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

Functions of the Gallbladder

The gallbladder stores and concentrates bile between meals. Gallbladder motor function is regulated by bile acids via the membrane bile acid receptor, TGR5, and by neurohormonal signals linked to digestion, for example, cholecystokinin and FGF15/19 intestinal hormones, which trigger gallbladder emptying and refilling, respectively. The cycle of gallbladder filling and emptying controls the flow of bile into the intestine and thereby the enterohepatic circulation of bile acids. The gallbladder also largely contributes to the regulation of bile composition by unique absorptive and secretory capacities. The gallbladder epithelium secretes bicarbonate and mucins, which both provide cytoprotection against bile acids. The reversal of fluid transport from absorption to secretion occurs together with bicarbonate secretion after feeding, predominantly in response to an adenosine 3',5'-cyclic monophosphate (cAMP)-dependent pathway triggered by neurohormonal factors, such as vasoactive intestinal peptide. Mucin secretion in the gallbladder is stimulated predominantly by calcium-dependent pathways that are activated by ATP present in bile, and bile acids. The gallbladder epithelium has the capacity to absorb cholesterol and provides a cholecystohepatic shunt pathway for bile acids. Changes in gallbladder motor function not only can contribute to gallstone disease, but also subserve protective functions in multiple pathological settings through the sequestration of bile acids and changes in the bile acid composition. Cholecystectomy increases the enterohepatic recirculation rates of bile acids leading to metabolic effects and an increased risk of nonalcoholic fatty liver disease, cirrhosis, and small-intestine carcinoid, independently of cholelithiasis. Among subjects with gallstones, cholecystectomy remains a priority in those at risk of gallbladder cancer, while others could benefit from gallbladder-preserving strategies. © 2016 American Physiological Society. Compr Physiol 6:1549-1577, 2016.

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.

A New Approach to Epithelial Isotonic Fluid Transport: An Osmosensor Feedback Model

A model for control of the transport rate and osmolarity of epithelial fluid (isotonic transport) is presented by using an analogy with the control of temperature and flow rate in a shower. The model brings recent findings and theory concerning the role of aquaporins in epithelia together with measurements of epithelial paracellular flow into a single scheme. It is not based upon osmotic equilibration across the epithelium but rather on the function of aquaporins as osmotic sensors that control the tonicity of the transported fluid by mixing cellular and paracellular flows, which may be regarded individually as hyper- and hypo-tonic fluids, to achieve near-isotonicity. The system is built on a simple feedback loop and the quasi-isotonic behavior is robust to the precise values of most parameters. Although the two flows are separate, the overall fluid transport rate is governed by the rate of salt pumping through the cell. The model explains many things: how cell pumping and paracellular flow can be coupled via control of the tight junctions; how osmolarity is controlled without depending upon the precise magnitude of membrane osmotic permeability; and why many epithelia have different aquaporins at the two membranes. The model reproduces all the salient features of epithelial fluid transport seen over many years but also indicates novel behavior that may provide a subject for future research and serve to distinguish it from other schemes such as simple osmotic equilibration. Isotonic transport is freed from constraints due to limited permeability of the membranes and the precise geometry of the system. It achieves near-isotonicity in epithelia in which partial water transport by co-transporters may be present, and shows apparent electro-osmotic effects. The model has been developed with a minimum of parameters, some of which require measurement, but the model is flexible enough for the basic idea to be extended both to complex systems of water and salt transport that still await a clear explanation, such as intestine and airway, and to systems that may lack aquaporins or use other sensors.

AQP and the control of fluid transport in a salivary gland

Experiments were performed with the perfused rat submandibular gland in vitro to investigate the nature of the coupling between transported salt and water by varying the osmolarity of the source bath and observing the changes in secretory volume flow. Glands were submitted to hypertonic step changes by changing the saline perfusate to one containing different levels of sucrose. The flow rate responded by falling to a lower value, establishing a new steady-state flow. The rate changes did not correspond to those expected from a system in which fluid production is due to simple osmotic equilibration, but were much larger. The changes were fitted to a model in which fluid production is largely paracellular, the rate of which is controlled by an osmosensor system in the basal membrane. The same experiments were done with glands from rats that had been bred to have very low levels of AQP5 (the principal aquaporin of the salivary acinar cell) in which little AQP5 is expressed at the basal membrane. In these rats, salivary secretion rates after hypertonic challenges were small and best modelled by simple osmotic equilibration. In rats which had intermediate AQP5 levels the changes in flow rate were similar to those of normal rats although their AQP5 levels were reduced.Finally, perfused normal glands were subject to retrograde ductal injection of salines containing different levels of Hg(2+) ions (0, 10 and 100 microM: ) which would act as inhibitors of AQP5 at the apical acinar membrane. The overall flow rates were progressively diminished with rising Hg(2+) concentration, but after hypertonic challenge the changes in flow rates were unchanged and similar to those of normal rats. All these results are difficult to explain by a cellular osmotic model but can be explained by a model in which paracellular flow is controlled by an osmosensor (presumably AQP5) present on the basal membrane.

What are aquaporins for?

The prime function of aquaporins (AQPs) is generally believed to be that of increasing water flow rates across membranes by raising their osmotic or hydraulic permeability. In addition, this applies to other small solutes of physiological importance. Notable applications of this 'simple permeability hypothesis' (SPH) have been epithelial fluid transport in animals, water exchanges associated with transpiration, growth and stress in plants, and osmoregulation in microbes. We first analyze the need for such increased permeabilities and conclude that in a range of situations at the cellular, subcellular and tissue levels the SPH cannot satisfactorily account for the presence of AQPs. The analysis includes an examination of the effects of the genetic elimination or reduction of AQPs (knockouts, antisense transgenics and null mutants). These either have no effect, or a partial effect that is difficult to explain, and we argue that they do not support the hypothesis beyond showing that AQPs are involved in the process under examination. We assume that since AQPs are ubiquitous, they must have an important function and suggest that this is the detection of osmotic and turgor pressure gradients. A mechanistic model is proposed--in terms of monomer structure and changes in the tetrameric configuration of AQPs in the membrane--for how AQPs might function as sensors. Sensors then signal within the cell to control diverse processes, probably as part of feedback loops. Finally, we examine how AQPs as sensors may serve animal, plant and microbial cells and show that this sensor hypothesis can provide an explanation of many basic processes in which AQPs are already implicated. Aquaporins are molecules in search of a function; osmotic and turgor sensors are functions in search of a molecule.

On the mechanism of fluid transport across corneal endothelium and epithelia in general

The mechanism by which fluid is transported across epithelial layers is still unclear. The prevalent idea is that fluid traverses these layers transcellularly, driven by local osmotic gradients secondary to electrolyte transport and utilizing the high osmotic permeability of aquaporins. However, recent findings that some aquaporin knockout mice epithelia transport fluid sow doubts on local osmosis. This review discusses recent evidence in corneal endothelium that points instead to electro-osmosis as the mechanism underlying fluid transport. In this concept, a local recirculating electrical current would result in electro-osmotic coupling at the level of the intercellular junctions, dragging fluid via the paracellular route. The text also mentions possible mechanisms for apical bicarbonate exit from endothelial cells, and discusses whether electro-osmosis could be a general mechanism.

Paracellular fluid transport by epithelia

The evidence that a major fraction of water crosses the paracellular route during isotonic fluid transfer is reviewed together with a description of the theory and experimental results derived from extracellular probe studies. Four transporting epithelia which have been studied using the method are gallbladder, intestine, Malpighian tubule, and salivary gland. It is concluded that paracellular probe flows are not due to simple convection generated by osmotic flow through the junctions but are generated by active fluid transport within the junction: a mechano-osmotic process. The geometry of the pathway involved would indicate that some salt accompanies the paracellular fluid, representing a hypo-osmotic flow. Transport of salt by the cell route, which may be accompanied by some water, represents a hypertonic flow. The problem then becomes one of balancing the two to produce an isotonic transportate. We suggest, using recent data from knockout mice, that some aquaporins are functioning in different epithelial tissues as osmo-comparators within a feedback loop that regulates the paracellular fluid flow rate. This results in an overall quasi-isotonic transport by the epithelium. The model is applied to forward-facing systems such as proximal tubule and backward-facing systems such as exocrine glands.

The paracellular component of water flow in the rat submandibular salivary gland

1. The pathway of water flow during salivary secretion by the isolated, perfused rat submandibular gland was examined using a family of homologous radiodextran molecules as probes of paracellular fluid transfer. 2. The secretion/perfusate ratio (S/P) of the secreted probes versus molecular radius during fluid secretion evoked by ACh could be resolved into two components: one that fitted a free-diffusion (Stokes-Einstein) curve and indicated diffusion through large channels, and a convective component that was linearly related to radius. 3. The convective component had a cut-off point at 0.5 nm (5 A) radius and an S/P intercept of near 1.0 at the radius of water, which indicates that most of the volume flow was paracellular. 4. The nature of such a paracellular flow is discussed together with the possible integration of this volume flow with the cellular transport of ions, resulting in an isotonic primary secretion from the gland.

Water does not flow across the tight junctions of MDCK cell epithelium

Although it has been known for decades that the tight junctions of fluid-transporting epithelia are leaky to ions, it has not been possible to determine directly whether significant transjunctional water movement also occurs. An optical microscopic technique was developed for the direct visualization of the flow velocity profiles within the lateral intercellular spaces of a fluid-absorptive, cultured renal epithelium (MDCK) and used to determine the velocity of the fluid flow across the tight junction. The flow velocity within the lateral intercellular spaces fell to near zero adjacent to the tight junction, showing that significant transjunctional flow did not occur, even when transepithelial fluid movement was augmented by imposition of osmotic gradients.

Routes and mechanism of fluid transport by epithelia

The mechanism of fluid transport by leaky epithelia and the route taken by the transported fluid are in dispute. A consideration of current mathematical models for coupling of solutes and water, as well as the methodologies for the study of fluid transport, shows that local osmosis best accounts for water movement. Although it seems virtually certain that the tight junctions are water permeable, the fraction of absorbed fluid that crosses the tight junction cannot yet be determined with confidence.

Comparative morphology of the gallbladder and biliary tract in vertebrates: variation in structure, homology in function and gallstones

A review of investigations on the morphology of the gallbladder and biliary tract in fish, reptiles, amphibians, birds, and mammals was performed. Scanning electron microscopy, transmission electron microscopy, and light microscopy observations by the authors were also included. Variations in the presence or absence of a gallbladder, surface epithelium of the gallbladder, and differences in the morphology of the biliary tract in vertebrates were reported. Many differences were diet-related. Despite some dissimilarities observed, analogous functioning of the biliary system was accomplished by its various components, with the biliary ducts performing the function of the gallbladder when this organ was absent. In addition, the occurrence of peculiar parasitism and gallstones among some cases of vertebrates, including humans, was presented.

The paracellular channel for water secretion in the upper segment of the Malpighian tubule of Rhodnius prolixus

Lumen to bath J12/C1 and bath to lumen J21/C2 fluxes per unit concentration of 19 probes with diameters (dm) ranging from 3.0-30.0 A (water, urea, erythritol, mannitol, sucrose, raffinose and 13 dextrans with dm 9.1-30.0 A) were measured during volume secretion (Jv) in the upper segment of the Malpighian Tubule of Rhodnius by perfusing lumen and bath with 14C or 3H-labeled probes. Jnet = (J12/C1-J21/C2) was studied as a function of Jv.Jv was varied by using different concentrations of 5-hydroxy tryptamine. Jnet for 3H-water was not different from Jv. We found: (i) A strong correlation between Jnet and Jv for 8 probes dm = 3.0-11.8 A (group a probes), indicating that the convective component of Jnet is more important than its diffusive component and than unstirred layers effects which are negligible. Therefore group a probes are solvent dragged as they cross the epithelium. (ii) There is no correlation between Jnet and Jv for 11 probes with dm = 11.8-30 A (group b). Therefore these probes must cross the epithelium by diffusion and not by solvent drag. (iii) In a plot of Jnet/Jv vs. dm group a probes show a steep linear relation with a slope = -0.111, while for group b probes the slope is -0.002. Thus there is a break between groups a and b in this plot. We tried to fit the data with models for restricted diffusion and convention through cylindrical or parallel slit pathways. We conclude that (i) group a probes are dragged by water through an 11.0 A-wide slit. (ii) Most of Jv must follow an extracellular noncytosolic pathway. (iii) Group b probes must diffuse through a 42 A-wide slit. (iv) A cylindrical pathway does not fit the data.

A mechanism for isotonic fluid flow through the tight junctions of Necturus gallbladder epithelium

During isotonic fluid flow, Necturus gallbladder epithelium mediates net fluxes of paracellular probes by a convective process. We show here that the paracellular system is modeled by permeation through three populations of channels: (i) convective parallel-sided ones of width 7.7 nm (ii) small diffusive ones of radius approximately 0.6 nm, and (ii) large diffusive ones of radius exceeding 50 nm. The reflexion coefficient of the convective channels is very low and the calculated osmotic flow rate is close to zero when compared with the observed fluid absorptive rate of 2 x 10(-6) cm/sec. Analysis reveals that the convective channels behave as though closed to back-diffusion of probes; if this is due to solvent drag then very high fluid velocities are required, acting through minute areas. There are no transjunctional gradients that could drive the flow, and so the fluid must be propelled through the channel by components of the junction. We propose a mechanism based upon an active junctional peristalsis which allows discrimination on the basis of molecular size, in which the channels are always occluded at some point and so back-diffusion cannot occur. There is no local gradient of salt distal to the junctions and therefore the osmotic permeability of the membranes is irrelevant. High fluid velocities are not required, and the flow can occur over a substantial fraction of the junction. The mechanism must involve motile and contractile elements associated with the junction for which there is already considerable evidence.