance method. Spin-lattice (T1) relaxation of water protons in samples of gallbladder tissue where the extracellular fluid contained 10 to 20 mM Mn2+ showed two exponential components. The fraction of the total water population responsible for the slower of the two was 24 +/- 2%. Both the size of the slow component, and the fact that it disappeared when the epithelial layer was removed from the tissue, suggest that it was due to water efflux from the epithelial cells. The rate constant of efflux was estimated to be 15.6 +/- 1.0 sec-1 which would be consistent with a diffusive membrane water permeability Pd of 1.6 X 10(-3) cm sec-1 and an osmotic permeability Pos of between 0.3 X 10(-4) and 1.4 X 10(-4) cm sec-1 osmolar-1. Using these data and a modified version of the standing-gradient model, we have reassessed the adequacy of a fluid transport theory based purely on transcellular osmotic water flow. We find that the model accounts satisfactorily for near-isosmotic fluid transport by the unilateral gallbladder preparation, but a substantial serosal diffusion barrier has to be included in order to account for the transport of fluid against opposing osmotic gradients.

Electro-osmosis and the reabsorption of fluid in renal proximal tubules

The lateral intercellular spaces (LIS) are believed to be the final common pathway for fluid reabsorption from the renal proximal tubule. We postulate that electrogenic sodium pumps in the lateral membranes produce an electrical potential within the LIS, that the lateral membranes bear a net negative charge, and that fluid moves parallel to these membranes because of Helmholtz-type electro-osmosis, the field-induced movement of fluid adjacent to a charged surface. Our theoretical analysis indicates that the sodium pumps produce a longitudinal electric field of the order of 1 V/cm in the LIS. Our experimental measurements demonstrate that the electrophoretic mobility of rat renal basolateral membrane vesicles is 1 micron/s per V/cm, which is also the electro-osmotic fluid velocity in the LIS produced by a unit electric field. Thus, the fluid velocity in the LIS due to electro-osmosis should be of the order of 1 micron/s, which is sufficient to account for the observed reabsorption of fluid from renal proximal tubules. Several experimentally testable predictions emerge from our model. First, the pressure in the LIS need not increase when fluid is transported. Thus, the LIS of mammalian proximal tubules need not swell during fluid transport, a prediction consistent with the observations of Burg and Grantham (1971, Membranes and Ion Transport, pp. 49-77). Second, the reabsorption of fluid is predicted to cease when the lumen is clamped to a negative voltage. Our analysis predicts that a voltage of -15 mV will cause fluid to be secreted into the Necturus proximal tubule, a prediction consistent with the observations of Spring and Paganelli (1972, J. Gen. Physiol., 60:181).

Effect of para-chloromercuribenzenesulfonic acid and temperature on cell water osmotic permeability of proximal straight tubules

The apparent Arrhenius energy of activation (Ea) of the water osmotic permeability (Pcos) of the basolateral plasma cell membrane of isolated rabbit proximal straight tubules has been measured under control conditions and after addition of 2.5 mM of the sulfhydryl reagent, para-chloromercuribenzenesulfonic acid (pCMBS), of mersalyl and of dithiothreitol. Ea (kcal/mol) was 3.2 +/- 1.4 (controls) and 9.2 +/- 2.2 (pCMBS), while Pcos decreased with pCMBS to 0.26 +/- 0.17 of its control value. Mersalyl also decreased Pcos both in vitro and in vivo (using therapeutical doses). These actions of pCMBS and mersalyl were quickly reverted with 5 mM dithiothreitol and prevented by 0.1 M thiourea. Ea for free viscous flow is 4.2 and greater than 10 for non-pore-containing lipid membranes. By analogy with these membranes and with red blood cells, where similar effects of pCMBS on Pos are observed, it is concluded that cell membranes of the proximal tubule are pierced by aqueous pores which are reversibly shut by pCMBS. Part of the action of mercurial diuretics can be explained by their action on Pcos.

Current-induced volume flow across bovine tracheal epithelium: evidence for sodium-water coupling

The passage of a constant current from lumen to serosa (Il-s), in the range 0.5-2.0 mA, across ouabain-treated bovine tracheal epithelium, induced a stable volume flow (Jv) toward the serosa, proportional to the current. No consistent Jv occurred when current was applied from serosa to lumen. When the standard K+ (6 mM) in the bathing solution was omitted or replaced by choline, Jv was in the same direction as, and proportional to, the current, both with Is-l and with Il-s. The electro-osmotic permeability beta was in the range of 10-15 microl h-1 cm-2 mA-1, i.e. 3-4 X 10(-6) cm s-1 mA-1. The fluxes of Na+, Cl- and mannitol were measured in current-clamp (1 mA, passed from serosa to lumen or lumen to serosa) or voltage-clamp (-20, 0 and +20 mV) conditions, with and without K+. Net transepithelial Na+ fluxes toward the cathode were either smaller than (with Is-l) or equal to (with Il-s) the net fluxes of Cl- toward the anode. The total transepithelial conductance (Gt) increased with the applied electrical gradient, both with Is-l and with Il-s, the change in Gt being larger with Il-s than with Is-l. This increase of Gt was less pronounced when K+ was omitted. The analyses of partial ionic conductances (GNa and GCl) and of the flux ratios indicate the existence of non-conductive diffusion for Cl- and also for Na+. The direction of the electrical gradient influenced the permeability ratio PNa/PCl. With Is-l, PNa/PCl was consistently lower than 0.7, i.e. the mobility ratio of Na+ and Cl- in solution. With Il-s, PNa/PCl was closer to 0.7. The highest Cl- selectivity of the epithelium was observed with Is-l in the presence of K+, i.e. under conditions which failed to induce any conspicuous Jv. The passage of current at 1 mA induced a net flux of mannitol toward the cathode, i.e. in the same direction as Na+ net flux and Jv. However, this mannitol flux was significant only in the absence of K+. These results indicate that Jv was predominantly coupled to the migration of Na+ along the electrical gradient, through a paracellular pathway.

Effect of amiloride on sodium and water reabsorption in the rabbit gall-bladder

The effects of the Na+-channel-blocking diuretic agent amiloride were assessed in the rabbit gall-bladder epithelium, a low-resistance epithelium with an isosmotic, coupled NaCl transport mechanism. Amiloride caused a rapid, reversible, and dose-dependent decrease in fluid absorption when applied from the mucosal side in concentrations between 8.8 X 10(-5) and 1.76 X 10(-3) M. These concentrations were without effect from the serosal side, suggesting an action of amiloride in the luminal cell membrane as in high-resistance epithelia. Amiloride did not affect the epithelial resistance or the passive serosa-to-mucosa Na+ flux, while net Na+ and water reabsorption were inhibited in parallel. Thus, amiloride did not affect the paracellular tight junction pathway, but inhibited a transcellular, coupled salt and water transport mechanism. The kinetics of the amiloride effect were of a Michaelis-Menten type. The dose of amiloride giving 50% inhibition of fluid absorption (ID50) was 4 X 10(-4) M, a value about three orders of magnitude higher than in high-resistance, Na+-retaining epithelia. The percentage inhibitory effect at each concentration of amiloride increased with increasing rate of spontaneous (control) fluid transport, reaching maximal responses fitting a Michaelis-Menten kinetic with an ID50 of 1.5 X 10(-4) M. No effects of changing the extracellular Na+ concentration between 51 and 145 mequiv/l on the maximal inhibitory effect of amiloride on Na+ and water reabsorption were observed. This suggests a non-competitive type of action of amiloride on a Na+-dependent isosmotic fluid transport mechanism. Removal of mucosal Ca2+ did not alter the effect of amiloride. The implications of these findings are discussed in relation to concepts concerning the mechanism of isosmotic salt and water transport. The data are compatible with the concept that amiloride interferes with a Na+-dependent formation and transcellular transport of isosmotic fluid volumes in a sequestered compartment in the epithelial cells.

Structure of Necturus gallbladder epithelium during transport at low external osmolarities

Gallbladders transport isotonically over a wide range of osmolarities. This ability has been assumed to depend on the geometry of the lateral intercellular spaces. We report that this geometry in the Necturus gallbladder varies extensively with the external osmolarity and depends in vitro on the integrity of the subepithelial tissues. The structure of the living epithelium was studied by Nomarski light microscopy while ultrastructural effects were revealed by electron microscopy. The short-term effects (less than 60 min) of low external osmolarities were: 1) the cells became bell-shaped with an increased cell height measured centrally, 2) lateral intercellular spaces lost their convoluted character; and 3) numerous membrane-bound cavities appeared in the cells. Furthermore, long-term exposure to the low external osmolarities caused an uneven density of epithelial cells. With subepithelial tissues intact, blistering of the epithelium cell layer was evident. Qualitative electron-microscopic data indicate that the membrane of the cavities was recruited from the basolateral cell membrane. This agrees well with light-microscopic observation that the cavities were initiated as invaginations of this cell membrane.

The effect of low external pH on properties of the paracellular pathway and junctional structure in isolated frog skin

1. Undirectional and net fluxes of Na, Cl and mannitol were measured across the isolated short-circuited frog skin when the mucosal surface was bathed with Ringer solution at normal (7.4) or low (2.5) pH. When this solution was changed from normal to low pH, there was a marked increase in both influx and backflux of Cl and mannitol. Na backflux increased markedly but Na influx did not, resulting in a substantial decrease in net flux. 2. In open-circuit conditions at low pH both undirectional fluxes of Na increased and the potential across the skin dropped by a third. 3. The total conductance, Gt and the short-circuit current, Isc increased when the mucosal solution was changed from normal to low pH. The structural integrity of the 'tight junctions' in the epithelium was disrupted by the low pH treatment and at least 50% of the junctions examined showed a separation of the two, previously apposed, membranes. 4. It has been shown previously that when a low pH solution bathes the mucosal surface the total and shunt conductance increase; the present results demonstrate that under these conditions the short-circuit current no longer provides a good estimate of the net Na flux. We present calculations to show that protons can be the carriers for the extra charge transfer. 5. Using our values for net Na flux in open circuit and published values for the solute-linked volume flow, Jv, it could be shown that the osmolarity of the absorbate decreased at low pH.

Effects of a small serosal hydrostatic pressure on sodium and water transport and morphology in rabbit gall-bladder

1. In order to investigate the mechanism of serosal pressure-induced inhibition of isosmotic fluid transport, the effect of 4.5 cm water serosal pressure on spontaneous water transfer (J(v)) in rabbit gall-bladders was measured (in the presence of a supporting soft nylon net on the mucosal side) in a modified Ussing chamber. This allowed unidirectional Na(+) fluxes ([Formula: see text] and [Formula: see text]), transepithelial potential difference and resistance (R(t)) to be measured simultaneously. The effects of the serosal pressure were also investigated by light and electron microscopy.2. During pressure application, R(t) increased due to a covering effect of the mucosal support. The serosal pressure caused a parallel decrease in J(v) and net Na(+) transport ([Formula: see text]) across the free epithelial surface of 80-85%. About 85% of the decrease in [Formula: see text] was due to a decrease in [Formula: see text].3. After inhibition of 93% of fluid absorption by serosal 10(-3)M-ouabain, pressure-induced change in J(v) was only 8% of the spontaneous fluid transport rate.4. Control Na(+) flux ratio ([Formula: see text]) was 3.5. The pressure-induced increase in steady-state [Formula: see text] of 30-35% therefore contributed little to the decrease in [Formula: see text]. Further, this increase in [Formula: see text] was completely prevented by mucosal 10(-3) M-amiloride.5. All pressure-induced effects on transport and electrical parameters were reversible.6. The light microscopical and scanning electron microscopical results showed that half of the epithelial surface was covered by the nylon net following serosal pressure application. Ruptures in the epithelium were not seen. Thin section and freeze fracture electron microscopy demonstrated continuous, well developed tight junctions both in control and experimental condition.7. It is concluded that a serosal pressure of only 4.5 cm water causes inhibition of a cellular active Na(+) and water transport with only minimal, if any, contribution from paracellular filtration. This would seem incompatible with the concept that an active ion transport mechanism localized in the basolateral cell membrane is responsible for transepithelial fluid transport. The possibility of a mechanical fluid transport mechanism via elements of a tubulo-cisternal endoplasmic reticulum is raised.

Epithelial cell volume modulation and regulation

Epithelial cell volume is a sensitive indicator of the balance between solute entry into the cell and solute exit. Solute accumulation in the cell leads to cell swelling because the water permeability of the cell membranes is high. Similarly, solute depletion leads to cell shrinkage. The rate of volume change under a variety of experimental conditions may be utilized to study the rate and direction of solute transport by an epithelial cell. The pathways of water movement across an epithelium may also be deduced from the changes in cellular volume. A technique for the measurement of the volume of living epithelial cells is described, and a number of experiments are discussed in which cell volume determination provided significant new information about the dynamic behavior of epithelia. The mechanism of volume regulation of epithelial cells exposed to anisotonic bathing solution is discussed and shown to involve the transient stimulation of normally dormant ion exchangers in the cell membrane.

Fluid reabsorption by Necturus proximal tubule perfused with solutions of normal and reduced osmolarity

Fluid absorption in Necturus proximal tubule was studied when the kidneys were perfused with solutions of different osmolarities. The rate of fluid absorption was inversely proportional to the perfusion fluid osmolarity, while Na uptake remained constant. No difference was detected between the collected and injected luminal fluid, i.e. reabsorption was isotonic at normal and reduced osmolarities. The transtubular osmotic permeability remained fairly constant under the different perfusion osmolarities. Using our experimental results to test various models based on osmotic equilibration across the tubule wall we show that none of these provides an adequate mechanism for fluid absorption in this epithelium.

Gallbladder epithelial cell hydraulic water permeability and volume regulation

The hydraulic water permeability (Lp) of the cell membranes of Necturus gallbladder epithelial cells was estimated from the rate of change of cell volume after a change in the osmolality of the bathing solution. Cell volume was calculated from computer reconstruction of light microscopic images of epithelial cells obtained by the "optical slice" technique. The tissue was mounted in a miniature Ussing chamber designed to achieve optimal optical properties, rapid bath exchange, and negligible unstirred layer thickness. The control solution contained only 80% of the normal NaCl concentration, the remainder of the osmolality was made up by mannitol, a condition that did not significantly decrease the fluid absorption rate in gallbladder sac preparations. The osmotic gradient ranged from 11.5 to 41 mosmol and was achieved by the addition or removal of mannitol from the perfusion solutions. The Lp of the apical membrane of the cell was 1.0 X 10(-3) cm/s . osmol (Posm = 0.055 cm/s) and that of the basolateral membrane was 2.2 X 10(-3) cm/s . osmol (Posm = 0.12 cm/s). These values were sufficiently high so that normal fluid absorption by Necturus gallbladder could be accomplished by a 2.4-mosmol solute gradient across the apical membrane and a 1.1-mosmol gradient across the basolateral membrane. After the initial cell shrinkage or swelling resulting from the anisotonic mucosal or serosal medium, cell volume returned rapidly toward the control value despite the fact that one bathing solution remained anisotonic. This volume regulatory response was not influenced by serosal ouabain or reduction of bath NaCl concentration to 10 mM. Complete removal of mucosal perfusate NaCl abolished volume regulation after cell shrinkage. Estimates were also made of the reflection coefficient for NaCl and urea at the apical cell membrane and of the velocity of water flow across the cytoplasm.

Intra- and transepithelial analytical techniques

Epithelia transport a variety of solutes and water. Study of such transport requires a determination of the driving forces responsible for transport, of the pathways through which transport occurs, and of the factors controlling such transport. Transepithelial driving forces are readily determined where the composition of the bathing media can be altered and electrical forces negated. Where substances move only through a paracellular pathway such manipulations may be adequate to define the permeability and selectivity of the pathways. For substances utilizing a cellular pathway, driving forces and permeabilities across the two dissimilar apical and basolateral cellular membranes must be determined. Where a substance can be shown to move across a membrane against its electrochemical potential gradient, the source of the energy for such movement must be assessed. This review focuses on the applicability and validity of a variety of techniques utilized for the study of epithelial transport to answer these questions. These include microelectrode techniques, chemical analyses, microprobe analysis, microscopy, and techniques for assessing the coupling of metabolism to transport.

Paracellular non-electrolyte permeation during fluid transport across rabbit gall-bladder epithelium

1. Mucosa-to-serosa fluxes of seven polar non-electrolytes were determined during isotonic fluid transport across the unilateral rabbit gall-bladder preparation in an attempt to estimate the contribution of the paracellular pathway to the total transepithelial water flow.2. (3)H- and (14)C-labelled non-electrolyte tracers appeared in the transported fluid at fractions (f(n)) of their mucosal concentration which were inversely related to molecular size: ethanediol, 0.80; thiourea, 0.55; glycerol, 0.16; erythritol, 0.11; mannitol, 0.05; sucrose, 0.05; inulin, 0.02. The mean volume flow rate was 78 mul. cm(-2) hr(-1).3. While the fluxes of the larger molecules were probably due to diffusion through a small but unrestricted paracellular ;shunt' permeability, the high f(n) values obtained for the smaller molecules indicate the existence of a substantial paracellular permeability restricted to molecules smaller than erythritol.4. Upper limits to the transcellular ethanediol and thiourea permeabilities, estimated from the time constants of tracer efflux from preloaded epithelial cells, were too low to account for more than a very small fraction of the transepithelial fluxes observed in the unilateral preparation.5. Comparison of the f(n) values with the predictions of a hydrodynamic model of paracellular permeation suggests that in order to account for the large fluxes of ethanediol and thiourea, considerably more than one half of the transepithelial water flow must follow the paracellular pathway.6. Following a reduction of the mucosal osmolality to 110 m-osmole kg(-1), the apparent non-electrolyte permeability of the epithelium increased steadily over a period of 4 hr. This seems to reflect an increase in the shunt permeability rather than a change in the selectivity of the restricted permeability.7. It is concluded that during isotonic fluid transport the bulk of the transepithelial water flow crossing the epithelium passes through paracellular channels of approximately 3 A radius which are probably located in the intercellular junction.

Volume flow, hydraulic conductivity and electrical properties across bovine tracheal epithelium in vitro: effect of histamine

Volume flow (Jv), potential difference (delta psi), short-circuit current (io) and electrical resistance (R) were measured simultaneously across bovine tracheal epithelium in vitro. Under basal conditions, with no applied hydrostatic or osmotic pressure gradients (delta P = 0, delta phi = 0), no spontaneous Jv was observed. delta psi was 31 +/- 2 mV (lumen negative ), io 161 +/- 8 microA cm-2 and R 202 +/- 9 omega cm2, n = 50. When a delta pi was applied, by adding 20 - 80 mM sucrose into the medium bathing either the luminal or the serosal side of the tissue, a linear relationship was found between delta pi and Jv toward the lumen or toward the serosa. The apparent hydraulic conductivity (apparent Lp) was 4.6 - 4.9 10(-6) cms-1 atm-1. Histamine 10(-4) M did not induce any spontaneous Jv under basal conditions and had no effect on io nor on R. However, histamine caused a 100% increase in Jv elicited by sucrose gradients. It was concluded that histamine exerts a selective action on the hydraulic conductivity of bovine tracheal epithelium. Experiments using H1-receptors antagonists (diphenhydramine, dimetindene, chloropyramine) and H2-antagonists (cimetidine, metiamide) or a H2-agonist (impromidine) showed that the increase of Lp induced by histamine was mediated via H2-receptors.