Solvent drag of large solutes indicates paracellular water flow in leaky epithelia

Is there a molecular basis for solvent drag in the renal proximal tubule?

The concept of solvent drag, i.e., water and solutes sharing the same pore and their transport being frictionally coupled, was first proposed in the early 1950s. During the following decades, it was applied to transport processes across cell membranes as well as transport along the paracellular pathway. Water-driven solute transport was proposed as the major mechanism for electrolyte and nutrient absorption in the small intestine and for Cl^- and HCO₃^- reabsorption in the renal proximal tubule. With the discovery of aquaporins as transcellular route for water transport and the claudin protein family as the major determinant of paracellular transport properties, new mechanistic insights in transepithelial water and solute transport are emerging and call for a reassessment of the solvent drag concept. Current knowledge does not provide a molecular basis for relevant solvent drag-driven, paracellular nutrient, and inorganic anion (re-)absorption. For inorganic cation transport, in contrast, solvent drag along claudin-2-formed paracellular channels appears feasible.

Mechanistic basis for loss of water balance in green tree frogs infected with a fungal pathogen

Chytridiomycosis, a lethal skin disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), disrupts skin function of amphibians, interfering with ionic and osmotic regulation. To regulate fungal loads, amphibians increase their rate of skin sloughing. However, sloughing also causes a temporary loss of ionic and osmotic homeostasis due to disruption of the skin, a key osmoregulatory organ. The combined effects of increased sloughing frequency and chytridiomycosis contribute to the high rates of mortality from Bd infections. However, the mechanisms responsible for the loss of cutaneous osmotic regulation remain unknown. We measured the changes in whole animal water uptake rates, in vitro transcutaneous water fluxes across the ventral skin, and the mRNA expression of epithelial water transport proteins (aquaporins, AQPs) and junctional proteins in Bd-infected and uninfected Litoria caerulea skin. We hypothesize that infected frogs would show reduction/inhibition in cutaneous water transporters responsible for regulating water balance, and sloughing would exacerbate cutaneous water fluxes. We found that infected, nonsloughing frogs had an impaired rate of water uptake and showed increased rates of in vitro water efflux across the ventral skin. In uninfected frogs, the expression of AQPs and junction genes increased significantly with sloughing, which may assist in regulating cutaneous water movements and barrier function in the newly exposed skin. In contrast, infected frogs did not show this postsloughing increase in AQP gene expression. The combination of increased sloughing frequency, impaired water uptake rates, and increased rates of water loss likely contributes to the loss of osmotic homeostasis in frogs infected with Bd.

Independence of net water flux from paracellular permeability in the intestine of Fundulus heteroclitus, a euryhaline teleost

Paracellular permeability and absorptive water flux across the intestine of the euryhaline killifish were investigated using in vitro gut sac preparations from seawater- and freshwater-acclimated animals. The permeability of polyethylene glycol (PEG), a well-established paracellular probe, was measured using trace amounts of radiolabelled oligomers of three different molecular sizes (PEG-400, PEG-900 and PEG-4000) at various times after satiation feeding. All three PEG molecules were absorbed, with permeability declining as a linear function of increasing hydrodynamic radius. Response patterns were similar in seawater and freshwater preparations, though water absorption and PEG-900 permeability were greater in the latter. Despite up to 4-fold variations in absorptive water flux associated with feeding and fasting (highest at 1-3 h, lowest at 12-24 h and intermediate at 1-2 weeks post-feeding), there were no changes in PEG permeability for any size oligomer. When PEG permeability was measured in the opposite direction (i.e. serosal to mucosal) from net water flux, it was again unchanged. HgCl(2) (10(-3) mol l(-1)), a putative blocker of aquaporins, eliminated absorptive water flux yet increased PEG-4000 permeability by 6- to 8-fold in both freshwater and seawater preparations. Experimentally raising the serosal osmolality by addition of 300 mmol l(-1) mannitol increased the absorptive water flux rate 10-fold, but did not alter PEG permeability. Under these conditions, HgCl(2) reduced absorptive water flux by 60% and again increased PEG permeability by 6- to 8-fold in both freshwater and seawater preparations. Clearly, there was no influence of solvent drag on PEG movement. The putative paracellular blocker 2,4,6-triaminopyrimidine (TAP, 20 mmol l(-1)) had no effect on net water flux or PEG permeability. We conclude that PEG and water move by separate pathways; absorptive water transport probably occurs via a transcellular route in the intestine of Fundulus heteroclitus.

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.

Beta-adrenergic activation of solute coupled water uptake by toad skin epithelium results in near-isosmotic transport

Transepithelial potential (V(T)), conductance (G(T)), and water flow (J(V)) were measured simultaneously with good time resolution (min) in isolated toad (Bufo bufo) skin epithelium with Ringer on both sides. Inside application of 5 microM isoproterenol resulted in the fast increase in G(T) from 1.2+/-0.3 to 2.4+/-0.4 mS x cm(-2) and slower increases in equivalent short circuit current, I(SC)(Eqv) = -G(T) x V(T), from 12.7+/-3.2 to 33.1+/-6.8 microA cm(-2), and J(V) from 0.72+/-0.17 to 3.01+/-0.49 nL cm(-2) s(-1). Amiloride in the outside solution abolished I(SC)(Eqv) (-1.6+/-0.1 microA cm(-2)) while J(V) decreased to 0.50+/-0.15 nL cm(-2) x s(-1), which is significantly different from zero. Isoproterenol decreased the osmotic concentration of the transported fluid, C(osm) approximately 2 x I(SC)(Eqv)/J(V), from 351+/-72 to 227+/-28 mOsm (Ringer's solution: 252.8 mOsm). J(V) depicted a saturating function of [Na+]out in agreement with Na+ self-inhibition of ENaC. Ouabain on the inside decreased I(SC)(Eqv) from 60+/-10 to 6.1+/-1.7 microA cm(-2), and J(V) from 3.34+/-0.47 to 1.40+/-0.24 nL cm(-2) x s(-1). Short-circuited preparations exhibited a linear relationship between short-circuit current and J(V) with a [Na+] of the transported fluid of 130+/-24 mM ([Na+]Ringer's solution = 117.4 mM). Addition of bumetanide to the inside solution reduced J(V). Water was transported uphill and J(V) reversed at an excess outside osmotic concentration, deltaC(S,rev) = 28.9+/-3.9 mOsm, amiloride decreased deltaC(S,rev) to 7.5+/-1.5 mOsm. It is concluded that water uptake is accomplished by osmotic coupling in the lateral intercellular space (lis), and hypothesized that a small fraction of the Na+ flux pumped into lis is recirculated via basolateral NKCC transporters.

Hans H. Ussing--scientific work: contemporary significance and perspectives

As a zoologist, Hans H. Ussing began his scientific career by studying the marine plankton fauna in East Greenland. This brought him in contact with August Krogh at the time George de Hevesy, Niels Bohr and Krogh planned the application of artificial radioactive isotopes for studying the dynamic state of the living organism. Following his studies of protein turnover of body tissues with deuterium-labeled amino acids, Ussing initiated a new era of studies of transport across epithelial membranes. Theoretical difficulties in the interpretation of tracer fluxes resulted in novel concepts such as exchange diffusion, unidirectional fluxes, flux-ratio equation, and solvent drag. Combining methods of biophysics with radioactive isotope technology, Ussing introduced and defined the phrases 'short-circuit current', 'active transport pathway' and 'shunt pathway', and with frog skin as experimental model, he unambiguously proved active transport of sodium ions. Conceived in his electric circuit analogue of frog skin, Ussing associated transepithelial ion fluxes with the hitherto puzzling 'bioelectric potentials'. The two-membrane hypothesis of frog skin initiated the study of epithelial transport at the cellular level and raised new questions about cellular mechanisms of actions of hormones and drugs. His theoretical treatment of osmotic water fluxes versus fluxes of deuterium labeled water resulted in the discovery of epithelial water channels. His discovery of paracellular transport in frog skin bridged studies of high and low resistance epithelia and generalized the description of epithelial transport. He devoted the last decade of his scientific life to solute-coupled water transport. He introduced the sodium recirculation theory of isotonic transport, and in an experimental study, he obtained the evidence for recirculation of sodium ions in toad small intestine. In penetrating analyses of essential aspects of epithelial membrane transport, Ussing provided insights of general applicability and powerful analytical methods for the study of intestine, kidney, respiratory epithelia, and exocrine glands-of equal importance to biology and medicine.

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.

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.

Paracellular transport of water and carbohydrates during intestinal perfusion of protamine in the rat

With these experiments, the authors' purpose was to determine whether the intestinal perfusion of protamine would successfully block paracellular transport without causing significant change in cardiovascular function. In anesthetized (50 mg x kg-1 sodium pentobarbital) rats (n=12), heart rate and mean arterial blood pressure were measured during perfusion (0.5 mL x min-1) of a carbohydrate-electrolyte solution through the small intestine. The carbohydrate-electrolyte solution contained 150 mM glucose, 150 mM fructose, 10 mM lactulose, 17 mEq sodium, 3 mEq potassium, and either 0.0, 0.1, 1.0, or 10 mg x mL-1 protamine. Osmolality of the 4 solutions ranged from 363 +/- 2 to 365 +/- 3 mOsm x kg-1. Core temperature was maintained at 37 degrees C in an environmental chamber. Heart rate and mean arterial blood pressure were constant during all intestinal perfusions. Forty-one percent of the perfused lactulose was absorbed. Absorption of glucose, fructose, and lactulose was significantly inhibited by 0.1 mg x mL-1 protamine, while water absorption was decreased 41 percent by 1.0 mg x mL-1 protamine. Water and lactulose absorption fell 75% with protamine, and glucose and fructose absorption fell 50%. Lactulose and fructose absorption did not decrease further when protamine dose rose to 10 mg x mL-1. These results indicate that 1) perfusion of protamine into the small intestine in doses that significantly affect intestinal transport does not significantly affect heart rate and mean arterial blood pressure; and 2) if the primary effect of protamine is to block paracellular movement of water and solute, the greater protamine inhibition of water and lactulose absorption is consistent with a greater paracellular transport of water and lactulose than for glucose and fructose.

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.

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.

Transport of inositol into the distal cauda epididymidis of the rat

The transport of [3H]myo-inositol into the lumen of the rat distal cauda epididymidis was studied by luminal perfusion in vitro. Entry was time and tubule length-dependent and saturable transport could be demonstrated with Vmax of 237 pmol/(30 min. cm) and K+ of 1 mM. Secretion of unlabeled inositol into the epididymal lumen was maintained for 5 hours at a concentration 6 times greater than that of the bathing solution. The turnover time of the epithelial pool of inositol was 4.5 hours, from which the intracellular concentration was estimated to be 26.6 mM. Transport was not reduced by metabolic inhibitors, and it was demonstrated that exchange diffusion across the basolateral membranes could drive the uptake in this region.

Solvent drag of sucrose during absorption indicates paracellular water flow in the rat kidney proximal tubule

Single convoluted proximal tubules of the rat kidney were lumen perfused in situ with isosmotic solutions containing C14-sucrose and H3-inulin as tracers, to evaluate whether the extracellular marker sucrose is entrained by water during proximal tubular reabsorption. Inulin was used as volume marker. The absorptive rate was varied by using as luminal perfusion fluids either a solution made up of (in mmole/l) 120 NaCl, 5 glucose, 25 NaHCO3 and altering the perfusion rate, or a solution containing 110 NaCl and 70 raffinose. Js, the net sucrose efflux is found to be a function of the net volume flow, Jv, such that at Jv = 0, Js is very small and at high rates of Jv, Js is over 60-fold the value observed at low Jv values. In addition, the transported to luminal sucrose concentrations decreased with Jv in a hyperbolic manner. Unstirred layers affect the diffusive component of Js, but only to a small extent. Therefore, the large remaining dependency of Js with Jv must be due to drag of sucrose by water, within the paracellular pathway. This leads to the conclusion that water flows through the paracellular pathway during absorption in the rat proximal tubule, in addition to transcellular water flow. Using equations for molecular sieving and the measured value of sigma s for sucrose of 0.76-0.91, it is calculated that the pathway where entrainment of solute by water occurs must be 1.0-1.1 nm wide. This calculation is only tentative since sigma s depends on the as yet unknown relative contribution of transcellular and paracellular pathways to transepithelial water osmotic permeability.

Comparison of transcellular and transepithelial water osmotic permeabilities (Pos) in the isolated proximal straight tubule (PST) of the rabbit kidney

Measurements of the water osmotic permeabilities of apical and basolateral membranes of PST cells and of the transepithelial permeability have been carried out using a very fast method with high temporal and spatial resolution. At 25 degrees C the values obtained are: 80.8 +/- 11.9 x 10(-4) cm3/s osmol cm2 of apical (luminal) surface area and 90.1 +/- 13.0 x 10(-4) cm3/s osmol cm2 of basement membrane area (no membrane invaginations taken in account). These values are higher than previously published values due to the use of a faster and more accurate volume measuring and recording system. The transepithelial water osmotic permeability at 25 degrees C is 77 +/- 11 in units of 10(-4) cm3/s osmol cm2 basement membrane area. The transcellular water osmotic permeability is 32 +/- 7 (same units), leaving a paracellular contribution of 45 +/- 10 (same units). In the presence of 2.5 mM parachloromercuribenzenesulfonate (pCMBS) the apical permeability is reduced with an incubation of 10-15 min to 23% of its control value and the basolateral permeability to 8% of its control value (after 25 min) but the transepithelial permeability is only reduced to about 1/2 of the control value. This leaves a transcellular permeability of 6 x 10(-4) cm3/s osmol cm2 of basement membrane area and a paracellular contribution of 33 +/- 6 (same units). These results indicate a significant contribution of the paracellular pathway to the transepithelial water osmotic permeabilities in PST.

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

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

Contribution of solvent drag through intercellular junctions to absorption of nutrients by the small intestine of the rat

The lumen of the small intestine in anesthetized rats was recirculated with 50 ml perfusion fluid containing normal salts, 25 mM glucose and low concentrations of hydrophilic solutes ranging in size from creatinine (mol wt 113) to Inulin (mol wt 5500). Ferrocyanide, a nontoxic, quadrupally charged anion was not absorbed; it could therefore be used as an osmotically active solute with reflection coefficient of 1.0 to adjust rates of fluid absorption, Jv, and to measure the coefficient of osmotic flow, Lp. The clearances from the perfusion fluid of all other test solutes were approximately proportional to Jv. From Lp and rates of clearances as a function of Jv and molecular size we estimate (a) the fraction of fluid absorption which passes paracellularly (approx. 50%), (b) coefficients of solvent drag of various solutes within intercellular junctions, (c) the equivalent pore radius of intercellular junctions (50 A) and their cross sectional area per unit path length (4.3 cm per cm length of intestine). Glucose absorption also varied as a function of Jv. From this relationship and the clearances of inert markers we calculate the rate of active transport of glucose, the amount of glucose carried paracellularly by solvent drag or back-diffusion at any given Jv and luminal glucose concentration and the concentration of glucose in the absorbate. The results indicate that solvent drag through paracellular channels is the principal route for intestinal transport of glucose or amino acids at physiological rates of fluid absorption and concentration. In the absence of luminal glucose the rate of fluid absorption and the clearances of all inert hydrophilic solutes were greatly reduced. It is proposed that Na-coupled transport of organic solutes from lumen to intercellular spaces provides the principal osmotic force for fluid absorption and triggers widening of intercellular junctions, thus promoting bulk absorption of nutrients by solvent drag. Further evidence for regulation of channel width is provided in accompanying papers on changes in electrical impedance and ultrastructure of junctions during Na-coupled solute transport.

Solute permeability of the alveolar capillary barrier

Images

Passive water flows driven across the isolated rabbit ileum by osmotic, hydrostatic and electrical gradients

Water flows generated by osmotic and hydrostatic pressure and electrical currents were measured in sheets of isolated rabbit ileum at 20 degrees C. Flows across the mucosal and serosal surfaces were monitored continuously by simultaneous measurement of tissue volume change (with an optical lever) and net water flows across one surface of the tissue (with a capacitance transducer). Osmotic gradients were imposed across the mucosal and serosal surfaces of the tissue separately, using probe molecules of various sizes from ethanediol (68 Da) to dextrans (161 000 Da). Flows across each surface were elicited with very short delay. The magnitudes of the flows were proportional to the osmotic gradient and related to the size of the probe molecule. Osmotic flow across the mucosal surface was associated with streaming potentials which were due to electro-osmotic water flow. The mucosal surface is a heteroporous barrier with narrow (0.7 nm radius, Lp (hydraulic conductivity) = (7.6 +/- 1.6) X 10(-9) cm s-1 cmH2O-1) cation-selective channels in parallel with wide neutral pores (ca. 6.5 nm radius, Lp = (2.3 +/- 0.2) X 10(-7) cm s-1 cmH2O-1) which admit large pressure-driven backflows from the submucosa to the lumen. There is additional evidence for a further set of narrow electroneutral pores less than 0.4 nm radius with Lp less than 7 X 10(-9) cm s-1 cmH2O-1. The serosal surface has neutral pores of uniform radius (ca. 6.5 nm), Lp = (7.6 +/- 1.6) X 10(-8) cm s-1 cmH2O-1. Hypertonic serosal solutions (100 mM-sucrose) cause osmotic transfer of fluid from isotonic mucosal solutions into the submucosa, expand it, and elevate the tissue pressure to 19.6 +/- 3.2 cmH2O (n = 4). Conversely, hypertonic mucosal solutions (100 mM-sucrose) draw fluid out of the submucosa in the presence of isotonic serosal solutions, collapse the submucosa, and lower the tissue pressure to -87.7 +/- 4.6 cmH2O (n = 5). Water flows coupled to cation movement could be generated across the mucosal surface in both directions by brief direct current pulses. The short latency of onset and cessation of flow (less than 2 s), absence of polarization potentials, and high electro-osmotic coefficients (range 50-520 mol water F-1), together with the presence of streaming potentials during osmotically generated water flows indicate electro-osmotic water flow through hydrated channels in the tight junctions and/or lateral intercellular spaces.(ABSTRACT TRUNCATED AT 400 WORDS)

Pathways for volume flow and volume regulation in leaky epithelia

Continuous pathways must pierce the cell membrane to be used by water during osmotic equilibration between proximal straight tubular cells and the external medium, because a) the water osmotic permeability coefficient of the basolateral plasma membrane, Poscb, is high; b) its activation energy, Ea, is as that of free water movement and c) pCMBS inhibits markedly (but reversibly) Poscb and increases Ea to values similar to those observed in lipid bilayers without pores. d) Preliminary measurements of Pd the water diffusive permeability coefficient using NMR indicate that Poscb/Pd is near 4 - 5. The following two observations indicate that a significant paracellular water flow must exist in leaky epithelia. Namely, a) large extracellular solutes are dragged by water in four leaky epithelia: gall bladder, Necturus proximal tubule, rat proximal tubule and Rhodnius malpighian tubule. b) The transcellular water osmotic permeability coefficient is smaller than the transepithelial values available in the rabbit proximal straight tubule. This requires a significant paracellular permeability.

Osmotic water permeability of the apical membrane of proximal straight tubular (PST) cells

Osmotic steps, delta C, were produced across the apical cell membrane of isolated rabbit PST by perfusing their lumens with double barreled micropipettes at a rate of 0.5-0.8 nl/s. delta C = 15-46 mOsmolar were induced with mannitol. Changes in luminal diameter were recorded as a function of time with a TV camera and an integrator-processor system with space and time resolutions of 0.03 micron and 0.0167 s (3). The tubules were bathed with oil. Outer tubule diameter was time invariant. Pcaos, the apical cell osmotic permeability was therefore calculated from cell volume changes with time in units of 10(-4) cm3/cm2 X s. Osmolar. Pcaos was independent of delta C. The mean is 22.8 +/- 1.3 (n = 55). With a basolateral permeability of 50.4 (3,12), the transcellular permeability is 14 (same units) smaller than the transepithelial values available. This leads to the conclusion that a significant paracellular water osmotic permeability must exist.

Active sodium transport and fluid secretion in the gall-bladder epithelium of Necturus

Intracellular Na, K and Cl activities (acNa, acK and acCl) and membrane potentials were measured in Necturus gall-bladder epithelium using double-barrelled ion-sensitive micro-electrodes. Mucosal membrane potential was about -55 mV and the mean control activities were acNa = 14.7 mM, acK = 91.6 mM and acCl = 20.3 mM. Replacing mucosal Na by K caused a fall in acNa that followed an exponential time course. The rate of change in acNa was linearly related to acNa above a certain value (congruent to 3 mM). acK and acCl both increased in K Ringer solution. From the change in all three ions the cell was estimated to swell at an initial rate of 0.13% s-1. From the initial rate of change in acNa, a net cell efflux of Na of 405 pmol cm-2 s-1 was calculated. Replacement of Na by Tris or choline led to a similar result. The transepithelial Na transport rate was for this group of animals 346 pmol cm-2 s-1. Ouabain (10(-3) M) produced an increase in acNa and acCl, whereas acK decreased. The cells were estimated to swell at an initial rate of 0.06% s-1. The initial Na influx after Na-pump inhibition was calculated to be 162 pmol cm-2 s-1. The parallel measure of the transepithelial rate of transport of Na gave a value of 189 pmol cm-2 s-1. Ouabain inhibited the decrease in acNa after replacement of Na by K by about 80%. A fast depolarization, ranging from 2 to 7 mV, occurred after the perfusion with ouabain. Em then slowly decreased from about 53 to 32 mV in 1 h. It is concluded that (a) the major fraction of the transepithelial transport of Na is transcellular and mediated by the Na pump, (b) the pumping rate is linearly dependent on internal Na within a certain range and (c) the Na pump is electrogenic under normal circumstances.

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

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

Ion activities in the lateral intercellular spaces of gallbladder epithelium transporting at low external osmolarities

The ion activities in the lateral spaces of the unilateral preparation of the gallbladder of Rana catesbiana were measured by double-barrelled ion-selective microelectrodes. The bladders were bathed in a saline solution with a low osmolarity (62 mOsm) containing, in mM: 27 Na+, 27 Cl-, 2 K+, 1 Ca++, 4 HCO3-. Working at reduced osmolarities had the advantage of an increased volume transport and of widened intercellular spaces. The reference barrel recorded an electrical potential of +2.7 mV in the spaces; they contained a solution similar to the external solution. The electrodes recorded a Na+ concentration of 27 mM, a K+ concentration of 1.7 mM, a Ca++ concentration of 0.69 mM and a Cl- concentration of 28.5 mM. In the spaces there was a lower resistance between the tip of the electrode and the serosal bath than that recorded with the tip in the lumen, and injection of fluorescent dye (11 A diameter) via the electrodes did not stain the cells. The concentrations in the secretion were similar to those in the spaces. The intracellular compartment had an apparent K+ concentration of 95 mM, and the concentrations of Na+ and Cl- were both about 5 mM. These data indicate that when the gallbladder is bathed with hypotonic solutions and is transporting fluid at approximately three or four times the normal rate, there are no significant osmotic gradients between the lumen and the lateral spaces. It is suggested that transcellular transport of water is implemented by a combination of high osmotic permeabilities across both mucosal and serosal cell membranes and low reflection coefficients (for K+ salts) at the serosal cell membranes.

Does water drag solutes through kidney proximal tubule?

Coupling of salt and water movements across kidney proximal tubules was studied in the presence of an induced transepithelial osmotic water flux. Convoluted proximal tubules from rabbit kidney were perfused in vitro with a control solution, with or without 50 mM/l of mannitol or raffinose in the both. Osmolalities of the perfused and collected fluids as well as the net water flux Jv were measured in each experiment. The net solute flux Js was calculated from the difference between the amount of total solutes delivered and collected at each end of the tubule. No apparent net transepithelial solute movements were detected in the presence of an osmotic water flux when active solute transport was inhibited either by an external to of 26 degrees C or by ouabain in the bath. The water flux observed was similar to that calculated assuming that only water crossed the epithelium, and no streaming potential was measured, whether or not active transport was blocked. It is concluded that the osmotic water flux through kidney proximal tubule does not drag a significant amount of solutes, probably because of the absence of convective solvent flux. This suggests the existence of different pathways for water and salt movement.

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

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

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

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

Salt-water coupling in leaky epithelia

The theory of quasi-isotonic transport by cellular osmosis (the standing-gradient theory) has been challenged on the grounds that the osmotic permeabilities of the mucosal and interspace membranes are too low; if they were as high as the theory requires then the osmotic permeability of the whole epithelium would be 2-3 orders of magnitude higher than observed. This objection has basically been accepted for it is now claimed that these enormous permeabilities do exist, but are masked by unstirred-layer effects; I show that this is incorrect because unstirred-layer corrections are small and that the situation has not changed since 1975. The view that the route of fluid transport is junctional is replacing the cellular theory, and trans-junctional water flows seem to account for major fractions of the flow in various epithelia. I argue on grounds of general theory that these are unlikely to be osmotic flows because the junctional pores cannot satisfy both the osmotic and diffusive properties required of them, but the basic osmotic theory is also rather vague here. Non-osmotic theories, if junctional flow is accepted, must be either electro-kinetic or peristaltic.