Sodium recirculation and isotonic transport in toad small intestine

Biophysical Analysis of a Minimalistic Kidney Model Expressing SGLT1 Reveals Crosstalk between Luminal and Lateral Membranes and a Plausible Mechanism of Isosmotic Transport

We extended our model of the S1 tubular segment to address the mechanisms by which SGLT1 interacts with lateral Na/K pumps and tight junctional complexes to generate isosmotic fluid reabsorption via tubular segment S3. The strategy applied allowed for simulation of laboratory experiments. Reproducing known experimental results constrained the range of acceptable model outputs and contributed to minimizing the free parameter space. (1) In experimental conditions, published Na and K concentrations of proximal kidney cells were found to deviate substantially from their normal physiological levels. Analysis of the mechanisms involved suggested insufficient oxygen supply as the cause and, indirectly, that a main function of the Na/H exchanger (NHE3) is to extrude protons stemming from mitochondrial energy metabolism. (2) The water path from the lumen to the peritubular space passed through aquaporins on the cell membrane and claudin-2 at paracellular tight junctions, with an additional contribution to water transport by the coupling of 1 glucose:2 Na:400 H2O in SGLT1. (3) A Na-uptake component passed through paracellular junctions via solvent drag in Na- and water-permeable claudin-2, thus bypassing the Na/K pump, in agreement with the findings of early studies. (4) Electrical crosstalk between apical rheogenic SGLT1 and lateral rheogenic Na/K pumps resulted in tight coupling of luminal glucose uptake and transepithelial water flow. (5) Isosmotic transport was achieved by Na-mediated ion recirculation at the peritubular membrane.

Salt-water acclimation of the estuarine crocodile Crocodylus porosus involves enhanced ion transport properties of the urodaeum and rectum

Estuarine crocodiles, Crocodylus porosus, inhabit freshwater, estuarine and marine environments. Despite being known to undertake extensive movements throughout and between hypo-osmotic and hyperosmotic environments, little is known about the role of the cloaca in coping with changes in salinity. We report here that, in addition to the well-documented functional plasticity of the lingual salt glands, the middle of the three cloacal segments (i.e. the urodaeum) responds to increased ambient salinity to enhance solute-coupled water absorption. This post-renal modification of urine serves to conserve water when exposed to hyperosmotic environments and, in conjunction with lingual salt gland secretions, enables C. porosus to maintain salt and water balance and thereby thrive in hyperosmotic environments. Isolated epithelia from the urodaeum of 70% seawater-acclimated C. porosus had a strongly enhanced short-circuit current (an indicator of active ion transport) compared with freshwater-acclimated crocodiles. This enhanced active ion absorption was driven by increased Na+/K+-ATPase activity, and possibly enhanced proton pump activity, and was facilitated by the apical epithelial Na+ channel (ENaC) and/or the apical Na+/H+ exchanger (NHE2), both of which are expressed in the urodaeum. NHE3 was expressed at very low levels in the urodaeum and probably does not contribute to solute-coupled water absorption in this cloacal segment. As C. porosus does not appear to drink water of salinities above 18 ppt, observations of elevated short-circuit current in the rectum as well as a trend for increased NHE2 expression in the oesophagus, the anterior intestine and the rectum suggest that dietary salt intake may stimulate salt and possibly water absorption by the gastrointestinal tract of C. porosus living in hyperosmotic environments.

Stationary and Nonstationary Ion and Water Flux Interactions in Kidney Proximal Tubule: Mathematical Analysis of Isosmotic Transport by a Minimalistic Model

Our mathematical model of epithelial transport (Larsen et al. Acta Physiol. 195:171–186, 2009) is extended by equations for currents and conductance of apical SGLT2. With independent variables of the physiological parameter space, the model reproduces intracellular solute concentrations, ion and water fluxes, and electrophysiology of proximal convoluted tubule. The following were shown:

Paracellular calcium transport across renal and intestinal epithelia

Calcium (Ca(2+)) is a key constituent in a myriad of physiological processes from intracellular signalling to the mineralization of bone. As a consequence, Ca(2+) is maintained within narrow limits when circulating in plasma. This is accomplished via regulated interplay between intestinal absorption, renal tubular reabsorption, and exchange with bone. Many studies have focused on the highly regulated active transcellular transport pathways for Ca(2+) from the duodenum of the intestine and the distal nephron of the kidney. However, comparatively little work has examined the molecular constituents creating the paracellular shunt across intestinal and renal epithelium, the transport pathway responsible for the majority of transepithelial Ca(2+) flux. More specifically, passive paracellular Ca(2+) absorption occurs across the majority of the intestine in addition to the renal proximal tubule and thick ascending limb of Henle's loop. Importantly, recent studies demonstrated that Ca(2+) transport through the paracellular shunt is significantly regulated. Therefore, we have summarized the evidence for different modes of paracellular Ca(2+) flux across renal and intestinal epithelia and highlighted recent molecular insights into both the mechanism of secondarily active paracellular Ca(2+) movement and the identity of claudins that permit the passage of Ca(2+) through the tight junction of these epithelia.

Functional characterization of water transport and cellular localization of three aquaporin paralogs in the salmonid intestine

Intestinal water absorption is greatly enhanced in salmonids upon acclimation from freshwater (FW) to seawater (SW); however, the molecular mechanism for water transport is unknown. We conducted a pharmacological characterization of water absorption in the rainbow trout intestine along with an investigation of the distribution and cellular localization of three aquaporins (Aqp1aa, -1ab, and -8ab) in pyloric caeca, middle (M), and posterior (P) intestine of the Atlantic salmon. In vitro iso-osmotic water absorption (J(v)) was higher in SW than FW-trout and was inhibited by (mmol L(-1)): 0.1 KCN (41%), 0.1 ouabain (72%), and 0.1 bumetanide (82%) suggesting that active transport, Na(+), K(+)-ATPase and Na(+), K(+), 2Cl(-)-co-transport are involved in establishing the driving gradient for water transport. J(v) was also inhibited by 1 mmol L(-1) HgCl(2), serosally (23% in M and 44% in P), mucosally (27% in M), or both (61% in M and 58% in P), suggesting involvement of both apical and basolateral aquaporins in water transport. The inhibition was antagonized by 5 mmol L(-1) mercaptoethanol. By comparison, 10 mmol L(-1) mucosal tetraethylammonium, an inhibitor of certain aquaporins, inhibited J(v) by 20%. In the presence of glucose, mucosal addition of phloridzin inhibited water transport by 20%, suggesting that water transport is partially linked to the Na(+)-glucose co-transporter. Using polyclonal antibodies against salmon Aqp1aa, -1ab, and -8ab, we detected Aqp1aa, and -1ab immunoreactivity in the brush border and sub-apical region of enterocytes in all intestinal segments. The Aqp8ab antibody showed a particularly strong immunoreaction in the brush border and sub-apical region of enterocytes throughout the intestine and also stained lateral membranes and peri-nuclear regions though at lower intensity. The present localization of three aquaporins in both apical and lateral membranes of salmonid enterocytes facilitates a model for transcellular water transport in the intestine of SW-acclimated salmonids.

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.

The involvement of H+-ATPase and carbonic anhydrase in intestinal HCO3- secretion in seawater-acclimated rainbow trout

Pyloric caeca and anterior intestine epithelia from seawater-acclimated rainbow trout exhibit different electrophysiological parameters with lower transepithelial potential and higher epithelial conductance in the pyloric caeca than the anterior intestine. Both pyloric caeca and the anterior intestine secrete HCO(3)(-) at high rates in the absence of serosal HCO(3)(-)/CO(2), demonstrating that endogenous CO(2) is the principal source of HCO(3)(-) under resting control conditions. Apical, bafilomycin-sensitive, H(+) extrusion occurs in the anterior intestine and probably acts to control luminal osmotic pressure while enhancing apical anion exchange; both processes with implications for water absorption. Cytosolic carbonic anhydrase (CAc) activity facilitates CO(2) hydration to fuel apical anion exchange while membrane-associated, luminal CA activity probably facilitates the conversion of HCO(3)(-) to CO(2). The significance of membrane-bound, luminal CA may be in part to reduce HCO(3)(-) gradients across the apical membrane to further enhance anion exchange and thus Cl(-) absorption and to facilitate the substantial CaCO(3) precipitation occurring in the lumen of marine teleosts. In this way, membrane-bound, luminal CA thus promotes the absorption of osmolytes and reduction on luminal osmotic pressure, both of which will serve to enhance osmotic gradients to promote intestinal water absorption.

Occludin and hydromineral balance in Xenopus laevis

To investigate the response of the tight junction (TJ) protein occludin to environmental change in an anuran amphibian, we examined occludin tissue distribution, immunolocalization and alterations in mRNA expression in African clawed frogs (Xenopus laevis) acclimated to brackish water (BW) conditions (from freshwater to 2 per thousand, 5 per thousand or 10 per thousand salt water). Occludin mRNA is widely expressed in Xenopus and is abundant in tissues involved in regulating salt and water balance, such as the gastrointestinal (GI) tract, kidney and urinary bladder. Immunohistochemical analyses revealed strong occludin immunolabelling in the apicolateral region of epithelia lining the GI tract and mRNA expression increased along the longitudinal axis of the gut. In kidney tissue, occludin was differentially expressed on the luminal side of the nephron tubule, appearing in the distal tubules and collecting ducts only. In response to BW acclimation, Xenopus exhibited a significant loss of tissue water as well as salinity-dependent elevations in serum osmolality as a result of increased urea levels followed by elevated serum Na(+) and Cl(-) levels. Tissue-specific alterations in the ionomotive enzyme Na(+),K(+)-ATPase were also observed in Xenopus in response to BW acclimation. Most notably, Na(+),K(+)-ATPase activity in the rectum increased in response to elevated environmental salt concentrations while renal activity decreased. Furthermore, acclimation to BW caused tissue-specific and salinity-dependent alterations in occludin mRNA expression within select Xenopus osmoregulatory organs. Taken together, these studies suggest that alterations in occludin, in conjunction with active transport processes, may contribute to amphibian hydromineral homeostasis during environmental change.

Local osmosis and isotonic transport

Osmotically driven water flow, u (cm/s), between two solutions of identical osmolarity, c(o) (300 mM: in mammals), has a theoretical isotonic maximum given by u = j/c(o), where j (moles/cm(2)/s) is the rate of salt transport. In many experimental studies, transport was found to be indistinguishable from isotonic. The purpose of this work is to investigate the conditions for u to approach isotonic. A necessary condition is that the membrane salt/water permeability ratio, epsilon, must be small: typical physiological values are epsilon = 10(-3) to 10(-5), so epsilon is generally small but this is not sufficient to guarantee near-isotonic transport. If we consider the simplest model of two series membranes, which secrete a tear or drop of sweat (i.e., there are no externally-imposed boundary conditions on the secretion), diffusion is negligible and the predicted osmolarities are: basal = c(o), intracellular approximately (1 + epsilon)c(o), secretion approximately (1 + 2epsilon)c(o), and u approximately (1 - 2epsilon)j/c(o). Note that this model is also appropriate when the transported solution is experimentally collected. Thus, in the absence of external boundary conditions, transport is experimentally indistinguishable from isotonic. However, if external boundary conditions set salt concentrations to c(o) on both sides of the epithelium, then fluid transport depends on distributed osmotic gradients in lateral spaces. If lateral spaces are too short and wide, diffusion dominates convection, reduces osmotic gradients and fluid flow is significantly less than isotonic. Moreover, because apical and basolateral membrane water fluxes are linked by the intracellular osmolarity, water flow is maximum when the total water permeability of basolateral membranes equals that of apical membranes. In the context of the renal proximal tubule, data suggest it is transporting at near optimal conditions. Nevertheless, typical physiological values suggest the newly filtered fluid is reabsorbed at a rate u approximately 0.86 j/c(o), so a hypertonic solution is being reabsorbed. The osmolarity of the filtrate c(F) (M) will therefore diminish with distance from the site of filtration (the glomerulus) until the solution being transported is isotonic with the filtrate, u = j/c(F).With this steady-state condition, the distributed model becomes approximately equivalent to two membranes in series. The osmolarities are now: c(F) approximately (1 - 2epsilon)j/c(o), intracellular approximately (1 - epsilon)c(o), lateral spaces approximately c(o), and u approximately (1 + 2epsilon)j/c(o). The change in c(F) is predicted to occur with a length constant of about 0.3 cm. Thus, membrane transport tends to adjust transmembrane osmotic gradients toward epsilonc(o), which induces water flow that is isotonic to within order epsilon. These findings provide a plausible hypothesis on how the proximal tubule or other epithelia appear to transport an isotonic solution.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

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.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Effects of carprofen on the integrity and barrier function of canine colonic mucosa

OBJECTIVE

To measure effects of carprofen on conductance and permeability to mannitol and histologic appearance in canine colonic mucosa.

SAMPLE POPULATION

Colonic mucosa from 13 mature mixed-breed dogs. Procedures-Sections of mucosa from the transverse colon and proximal and distal portions of the descending colon were obtained immediately after dogs were euthanized. Sections were mounted in Ussing chambers. Carprofen (400 microg/mL) was added to the bathing solution for treated sections. Conductance was calculated at 15-minute intervals for 240 minutes. Flux of mannitol was calculated for three 1-hour periods. Histologic examination of sections was performed after experiments concluded. Conductance was graphed against time for each chamber, and area under each curve was calculated. Conductance X time, flux of mannitol, and frequency distribution of histologic findings were analyzed for an effect of region and carprofen.

RESULTS

Carprofen significantly increased mean conductance X time, compared with values for control (untreated) sections for all regions of colon. Carprofen significantly increased mean flux of mannitol from period 1 to period 2 and from period 2 to period 3 for all regions of colon. Carprofen caused a significant proportion of sections to have severe sloughing of cells and erosions involving >or= 10% of the epithelium, compared with control sections.

CONCLUSIONS AND CLINICAL RELEVANCE

Carprofen increased in vitro conductance and permeability to mannitol in canine colonic mucosa. Carprofen resulted in sloughing of cells and erosion of the colonic mucosa. These findings suggested that carprofen can compromise the integrity and barrier function of the colonic mucosa of dogs.

Application of the Na+ recirculation theory to ion coupled water transport in low- and high resistance osmoregulatory epithelia

The theory of Na+ recirculation for isosmotic fluid absorption follows logically from Hertz's convection-diffusion equation applied to the exit of water and solutes from the lateral intercellular space. Experimental evidence is discussed indicating Na+ recirculation based upon the following approaches: (i) An isotope tracer method in small intestine. Simultaneous measurement of water flow and ion transport in toad skin epithelium demonstrating, (ii) occasional hyposmotic absorbates, and (iii) reduced fluid absorption in the presence of serosal bumetanide. (iv) Studies of the metabolic cost of net Na+ absorption demonstrating an efficiency that is lower than the 18 Na+ per O2 consumed given by the stoichiometry of the Na+/K+-pump. Mathematical modeling 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, the adverse relationship between hydraulic permeability and the concentration difference needed to reverse transepithelial water flow, and in a non-contradictory way the wide range of metabolic efficiencies from above to below 18 Na+/O2. Certain types of observations are poorly or not at all reproduced by the model. It is discussed that such lack of agreement between model and experiment is due to cellular regulations of ion permeabilities that are not incorporated in the modeling. Clarification of these problems requires further experimental studies.

Interaction between transcellular and paracellular water transport pathways through Aquaporin 5 and the tight junction complex

To investigate potential physiological interactions between the transcellular and paracellular pathways of water transport, we asked whether targeted deletion of Aquaporin 5 (AQP5), the major transcellular water transporter in salivary acinar cells, affected paracellular transport of 4-kDa FITC-labeled dextran (FITC-D), which is transported through the paracellular but not the transcellular route. After i.v. injection of FITC-D into either AQP5 wild-type or AQP5-/- mice and saliva collection for fixed time intervals, we show that the relative amount of FITC-D transported in the saliva of AQP5-/- mice is half that in matched AQP5+/+ mice, indicating a 2-fold decrease in permeability of the paracellular barrier in mice lacking AQP5. We also found a significant difference in the proportion of transcellular vs. paracellular transport between male and female mice. Freeze-fracture electron microscopy revealed an increase in the number of tight junction strands of both AQP5+/+ and AQP5-/- male mice after pilocarpine stimulation but no change in strand number in female mice. Average acinar cell volume was increased by approximately 1.4-fold in glands from AQP5-/- mice, suggesting an alteration in the volume-sensing machinery of the cell. Western blots revealed that expression of Claudin-7, Claudin-3, and Occludin, critical proteins that regulate the permeability of the tight junction barrier, were significantly decreased in AQP5-/- compared with AQP5+/+ salivary glands. These findings reveal the existence of a gender-influenced molecular mechanism involving AQP5 that allows transcellular and paracellular routes of water transport to act in conjunction.

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.

Intestinal anion exchange in marine fish osmoregulation

Despite early reports, dating back three quarters of a century, of high total CO(2) concentrations in the intestinal fluids of marine teleost fishes, only the past decade has provided some insight into the functional significance of this phenomenon. It is now being recognized that intestinal anion exchange is responsible for high luminal HCO(3)(-) and CO(3)(2-) concentrations while at the same time contributing substantially to intestinal Cl(-) and thereby water absorption, which is vital for marine fish osmoregulation. In species examined to date, the majority of HCO(3)(-) secreted by the apical anion exchange process is derived from hydration of metabolic CO(2) with the resulting H(+) being extruded via a Na(+):H(+) exchange mechanism in the basolateral membrane. The basolateral H(+) extrusion is critical for the apical anion exchange and relies on the Na(+) gradient established by the Na(+)-K(+)-ATPase. This enzyme thereby ultimately fuels the secondary active transport of HCO(3)(-) and Cl(-) by the apical anion exchanger. High cellular HCO(3)(-) concentrations (>10 mmol l(-1)) are required for the anion exchange process and could be the result of both a high metabolic activity of the intestinal epithelium and a close association of the anion exchange protein and the enzyme carbonic anhydrase. The anion exchange activity in vivo is likely most pronounced in the anterior segment and results in net intestinal acid absorption. In contrast to other water absorbing vertebrate epithelia, the marine teleost intestine absorbs what appears to be a hypertonic fluid to displace diffusive fluid loss to the marine environment.

Fluid transport and ion fluxes in mammalian kidney proximal tubule: a model analysis of isotonic transport

AIM

By mathematical modelling, we analyse conditions for near-isotonic and isotonic transport by mammalian kidney proximal tubule.

METHODS

The model comprises compliant lateral intercellular space (lis) and cells, and infinitely large luminal and peritubular compartments with diffusible species: Na+, K+, Cl- and an intracellular non-diffusible anion. Unknown model variables are solute concentrations, electrical potentials, volumes and hydrostatic pressures in cell and lis, and transepithelial potential. We used data mainly from rat proximal tubule to model epithelial cells and interspace with luminal and peritubular baths of identical composition.

RESULTS

The model of the tubular epithelium with physiological water permeability and paracellular electrical resistance generates solute coupled water uptake with an approx. 3% hypertonic absorbate. This function remains unperturbed following 'blocking' of apical water channels and in 'aquaporin-null' simulation. Reduced rate of volume reabsorption in AQP(-/-) mice would also require decreased apical sodium permeability. Paracellular convection accounts for approx. 36% of the net Na+ absorption, and the model epithelium accomplishes uphill water transport similar to rat proximal tubule. Na+ recirculation is required for truly isotonic transport. The tonicity of the absorbate and the recirculation flux depend critically on ion permeabilities of interspace basement membrane.

CONCLUSION

Our model based on solute-solvent coupling in lateral space simulates major physiological features of proximal tubule, including significantly lower water permeability of the AQP1-null preparation, and a ratio of net sodium uptake and oxygen consumption exceeding that predicted from stoichiometry of the Na+/K+-pump. Physical properties of interspace basement membrane are critical for obtaining near-isotonic and truly isotonic transport.

Short-term distribution of 134Cs in relation to 51Cr-EDTA after intravenous dose in goats

AIM

The aim of this study was to gather information about the short-term rate of caesium uptake (incorporation) in different animal tissues and explain them with known physiological mechanisms affecting ion distribution.

METHODS

Six goats were given an intravenous bolus containing (134)Cs as a tracer and (51)Cr-EDTA as an extracellular marker. After 30 min, the animals were killed and the activity concentration of radioactive isotopes in different tissues and fluid compartments were measured.

RESULTS

The highest relative activity concentration of (134)Cs was found in kidney cortex, with a tissue/plasma-ratio around 50. In urine, the ratio varied between 5 and 28. In the salivary gland, cardiac muscle and small intestine the ratio was around 11, 7 and 6, respectively. The contents of small intestine had an average activity concentration five times that of plasma. In skeletal muscle the terminal activity concentration was surprisingly low, with a tissue/plasma ratio mostly far less than unity. Even in connective tissue and cartilage the terminal activity concentration was generally higher than in skeletal muscle.

CONCLUSION

The rate of uptake of caesium varies widely from tissue to tissue. Many of these differences can be explained with differences in Na,K-ATPase activity. Also, perfusion and accessibility play a role in some tissues, like brain and possibly part of the skeletal muscles. The short-term distribution of caesium differs distinctly from the long-term distribution reported in literature.

Postmortem examination of transepithelial ion currents in rabbit colon and trachea in relation to temperature of storage and its importance for interlethal reactions

Electrical phenomena resulting from transepithelial ion transport have been a subject of clinical, physiological, pharmacologic and toxicologic studies. These examinations concern mainly electric phenomena in live organisms. The changes of transepithelial ion pathways which take place postmortem have not been yet established. The aim of the study was an attempt to trace variability of electrophysiological parameters related to transepithelial ion transport in specimens of rabbit trachea and colon depending on temperature at which specimens were stored after death. It was observed that postmortem electric phenomena in epithelium of airways and alimentary tract of rabbit occur well-ordered but with slower course in trachea samples and in tissues which were preserved at low temperature after death.

Prolactin directly stimulated the solvent drag-induced calcium transport in the duodenum of female rats

Prolactin has been reported to stimulate the calcium absorption of the duodenum where three components of the active calcium transport, namely transcellular active, voltage-dependent and solvent drag-induced calcium transport, were identified. It was known that the transcellular active, but not the voltage-dependent, duodenal calcium transport was directly stimulated by prolactin. The present study thus aimed to evaluate the direct action of prolactin on the solvent drag-induced duodenal calcium transport by using the Ussing chamber technique. The jejunum was used as a reference for the existence of solvent drag and the widening of tight junction induced by cytochalasin E. Results showed that the solvent drag-induced calcium transport existed in both intestinal segments, but the magnitude was significantly greater in the duodenum (29.27+/-2.27 vs. 17.31+/-1.65 nmol h(-1) cm(-2), P<0.001). We further demonstrated that 200, 600 and 800, but not 1000 ng/ml, prolactin significantly promoted the solvent drag-induced duodenal calcium transport in a dose-response manner, i.e. from the control value of (nmol h(-1) cm(-2)) 24.31+/-2.36 to 45.42+/-3.47 (P<0.01), 63.82+/-5.28 (P<0.001) and 53.93+/-5.41 (P<0.01), respectively. However, prolactin did not manifest any effect on the jejunum. Because the paracellular transport was suggested to be size-selective as well as charge-selective, further experiments were designed to evaluate the mechanism by which prolactin stimulated the solvent drag-induced calcium transport. The duodenum was exposed to 20 microM cytochalasin E, 600 ng/ml prolactin or the combination of both in the presence of a paracellular marker 3H-mannitol, while the jejunum was a positive reference. The results showed that, in the jejunum, cytochalasin E alone and cytochalasin E plus prolactin significantly increased the mannitol fluxes from (micromol h(-1) cm(-2)) 0.29+/-0.04 to 0.49+/-0.03 (P<0.05) and 0.48+/-0.05 (P<0.05), respectively, while having no effect on the calcium fluxes. Prolactin alone had no effect on the jejunal calcium flux. In the duodenum, neither mannitol nor calcium fluxes were enhanced by cytochalasin E, however, prolactin still increased the solvent drag-induced calcium flux from 27.74+/-2.41 to 51.03+/-4.35 nmol h(-1) cm(-2) (P<0.001). It was concluded that prolactin directly stimulated the solvent drag-induced duodenal calcium transport in a dose-response and biphasic manner without the widening of tight junction.

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.

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.