Gallbladder epithelial cell hydraulic water permeability and volume regulation

Epithelial transport in The Journal of General Physiology

JGP hosts key papers that shaped the epithelial transport field.

Epithelia define the boundaries of the body and often transfer solutes and water from outside to inside (absorption) or from inside to outside (secretion). Those processes involve dual plasma membranes with different transport components that interact with each other. Understanding those functions has entailed breaking down the problem to analyze properties of individual membranes (apical vs. basolateral) and individual transport proteins. It also requires understanding of how those components interact and how they are regulated. This article outlines the modern history of this research as reflected by publications in The Journal of General Physiology.

Methods for cell volume measurement

Volume is an essential characteristic of a cell, and this review describes the main methods of its measurement that have been used in the past several decades. The discussed methods include various implementations of light scattering, estimates based on one or two cell dimensions, surface scanning, fluorescence confocal and transmission slice-by-slice imaging, intracellular volume markers, displacement of extracellular solution, quantitative phase imaging, radioactive methods, and some others. Suitability of these methods to some typical samples and applications is discussed. © 2017 International Society for Advancement of Cytometry.

Cerebrospinal Fluid Secretion by the Choroid Plexus

The choroid plexus epithelium is a cuboidal cell monolayer, which produces the majority of the cerebrospinal fluid. The concerted action of a variety of integral membrane proteins mediates the transepithelial movement of solutes and water across the epithelium. Secretion by the choroid plexus is characterized by an extremely high rate and by the unusual cellular polarization of well-known epithelial transport proteins. This review focuses on the specific ion and water transport by the choroid plexus cells, and then attempts to integrate the action of specific transport proteins to formulate a model of cerebrospinal fluid secretion. Significant emphasis is placed on the concept of isotonic fluid transport across epithelia, as there is still surprisingly little consensus on the basic biophysics of this phenomenon. The role of the choroid plexus in the regulation of fluid and electrolyte balance in the central nervous system is discussed, and choroid plexus dysfunctions are described in a very diverse set of clinical conditions such as aging, Alzheimer's disease, brain edema, neoplasms, and hydrocephalus. Although the choroid plexus may only have an indirect influence on the pathogenesis of these conditions, the ability to modify epithelial function may be an important component of future therapies.

Efficiency of primary saliva secretion: an analysis of parameter dependence in dynamic single-cell and acinus models, with application to aquaporin knockout studies

Secretion from the salivary glands is driven by osmosis following the establishment of osmotic gradients between the lumen, the cell and the interstitium by active ion transport. We consider a dynamic model of osmotically driven primary saliva secretion and use singular perturbation approaches and scaling assumptions to reduce the model. Our analysis shows that isosmotic secretion is the most efficient secretion regime and that this holds for single isolated cells and for multiple cells assembled into an acinus. For typical parameter variations, we rule out any significant synergistic effect on total water secretion of an acinar arrangement of cells about a single shared lumen. Conditions for the attainment of isosmotic secretion are considered, and we derive an expression for how the concentration gradient between the interstitium and the lumen scales with water- and chloride-transport parameters. Aquaporin knockout studies are interpreted in the context of our analysis and further investigated using simulations of transport efficiency with different membrane water permeabilities. We conclude that recent claims that aquaporin knockout studies can be interpreted as evidence against a simple osmotic mechanism are not supported by our work. Many of the results that we obtain are independent of specific transporter details, and our analysis can be easily extended to apply to models that use other proposed ionic mechanisms of saliva secretion.

Fluid Transport Across Leaky Epithelia: Central Role of the Tight Junction and Supporting Role of Aquaporins

The mechanism of epithelial fluid transport remains unsolved, which is partly due to inherent experimental difficulties. However, a preparation with which our laboratory works, the corneal endothelium, is a simple leaky secretory epithelium in which we have made some experimental and theoretical headway. As we have reported, transendothelial fluid movements can be generated by electrical currents as long as there is tight junction integrity. The direction of the fluid movement can be reversed by current reversal or by changing junctional electrical charges by polylysine. Residual endothelial fluid transport persists even when no anions (hence no salt) are being transported by the tissue and is only eliminated when all local recirculating electrical currents are. Aquaporin (AQP) 1 is the only AQP present in these cells, and its deletion in AQP1 null mice significantly affects cell osmotic permeability (by ∼40%) but fluid transport much less (∼20%), which militates against the presence of sizable water movements across the cell. In contrast, AQP1 null mice cells have reduced regulatory volume decrease (only 60% of control), which suggests a possible involvement of AQP1 in either the function or the expression of volume-sensitive membrane channels/transporters. A mathematical model of corneal endothelium we have developed correctly predicts experimental results only when paracellular electro-osmosis is assumed rather than transcellular local osmosis. Our evidence therefore suggests that the fluid is transported across this layer via the paracellular route by a mechanism that we attribute to electro-osmotic coupling at the junctions. From our findings we have developed a novel paradigm for this preparation that includes 1) paracellular fluid flow; 2) a crucial role for the junctions; 3) hypotonicity of the primary secretion; and 4) an AQP role in regulation rather than as a significant water pathway. These elements are remarkably similar to those proposed by the laboratory of Adrian Hill for fluid transport across other leaky epithelia.

A mathematical model of calcium-induced fluid secretion in airway epithelium

Regulation of periciliary liquid (PCL) depth is of central importance to mucociliary clearance by the airway epithelium. Without adequate hydration mucociliary transport would cease, leading to build up of mucus in the airways, and impairing the clearance of any trapped inhaled particulates. Airway epithelial cells are known to release ATP under a number of stress conditions. Cell surface receptors bind ATP and trigger an intracellular calcium response which regulates the gating of specific ion channels on the apical and basolateral cell membranes. This shifts the electrochemical balance, resulting in the accumulation of Na(+) and Cl(-) in the periciliary liquid, and providing an osmotic driving force for water flux. In this study, we present a mathematical model of a single airway epithelial cell which describes the fluid secretion elicited after a rise in intracellular calcium. The model provides a basis to quantitatively analyse the influence of intracellular calcium signalling on fluid movement. The model demonstrates behaviour consistent with a number of experimental data on manipulating periciliary liquid volume and tonicity, and provides a quantitative basis for analysing the role of the different membrane ion channels in determining water flux following different physiological stimuli.

Very high aquaporin-1 facilitated water permeability in mouse gallbladder

Water transport across gallbladder epithelium is driven by osmotic gradients generated from active salt absorption and secretion. Aquaporin (AQP) water channels have been proposed to facilitate transepithelial water transport in gallbladder and to modulate bile composition. We found strong AQP1 immunofluorescence at the apical membrane of mouse gallbladder epithelium. Transepithelial osmotic water permeability (Pf) was measured in freshly isolated gallbladder sacs from the kinetics of luminal calcein self-quenching in response to an osmotic gradient. Pf was very high (0.12 cm/s) in gallbladders from wild-type mice, cAMP independent, and independent of osmotic gradient size and direction. Although gallbladders from AQP1 knockout mice had similar size and morphology to those from wild-type mice, their Pf was reduced by approximately 10-fold. Apical plasma membrane water permeability was greatly reduced in AQP1-deficient gallbladders, as measured by cytoplasmic calcein quenching in perfluorocarbon-filled, inverted gallbladder sacs. However, neither bile osmolality nor bile salt concentration differed in gallbladders from wild-type vs. AQP1 knockout mice. Our data indicate constitutively high water permeability in mouse gallbladder epithelium involving transcellular water transport through AQP1. The similar bile salt concentration in gallbladders from AQP1 knockout mice argues against a physiologically important role for AQP1 in mouse gallbladder.

'What controls aqueous humour outflow resistance?'

The bulk of aqueous humour outflow resistance is generated in or near the inner wall endothelium of Schlemm's canal in normal eyes, and probably also in glaucomatous eyes. Fluid flow through this region is controlled by the location of the giant vacuoles and pores found in cells of the endothelium of Schlemm's canal, but the flow resistance itself is more likely generated either in the extracellular matrix of the juxtacanalicular connective tissue or the basement membrane of Schlemm's canal. Future studies utilizing in vitro perfusion studies of inner wall endothelial cells may give insights into the process by which vacuoles and pores form in this unique endothelium and why inner wall pore density is greatly reduced in glaucoma.

Water permeability of capillaries in the subfornical organ of rats determined by Gd-DTPA(2-) enhanced 1H magnetic resonance imaging

The water permeability of capillaries in the subfornical organ (SFO) of rat was measured by a (1)H nuclear magnetic resonance method in combination with a venous injection of a relaxation reagent, gadolinium-diethylene triamine-N,N,N',N",N"-pentaacetic acid (Gd-DTPA(2-)), which could not pass through the blood-brain barrier (BBB). Judging from results of Gd-DTPA(2-) dose dependency in the intact brain and the BBB-permeabilized brain, Gd-DTPA(2-) could not have leaked out from the capillaries in the cortex, thalamus or SFO, but it could have been extravasated in the posterior lobe of the pituitary gland. The longitudinal (T(1)) relaxation time of water in the SFO region was measured by inversion-recovery magnetic resonance imaging at 4.7 T. The T(1) relaxation rates (1/T(1)) before and after Gd-DTPA(2-) infusion were 0.70 +/- 0.02 s(-1) (mean +/- S.E.M., n = 9) and 1.53 +/- 0.11 s(-1) (n = 9), respectively. The rate constant for water influx to the capillaries was estimated to be 0.84 +/- 0.11 s(-1) (n = 9) which corresponds with a diffusive membrane permeability (P(d)) of 3.7 x 10(-3) cm s(-1). Compared with values found in the literature available on this subject, this P(d) value for the capillaries in the SFO was the same order of magnitude as that for transmembrane permeability of water for the vasa recta, and it may be 10-100 times larger than that of the blood-brain barrier in the cortex. Areas of the cortex and thalamus showed minimal changes in the T(1) relaxation rate (ca 0.09 s(-1)), but these values were not statistically significant and they corresponded to P(d) values much smaller than those found in the SFO. From these results, we conclude that the capillaries in the SFO have one of the highest water permeability values among all of the capillaries in the brain. It is also suggested that this magnetic resonance imaging, based on T(1) relaxation rate, is a useful method to detect local water permeability in situ.

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.

Calcium regulates estrogen increase in permeability of cultured CaSki epithelium by eNOS-dependent mechanism

Estrogen increases baseline transepithelial permeability across CaSki cultures and augments the increase in permeability in response to hypertonic gradients. In estrogen-treated cells, lowering cytosolic calcium abrogated the hypertonicity-induced augmented increase in permeability and decreased baseline permeability to a greater degree than in estrogen-deprived cells. Steady-state levels of cytosolic calcium in estrogen-deprived cells were higher than in estrogen-treated cells. Increases in extracellular calcium increased cytosolic calcium more in estrogen-deprived cells than in estrogen-treated cells. However, in estrogen-treated cells, increasing cytosolic calcium was associated with greater increases in permeability in response to hypertonic gradients than in estrogen-deprived cells. Lowering cytosolic calcium blocked the estrogen-induced increase in nitric oxide (NO) release and in the in vitro conversion of L-[(3)H]arginine to L-[(3)H]citrulline. Treatment with estrogen upregulated mRNA of the NO synthase isoform endothelial nitric oxide synthase (eNOS). These results indicate that cytosolic calcium mediates the responses to estrogen and suggest that the estrogen increase in permeability and the augmented increase in permeability in response to hypertonicity involve an increase in NO synthesis by upregulation of the calcium-dependent eNOS.

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.

Structural correlates of the transepithelial water transport

Transepithelial permeability is one of the fundamental problems in cell biology. Epithelial cell layers protect the organism from its environment and form a selective barrier to the exchange of molecules between the lumen of an organ and an underlying tissue. This chapter discusses some problems and analyzes the participation of intercellular junctions in the paracellular transport of water, migration of intramembrane particles in the apical membrane during its permeability changes for isotonic fluid in cells of leaky epithelia, insertion of water channels into the apical membrane and their cytoplasmic sources in cells of tight epithelia under ADH (antidiuretic hormone)-induced water flows, the osmoregulating function of giant vacuoles in the transcellular fluxes of hypotonic fluid across tight epithelia, and the role of actin filaments and microtubules in the transcellular transport of water across epithelia.

Osmotic water permeabilities of cultured, well-differentiated normal and cystic fibrosis airway epithelia

Current hypotheses describing the function of normal airway surface liquid (ASL) in lung defense are divergent. One theory predicts that normal airways regulate ASL volume by modulating the flow of isosmotic fluid across the epithelium, whereas an alternative theory predicts that ASL is normally hyposmotic. These hypotheses predict different values for the osmotic water permeability (P(f)) of airway epithelia. We measured P(f) of cultures of normal and cystic fibrosis (CF) airway epithelia that, like the native tissue, contain columnar cells facing the lumen and basal cells that face a basement membrane. Xz laser scanning confocal microscopy recorded changes in epithelial height and transepithelial volume flow in response to anisosmotic challenges. With luminal hyperosmotic challenges, transepithelial and apical membrane P(f) are relatively high for both normal and CF airway epithelia, consistent with an isosmotic ASL. Simultaneous measurements of epithelial cell volume and transepithelial water flow revealed that airway columnar epithelial cells behave as osmometers whose volume is controlled by luminal osmolality. Basal cell volume did not change in these experiments. When the serosal side of the epithelium was challenged with hyperosmotic solutions, the basal cells shrank, whereas the lumen-facing columnar cells did not. We conclude that (a) normal and CF airway epithelia have relatively high water permeabilities, consistent with the isosmotic ASL theory, and the capacity to restore water on airway surfaces lost by evaporation, and (b) the columnar cell basolateral membrane and tight junctions limit transepithelial water flow in this tissue.

NO increases permeability of cultured human cervical epithelia by cGMP-mediated increase in G-actin

Human cervical epithelial cells express mRNA for the nitric oxide (NO) synthase (NOS) isoforms ecNOS, bNOS, and iNOS and release NO into the extracellular medium. N(G)-nitro-L-arginine methyl ester (L-NAME), an NOS inhibitor, and Hb, an NO scavenger, decreased paracellular permeability; in contrast, the NO donors sodium nitroprusside (SNP) and N-(ethoxycarbonyl)-3-(4-morpholinyl)sydnonimine increased paracellular permeability across cultured human cervical epithelia on filters, suggesting that NO increases cervical paracellular permeability. The objective of the study was to understand the mechanisms of NO action on cervical paracellular permeability. 8-Bromo-cGMP (8-BrcGMP) also increased permeability, and the effect was blocked by KT-5823 (a blocker of cGMP-dependent protein kinase), but not by LY-83583 (a blocker of guanylate cyclase). In contrast, LY-83583 and KT-5823 blocked the SNP-induced increase in permeability. Treatment with SNP increased cellular cGMP, and the effect was blocked by Hb and LY-83583, but not by KT-5823. Neither SNP nor 8-BrcGMP had modulated cervical cation selectivity. In contrast, both agents increased fluorescence from fura 2-loaded cells in the Ca(2+)-insensitive wavelengths, indicating that SNP and 8-BrcGMP stimulate a decrease in cell size and in the resistance of the lateral intercellular space. Neither SNP nor 8-BrcGMP had an effect on total cellular actin, but both agents increased the fraction of G-actin. Hb blocked the SNP-induced increase in G-actin, and KT-5823 blocked the 8-BrcGMP-induced increase in G-actin. On the basis of these results, it is suggested that NO acts on guanylate cyclase and stimulates an increase in cGMP; cGMP, acting via cGMP-dependent protein kinase, shifts actin steady-state toward G-actin; this fragments the cytoskeleton and renders cells more sensitive to decreases in cell size and resistance of the lateral intercellular space and, hence, to increases in permeability. These results may be important for understanding NO regulation of transcervical paracellular permeability and secretion of cervical mucus in the woman.

Volume regulatory decrease in UMR-106.01 cells is mediated by specific alpha1 subunits of L-type calcium channels

An early cellular response of osteoblasts to swelling is plasma membrane depolarization, accompanied by a transient increase in intracellular calcium ([Ca2+]i), which initiates regulatory volume decrease (RVD). The authors have previously demonstrated a hypotonically induced depolarization of the osteoblast plasma membrane, sufficient to open L-type Ca channels and mediate Ca2+ influx. Herein is described the initiation of RVD in UMR-106.01 cells, mediated by hypotonically induced [Ca2+]i transients resulting from the activation of specific isoforms of L-type Ca channels. The authors further demonstrate that substrate interaction determines which specific alpha1 Ca channel subunit isoform predominates and mediates Ca2+ entry and RVD. Swelling-induced [Ca2+]i transients, and RVD in cells grown on a type I collagen matrix, are inhibited by removal of Ca from extracellular solutions, dihydropyridines, and antisense oligodeoxynucleotides directed exclusively to the alpha1C isoform of the L-type Ca channel. Ca2+ transients and RVD in cells grown on untreated glass cover slips were inhibited by similar maneuvers, but only by antisense oligodeoxynucleotides directed to the alpha1S isoform of the L-type Ca channel. This represents the first molecular identification of the Ca channels that transduce the initiation signal for RVD by osteoblastic cells.

Stretch-activated single K+ channels account for whole-cell currents elicited by swelling

Functionally significant stretch-activated ion channels have been clearly identified in excitable cells. Although single-channel studies suggest their expression in other cell types, their activity in the whole-cell configuration has not been shown. This discrepancy makes their physiological significance doubtful and suggests that their mechanical activation is artifactual. Possible roles for these molecules in nonexcitable cells are acute cell-volume regulation and, in epithelial cells, the complex adjustment of ion fluxes across individual cell membranes when the rate of transepithelial transport changes. We report the results of experiments on isolated epithelial cells expressing in the basolateral membrane stretch-activated K+ channels demonstrable by the cell-attached patch-clamp technique. In these cells, reversible whole-cell currents were elicited by both isosmotic and hyposmotic cell swelling. Cation selectivity and block by inorganic agents were the same for single-channel and whole-cell currents, indicating that the same entity underlies single-channel and whole-cell currents and that the single-channel events are not artifactual. In these cells, when the rate of apical-membrane NaCl entry increases, the cell Na+ content and volume also increase, stimulating the Na+,K+-ATPase at the basolateral membrane, i.e., both Na+ extrusion and K+ uptake increase. We speculate that, under these conditions, the parallel activation of basolateral K+ channels (by the swelling) elevates conductive K+ loss, tending to maintain the cell K+ content constant ("pump-leak parallelism"). This study describes a physiologically relevant stretch-activated channel, at both the single-channel and whole-cell levels, in a nonneural cell type.

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.

Enhancing clinical efficacy of oral rehydration therapy: is low osmolality the key?

Many empirical clinical trials have used complex carbohydrate as substrate in oral rehydration solutions (ORSs) instead of glucose and have shown a number of important clinical benefits. Foremost among these are reduced stool volumes, shorter duration of diarrheal illness, and lower ORS intake. The underlying mechanisms to explain this clinical advantage have not been fully established, but a number of possible factors have been proposed: (1) increased substrate availability, (2) a "kinetic advantage" for glucose absorption by glucose polymer, (3) differential handling of glucose monomer and polymer by the small intestine, (4) low osmolality, (5) a separate effect of peptides and amino acids on solute-linked sodium absorption, (6) an antisecretory moiety in rice, and (6) enhanced mucosal repair and regeneration by luminal nutrients. In this report, we assess the relative contribution of these factors using evidence from laboratory-based studies, mainly in disease-related intestinal perfusion systems in animals and humans, and the relevant clinical studies available to date. We advance the hypothesis that of all the possible mechanisms proposed to underlie the enhanced clinical efficacy of complex carbohydrate ORSs, their hypotonicity plays the dominant role. If confirmed, this concept could guide future development of glucose and complex carbohydrate-based ORSs.

NMDA receptor activation inhibits neuronal volume regulation after swelling induced by veratridine-stimulated Na+ influx in rat cortical cultures

Neurons and glia experience rapid fluctuations in transmembrane solute and water fluxes during normal brain activity. Cell volume must be regulated under these conditions to maintain optimal neural function. Almost nothing is known, however, about how brain cells respond to volume challenges induced by changes in transmembrane solute flux. As such, we characterized the volume-regulatory mechanisms of cultured cortical neurons swollen by veratridine-stimulated Na+ influx. Exposure of cortical neurons to 100 microM veratridine for 10-15 min caused a 1.8- to 2-fold increase in cell volume that persisted for at least 90 min. This volume increase was blocked by extracellular Na+ removal or by exposure to 5 microM tetrodotoxin, indicating that swelling is a result of Na+ entry via Na+ channels. Treatment of cells with veratridine together with various NMDA receptor antagonists had no effect on the magnitude of swelling. NMDA receptor antagonist-treated cells, however, underwent nearly complete volume recovery within 50-70 min after veratridine exposure. This recovery suggests that NMDA receptor activation disrupts neuronal osmoregulatory pathways. Volume regulation was blocked by Ba2+, quinidine, or 5-nitro-2-(3-phenylpropylamino) benzoic acid, indicating that swelling activates volume regulatory K+ and Cl- channels. Veratridine also caused a rapid, transient increase in intracellular Ca2+. Extracellular Ca2+ removal or intracellular Ca2+ chelation prevented or dramatically reduced veratridine-induced increases in intracellular Ca2+ and completely blocked volume recovery. These findings indicate that increases in Ca2+ during cell swelling induced by Na+ influx are required for activation of neuronal volume-regulatory pathways.

Cell swelling activates the K+ conductance and inhibits the Cl- conductance of the basolateral membrane of cells from a leaky epithelium

Necturus gallbladder epithelial cells bathed in 10 mM HCO3/1% CO2 display sizable basolateral membrane conductances for Cl- (GClb) and K+ (GKb). Lowering the osmolality of the apical bathing solution hyperpolarized both apical and basolateral membranes and increased the K+/Cl- selectivity of the basolateral membrane. Hyperosmotic solutions had the opposite effects. Intracellular free-calcium concentration ([Ca2+]i) increased transiently during hyposmotic swelling (peak at approximately 30 s, return to baseline within approximately 90 s), but chelation of cell Ca2+ did not prevent the membrane hyperpolarization elicited by the hyposmotic solution. Cable analysis experiments showed that the electrical resistance of the basolateral membrane decreased during hyposmotic swelling and increased during hyperosmotic shrinkage, whereas the apical membrane resistance was unchanged in hyposmotic solution and decreased in hyperosmotic solution. We assessed changes in cell volume in the epithelium by measuring changes in the intracellular concentration of an impermeant cation (tetramethylammonium), and in isolated polarized cells measuring changes in intracellular calcein fluorescence, and observed that these epithelial cells do not undergo measurable volume regulation over 10-12 min after osmotic swelling. Depolarization of the basolateral membrane voltage (Vcs) produced a significant increase in the change in Vcs elicited by lowering basolateral solution [Cl-], whereas hyperpolarization of Vcs had the opposite effect. These results suggest that: (a) Hyposmotic swelling increases GKb and decreases GClb. These two effects appear to be linked, i.e., the increase in GKb produces membrane hyperpolarization, which in turn reduces GClb. (b) Hyperosmotic shrinkage has the opposite effects on GKb and GClb. (c) Cell swelling causes a transient increase in [Ca2+]i, but this response may not be necessary for the increase in GKb during cell swelling.

NMDA receptor activation inhibits neuronal volume regulation after swelling induced by veratridine-stimulated Na+ influx in rat cortical cultures

Neurons and glia experience rapid fluctuations in transmembrane solute and water fluxes during normal brain activity. Cell volume must be regulated under these conditions to maintain optimal neural function. Almost nothing is known, however, about how brain cells respond to volume challenges induced by changes in transmembrane solute flux. As such, we characterized the volume-regulatory mechanisms of cultured cortical neurons swollen by veratridine-stimulated Na+ influx. Exposure of cortical neurons to 100 microM veratridine for 10-15 min caused a 1.8- to 2-fold increase in cell volume that persisted for at least 90 min. This volume increase was blocked by extracellular Na+ removal or by exposure to 5 microM tetrodotoxin, indicating that swelling is a result of Na+ entry via Na+ channels. Treatment of cells with veratridine together with various NMDA receptor antagonists had no effect on the magnitude of swelling. NMDA receptor antagonist-treated cells, however, underwent nearly complete volume recovery within 50-70 min after veratridine exposure. This recovery suggests that NMDA receptor activation disrupts neuronal osmoregulatory pathways. Volume regulation was blocked by Ba2+, quinidine, or 5-nitro-2-(3-phenylpropylamino) benzoic acid, indicating that swelling activates volume regulatory K+ and Cl- channels. Veratridine also caused a rapid, transient increase in intracellular Ca2+. Extracellular Ca2+ removal or intracellular Ca2+ chelation prevented or dramatically reduced veratridine-induced increases in intracellular Ca2+ and completely blocked volume recovery. These findings indicate that increases in Ca2+ during cell swelling induced by Na+ influx are required for activation of neuronal volume-regulatory pathways.

Nucleotide receptor-mediated decrease of tight-junctional permeability in cultured human cervical epithelium

Extracellular ATP changes the transepithelial electrical conductance (GT) across cultures of human cervical cells acutely, in a biphasic manner that is characterized by a rapid increase (phase I) followed by a sustained decrease in GT (phase II). We tested the hypothesis that the phase II response is mediated by decreases in the permeability of tight junctions. We studied the effect of ATP on the relative mobilities of Cl- vs. Na+ (uCl/uNa) as calculated from changes in the dilution potential (Vdil). Vdil was induced by lowering NaCl from 130 to 10 mM in either the luminal or subluminal solutions bathing filters containing cells. uCl/uNa was 1.27 across cervical cultures and 1.34 across blank filters, compared with a level of 1.52 in free solution. Increases in GT induced by transepithelial hydrostatic or hypertonic gradients (which increase permeability of lateral intercellular space) had no effect on uCl/uNa. Increases in GT induced by lowering extracellular Ca2+ to < 0.1 mM increased uCl/uNa to levels obtained in blank filters, indicating abrogation of tight-junctional resistance. Phase I response and ionomycin (which produces a sustained phase I-like increase in GT) had no effect on uCl/uNa. The phase II response, however, decreased uCl/uNa from 1.27 to 1.24, and the effect could be abrogated by lowering extracellular Ca2+. These results indicate that phase II decreases in GT across cultured human cervical epithelium are mediated by acute decreases in tight-junctional permeability.

Volume changes in single N1E-115 neuroblastoma cells measured with a fluorescent probe

A non-invasive microspectrofluorimetric technique was used to investigate experimentally induced changes in cell water volume in single N1E-115 murine neuroblastoma cells, using calcein, a derivative of fluorescein, as a marker of the intracellular water compartment. The osmotic behavior of N1E-115 cells exposed to media of various osmolalities was studied. Exposure to hyperosmotic (up to +28%) or hyposmotic (up to -17%) solutions produced reversible decreases and increases in cell water volume, respectively, which agreed with near-osmometric behavior. Increases in [Ca2+]i produced by exposing the cells to the ionophore ionomycin (1 microM) in isosmotic medium, resulted in a gradual decrease in cell water volume. Cells shrank to 40 +/- 7% (n = 7) below their initial water volume at an initial rate of -1.2 +/- 0.2%/min. It is concluded that N1E-115 cells are endowed with Ca2+-sensitive mechanisms for volume control, which can produce cell shrinkage when activated under isosmotic conditions. Because the technique used for measuring cell water volume changes is new, we describe it in detail. It is based on the principle that relative cell water volume in single cells can be measured by introducing an impermeant probe into cells and measuring its changes in concentration. If the intracellular content of the probe is constant, changes in its concentration reflect changes in cell water volume. Calcein was used as the probe because its fluorescence intensity is directly proportional to its concentration and independent of changes in the concentration of native intracellular ions within the physiological range. Because calcein is two to three times more fluorescent that other fluorophores such as 2,7,-bis-[2-carboxyethyl]-5-[and 6]-carboxyfluorescein or Fura-2, and it is used at its peak excitation and emission wavelengths, it has a better signal to noise ratio and baseline stability than the other dyes. Calcein can also be esterified allowing for cell loading and because of the possibility of reducing the intensity of the excitation light, measurements can be performed producing minimal photodynamic damage. The technique allows for measurements of cell water volume changes of < 5% and it can be applied to single cells which can be grown or affixed to a rigid substratum, e.g., a coverslip.

Apical membrane sodium and chloride entry during osmotic swelling of renal (A6) epithelial cells

To assess the role of chloride in cell volume and sodium transport regulation, we measured cell height changes (CH), transepithelial chloride and sodium fluxes, and intracellular chloride content during challenge with hyposmotic solutions under open circuit (OC) conditions. CH maximally increased following hyposmotic challenge within approximately 5 minutes. The change in CH was smaller under short circuit (SC) conditions or following replacement of chloride in the mucosal solution by gluconate or cyclamate (Cl(-)-freem). When corrected for the osmotically inactive cell volume (30 +/- 2%), delta CH for controls (OC) were greater than predicted for an ideal osmometer. In contrast, delta CH for Cl(-)-freem or SC conditions were similar to that predicted for an ideal osmometer. Na+ and Cl- mucosa-to-serosa fluxes increased following hyposmotic challenge. Chloride fluxes increased maximally within 5 min, then decreased. In contrast, the Na+ flux increased slowly and reached a steady state after approximately 25 min. Under isosmotic conditions, exposure to Cl(-)-freem solutions led to decreases in the transepithelial conductance, Na+ flux, and CH. Chloride permeabilities in the apical and basolateral membranes were detected using the fluorescent intracellular chloride indicator MQAE. The results indicate that during osmotic swelling, the entry of both sodium and chloride is increased. The time courses of these increases differ, suggesting distinct mechanisms for the osmotic regulation of these apical membrane transport processes.

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.

Selective response of human airway epithelia to luminal but not serosal solution hypertonicity. Possible role for proximal airway epithelia as an osmolality transducer

The response of cultured human nasal epithelia to hypertonic bathing solutions was tested using ion-selective microelectrode and quantitative microscopy. Raised luminal, but not serosal, osmolality (+/- 150 mM mannitol) decreased Na+ absorption but did not induce Cl- secretion. Raised luminal osmolality increased cell Cl- activity, Na+ activity, and transepithelial resistance and decreased both apical and basolateral membrane potentials and the fractional resistance of the apical membrane; equivalent circuit analysis revealed increases in apical, basolateral, and shunt resistances. Prolonged exposure (10 min) to 430 mosM luminal solution elicited no regulation of any parameter. Optical measurements revealed a reduction in the thickness of preparations only in response to luminal hypertonic solutions. We conclude that (a) airway epithelial cells exhibit asymmetric water transport properties, with the apical membrane water permeability exceeding that of the basolateral membrane; (b) the cellular response to volume loss is a deactivation of the basolateral membrane K+ conductance and the apical membrane Cl- conductance; (c) luminal hypertonicity slows the rate of Na+ absorption but does not induce Cl- secretion; and (d) cell volume loss increases the resistance of the paracellular path. We speculate that these properties configure human nasal epithelium to behave as an osmotic sensor, transducing information about luminal solutions to the airway wall.

Transcellular water transport in lung alveolar epithelium through mercury-sensitive water channels

The movement of water between the air space and capillary compartments is important for the maintenance of air space hydration during respiration and for reabsorption of excess alveolar fluid. We have obtained immunocytochemical and functional evidence that plasma-membrane water channels are responsible for water transport in the intact lung. Northern and quantitative immunoblot analysis showed high expression of CHIP28 (channel-forming integral membrane protein of 28 kDa) water channels in rat lung; immunocytochemistry showed CHIP28 localization to epithelial cell plasma membranes. Stopped-flow light scattering measurements of osmotic water permeability (Pf) in freshly isolated rat alveolar type II epithelial cells indicated a high Pf of 0.015 +/- 0.002 cm/s (10 degrees C) that was weakly temperature-dependent (activation energy, 4 kcal/mol) and reversibly inhibited by 78 +/- 4% by 0.5 mM HgCl2. An in situ-perfused sheep lung model was used to determine the route for water movement in intact lung. Blood-to-air-space water transport was measured by sampling air space fluid after instillation into distal air spaces of hyperosmolar saline (900 mOsm) containing radioiodinated albumin and [14C]mannitol. In seven sets of experiments, air space osmolality and radioiodinated albumin equilibrated with a t1/2 of 0.85 +/- 0.1 min. In the contralateral lung perfused with 0.5 mM HgCl2, t1/2 increased to 2.7 +/- 0.4 min; the inhibitory effect of HgCl2 was fully reversed by 5 mM 2-mercaptoethanol. These results provide direct evidence for transcellular movement of water across the alveolar epithelium in intact lung through mercury-sensitive water channels.

Tight-junction tightness of Necturus gall bladder epithelium is not regulated by cAMP or intracellular Ca2+. I. Microscopic and general electrophysiological observations

Following the publications by Duffey et al. [Nature 294:451 (1981)] and Palant et al. [Am J Physiol 245: C203 (1983)] it is generally accepted that tight-junction tightness of Necturus gall bladder epithelium is up-regulated by cAMP-mediated and Ca(2+)-mediated stimulation. This conclusion was mainly based on observed increases in transepithelial resistance (Rt). However, since in leaky epithelia Rt cannot be simply equated with the tight junction resistance (Rj), but may include large contributions from the lateral space resistance (Rlis), we asked whether the observed increases in Rt resulted indeed from Rj or whether Rlis also increased. The experiments were performed on Necturus gall bladders using forskolin or the Ca2+ ionophore A23187 as stimulants. Forskolin (2 mumol/l) had a biphasic effect. In the first 5 min Rt decreased from 128 +/- 13 to 119 +/- 14 omega cm2 (P < 0.05, n = 10) which probably reflects stimulation of an apical cell membrane Cl- conductance (see accompanying paper). Subsequently Rt increased in approximately 30 min to 184 +/- 20 omega cm2 and then remained fairly constant. Simultaneously the lateral spaces collapsed. If the spaces were now transiently opened by passing mucosa-positive direct current across the epithelium, Rt fell transiently to 111 +/- 7 omega cm2, but returned gradually to its elevated level when the spaces collapsed again. When the spaces were constantly dilated by a serosa-positive hydrostatic pressure of 1 cm H2O, forskolin neither affected the space width nor increased Rt, and current passage was virtually ineffective, although the cells depolarized in response to forskolin as usual.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of volume and water permeability of plated cells from measurements of light scattering

Measurements of cell membrane water osmotic permeabilities can be inaccurate because of the technical difficulties inherent to cell volume measurements and because of the presence of an unstirred water layer in contact with the cells. We detail here a method we have developed to quantify transient changes in cell volumes from the intensity of light scattered by cells. For this, we theorize how an unstirred layer originates in a perfusing chamber, and we calculate values for both cell membrane water osmotic permeability and unstirred layer thickness from time transient changes in scattered light. We apply a computer algorithm that finds the best correspondence between experimental data and estimated values. This is done by solving a differential equation governing cell volume changes by numerical integration (Runge-Kutta) and iterating the procedure varying the test values of osmotic permeability and unstirred layer thickness until the best fit is achieved. We exemplify this procedure with experimental results obtained in adherent cultured cells.

External Ca effect on water permeability, regulatory volume decrease, and extracellular space in barnacle muscle cells

The effect of extracellular Ca2+ (Cao) on sarcolemmal hydraulic water permeability (L'p), regulatory volume decrease (RVD), and extracellular space (ECS) was studied in barnacle muscle cells. Absence or presence of Cao had no effect on L'p [0 Cao = 2.762 +/- 0.098 x 10(-5), and 11 mM Cao = 2.720 +/- 0.222 x 10(-5) cm.kg.s-1 x osmol x 1-kgH2O-1]. Likewise, cells exposed to anisosmotic media (for < 30 min) behaved as osmometers in 0 and 11 mM Cao, showing similar slopes and intercepts in van't Hoff plots. At longer incubation times, however, hyposmotic conditions promoted a Cao-dependent RVD. The relationship between Cao and the percentage of cells responding with RVD to a hyposmotic challenge was sigmoidal (half-maximal Cao = 4.83 mM). The mean rate of RVD (40 nl/min) was independent of the level of swelling in response to hyposmotic challenges. However, the magnitude of RVD increased with larger hyposmotic challenges. Both the presence of Cao and hypotonicity reduced the "apparent" ECS by 47 +/- 6 and 39 +/- 6%, respectively. Three-dimensional reconstruction of autoradiographs of the cells was made to interpret these results.

Convective fluid flow through the paracellular system of Necturus gall-bladder epithelium as revealed by dextran probes

1. Bidirectional paracellular fluxes using radioactive dextrans as inert molecular probes have been measured across Necturus gall-bladder epithelium during conditions of normal fluid absorption. There is a net flux at all radii analysed (0.4-2.2 nm) in the direction of fluid absorption. 2. The net flux is substantial at all radii within the range. The data extraplate to 2 x 10(-6) cm s-1 at zero probe radius, which is very close to the rate of epithelial fluid absorption. 3. The unstirred layers at the epithelial faces during transport have been determined; their contribution to the net fluxes is negligible. 4. Two possible mechanisms for the net flow of probes are considered: (i) that the probes diffuse across the junctions and are then entrained in a local osmotic flow along the interspaces and subepithelium; (ii) that the probes are entrained in volume flow across the junctions and the emergent solution subsequently passes through the interspaces and subepithelium. Model calculations clearly rule out mechanism (i) in which the maximum net flow obtainable is less than 10% of that observed. In addition the presence of leak paths shunting the junctions is not compatible with the observed fluxes. With mechanism (ii) the net flows are correctly predicted with all the fluid flow being transjunctional. The fluid absorption is therefore entirely paracellular. 5. The slope of the net flow curve shows no apparent change in magnitude over the range of the probe radii, indicating that effectively only one population of convective channels is present with parallel walls separated by about 7.7 nm. This agrees with the width previously determined by electron microscopy. 6. If the fluid absorption is junctional then the cellular route offers little if any relative contribution. The hydraulic conductivity of the junctions is not high enough, or the osmotic permeability of the membranes low enough, to accommodate this by osmosis and therefore the junctional fluid absorption must be non-osmotic.

Molecular aspects of water transport

Due to its fundamental importance, the movement of water across cell membranes has been an active area of research for more than 100 years. This subject is central to consideration of normal water metabolism by terrestrial animals, as well as derangements in overall water balance that are frequently encountered by nephrologists in the care of their patients. The objective of this review is to discuss the most basic aspects of cell membrane water permeability and provide a framework for these data in the context of the care of pediatric patients with renal disease. While the water permeability of most cell membranes can be accounted for by the diffusion of water across the lipid bilayer, other cells, including the red blood cell and certain epithelial cells that line the proximal and collecting tubules of the kidney and the urinary bladder of amphibians, possess specialized water channels. Water channels are composed of specialized proteins that create aqueous pores across cell membrane. Currently, there are active research efforts to isolate and characterize water channel proteins from these cell types. Data concerning the distribution, permeability and function of these various water channels will greatly enhance our knowledge of how water is transported across cell membranes.

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.

Pseudo-streaming potentials in Necturus gallbladder epithelium. II. The mechanism is a junctional diffusion potential

The mechanisms of apparent streaming potentials elicited across Necturus gallbladder epithelium by addition or removal of sucrose from the apical bathing solution were studied by assessing the time courses of: (a) the change in transepithelial voltage (Vms). (b) the change in osmolality at the cell surface (estimated with a tetrabutylammonium [TBA+]-selective microelectrode, using TBA+ as a tracer for sucrose), and (c) the change in cell impermeant solute concentration ([TMA+]i, measured with an intracellular double-barrel TMA(+)-selective microelectrode after loading the cells with TMA+ by transient permeabilization with nystatin). For both sucrose addition and removal, the time courses of Vms were the same as the time courses of the voltage signals produced by [TMA+]i, while the time courses of the voltage signals produced by [TBA+]o were much faster. These results suggest that the apparent streaming potentials are caused by changes of [NaCl] in the lateral intercellular spaces, whose time course reflects the changes in cell water volume (and osmolality) elicited by the alterations in apical solution osmolality. Changes in cell osmolality are slow relative to those of the apical solution osmolality, whereas lateral space osmolality follows cell osmolality rapidly, due to the large surface area of lateral membranes and the small volume of the spaces. Analysis of a simple mathematical model of the epithelium yields an apical membrane Lp in good agreement with previous measurements and suggests that elevations of the apical solution osmolality elicit rapid reductions in junctional ionic selectivity, also in good agreement with experimental determinations. Elevations in apical solution [NaCl] cause biphasic transepithelial voltage changes: a rapid negative Vms change of similar time course to that of a Na+/TBA+ bi-ionic potential and a slow positive Vms change of similar time course to that of the sucrose-induced apparent streaming potential. We conclude that the Vms changes elicited by addition of impermeant solute to the apical bathing solution are pseudo-streaming potentials, i.e., junctional diffusion potentials caused by salt concentration changes in the lateral intercellular spaces secondary to osmotic water flow from the cells to the apical bathing solution and from the lateral intercellular spaces to the cells. Our results do not support the notion of junctional solute-solvent coupling during transepithelial 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.