The contributions of diffusion and flow to the passage of D2O through living membranes; effect of neurohypophyseal hormone on isolated anuran skin

Microbial membrane transport proteins and their biotechnological applications

Because of the hydrophobic nature of the membrane lipid bilayer, the majority of the hydrophilic solutes require special transportation mechanisms for passing through the cell membrane. Integral membrane transport proteins (MTPs), which belong to the Major Intrinsic Protein Family, facilitate the transport of these solutes across cell membranes. MTPs including aquaporins and carrier proteins are transmembrane proteins spanning across the cell membrane. The easy handling of microorganisms enabled the discovery of a remarkable number of transport proteins specific to different substances. It has been realized that these transporters have very important roles in the survival of microorganisms, their pathogenesis, and antimicrobial resistance. Astonishing features related to the solute specificity of these proteins have led to the acceleration of the research on the discovery of their properties and the development of innovative products in which these unique properties are used or imitated. Studies on microbial MTPs range from the discovery and characterization of a novel transporter protein to the mining and screening of them in a large transporter library for particular functions, from simulations and modeling of specific transporters to the preparation of biomimetic synthetic materials for different purposes such as biosensors or filtration membranes. This review presents recent discoveries on microbial membrane transport proteins and focuses especially on formate nitrite transport proteins and aquaporins, and advances in their biotechnological applications.

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

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

Plasticity of skin water permeability and skin thickness in the amphibious mangrove rivulus Kryptolebias marmoratus

The skin of amphibious fishes is a multipurpose organ, important for gas and ion exchange and nitrogen excretion when fish are out of water (emersed). We tested the hypothesis that skin permeability is altered to maintain water balance through changes in water permeability and skin thickness during salinity acclimation and/or when fish emerse, using the euryhaline, amphibious fish Kryptolebias marmoratus as a model. We first recorded the behaviour of fish out of water to determine which part of the cutaneous surface was in contact with the substrate. Fish spent about 70% of their time on their ventral surface when out of water. Osmotic permeability of the skin was assessed in fish acclimated to 0.3 or 45‰ using ³H₂O fluxes in an in vitro micro-Ussing chamber setup. In freshwater-acclimated fish, ³H₂O influx across the skin was significantly higher compared to hypersaline-acclimated fish, with no significant changes in efflux. Prolonged emersion (7 days) resulted in an increase in skin ³H₂O influx, but not efflux in fish acclimated to a moist 45‰ substrate. In a separate experiment, dorsal epidermal skin thickness increased while the ventral dermis thickness decreased in fish emersed for over a week. However, there was no link between regional skin thickness and water flux in our experiments. Taken together, these findings suggest that K. marmoratus alter skin permeability to maximize water uptake while emersed in hypersaline conditions, adjustments that probably help them survive months of emersion during the dry season when drinking to replace water loss is not possible.

The Roles of Aquaporins in Plant Stress Responses

Aquaporins are membrane channel proteins ubiquitously present in all kingdoms of life. Although aquaporins were originally discovered as water channels, their roles in the transport of small neutral solutes, gasses, and metal ions are now well established. Plants contain the largest number and greatest diversity of aquaporin homologs with diverse subcellular localization patterns, gating properties, and solute specificity. The roles of aquaporins in physiological functions throughout plant growth and development are well known. As an integral regulator of plant–water relations, they are presumed to play an important role in plant defense responses against biotic and abiotic stressors. This review highlights involvement of various aquaporin homologs in plant stress responses against a variety of environmental stresses that disturb plant cell osmotic balance and nutrient homeostasis.

The emergence of the concept of tight junctions and physiological regulation by ouabain

The exchange of substances between metazoan and the environment takes place across transporting epithelia that have two fundamental differentiated features: tight junctions (TJ) and apical/basolateral polarity. Usually, reviews of the structure and function of transporting epithelia follow a historical description of major biological findings, but seldom refer to the fact that it also required fundamental theoretical changes in the physics and chemistry involved. We make a brief description of the concatenation of both types of achievements, in which it becomes clear that the major source of conflicts was the enzyme Na(+),K(+)-ATPase (also referred to as "the pump"), because of its intrinsic mechanisms and its asymmetric expression on one side of epithelial cells only (polarity). This enzyme is also the receptor of the newly recognized hormone ouabain, whose chief function is to modulate cell contacts, such as TJs, several types of cell-cell contacts participating in polarization (as gauged through ciliogenesis).

Assessment of the requirement for aquaporins in the thylakoid membrane of plant chloroplasts to sustain photosynthetic water oxidation

Oxygenic photosynthetic organisms use sunlight energy to oxidize water to molecular oxygen. This process is mediated by the photosystem II complex at the lumenal side of the thylakoid membrane. Most research efforts have been dedicated to understanding the mechanism behind the unique water oxidation reactions, whereas the delivery pathways for water molecules into the thylakoid lumen have not yet been studied. The most common mechanisms for water transport are simple diffusion and diffusion facilitated by specialized channel proteins named aquaporins. Calculations using published data for plant chloroplasts indicate that aquaporins are not necessary to sustain water supply into the thylakoid lumen at steady state photosynthetic rates. Yet, arguments for their presence in the plant thylakoid membrane and beneficial action are presented.

Chemosensory function of amphibian skin: integrating epithelial transport, capillary blood flow and behaviour

Terrestrial anuran amphibians absorb water across specialized regions of skin on the posterioventral region of their bodies. Rapid water absorption is mediated by the insertion of aquaporins into the apical membrane of the outermost cell layer. Water moves out of the epithelium via aquaglyceroporins in the basolateral membrane and into the circulation in conjunction with increased capillary blood flow to the skin and aquaporins in the capillary endothelial cells. These physiological responses are activated by intrinsic stimuli relating to the animals' hydration status and extrinsic stimuli relating to the detection of osmotically available water. The integration of these processes has been studied using behavioural observations in conjunction with neurophysiological recordings and studies of epithelial transport. These studies have identified plasma volume and urinary bladder stores as intrinsic stimuli that activate the formation of angiotensin II (AII) to stimulate water absorption behaviour. The coordinated increase in water permeability and capillary blood flow appears to be mediated primarily by sympathetic stimulation of beta adrenergic receptors, although the neurohypopyseal hormone arginine vasotocin (AVT) may also play a role. Extrinsic stimuli relate primarily to the ionic and osmotic properties of hydration sources. Toads avoid NaCl solutions that have been shown to be harmful in acute exposure, approx. 200-250 mm. The avoidance is partially attenuated by amiloride raising the hypothesis that the mechanism for salt detection by toads resembles that for salt taste in mammals that take in water by mouth. In this model, depolarization of the basolateral membrane of taste cells is coupled to afferent neural stimulation. In toad skin we have identified innervation of skin epithelial cells by branches of spinal nerves and measured neural responses to NaCl solutions that elicit behavioural avoidance. These same concentrations produce depolarization of the basolateral membrane in isolated epithelial preparations. As with salt taste in mammals, the neural responses and depolarization of basolateral membrane potential are partially inhibited by amiloride. In addition, toads are more tolerant of sodium gluconate solution which is consistent with the phenomenon in mammalian taste physiology termed the anion paradox in which sodium salts with larger molecular weight anions produce a reduced intensity of salt taste. Finally, toads also avoid concentrated solutions of a non-electrolyte, mannitol, which differs from NaCl solutions in not affecting transepithelial conductance and requires a longer time to depolarize the basolateral membrane. Osmotic stimuli may mediate sensory processes for longer term detection of conditions with low water potential while ionic stimuli are more important for shorter term analysis of rehydration sources.

The effect of antidiuretic hormone on human sweating

1. Changes in insensible perspiration and sweating were followed in normal subjects by continuously monitoring total body weight loss in environmental temperatures of 18, 29 and 37 degrees C.2. Pharmacological doses of ADH had no effect on cutaneous water loss at 18 degrees C.3. Pharmacological doses of ADH are capable of increasing the rate of cutaneous water loss in human subjects who are close to or above the thermal sweating threshold.4. Physiological doses of ADH had no effect on cutaneous water loss in either cool or hot environments.5. At normal rates of secretion in the body, ADH probably does not influence human sweat secretion.

Water relations of tetrapod integument

The vertebrate integument represents an evolutionary compromise between the needs for mechanical protection and those of sensing the environment and regulating the exchange of materials and energy. Fibrous keratins evolved as a means of strengthening the integument while simultaneously providing a structural support for lipids, which comprise the principal barrier to cutaneous water efflux in terrestrial taxa. Whereas lipids are of fundamental importance to water barriers, the efficacy of these barriers depends in many cases on structural features that enhance or maintain the integrity of function. Amphibians are exceptional among tetrapods in having very little keratin and a thin stratum corneum. Thus, effective lipid barriers that are present in some specialized anurans living in xeric habitats are external to the epidermis, whereas lipid barriers of amniotes exist as a lipid-keratin complex within the stratum corneum. Amphibians prevent desiccation of the epidermis and underlying tissues either by evaporating water from a superficial aqueous film, which must be replenished, or by shielding the stratum corneum with superficial lipids. Water barrier function in vertebrates generally appears to be relatively fixed, although various species have`plasticity' to adjust the barrier effectiveness facultatively. While it is clear that both phenotypic plasticity and genetic adaptation can account for covariation between environment and skin resistance to water efflux, studies of the relative importance of these two phenomena are few. Fundamental mechanisms for adjusting the skin water barrier include changes in barrier thickness, composition and physicochemical properties of cutaneous lipids,and/or geometry of the barrier within the epidermis. While cutaneous lipids have been studied extensively in the contexts of disease and cosmetics,relatively little is known about the processes of permeability barrier ontogenesis related to adaptation and environment. Advances in such knowledge have didactic significance for understanding vertebrate evolution as well as practical application to clinical dermatology.

Role of villus microcirculation in intestinal absorption of glucose: coupling of epithelial with endothelial transport

Capillaries in jejunal villi can absorb nutrients at rates several hundred times greater (per gram tissue) than capillaries in other tissues, including contracting skeletal muscle and brain. We here present an integrative hypothesis to account for these exceptionally large trans-endothelial fluxes and their relation to epithelial transport. Equations are developed for estimating concentration gradients of glucose across villus capillary walls, along paracellular channels and across subjunctional lateral membranes of absorptive cells. High concentrations of glucose discharged across lateral membranes to subjunctional intercellular spaces are delivered to abluminal surfaces of villus capillaries by convection-diffusion in intercellular channels without significant loss of concentration. Post-junctional paracellular transport thus provides the series link between epithelial and endothelial transport and makes possible the large trans-endothelial concentration gradients required for absorption to blood. Our analysis demonstrates that increases of villus capillary blood flow and permeability-surface area product (PS) are essential components of absorptive mechanisms: epithelial transport of normal digestive loads could not be sustained without concomitant increases in capillary blood flow and PS. The low rates of intestinal absorption found in anaesthetised animals may be attributed to inhibition of normal villus microvascular responses to epithelial transport.

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.

Importance of intracellular water apparent diffusion to the measurement of membrane permeability

The exchange of water across biological membranes is of fundamental significance to both animal and plant physiology. Diffusional membrane permeability (P(d)) for the Xenopus oocyte, an important model system for water channel investigation, is typically calculated from intracellular water pre-exchange lifetime, cell volume, and cell surface area. There is debate, however, whether intracellular water motion affects water lifetime, and thereby P(d). Mathematical modeling of water transport is problematic because the intracellular water diffusion rate constant (D) for cells is usually unknown. The measured permeability may be referred to as the apparent diffusional permeability, P, to acknowledge this potential error. Herein, we show that magnetic resonance (MR) spectroscopy can be used to measure oocyte water exchange with greater temporal resolution and higher signal-to-noise ratio than other methods. MR imaging can be used to assess both oocyte geometry and intracellular water diffusion for the same single cells. MR imaging is used to confirm the dependence of intracellular water lifetime on intracellular diffusion. A model is presented to relate intracellular lifetime to true membrane diffusional permeability. True water diffusional permeability (2.7 +/- 0.4 microm/s) is shown to be 39 +/- 6% greater than apparent diffusional permeability for 8 oocytes. This discrepancy increases with cell size and permeability (such as after water channel expression) and decreases with increasing intracellular water D.

Transport of volatile solutes through AQP1

For almost a century it was generally assumed that the lipid phases of all biological membranes are freely permeable to gases. However, recent observations challenge this dogma. The apical membranes of epithelial cells exposed to hostile environments, such as gastric glands, have no demonstrable permeability to the gases CO2 and NH3. Additionally, the water channel protein aquaporin 1 (AQP1), expressed at high levels in erythrocytes, can increase membrane CO2 permeability when expressed in Xenopus oocytes. Similarly, nodulin-26, which is closely related to AQP1, can act as a conduit for NH3. A key question is whether aquaporins, which are abundant in virtually every tissue that transports O2 and CO2 at high levels, ever play a physiologically significant role in the transport of small volatile molecules. Preliminary data are consistent with the hypothesis that AQP1 enhances the reabsorption of HCO3- by the renal proximal tubule by increasing the CO2 permeability of the apical membrane. Other preliminary data on Xenopus oocytes heterologously expressing the electrogenic Na+-HCO3- cotransporter (NBC), AQP1 and carbonic anhydrases are consistent with the hypothesis that the macroscopic cotransport of Na+ plus two HCO3- occurs as NBC transports Na+ plus CO3(2-) and AQP1 transports CO2 and H2O. Although data - obtained on AQP1 reconstituted into liposomes or on materials from AQP1 knockout mice - appear inconsistent with the model that AQP1 mediates substantial CO2 transport in certain preparations, the existence of unstirred layers or perfusion-limited conditions may have masked the contribution of AQP1 to CO2 permeability.

Plant aquaporins

Aquaporins are ubiquitous membrane channel proteins that facilitate and regulate the permeation of water across biological membranes. Aquaporins are members of the MIP family and some of them seem to be also able to transport other molecules such as urea or glycerol. In the plant kingdom, a single plant expresses a considerably large number of MIP homologues. These homologues can be subdivided into four groups (PIP, TIP, NIP, SIP) with highly conserved amino acid sequences and intron positions in each group. Since their discovery, advancing knowledge of their structure led to an understanding of the basic features of the water transport mechanism. An optimal water balance is essential to the homeostasis of most organisms, and aquaporins may be one of the mechanisms involved under changing environmental and developmental conditions. In fact, this may be one reason for the abundance and diversity of aquaporins, in particular in plants. In addition, exposure to different types of stress alters water relations and thus, aquaporins may be involved in stress responses as well. The transcriptional and/or post-translational regulation of aquaporins would determine changes in membrane water permeability. Both phosphorylation and translocation to/from vesicles have been reported as post-translational mechanisms. However, translocation in plants has not yet been shown. Although significant advances have been achieved, complete understanding of aquaporin function and regulation remains elusive.

Cell volume kinetics of adherent epithelial cells measured by laser scanning reflection microscopy: determination of water permeability changes of renal principal cells

The water channel aquaporin-2 (AQP2), a key component of the antidiuretic machinery in the kidney, is rapidly regulated by the antidiuretic hormone vasopressin. The hormone exerts its action by inducing a translocation of AQP2 from intracellular vesicles to the cell membrane. This step requires the elevation of intracellular cyclic AMP. We describe here a new method, laser scanning reflection microscopy (LSRM), suitable for determining cellular osmotic water permeability coefficient changes in primary cultured inner medullary collecting duct (IMCD) cells. The recording of vertical-reflection-mode x-z-scan section areas of unstained, living IMCD cells proved useful and valid for the investigation of osmotic water permeability changes. The time-dependent increases of reflection-mode x-z-scan section areas of swelling cells were fitted to a single-exponential equation. The analysis of the time constants of these processes indicates a twofold increase in osmotic water permeability of IMCD cells after treatment of the cells both with forskolin, a cyclic AMP-elevating agent, and with Clostridium difficile toxin B, an inhibitor of Rho proteins that leads to depolymerization of F-actin-containing stress fibers. This indicates that both agents lead to the functional insertion of AQP2 into the cell membrane. Thus, we have established a new functional assay for the study of the regulation of the water permeability at the cellular level.

The mechanisms of aquaporin control in the renal collecting duct

The antidiuretic hormone arginine-vasopressin (AVP) regulates water reabsorption in renal collecting duct principal cells. Central to its antidiuretic action in mammals is the exocytotic insertion of the water channel aquaporin-2 (AQP2) from intracellular vesicles into the apical membrane of principal cells, an event initiated by an increase in cAMP and activation of protein kinase A. Water is then reabsorbed from the hypotonic urine of the collecting duct. The water channels aquaporin-3 (AQP3) and aquaporin-4 (AQP4), which are constitutively present in the basolateral membrane, allow the exit of water from the cell into the hypertonic interstitium. Withdrawal of the hormone leads to endocytotic retrieval of AQP2 from the cell membrane. The hormone-induced rapid redistribution between the interior of the cell and the cell membrane establishes the basis for the short term regulation of water permeability. In addition water channels (AQP2 and 3) of principal cells are regulated at the level of expression (long term regulation). This review summarizes the current knowledge on the molecular mechanisms underlying the short and long term regulation of water channels in principal cells. In the first part special emphasis is placed on the proteins involved in short term regulation of AQP2 (SNARE proteins, Rab proteins, cytoskeletal proteins, G proteins, protein kinase A anchoring proteins and endocytotic proteins). In the second part, physiological and pathophysiological stimuli determining the long term regulation are discussed.

Structural correlates of the transepithelial water transport

Transepithelial permeability is one of the fundamental problems in cell biology. Epithelial cell layers protect the organism from its environment and form a selective barrier to the exchange of molecules between the lumen of an organ and an underlying tissue. This chapter discusses some problems and analyzes the participation of intercellular junctions in the paracellular transport of water, migration of intramembrane particles in the apical membrane during its permeability changes for isotonic fluid in cells of leaky epithelia, insertion of water channels into the apical membrane and their cytoplasmic sources in cells of tight epithelia under ADH (antidiuretic hormone)-induced water flows, the osmoregulating function of giant vacuoles in the transcellular fluxes of hypotonic fluid across tight epithelia, and the role of actin filaments and microtubules in the transcellular transport of water across epithelia.

The entrance of water into beef and dog red cells

The rate constants for diffusion of THO across the red cell membrane of beef and dog, and the rate of entrance of water into the erythrocytes of these species under an osmotic pressure gradient have been measured. For water entrance into the erythrocyte by diffusion the rate constants are 0.10 ± 0.02 msec.^-1 (beef) and 0.14 ± 0.03 msec.^-1 (dog); the permeability coefficients for water entrance under a pressure gradient of 1 osmol./cm³ are 0.28 See PDF for Equation These values permit the calculation of an equivalent pore radius for the erythrocyte membrane of 4.1 A for beef and 7.4 A for dog. In the beef red cell the change in THO diffusion due to osmotically produced cell volume shifts has been studied. The resistance to THO diffusion increases as the cell volume increases. At the maximum volume, (1.06 times normal), THO diffusion is decreased to 0.84 times the normal rate. This change in diffusion is attributed to swelling of the cellular membrane.

The Escherichia coli aquaporin-Z water channel

The membrane pathway of the rapid fluxes of water by which microorganisms adapt promptly to abrupt changes in environmental osmolality have begun to be understood since the discovery of the Escherichia coli aquaporin-Z water channel, AqpZ. As in animals and plants, aquaporins are variously represented among microorganisms, in which 31 homologous genes have already been identified in eubacteria, Archaea, fungi and protozoa. The AqpZ channel is selectively permeable to water, although other functions are not excluded. Consistent with a conservation over the course of evolution, AqpZ and AQP1, a human counterpart, share similar structures. The aqpZ gene is growth phase and osmotically regulated. AqpZ has a role in both the short- and the long-term osmoregulatory response and is required by rapidly growing cells. AqpZ-like proteins seem to be necessary for the virulence expressed by some pathogenic bacteria. Microbial aquaporins are also likely to be involved in spore formation and/or germination. Additional roles may still be unknown. The use of AqpZ as a model system will continue to provide insight into the understanding of the importance of aquaporins.

Effect of PCMBS on CO2 permeability of Xenopus oocytes expressing aquaporin 1 or its C189S mutant

A recent study on Xenopus oocytes [N. L. Nakhoul, M. F. Romero, B. A. Davis, and W. F. Boron. Am. J. Physiol. 274 (Cell Physiol. 43): C543-548, 1998] injected with carbonic anhydrase showed that expressing aquaporin 1 (AQP1) increases by approximately 40% the rate at which exposing the cell to CO2 causes intracellular pH to fall. This observation is consistent with several interpretations. Overexpressing AQP1 might increase apparent CO2 permeability by 1) allowing CO2 to pass through AQP1, 2) stimulating injected carbonic anhydrase, 3) enhancing the CO2 solubility of the membrane's lipid, or 4) increasing the expression of a native "gas channel." The purpose of the present study was to distinguish among these possibilities. We found that expressing the H2O channel AQP1 in Xenopus oocytes increases the CO2 permeability of oocytes in an expression-dependent fashion, whereas expressing the K+ channel ROMK1 has no effect. The mercury derivative p-chloromercuriphenylsulfonic acid (PCMBS), which inhibits the H2O movement through AQP1, also blocks the AQP1-dependent increase in CO2 permeability. The mercury-insensitive C189S mutant of AQP1 increases the CO2 permeability of the oocyte to the same extent as does the wild-type channel. However, the C189S-dependent increase in CO2 permeability is unaffected by treatment with PCMBS. These data rule out options 2-4 listed above. Thus our results suggest that CO2 passes through the pore of AQP1 and are the first data to demonstrate that a gas can enter a cell by a means other than diffusing through the membrane lipid.

Experimental study of the independence of diffusion and hydrodynamic permeability coefficients in collodion membranes

The two parameters usually invoked when discussing transport across membranes are the "diffusion permeability coefficient" and the "hydrodynamic permeability coefficient." In this study the magnitude of these two coefficients is established experimentally for collodion membranes of differing porosities. The hydrodynamic permeability is predominant while convergence of the two permeabilities tends to obtain as the membranes become less coarse. The flux data obtained are used to calculate "average pore diameter" and the meaningfulness of these calculations is interpreted. The relationship between the two coefficients and transport across membranes as treated by the system of irreversible thermodynamics is discussed.

Permeability of the isolated toad bladder to solutes and its modification by vasopressin

Measurements have been made of the permeability of the isolated urinary bladder of the toad to a number of small solute molecules, in the presence and absence of vasopressin. Vasopressin has a strikingly specific effect on increasing permeability of the bladder to a group of small, uncharged amides and alcohols while penetration by other small molecules and ions is unaffected. The movement of urea is passive, as indicated by equal flux rates in the two directions. The reflection coefficients for chloride and thiourea indicate a high degree of impermeability of the bladder to these solutes even in the presence of large net movements of water. The low concentration of thiourea in the tissue water when this compound is added to the mucosal bathing medium indicates that the major permeability barrier to thiourea is at the mucosal surface of the bladder. The findings can be accounted for by a double permeability barrier consisting of a fine selective diffusion barrier and a porous barrier in series. The former would constitute the permeability barrier to most small solutes while the latter would be the rate-limiting barrier for water and the amides. It would be the porous barrier which is affected by vasopressin. Reasons are presented which require both barriers to be contained in or near the plasma membrane at the mucosal surface of the bladder.

Regulation of water channel activity of aquaporin 1 by arginine vasopressin and atrial natriuretic peptide

Aquaporin 1 (AQP1), a six-transmembrane domain protein that functions as a water channel, is present in many fluid secreting and absorbing tissues such as kidney, brain, heart, and eye. It is believed that among the five known mammalian aquaporins, kidney aquaporin (AQP2) is the only water channel that is regulated by arginine vasopressin (AVP). The present data suggest that AQP1 may also be regulated by AVP. The application of AVP to Xenopus oocytes injected with AQP1 cRNA increased the membrane permeability to water. In addition, our data reveal that atrial natriuretic peptide (ANP), a peptide hormone that plays an important role in the regulation of body fluid homeostasis, blocks the AQP1-mediated increase in water permeability. Incubation with 8-bromo-cAMP or direct 8-bromo-cAMP injection into oocytes expressing AQP1 cRNA significantly increased membrane permeability to water, suggesting that stimulation of AQP1 activity by AVP may involve a cAMP-dependent mechanism. Regulation of water permeability by AVP and ANP has potential relevance to active water transport in a variety of tissues that express AQP1 including kidney, brain, and eye.

AQUAPORINS AND WATER PERMEABILITY OF PLANT MEMBRANES

The mechanisms of plant membrane water permeability have remained elusive until the recent discovery in both vacuolar and plasma membranes of a class of water channel proteins named aquaporins. Similar to their animal counterparts, plant aquaporins have six membrane-spanning domains and belong to the MIP superfamily of transmembrane channel proteins. Their very high efficiency and selectivity in transporting water molecules have been mostly characterized using heterologous expression in Xenopus oocytes. However, techniques set up to measure the osmotic water permeability of plant membranes such as transcellular osmosis, pressure probe measurements, or stopped-flow spectrophotometry are now being used to analyze the function of plant aquaporins in their native membranes. Multiple mechanisms, at the transcriptional and posttranslational levels, control the expression and activity of the numerous aquaporin isoforms found in plants. These studies suggest a general role for aquaporins in regulating transmembrane water transport during the growth, development, and stress responses of plants. Future research will investigate the integrated function of aquaporins in long-distance water transport and cellular osmoregulation.