Fluid transport and the dimensions of cells and interspaces of living Necturus gallbladder

Mitochondrial dynamics regulate cell morphology in the developing cochlea

In multicellular tissues, the size and shape of cells are intricately linked with their physiological functions. In the vertebrate auditory organ, the neurosensory epithelium develops as a mosaic of sensory hair cells (HCs), and their glial-like supporting cells, which have distinct morphologies and functional properties at different frequency positions along its tonotopic long axis. In the chick cochlea, the basilar papilla (BP), proximal (high-frequency) HCs, are larger than their distal (low-frequency) counterparts, a morphological feature essential for sound perception. Mitochondrial dynamics, which constitute the equilibrium between fusion and fission, regulate differentiation and functional refinement across a variety of cell types. We investigate this as a potential mechanism for regulating the shape of developing HCs. Using live imaging in intact BP explants, we identify distinct remodelling of mitochondrial networks in proximal compared with distal HCs. Manipulating mitochondrial dynamics in developing HCs alters their normal morphology along the proximal-distal (tonotopic) axis. Inhibition of the mitochondrial fusion machinery decreased proximal HC surface area, whereas promotion of fusion increased the distal HC surface area. We identify mitochondrial dynamics as a key regulator of HC morphology in developing inner ear epithelia.

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.

Reconciling the Krogh and Ussing interpretations of epithelial chloride transport - presenting a novel hypothesis for the physiological significance of the passive cellular chloride uptake

In 1937, August Krogh discovered a powerful active Cl(-) uptake mechanism in frog skin. After WWII, Hans Ussing continued the studies on the isolated skin and discovered the passive nature of the chloride uptake. The review concludes that the two modes of transport are associated with a minority cell type denoted as the γ-type mitochondria-rich (MR) cell, which is highly specialized for epithelial Cl(-) uptake whether the frog is in the pond of low [NaCl] or the skin is isolated and studied by Ussing chamber technique. One type of apical Cl(-) channels of the γ-MR cell is activated by binding of Cl(-) to an external binding site and by membrane depolarization. This results in a tight coupling of the uptake of Na(+) by principal cells and Cl(-) by MR cells. Another type of Cl(-) channels (probably CFTR) is involved in isotonic fluid uptake. It is suggested that the Cl(-) channels serve passive uptake of Cl(-) from the thin epidermal film of fluid produced by mucosal glands. The hypothesis is evaluated by discussing the turnover of water and ions of the epidermal surface fluid under terrestrial conditions. The apical Cl(-) channels close when the electrodiffusion force is outwardly directed as it is when the animal is in the pond. With the passive fluxes eliminated, the Cl(-) flux is governed by active transport and evidence is discussed that this is brought about by an exchange of cellular HCO(3) (-) with Cl(-) of the outside bath driven by an apical H(+) V-ATPase.

Lateral cell membranes and shunt resistance in rabbit esophageal epithelium

BACKGROUND AND AIMS

The structures that contribute to shunt resistance (Rs) in esophageal epithelium are incompletely understood, with 35-40% of Rs known to be calcium-dependent, reflecting the role of e-cadherin. Two calcium-independent candidates for the remaining approximately 60% of Rs have been identified: the glycoprotein matrix (GPM) within stratum corneum of esophageal epithelium, and the lateral cell membranes (LCMs) from neighboring cells.

METHODS

To determine the contribution of GPM and LCMs to Rs, rabbit esophageal epithelium was mounted in Ussing chambers so that transepithelial resistance (R(T)), a marker of Rs, could be monitored during luminal exposure to either glycosidases for disruption of the GPM or to hypertonic urea for separation of the LCMs.

RESULTS

Glycosidases had no effect on R(T). In contrast, hypertonic urea reduced R(T), increased fluorescein flux and widened the intercellular spaces. That urea reduced R(T), and so Rs, by widening the intercellular spaces, and not by altering the e-cadherin-dependent apical junctional complex, was supported by the ability of: (a) calcium-free solution to reduce R(T) beyond that produced by urea, (b) hypertonic urea to reduce R(T) beyond that produced by calcium free solution, (c) hypertonic sucrose to collapse the intercellular spaces and raise R(T), and (d) empigen, a zwitterionic detergent, to non-osmotically widen the intercellular spaces and reduce R(T).

CONCLUSION

These data indicate that the LCMs from neighboring cells are a major contributor to shunt resistance in esophageal epithelium. As resistor, they are distinguishable from the apical junctional complex by their sensitivity to (luminal) hypertonicity and insensitivity to removal of calcium.

The lateral intercellular space as osmotic coupling compartment in isotonic transport

Solute-coupled water transport and isotonic transport are basic functions of low- and high-resistance epithelia. These functions are studied with the epithelium bathed on the two sides with physiological saline of similar composition. Hence, at transepithelial equilibrium water enters the epithelial cells from both sides, and with the reflection coefficient of tight junction being larger than that of the interspace basement membrane, all of the water leaves the epithelium through the interspace basement membrane. The common design of transporting epithelia leads to the theory that an osmotic coupling of water absorption to ion flow is energized by lateral Na(+)/K(+) pumps. We show that the theory accounts quantitatively for steady- and time dependent states of solute-coupled fluid uptake by toad skin epithelium. Our experimental results exclude definitively three alternative theories of epithelial solute-water coupling: stoichiometric coupling at the molecular level by transport proteins like SGLT1, electro-osmosis and a 'junctional fluid transfer mechanism'. Convection-diffusion out of the lateral space constitutes the fundamental problem of isotonic transport by making the emerging fluid hypertonic relative to the fluid in the lateral intercellular space. In the Na(+) recirculation theory the 'surplus of solutes' is returned to the lateral space via the cells energized by the lateral Na(+)/K(+) pumps. We show that this theory accounts quantitatively for isotonic and hypotonic transport at transepithelial osmotic equilibrium as observed in toad skin epithelium in vitro. Our conclusions are further developed for discussing their application to solute-solvent coupling in other vertebrate epithelia such as small intestine, proximal tubule of glomerular kidney and gallbladder. Evidence is discussed that the Na(+) recirculation theory is not irreconcilable with the wide range of metabolic cost of Na(+) transport observed in fluid-transporting epithelia.

Restoration of barrier function in injured intestinal mucosa

Mucosal repair is a complex event that immediately follows acute injury induced by ischemia and noxious luminal contents such as bile. In the small intestine, villous contraction is the initial phase of repair and is initiated by myofibroblasts that reside immediately beneath the epithelial basement membrane. Subsequent events include crawling of healthy epithelium adjacent to the wound, referred to as restitution. This is a highly regulated event involving signaling via basement membrane integrins by molecules such as focal adhesion kinase and growth factors. Interestingly, however, ex vivo studies of mammalian small intestine have revealed the importance of closure of the interepithelial tight junctions and the paracellular space. The critical role of tight junction closure is underscored by the prominent contribution of the paracellular space to measures of barrier function such as transepithelial electrical resistance. Additional roles are played by subepithelial cell populations, including neutrophils, related to their role in innate immunity. The net result of reparative mechanisms is remarkably rapid closure of mucosal wounds in mammalian tissues to prevent the onset of sepsis.

A primary culture of mouse proximal tubular cells, established on collagen-coated membranes

A simple method is described to establish primary cultures of kidney proximal tubule cells (PTC) on membranes. The permeable membranes represent a unique culture surface, allowing a high degree of differentiation since both apical and basolateral membranes are accessible for medium. Proximal tubule (PT) segments from collagenase-digested mouse renal cortices were grown for 7 days, by which time cells were organized as a confluent monolayer. Electron microscopic evaluation revealed structurally polarized epithelial cells with numerous microvilli, basolateral invaginations, and apical tight junctions. Immunoblotting for markers of distinct parts of the nephron demonstrated that these primary cultures only expressed PT-specific proteins. Moreover immunodetection of distinct components of the receptor-mediated endocytic pathway and uptake of FITC-albumin indicated that these cells expressed a functional endocytotic apparatus. In addition, primary cultures possessed the PT brush-border enzymes, alkaline phosphatase, and gamma-glutamyl-transferase, and a phloridzin-sensitive sodium-dependent glucose transport at their apical side. Electrophysiological measurements show that the primary cultured cells have a low transepithelial resistance and high short-circuit current that was completely carried by Na(+) similar to a leaky epithelium like proximal tubule cells. This novel method established well-differentiated PTC cultures.

Na+ Recirculation and Isosmotic Transport

The Na(+) recirculation theory for solute-coupled fluid absorption is an expansion of the local osmosis concept introduced by Curran and analyzed by Diamond & Bossert. Based on studies on small intestine the theory assumes that the observed recirculation of Na(+) serves regulation of the osmolarity of the absorbate. Mathematical modeling reproducing bioelectric and hydrosmotic properties of small intestine and proximal tubule, respectively, predicts a significant range of observations such as isosmotic transport, hyposmotic transport, solvent drag, anomalous solvent drag, the residual hydraulic permeability in proximal tubule of AQP1 (-/-) mice, and the inverse relationship between hydraulic permeability and the concentration difference needed to reverse transepithelial water flow. The model reproduces the volume responses of cells and lateral intercellular space (lis) following replacement of luminal NaCl by sucrose as well as the linear dependence of volume absorption on luminal NaCl concentration. Analysis of solvent drag on Na(+) in tight junctions provides explanation for the surprisingly high metabolic efficiency of Na(+) reabsorption. The model predicts and explains low metabolic efficiency in diluted external baths. Hyperosmolarity of lis is governed by the hydraulic permeability of the apical plasma membrane and tight junction with 6-7 mOsm in small intestine and < or = 1 mOsm in proximal tubule. Truly isosmotic transport demands a Na(+) recirculation of 50-70% in small intestine but might be barely measurable in proximal tubule. The model fails to reproduce a certain type of observations: The reduced volume absorption at transepithelial osmotic equilibrium in AQP1 knockout mice, and the stimulated water absorption by gallbladder in diluted external solutions. Thus, it indicates cellular regulation of apical Na(+) uptake, which is not included in the mathematical treatment.

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.

Analysis of the sodium recirculation theory of solute-coupled water transport in small intestine

Our previous mathematical model of solute-coupled water transport through the intestinal epithelium is extended for dealing with electrolytes rather than electroneutral solutes. A 3Na+-2K+ pump in the lateral membranes provides the energy-requiring step for driving transjunctional and translateral flows of water across the epithelium with recirculation of the diffusible ions maintained by a 1Na+-1K+-2Cl- cotransporter in the plasma membrane facing the serosal compartment. With intracellular non-diffusible anions and compliant plasma membranes, the model describes the dependence on membrane permeabilities and pump constants of fluxes of water and electrolytes, volumes and ion concentrations of cell and lateral intercellular space (lis), and membrane potentials and conductances. Simulating physiological bioelectrical features together with cellular and paracellular fluxes of the sodium ion, computations predict that the concentration differences between lis and bathing solutions are small for all three ions. Nevertheless, the diffusion fluxes of the ions out of lis significantly exceed their mass transports. It is concluded that isotonic transport requires recirculation of all three ions. The computed sodium recirculation flux that is required for isotonic transport corresponds to that estimated in experiments on toad small intestine. This result is shown to be robust and independent of whether the apical entrance mechanism for the sodium ion is a channel, a SGLT1 transporter driving inward uphill water flux, or an electroneutral Na+-K+-2Cl- cotransporter.

A mathematical model of solute coupled water transport in toad intestine incorporating recirculation of the actively transported solute

A mathematical model of an absorbing leaky epithelium is developed for analysis of solute coupled water transport. The non-charged driving solute diffuses into cells and is pumped from cells into the lateral intercellular space (lis). All membranes contain water channels with the solute passing those of tight junction and interspace basement membrane by convection-diffusion. With solute permeability of paracellular pathway large relative to paracellular water flow, the paracellular flux ratio of the solute (influx/outflux) is small (2-4) in agreement with experiments. The virtual solute concentration of fluid emerging from lis is then significantly larger than the concentration in lis. Thus, in absence of external driving forces the model generates isotonic transport provided a component of the solute flux emerging downstream lis is taken up by cells through the serosal membrane and pumped back into lis, i.e., the solute would have to be recirculated. With input variables from toad intestine (Nedergaard, S., E.H. Larsen, and H.H. Ussing, J. Membr. Biol. 168:241-251), computations predict that 60-80% of the pumped flux stems from serosal bath in agreement with the experimental estimate of the recirculation flux. Robust solutions are obtained with realistic concentrations and pressures of lis, and with the following features. Rate of fluid absorption is governed by the solute permeability of mucosal membrane. Maximum fluid flow is governed by density of pumps on lis-membranes. Energetic efficiency increases with hydraulic conductance of the pathway carrying water from mucosal solution into lis. Uphill water transport is accomplished, but with high hydraulic conductance of cell membranes strength of transport is obscured by water flow through cells. Anomalous solvent drag occurs when back flux of water through cells exceeds inward water flux between cells. Molecules moving along the paracellular pathway are driven by a translateral flow of water, i.e., the model generates pseudo-solvent drag. The associated flux-ratio equation is derived.

The role of growth hormone in the adaptability of Atlantic salmon (Salmo salar L.) to seawater: effects on the morphology of the mucosa of the middle intestine

Ultrastructural modifications of the middle intestine of the salmon, Salmo salar, induced by transfer to seawater have been studied in two groups of fish: the first group received sham treatment and the second was treated with ovine growth hormone (oGH). In sham-treated fish during the first 2 days in seawater, significant distension of the intercellular spaces was observed between the apical tight junction and the basement membrane. In the basal part of the enterocytes, tubular invaginations in the intercellular spaces were closely associated with mitochondria. In oGH-implanted fish, we observed no signs of modification of the ultrastructure of the mucosa. There were no dilatations of the intercellular spaces and no infoldings in the basal part of the enterocytes. After 7 days in seawater, the mucosa of the intestine of sham- and oGH-treated fish was quite similar. The effects of oGH treatment were clear, and treatment seemed to provoke "pre-adaptation" of the intestinal mucosa before exposure to high salinity to maintain the morphology of the middle intestine of Atlantic salmon abruptly transferred to seawater.

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.

Long-term effects of tumor necrosis factor on LLC-PK1 transepithelial resistance

Renal epithelial LLC-PK1 cell sheets incubated with tumor necrosis factor (TNF) undergo an acute, spontaneous, and rapidly reversible decrease in transepithelial resistance (TER). (Mullin et al., 1992). However, 24 to 72 h following TNF exposure, TER across the cell sheet increases 2-fold. This later effect of TNF is also reversible, albeit slowly. The TER of TNF-treated cell sheets then declines toward initial levels between 72 and 144 h following exposure to the cytokine. Whereas the long-term increase in TER following TNF exposure is not associated with a decreased transepithelial 14C-mannitol flux (size selectivity), the charge (anionic) selectivity of the LLC-PK1 tight junction is decreased. Basal-lateral (ouabain and bumetanide-insensitive) Rb+ and apical Na+-dependent alpha-methylglucoside (AMG) uptake into the cell are both reduced in cultures exposed to TNF 24 h earlier. Correspondingly, this long-term effect on TER is accompanied by a 30% decrease in short circuit current (iscc). Along with an observed increase in basal-lateral methylamino-isobutyric acid (MeAIB) influx into the cells, an increased incorporation of [3H]-thymidine into DNA indicates increased cell cycling after exposure to TNF. While the increase in cell cycling is not sustained for the duration of the elevation in TER, it does appear to initiate a sequence of events that lead to the sustained increase in TER. A decrease in the lateral intercellular space, observed between these epithelial cells after long-term TNF exposure, may be a mechanism for the elevated TER following from the mitogenesis and/or transport changes. This overall long-term tightening of an epithelium in response to TNF may function, in part, as a compensatory action of the epithelium to reestablish its effectiveness as a physiological barrier, following the acute effect of TNF.

Regulation of apical membrane ion transport in Necturus gallbladder

Na and Cl movement through the apical membrane of Necturus gallbladder epithelium was investigated using electrophysiological and light microscopic measurements. Changes in membrane potential difference, fractional resistance of the apical membrane, and transepithelial resistance caused by changes in apical bath Cl concentration revealed the presence of a Cl conductance in the apical membrane of control tissues that was apparently not present in the preparations studied by other investigators. This Cl conductance was blocked by bumetanide (10(-5) M) or by the inhibitor of adenosine 3',5'-cyclic monophosphate (cAMP) action, the Rp isomer of adenosine 3',5'-cyclic monophosphorothioate (Rp-cAMPS; 0.5 mM). Treatment of the tissues with Rp-cAMPS also eliminated bumetanide-sensitive cell swelling in the presence of ouabain and activated an amiloride-sensitive swelling, changes consistent with inhibition of NaCl cotransport and the activation of Na-H and Cl-HCO3 exchange. We conclude that the mode of NaCl entry into Necturus gallbladder epithelial cells is determined by the level of cAMP. When cAMP levels are high, entry occurs by NaCl cotransport; when cAMP levels are low, parallel exchange of Na-H and Cl-HCO3 predominates. These observations explain the previous disagreements about the mode of NaCl entry into Necturus gallbladder epithelial cells.

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.

Upregulation of inositol transport mediates inositol accumulation in hyperosmolar brain cells

Attempts to understand brain volume regulation have been greatly hampered by the structural complexity of the mammalian central nervous system, indicating a need for the investigation of cultured brain cell lines whose behavior reflects that observed in situ. We demonstrate here that rat C6 glioma cells exhibit a pattern of hyperosmolar volume regulation qualitatively similar to that of the intact brain. Chronic (2-6 days) acclimation of C6 cells to high NaCl media (440 or 590 mosM) resulted in a 46-133 mM increase in cellular inositol, a known major brain osmolyte. C6 cells exposed acutely to 440 mosM medium shrank abruptly and then underwent a complete regulatory volume increase (RVI) within 4 h. Inositol levels began to increase after 10 h of hyperosmolar stress and reached maximal values by 24 h, suggesting that RVI is initially mediated by inorganic ion uptake. [3H]inositol uptake measurements revealed a sevenfold stimulation of phlorizin-inhibitable inositol transport in hyperosmotic cells. The enhancement of inositol transport paralleled the rise in cellular inositol content. Phlorizin reduced inositol accumulation in hyperosmolar cells by 44%. Our studies provide the first demonstration of RVI and organic osmolyte accumulation in a cultured brain cell line.

Effects of changes in mucosal solution Cl- or K+ concentration on cell water volume of Necturus gallbladder epithelium

An electrophysiologic technique was used to measure changes in cell water volume in response to isosmotic luminal solution ion replacement. Intracellular Cl- activity (aCl-i) was measured and net flux determined from the changes in volume and activity. Reduction of luminal solution [Cl-] from 98 to 10 mM (Cl- replaced with cyclamate) resulted in a large fall in aCl-i with no significant change in cell water volume. Elevation of luminal solution [K+] from 2.5 to 83.5 mM (K+ replaced Na+) caused a small increase in aCl-i with no change in cell water volume. Exposure of the Necturus gallbladder epithelium to agents that increase intracellular cAMP levels (forskolin and/or theophylline) induces an apical membrane electrodiffusive Cl- permeability accompanied by a fall in aCl-i and cell shrinkage. In stimulated tissues, reduction of luminal solution [Cl-] resulted in a large fall in aCl-i and rapid cell shrinkage, whereas elevation of luminal solution [K+] caused a large, rapid cell swelling with no significant change in aCl-i. The changes in cell water volume of stimulated tissues elicited by lowering luminal solution [Cl-] or by elevating luminal solution [K+] were reduced by 60 and 70%, respectively, by addition of tetraethylammonium (TEA+) to the luminal bathing solution. From these results, we conclude that: (a) In control tissues, the fall in aCl-i upon reducing luminal solution [Cl-], without concomitant cell shrinkage, indicates that the Cl- entry mechanism is electroneutral (Cl-/HCO3-) exchange. (b) Also in control tissues, the small increase in aCl-i upon elevating luminal solution [K+] is consistent with the recent demonstration of a basolateral Cl- conductance. (c) The cell shrinkage elicited by elevation of intracellular cAMP levels results from conductive loss of Cl- (and probably K+). (d) Elevation of cAMP inhibits apical membrane Cl-/HCO-3-exchange activity by 70%. (e) The cell shrinkage in response to the reduction of mucosal solution [Cl-] in stimulated tissues results from net K+ and Cl- efflux via parallel electrodiffusive pathways. (f) A major fraction of the K+ flux is via a TEA(+)-sensitive apical membrane K+ channel.

Regulation of cellular volume in rabbit ventricular myocytes: bumetanide, chlorothiazide, and ouabain

Video microscopy was used to study the regulation of cell volume in isolated rabbit ventricular myocytes. Myocytes rapidly (less than or equal to 2 min) swelled and shrank in hyposmotic and hyperosmotic solutions, respectively, and this initial volume response was maintained without a regulatory volume decrease or increase for 20 min. Relative cell volumes (normalized to isosmotic solution, 1T) were as follows: 1.41 +/- 0.01 in 0.6T, 1.20 +/- 0.04 in 0.8T, 0.71 +/- 0.04 in 1.8T, and 0.57 +/- 0.03 in 2.6T. These volume changes were significantly less than expected if all of the measured volume was osmotically active water. Changes in width and thickness were significantly greater than changes in cell length. The idea that cotransport contributes to cell volume regulation was tested by inhibiting Na(+)-K(+)-2Cl- cotransport with bumetanide (BUM) and Na(+)-Cl- cotransport with chlorothiazide (CTZ). Under isotonic conditions, a 10-min exposure to BUM (1 microM), CTZ (100 microM), or BUM (10 microM) plus CTZ (100 microM) decreased relative cell volume to 0.87 +/- 0.01, 0.86 +/- 0.02, and 0.82 +/- 0.04, respectively. BUM plus CTZ also modified the response to osmotic stress. Swelling in 2.6T medium was 76% greater and shrinkage in 0.6T medium was 29% less than in the absence of diuretics. In contrast to the rapid effects of diuretics, inhibition of the Na(+)-K+ pump with 10 microM ouabain for 20 min did not affect cell volume in 1T solution. Nevertheless, ouabain decreased swelling in 0.6T medium by 52% and increased shrinkage in 1.8T medium by 34%. These data suggest that under isotonic conditions Na(+)-K(+)-2Cl- and Na(+)-Cl- cotransport are critical in establishing cell volume, but osmoregulation can compensate for Na(+)-K+ pump inhibition for at least 20 min. Under anisotonic conditions, the Na(+)-K+ pump and Na(+)-K(+)-2Cl- and/or Na(+)-Cl- cotransport are important in myocyte volume regulation.

Quantitative assessment of canalicular bile formation in isolated hepatocyte couplets using microscopic optical planimetry

Isolated rat hepatocyte couplets (IRHC) are primary units of bile secretion that accumulate fluid in an enclosed canalicular space with time in culture. We have quantitated the rate of canalicular secretion in IRHC cultured for 4-8 h by measuring the change in canalicular space volume by video-microscopic optical planimetry using high resolution Nomarski optics. Electron microscopic morphometric studies revealed significant increases in canalicular membrane area after 4-6 h in culture. Canalicular secretion in basal L-15 medium (3.8 +/- 1.3 fl/min) increased significantly with the choleretic bile salts (10 microM), taurocholate, and ursodeoxycholate (14 +/- 7 fl/min each). Secretion rates after exposure to bile acids correlated directly with the canalicular surface area before stimulation. In contrast, expansion times after stimulation varied inversely with initial canalicular volumes. Ursodeoxycholic acid failed to produce a hypercholeresis at 10-, 100-, or 200-microM concentrations compared with taurocholate, either in normal or taurine-depleted IRHC. The present findings establish that rates of canalicular bile secretion can be quantitated in IRHC by serial optical planimetry, both in the basal state and after stimulation with bile acids. Furthermore, ursodeoxycholate does not acutely induce hypercholeresis at the canalicular level in this model. Rather, both taurocholic and ursodeoxycholic acids induced secretion in proportion to the surface area of the canalicular membrane. The IRHC are a useful model to identify canalicular choleretics and for studies of canalicular bile formation.

Epithelial cell surface morphology in the endolymphatic sac: a scanning electron microscopic study in the mouse

The apical, lateral and basal surface structures of the epithelial cells in the murine endolymphatic sac were studied using the freeze-cracking technique and scanning electron microscopy. In this way, it was possible to visualize the luminal surface and the interior of the cell simultaneously. The light epithelial cells, with their smooth rounded nuclei and many mitochondria, had numerous microvilli on their apical cell surface, whereas the dark epithelial cells, with indented nuclei, had only a few such microvilli. The lateral surfaces of these cells were flat, with few projections facing a dilated lateral intercellular space.

Absence of significant cellular dilution during ADH-stimulated water reabsorption

Water reabsorption across many "tight" urinary epithelia is driven by large transepithelial osmotic gradients and is controlled by antidiuretic hormone (ADH). Numerous investigators have concluded that ADH-induced water reabsorption causes large apparent increases in cell volume with concomitant cytoplasmic dilution. A central question in renal physiology has been how cellular homeostasis is maintained in tight urinary epithelia during antidiuresis. Previous direct measurements of cell membrane permeability to water and the present direct measurements of cell volume in collecting tubules of rabbit kidney cortex by quantitative light microscopy show that cell volume does not change significantly during transcellular water flow. Fluid transported across the epithelium accumulated in lateral and basal intercellular spaces; the effect was an increase in cell height and tubule wall thickness accompanied by maintenance of nearly constant cell volume. The stability of cell volume is a consequence of the relatively high water permeability of the blood-facing cell membrane.

Cell membrane water permeability of rabbit cortical collecting duct

The water permeability (Posm) of the cell membranes of isolated perfused rabbit cortical collecting ducts was measured by quantitative light microscopy. Water permeability of the basolateral membrane, corrected for surface area, was 66 microns X sec-1 for principal cells and 62.3 microns X sec-1 for intercalated cells. Apical membrane Posm values corrected for surface area, were 19.2 and 25 microns X sec-1 for principal and intercalated cells, respectively, in the absence of antidiuretic hormone (ADH). Principal and intercalated cells both responded to ADH by increasing Posm of their apical membranes to 92.2 and 86.2 microns X sec-1, respectively. The ratio of the total basolateral cell membrane osmotic water permeability to that of the apical cell membrane was approximately 27:1 in the absence of ADH and approximately 7:1 in the presence of the hormone for both cell types. This asymmetry in water permeability is most likely due to the fact that basolateral membrane surface area is at least 7 to 8 times greater than that of the apical membrane. Both cell types exhibited volume regulatory decrease when exposed to dilute serosal bathing solutions. Upon exposure to a hyperosmotic serosal bath (390 mosM), principal cells did not volume regulate while two physiologically distinct groups of intercalated cells were observed. One group of intercalated cells failed to volume regulate; the second group showed almost complete volume regulatory increase behavior.

Transcellular sodium fluxes and pump activity in Necturus gall-bladder epithelial cells

1. Transepithelial Na transport in Necturus was determined by measuring the rate of isotonic fluid flow. The rate at 20 degrees C was equivalent to 175 pmol cm-2 s-1. 2. Ouabain was effective in Necturus, binding to the Na pump in gall-bladder cells with a mean rate constant of 5.4 X 10(3) M-1 s-1. Measurement of the diffusive time constant of the free space for [3H]ouabain shows that the pump must be fully inhibited within 20 s when ouabain is applied to the serosa at 10(-3) M. 3. The serosal Na efflux from loaded cells was inhibited 36% by ouabain equal to a flux of 73 pmol cm-2 s-1. The remaining flux could not be attributed to either exchange diffusion or electrodiffusion induced by ouabain. 4. The transepithelial potential was 0.3 mV serosa positive. The short-circuit current measured was 6.33 +/- 1.9 microA cm-2, equal to a positive univalent ion flux of 65.6 pmol cm-2 s-1 or 38% of the net Na transfer. The current was inhibited within 1-5 min by 5 X 10(-5) M-amiloride. 5. Fluid secretion was immediately inhibited 34% by ouabain, equivalent to an isotonic transport of Na of 59.7 pmol cm-2 s-1. Thereafter it continued for at least an hour, sometimes declining slowly. Amiloride had little effect (13%). 6. The Na pump rate was measured by titrating the cell content with tracer Na at different times after ouabain treatment. The initial slope was equal to a rate of 61.6 pmol cm-2 s-1 or 35% of the net flux at time zero. 7. The Na pump rate has also been measured by recording the rise in cell Na activity with ion-specific micro-electrodes, and correcting for swelling effects. The Na pump rate was very similar to that estimated from the rise in tracer Na content, equal to 59.3 pmol cm-2 s-1 or 31.4% of the transepithelial rate. Examination of the same experiment in the literature shows a closely similar value, about one-third of that expected from fluid secretion or net flux measurements. 8. A scheme is proposed to explain the results, which requires a flow of NaCl through a parallel pathway of small Na content involving exchange en route with the cytoplasmic Na.

Carbonic anhydrase inhibition and cell volume regulation in Necturus gallbladder

Gallbladder epithelial cells transport salt and water isotonically as the renal proximal tubule. The cells also have the property of regulating their cell volume in response to osmotic stress (Fisher et al. 1981, Persson & Spring 1982, Fisher & Spring 1984, Foskett & Spring 1985). The volume-regulating phenomenon is the result of a balance between cell uptake of salt and water at the luminal membrane and exit at the basolateral membrane. Different properties regarding volume regulatory increase and decrease have been found (Eriksson & Spring 1982 and Larson & Spring 1983). The present study links fluid transport and volume regulatory increase of the cell. First we concluded from histological techniques that carbonic anhydrase is present in the cell membrane or in the vicinity of the epithelial cells. Then we measured a decreased net fluid transport in the presence of increasing concentrations of the carbonic anhydrase inhibitor acetazolamide. We showed that the volume regulatory increase is substantially slowed down and that the steady-state volume of the cells changed when carbonic anhydrase was inhibited. Our conclusion is that the rate of CO2 hydration was a limiting step, at carbonic anhydrase inhibition, in both the net transfer of salt and water and also in the ability of the cells to efficiently regulate their volume.

Influence of glucose absorption on ion activities in cells and submucosal space in goldfish intestine

Mucosal glucose addition evokes in goldfish intestinal epithelium a fast depolarization of the mucosal membrane potential (delta psi mc = 12 mV) followed by a slower repolarization (delta psi mc = -7 mV). The intracellular sodium activity, aiNa+, rises from 13.2 +/- 2.4 meq/l by 6.7 +/- 0.5 meq/l within 5 min, aiCl- rises about 3 meq/l above the control value of 37.7 +/- 2.2 meq/l, while aiK is constant (97.7 +/- 7.4 meq/l). The potassium activity measured in the submucosal interstitium near the basal side of the cells (asK+) is 5.2 +/- 0.2 meq/l in non-absorbing tissue compared to 4.2 meq/l in the bathing solution and shows a transient increase due to glucose absorption (1.1 +/- 0.1 meq/l). In chloride-free media asK+ = 4.2 +/- 0.1 meq/l and psi mc hyperpolarizes by -13 mV. The depolarization due to glucose absorption increases (delta psi mc = 14.1 +/- 1.4) and the repolarization (delta psi repolmc) disappears. In addition, aiNa+ rises from 16.3 +/- 2.4 meq/l by 9.9 +/- 1.5 meq/l within 5 min, aiK+ remains constant and equal to the value in chloride containing solutions (88.5 +/- 2.8 meq/l); asK+ increases transiently (1.1 +/- 0.1 meq/l). Serosal Ba2+ (5 mM) depolarizes psi mc (+14.2 +/- 1.0 mV) and abolishes the repolarization. Increased serosal or mucosal potassium activity depolarizes psi mc and abolishes the repolarization. These effects are discussed in terms of changes of ion activities, the basolateral potassium conductance, the influence of intracellular Ca2+, the functional state of the Na/K-pump, and modulation of membrane permeabilities by extracellular potassium.

Effects of Ca2+ and furosemide on Cl- transport and O2 uptake in rat parotid acini

Stimulation-induced changes in Cl- content and O2 consumption of collagenase-isolated rat parotid acini were measured. In less than 10 s, carbachol caused a net Cl- efflux, corresponding to approximately 50% of the Cl- content, and increased the O2 uptake by 100%. The increase was inhibitable by ouabain and was dependent on the presence of extracellular Ca2+. Furosemide reduced the unstimulated 36Cl- uptake and prevented the reuptake of Cl- after carbachol-induced release. This suggests that a cotransport system is operating in both the unstimulated and stimulated states. Furthermore, furosemide inhibited the stimulated ouabain-sensitive O2 uptake. Raising intracellular Ca2+ by the calcium ionophore A23187 evoked the same pattern of Cl- loss and O2 uptake as carbachol. Our results are compatible with the following hypothesis. Carbachol raises intracellular Ca2+, causing an increased Cl- permeability of the luminal membrane, resulting in a net Cl- efflux. A subsequently enhanced influx of Cl- and Na+ via a furosemide-sensitive cotransport system increases intracellular Na+. This stimulates the Na+-K+-ATPase and thereby the O2 consumption.

Structural changes associated with fluid absorption by dog tracheal epithelium

During fluid absorption induced by amphotericin B, the lateral intercellular spaces (LIS) of the dog tracheal epithelium were widely dilated as compared to untreated time controls. When fluid absorption was inhibited by ouabain, or by replacement of luminal Na by choline, amphotericin B failed to cause dilation of the LIS. These data suggest that, as in other epithelia, a significant amount of transepithelial fluid flow passes down the LIS, and that these spaces may provide the local osmotic compartment which is responsible for linking transepithelial fluid movement to active ion transport.

Potassium induced changes in cell volume of gallbladder epithelium

The mechanisms of transmembrane K and anion movements were investigated by measurement of the changes in cell volume, apical membrane potential difference, and intracellular K activity resulting from exposure of Necturus gallbladder to solutions with increased K concentration. Cell swelling occurred when the tissue was exposed bilaterally to 25 mmol/l K. This swelling was both Cl and HCO3 dependent, but was not blocked by DIDS or bumetanide. Unilateral tenfold increases in extracellular K concentration did not cause cell swelling; addition of 5 mmol/l Ba to the contralateral cell surface resulted in cell volume increases comparable to those seen with bilateral K increase. Complete blockage of K channels by Ba could be demonstrated electrophysiologically at normal extracellular K concentrations but not in the presence of increased K. Our results were consistent with the passive movement of K through Ba-sensitive channels in both cell membranes. We were unable to detect other mechanisms for transmembrane K movement. The cell swelling caused by exposure to 25 mmol/l K was not due to intracellular K accumulation and may be related to the effects of membrane depolarization on voltage sensitive anion transport processes.

Cyclic AMP-induced changes in membrane conductance of Necturus gallbladder epithelial cells

Enhanced cellular cAMP levels have been shown to increase apical membrane Cl- and HCO3- conductances in epithelia. We found that the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX) increases cAMP levels in Necturus gallbladder. We used conventional open-tip and double-barreled Cl- -selective microelectrodes to study the effects of IBMX on membrane conductances and intracellular Cl- activities in gallbladders mounted in a divided chamber and bathed with Ringer's solutions at 23 degrees C and pH 7.4. In HCO3- -free media, 0.1 mM IBMX added to the mucosal medium depolarized the apical membrane potential Va, decreased the fractional resistance FR, and significantly reduced intracellular Cl- activity (aCli). Under control conditions, aCli was above the value corresponding to passive distribution across the apical cell membrane. In media containing 25 mM HCO3-, IBMX caused a small transient hyperpolarization of Va followed by a depolarization not significantly different from that observed in HCO3- -free Ringer's. Removal of mucosal Cl-, Na+ or Ca2+ did not affect the IBMX-induced depolarization in Va. The basolateral membrane of Necturus gallbladder is highly K+ permeable. Increasing serosal K+ from 2.5 to 80 mM, depolarized Va. Mucosal IBMX significantly reduced this depolarization. Addition of 10 mM Ba2+, a K+ channel blocker, to the serosal medium depolarized Va and, essentially, blocked the depolarization induced by IBMX. These results indicate that mucosal IBMX increases apical HCO3- conductance and decreases basolateral K+ conductance in gallbladder epithelial cells via a cAMP-dependent mechanism. The latter effect, not previously reported in epithelial tissues, appears to be the major determinant of the IBMX-induced depolarization of Va.

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.

Changes in cell volume measured with an electrophysiologic technique

Epithelial cells of the gallbladder of Necturus maculosus were loaded with tetramethylammonium (Me4N+) by transient exposure of the apical (lumen-facing) surface to a solution of high Me4N+ concentration containing also the polyene antibiotic nystatin. Upon removal of nystatin, in the continued presence of Me4N+, spontaneous restoration of the native ionic permeability of the apical cell membrane was observed. At this time, external Me4N+ was removed; intracellular [Me4N+] measured with ion-sensitive microelectrodes was 2-15 mM and remained unchanged for several hours. Changes in cell volume were estimated from the changes in intracellular [Me4N+] produced by alterations in the osmolality of the mucosal bathing solution. Assuming that such changes are caused entirely by water fluxes across the apical membrane, the minimum value of its hydraulic permeability coefficient (Lp) was 1-3 X 10(-3) cm.sec-1.(osmoles/kg)-1, suggesting that an osmolality difference across the apical membrane as small as 1-3 milliosmoles/kg could explain the average rate of transepithelial water transport. These results agree with optical measurements [Persson, B. O. & Spring, K. R. (1982) J. Gen. Physiol. 79, 481-505]. The effective thickness of the apical unstirred layer was estimated from the time courses of both the apical membrane voltage and the response of an extracellular K+-sensitive microelectrode to an increase in [K+] in the mucosal bath. Since changes in concentration of the osmotically active solute at the membrane surface were thus shown to be significantly delayed by diffusion, the Lp value, calculated assuming a step-change in osmolality, is an underestimate.

Effects of mucosal sodium removal on cell volume in Necturus gallbladder epithelium

Necturus gallbladder epithelium transports sodium and chloride by a process that first involves the cellular entry of each ion across the apical membrane in an electrically silent process. In this paper we present results from cell volume and fluid flux measurements in the presence of different inhibitors and at normal and reduced sodium concentrations, which bear on the process by which ionic entry is effected. We find that reduction of mucosal sodium to a concentration of 10 mM has no effect on either cell volume or on the rate of transepithelial fluid transport, whereas the complete removal of sodium causes a significant decrease in cell volume in addition to its known inhibitory effect on fluid transport. Amiloride had no effect on cell volume at normal sodium concentrations but markedly reduced it when the sodium concentration was reduced to 10 mM. Amiloride, bumetanide, and dipyridamole markedly and reversibly inhibited fluid transport. Finally, the addition of ouabain to the serosal medium induced cell swelling, which was prevented by the removal of potassium from the mucosal medium. These results indicate that the process of sodium entry at the apical membrane is complicated and likely includes both cotransport (NaCl or Na-K-2Cl) and parallel exchange (Na-H and Cl-HCO3) transport mechanisms, and that the proportion of NaCl transported by the different mechanisms varies with the conditions.

Protamine reversibly decreases paracellular cation permeability in Necturus gallbladder

Protamine, a naturally occurring arginine-rich polycationic protein (pI 9.7 to 12), was tested in Necturus gallbladder using a transepithelial AC-impedance technique. Protamine sulfate or hydrochloride (100 micrograms/ml = 20 microM), dissolved in the mucosal bath, increased transepithelial resistance by 89% without affecting the resistance of subepithelial layers. At the same time, transepithelial voltage (psi ms) turned from slightly mucosa-positive values to mucosa-negative values of approximately +1 to -5 mV. The effect of protamine on transepithelial resistance was minimal at concentrations below 5 micrograms/ml but a maximum response was achieved between 10 and 20 micrograms/ml. Resistance started to increase within 1 min and was maximal after 10 min. These effects were not inhibited by serosal ouabain (5 X 10(-4) M) but could be readily reversed by mucosal heparin. The sequence of protamine effect and heparin reversal could be repeated several times in the same gallbladder. Mucosal heparin, a strong negatively charged mucopolysaccharide, or serosal protamine were without effect. Mucosal protamine reversibly decreased the partial ionic conductance of K and Na by a factor of 3, but did not affect Cl conductance. Net water transport from mucosa to serosa was reversibly increased by 60% by protamine. We conclude that protamine reversibly decreases the conductance of the cation-selective pathway through the tight junction. Although this effect is similar to that reported for 2,4,6-triamino-pyrimidinium (TAP), the mechanism of action may differ. We propose that protamine binds to the apical cell membrane and induces a series of intracellular events which leads to a conformational alteration of the tight junction structure resulting in decreased cationic permeability.