Na+ Recirculation and Isosmotic Transport

Gulf toadfish (Opsanus beta) urinary bladder ion and water transport is enhanced by acclimation to higher salinity to serve water balance

Marine teleosts experience ion gain and water loss in their natural habitats. Among other tissues, the urinary bladder epithelium of marine fishes has been shown to actively transport ions to facilitate water absorption. However, transport properties of the urinary bladder epithelium of marine fishes and its plasticity in altered ambient salinities is relatively under-investigated. We describe urinary bladder epithelium electrophysiology, water flux, and expressions of ion transporters in urinary bladder tissue of Gulf toadfish (Opsanus beta) acclimated to either 35 ppt or 60 ppt seawater. Water absorption in bladder sac preparations increased ∼350% upon acclimation to 60 ppt. Increases in water transport coincided with a significant ∼137% increase in urinary bladder tissue mucosal-to-serosal short circuit current (Isc) and a ∼56% decrease in tissue membrane resistance. Collectively, these metrics indicate that an active electrogenic system facilitates water absorption via Na+ (and Cl-) transport in urinary bladder tissue. Furthermore, pharmacological inhibition of urinary bladder tissue Isc and expression of a suite of ion transporters and channels previously unidentified in this tissue provide mechanistic insights into the transport processes responsible for water flux. Analysis of water transport to overall Gulf toadfish water balance reveals a modest water conservation role for the urinary bladder of ∼0.5% of total water absorption in 35 ppt and 1.9% in 60 ppt acclimated toadfish. These results emphasize that electrogenic ion transport facilitates water-absorptive properties of the urinary bladder in Gulf toadfish-a process that is regulated to facilitate water homeostasis.NEW & NOTEWORTHY Novel experiments showcasing increased urinary bladder water absorption, ion transport, and altered channel/transporter expression in a marine fish acclimated to high salinities. Our results provide additional and noteworthy mechanistic insight into the ionoregulatory processes controlling water transport at the level of the urinary bladder in marine teleosts. Experimental outcomes are applied to whole organism-level water transport values, and the relative importance of marine teleost urinary bladder function to overall organism water conservatory measures is discussed.

How Münch's adaptation of Pfeffer's circulating water flow became the pressure-flow theory, and the resulting problems - A historical perspective

Long-distance transport of photoassimilates in the phloem of vascular plants occurs as bulk flow in sieve tubes. These tubes are arrays of cells that lose nuclei, cytoskeleton, and some organelles when they differentiate into mature sieve elements. Symplasmic continuity is achieved by perforations that turn the cell walls between adjoining sieve elements into sieve plates. These structural features are interpreted as adaptations that reduce the resistance sieve tubes offer to cytoplasmic bulk flow. According to the common reading of Ernst Münch's pressure-flow theory, the driving forces for these flows are osmotically generated gradients of hydrostatic pressure along the sieve tubes. However, the significance of pressure gradients in the flow direction has also been questioned. Münch himself stated that no detectable pressure gradients existed between the linked osmotic cells that he used to demonstrate the validity of his ideas, and the earliest explanation of osmotically driven flows by Wilhelm Pfeffer, on which Münch based his theory, explicitly claimed the absence of pressure gradients. To resolve the apparent contradiction, we here reconstruct the history of the idea that osmotically driven transport processes in organisms necessarily require steps or gradients of hydrostatic pressure along the transport route. Our analysis leads us to conclude that some defects of overly simplifying interpretations of Münch's ideas (such as the sieve plate fallacy) could be avoided if our descriptions of his theory in textbooks and the scientific literature would follow the logics of the theory's earliest formulations more closely.

Intestinal osmoregulatory mechanisms differ in Mediterranean and Atlantic European sea bass: A focus on hypersalinity

European sea bass (Dicentrarchus labrax) migrate towards habitats where salinity can reach levels over 60‰, notably in Mediterranean lagoons. D. labrax are genetically subdivided in Atlantic and Mediterranean lineages and have evolved in slightly different salinities. We compared Atlantic and West-Mediterranean populations regarding their capacity to tolerate hypersalinity with a focus on the involvement of the intestine in solute-driven water reabsorption. Fish were analyzed following a two-week transfer from seawater (SW, 36‰) to either SW or hypersaline water (HW, 55‰). Differences among lineages were observed in posterior intestines of fish maintained in SW regarding NKA activities and mRNA expressions of nkaα1a, aqp8b, aqp1a and aqp1b with systematic higher levels in Mediterranean sea bass. High salinity transfer triggered similar responses in both lineages but at different magnitudes which may indicate slight different physiological strategies between lineages. High salinity transfer did not significantly affect the phenotypic traits measured in the anterior intestine. In the posterior intestine however, the size of enterocytes and NKA activity were higher in HW compared to SW. In this tissue, nka-α1a, nkcc2, aqp8ab and aqp8aa mRNA levels were higher in HW compared to SW as well as relative protein expression of AQP8ab. For aqp1a, 1b, 8aa and 8b, an opposite trend was observed. The sub-apical localization of AQP8ab in enterocytes suggests its role in transepithelial water reabsorption. Strong apical NKCC2/NCC staining indicates an increased Na⁺ and Cl^- reuptake by enterocytes which could contribute to solute-coupled water reuptake in cells where AQP8ab is expressed.

The digestive tract as an essential organ for water acquisition in marine teleosts: lessons from euryhaline eels

Adaptation to a hypertonic marine environment is one of the major topics in animal physiology research. Marine teleosts lose water osmotically from the gills and compensate for this loss by drinking surrounding seawater and absorbing water from the intestine. This situation is in contrast to that in mammals, which experience a net osmotic loss of water after drinking seawater. Water absorption in fishes is made possible by (1) removal of monovalent ions (desalinization) by the esophagus, (2) removal of divalent ions as carbonate (Mg/CaCO₃) precipitates promoted by HCO₃^- secretion, and (3) facilitation of NaCl and water absorption from diluted seawater by the intestine using a suite of unique transporters. As a result, 70-85% of ingested seawater is absorbed during its passage through the digestive tract. Thus, the digestive tract is an essential organ for marine teleost survival in the hypertonic seawater environment. The eel is a species that has been frequently used for osmoregulation research in laboratories worldwide. The eel possesses many advantages as an experimental animal for osmoregulation studies, one of which is its outstanding euryhalinity, which enables researchers to examine changes in the structure and function of the digestive tract after direct transfer from freshwater to seawater. In recent years, the molecular mechanisms of ion and water transport across epithelial cells (the transcellular route) and through tight junctions (the paracellular route) have been elucidated for the esophagus and intestine. Thanks to the rapid progress in analytical methods for genome databases on teleosts, including the eel, the molecular identities of transporters, channels, pumps and junctional proteins have been clarified at the isoform level. As 10 y have passed since the previous reviews on this subject, it seems relevant and timely to summarize recent progress in research on the molecular mechanisms of water and ion transport in the digestive tract in eels and to compare the mechanisms with those of other teleosts and mammals from comparative and evolutionary viewpoints. We also propose future directions for this research field to achieve integrative understanding of the role of the digestive tract in adaptation to seawater with regard to pathways/mechanisms including the paracellular route, divalent ion absorption, metabolon formation and cellular trafficking of transporters. Notably, some of these have already attracted practical attention in laboratories.

Diuretic state affects ascending thin limb tight junctions

The nephron segments in the inner medulla are part of the urine concentrating mechanism. Depending on the diuretic state, they are facing a large range of extracellular osmolality. We investigated whether water homeostasis affects tubular transport and permeability properties in inner medullary descending thin limb (IMdTL) and ascending thin limb (IMaTL). Three experimental groups of rats under different diuretic states were investigated on metabolic cages: waterload, furosemide-induced diuresis, and control (antidiuresis). Urine production and osmolalities reflected the 3-day treatment. To functionally investigate tubular epithelial properties, we performed experiments in freshly isolated inner medullary thin limbs from these animals. Tubular segments were acutely dissected and investigated for trans- and paracellular properties by in vitro perfusion and electrophysiological analysis. IMdTL and IMaTL were distinguished by morphological criteria. We confirmed absence of transepithelial electrogenic transport in thin limbs. Although diffusion potential measurements showed no differences between treatments in IMdTLs, we observed increased paracellular cation selectivity under waterload in IMaTLs. NaCl diffusion potential was -5.64 ± 1.93 mV under waterload, -1.99 ± 1.72 mV under furosemide-induced diuresis, and 0.27 ± 0.40 mV under control. The corresponding permeability ratio P_Na/Cl was 1.53 ± 0.21 (waterload), 1.22 ± 0.18 (furosemide-induced diuresis), and 0.99 ± 0.02 (control), respectively. Claudins are main constituents of the tight junction responsible for paracellular selectivity; however, immunofluorescence did not show qualitative differences in claudin 4, 10, and 16 localization. Our results show that IMaTLs change tight junction properties in response to diuretic state to allow adaptation of NaCl reabsorption.

Beyond Ussing's chambers: contemporary thoughts on integration of transepithelial transport

In the mid-20th century, Hans Ussing developed a chamber that allowed for the simultaneous measurement of current and labeled probe flux across epithelia. Using frog skin as a model, Ussing used his results to propose mechanisms of transcellular Na(+) and K(+) transport across apical (exterior/luminal) and basolateral (interior) membranes that is essentially unchanged today. Others took advantage of Ussing's chambers to study mucosal tissues, including bladder and intestines. It quickly became clear that, in some tissues, passive paracellular flux, i.e., across the tight junction, was an important component of overall transepithelial transport. Subsequent work demonstrated that activation of the apical Na(+)-glucose cotransporter SGLT1 regulated paracellular permeability such that intestinal paracellular transport could coordinate with and amplify transcellular transport. Intermediates in this process include activation of p38 MAPK, the apical Na(+)/H(+) exchanger NHE3, and myosin light chain kinase (MLCK). Investigators then focused on these processes in disease. They found that TNF induces barrier dysfunction via MLCK activation and downstream caveolin-1-dependent endocytosis of the tight junction protein occludin. TNF also inhibited NHE3, and both barrier loss and PKCα-dependent NHE3 inhibition were required for TNF-induced acute diarrhea, emphasizing the interplay between transcellular and paracellular transport. Finally, studies using immune-mediated inflammatory bowel disease models showed that mice lacking epithelial MLCK were initially protected, but became ill as epithelial damage progressed and provided a tight junction-independent means of barrier loss. None of these advances would have been possible without the insights provided by Ussing and others using Ussing's ingenious, and still useful, chambers.

Short-term functional adaptation of aquaporin-1 surface expression in the proximal tubule, a component of glomerulotubular balance

Transepithelial water flow across the renal proximal tubule is mediated predominantly by aquaporin-1 (AQP1). Along this nephron segment, luminal delivery and transepithelial reabsorption are directly coupled, a phenomenon called glomerulotubular balance. We hypothesized that the surface expression of AQP1 is regulated by fluid shear stress, contributing to this effect. Consistent with this finding, we found that the abundance of AQP1 in brush border apical and basolateral membranes was augmented >2-fold by increasing luminal perfusion rates in isolated, microperfused proximal tubules for 15 minutes. Mouse kidneys with diminished endocytosis caused by a conditional deletion of megalin or the chloride channel ClC-5 had constitutively enhanced AQP1 abundance in the proximal tubule brush border membrane. In AQP1-transfected, cultured proximal tubule cells, fluid shear stress or the addition of cyclic nucleotides enhanced AQP1 surface expression and concomitantly diminished its ubiquitination. These effects were also associated with an elevated osmotic water permeability. In sum, we have shown that luminal surface expression of AQP1 in the proximal tubule brush border membrane is regulated in response to flow. Cellular trafficking, endocytosis, an intact endosomal compartment, and controlled protein stability are the likely prerequisites for AQP1 activation by enhanced tubular fluid shear stress, serving to maintain glomerulotubular balance.

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.

Aquaporin expression in the Japanese medaka (Oryzias latipes) in freshwater and seawater: challenging the paradigm of intestinal water transport?

We investigated the salinity-dependent expression dynamics of seven aquaporin paralogs (aqp1a, aqp3a, aqp7, aqp8ab, aqp10a, aqp10b and aqp11a) in several tissues of euryhaline Japanese medaka (Oryzias latipes). All paralogs except aqp7 and aqp10a had a broad tissue distribution, and several were affected by salinity in both osmoregulatory and non-osmoregulatory tissues. In the intestine, aqp1a, aqp7, aqp8ab and aqp10a decreased upon seawater (SW) acclimation in both long-term acclimated fish and during 1-3 days of the transition period. In the gill, aqp3a was lower and aqp10a higher in SW than in freshwater (FW). In the kidney no aqps were affected by salinity. In the skin, aqp1a and aqp3a were lower in SW than in FW. In the liver, aqp8ab and aqp10a were lower in SW than in FW. Furthermore, six Na(+),K(+)-ATPase α-subunit isoform transcripts were analysed in the intestine but none showed a consistent response to salinity, suggesting that water transport is not regulated at this level. In contrast, mRNA of the Na(+),K(+),2Cl(-)-cotransporter type-2 strongly increased in the intestine in SW compared with FW fish. Using custom-made antibodies, Aqp1a, Aqp8ab and Aqp10a were localized in the apical region of enterocytes of FW fish. Apical staining intensity strongly decreased, vanished or moved to subapical regions, when fish were acclimated to SW, supporting the lower mRNA expression in SW. Western blots confirmed the decrease in Aqp1a and Aqp10a in SW. The strong decrease in aquaporin expression in the intestine of SW fish is surprising, and challenges the paradigm for transepithelial intestinal water absorption in SW fishes.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

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.

Functional characterization of the vertebrate primary ureter: structure and ion transport mechanisms of the pronephric duct in axolotl larvae (Amphibia)

BACKGROUND

Three kidney systems appear during vertebrate development: the pronephroi, mesonephroi and metanephroi. The pronephric duct is the first or primary ureter of these kidney systems. Its role as a key player in the induction of nephrogenic mesenchyme is well established. Here we investigate whether the duct is involved in urine modification using larvae of the freshwater amphibian Ambystoma mexicanum (axolotl) as model.

RESULTS

We investigated structural as well as physiological properties of the pronephric duct. The key elements of our methodology were: using histology, light and transmission electron microscopy as well as confocal laser scanning microscopy on fixed tissue and applying the microperfusion technique on isolated pronephric ducts in combination with single cell microelectrode impalements. Our data show that the fully differentiated pronephric duct is composed of a single layered epithelium consisting of one cell type comparable to the principal cell of the renal collecting duct system. The cells are characterized by a prominent basolateral labyrinth and a relatively smooth apical surface with one central cilium. Cellular impalements demonstrate the presence of apical Na+ and K+ conductances, as well as a large K+ conductance in the basolateral cell membrane. Immunolabeling experiments indicate heavy expression of Na+/K+-ATPase in the basolateral labyrinth.

CONCLUSIONS

We propose that the pronephric duct is important for the subsequent modification of urine produced by the pronephros. Our results indicate that it reabsorbs sodium and secretes potassium via channels present in the apical cell membrane with the driving force for ion movement provided by the Na+/K+ pump. This is to our knowledge the first characterization of the pronephric duct, the precursor of the collecting duct system, which provides a model of cell structure and basic mechanisms for ion transport. Such information may be important in understanding the evolution of vertebrate kidney systems and human diseases associated with congenital malformations.

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.

Frequency spectrum of transepithelial potential difference reveals transport-related oscillations

How epithelia transport fluid is a fundamental issue that is unresolved. Explanations offered include molecular engines, local transcellular osmosis, local paracellular osmosis, and paracellular fluid transport. On the basis of experimental and theoretical work done on corneal endothelium, a fluid transporting epithelium, we suggest electroosmotic coupling at the level of the intercellular junctions driven by the transendothelial electrical potential difference as an explanation of paracellular fluid transport. We collect frequency spectra of that potential difference in real-time. For what we believe is the first time for any epithelium, we report that, unexpectedly, the potential difference displays oscillations at many characteristic frequencies. We also show that on both stimulating cell activity and inhibiting ion transport mechanisms, there are corresponding changes in the oscillations amplitudes that mirror changes known previously in rates of fluid transport. We believe these findings provide a novel tool to study the kinetics of electrogenic elements such as channels and transporters, which from this evidence would give rise to current oscillations with characteristic periods going from 150 ms to 8 s.

Regulation of paracellular Na+ and Cl(-) conductances by hydrostatic pressure

The effect of hydrostatic pressure on the paracellular ion conductance (Gp) composed of the Na(+) conductance (G(Na)) and the Cl(-) conductance (G(Cl)) has been Investigated. Gp, G(Na) and G(Cl) were time-dependently increased after applying an osmotic gradient generated by NaCl with basolateral hypotonicity. Hydrostatic pressure (1-4cm H2O) applied from the basolateral side enhanced the osmotic gradient-induced increase in Gp, G(Na) and G(Cl) in a magnitude-dependent manner, while the hydrostatic pressure applied from the apical side diminished the osmotic gradient-induced increase in Gp, G(Na) and G(Cl). How the hydrostatic pressure influences Gp, G(Na) and G(Cl) under an isosmotic condition was also investigated. Gp, G(Na) and G(Cl) were stably constant under a condition with basolateral application of sucrose canceling the NaCl-generated osmotic gradient (an isotonic condition). Even under this stable condition, the basolaterally applied hydrostatic pressure drastically elevated Gp, G(Na) and G(Cl), while apically applied hydrostatic pressure had little effect on Gp, G(Na) or G(Cl). Taken together, these observations suggest that certain factors controlled by the basolateral osmolality and the basolaterally applied hydrostatic pressure mainly regulate the Gp, G(Na) and G(Cl).

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.

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.

1,25-dihydroxyvitamin D(3) rapidly stimulates the solvent drag-induced paracellular calcium transport in the duodenum of female rats

A calcium-regulating hormone 1alpha,25-dihydroxyvitamin D(3) (1,25-[OH](2)D(3)) has been known to rapidly stimulate the transcellular active calcium transport in the chick duodenum. However, its effects on the solvent drag-induced paracellular calcium transport, which normally contributes approximately 70% of the total active calcium transport, and the underlying mechanism were unknown. The present study aimed to investigate the rapid nongenomic actions of physiological concentrations of 1,25-(OH)(2)D(3), i.e., 1, 10, and 100 nmol/l, on the duodenal calcium absorption in female rats. Quantitative real-time PCR revealed strong expressions of the classical vitamin D receptor (VDR) and the membrane-associated rapid response steroid binding receptors (MARRS) in both small and large intestines. By using the Ussing chamber technique, we found that duodenal epithelia acutely exposed to 10 and 100 nmol/l 1,25-(OH)(2)D(3) rapidly increased the solvent drag-induced calcium transport, but not the transcellular calcium transport, in a dose-response manner. On the other hand, 3-day daily injections of 1,25-(OH)(2)D(3) enhanced the transcellular active duodenal calcium transport. The 1,25-(OH)(2)D(3)-stimulated solvent drag-induced transport was abolished by the phosphatidylinositol 3-kinase (PI3K) inhibitors, 200 nmol/l wortmannin and 75 micromol/l LY294002, as well as PKC (1 micromol/l GF109203X) and MEK inhibitors (10 micromol/l U0126). Although 100 nmol/l 1,25-(OH)(2)D(3) did not alter the transepithelial mannitol flux, indicating no widening of the tight junction, it decreased the transepithelial resistance and increased both sodium and chloride permeability through the paracellular channel. We conclude that 1,25-(OH)(2)D(3) uses the nongenomic signaling pathways involving PI3K, PKC, and MEK to rapidly enhance the solvent drag-induced calcium transport, partly by altering the charge-selective property of the duodenal epithelium at least for the pathways involving PI3K and MEK.

Branchial expression patterns of claudin isoforms in Atlantic salmon during seawater acclimation and smoltification

In euryhaline teleosts, permeability changes in gill epithelia are essential during acclimation to changed salinity. This study examined expression patterns of branchial tight junction proteins called claudins, which are important determinants of ion selectivity and general permeability in epithelia. We identified Atlantic salmon genes belonging to the claudin family by screening expressed sequence tag libraries available at NCBI, and classification was performed with the aid of maximum likelihood analysis. In gill libraries, five isoforms (10e, 27a, 28a, 28b, and 30) were present, and quantitative PCR analysis confirmed tissue-specific expression in gill when compared with kidney, intestine, heart, muscle, brain, and liver. Expression patterns during acclimation of freshwater salmon to seawater (SW) and during the smoltification process were examined. Acclimation to SW reduced the expression of claudin 27a and claudin 30 but had no overall effect on claudin 28a and claudin 28b. In contrast, SW induced a fourfold increase in expression of claudin 10e. In accord, a peak in branchial claudin 10e was observed during smoltification in May, coinciding with optimal SW tolerance. Smoltification induced no significant changes in expression of the other isoforms. This study demonstrates the expression of an array of salmon claudin isoforms and shows that SW acclimation involves inverse regulation, in the gill, of claudin 10e vs. claudin 27a and 30. It is possible that claudin 10e is an important component of cation selective channels, whereas reduction in claudin 27a and 30 may change permeability conditions in favor of the ion secretory mode of the SW gill.

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.