Osmotic Sensor Research Articles

Response and defense mechanisms of the earthworms Eisenia foetida to natural saline soil stress

Salinization of soil is a serious global environmental issue, particularly in agricultural lands. Saline farmland not only endangers grain production but also affects the survival of soil fauna. Earthworms, as soil ecosystem engineers, play a crucial role in maintaining soil health and enhancing global agricultural production. However, the response of earthworms to natural saline soil stress remains poorly understood. To explore this, we investigated the effects of natural saline soil from Dongying City, Shandong Province, China, on the growth, survival, reproduction, antioxidation, and defense-related gene expression of the earthworm Eisenia foetida. Our findings demonstrate that the growth rate, survival rate, and cocoon production of E. foetida decrease under exposure to natural saline soil in a dose-dependent manner. Elevated levels of DNA damage in coelomocytes and increased reactive oxygen species (ROS) were observed. Additionally, antioxidant enzymes, such as superoxide dismutase (SOD) and catalase (CAT), increased under stress. The mRNA levels of Cyp450 and Hsp70 also rose in response to saline soil exposure. Furthermore, the activity of Na+/K+-ATPase and the expression of the osmotic sensor gene wnk-1 were elevated. In conclusion, our findings indicate that natural saline soil induces antioxidant and osmotic stress in earthworms E. foetida, highlighting the detrimental effects and defense mechanisms of soil fauna under such conditions.

Genomewide identification and analysis of the OSCA gene family in barley (Hordeum vulgare L.)

The hyperosmolality-gated calcium-permeable channels (OSCA) are considered to be osmotic sensors that play an important role in the early stages of hyperosmotic stress response. We analysed the physicochemical properties, chromosome distribution, phylogeny, gene structure and expression pattern of the OSCA gene family in barley, and investigated the expression of HvOSCA genes in barley under drought stress by quantitative real-time polymerase chain reaction (qRT-PCR). Finally, 14 members of the OSCA gene family were identified from barley, all containing the RSN1_7TM domain and often accompanied by the RSN1_TM and PHM7_cyt domains. According to the phylogenetic relationship, they were divided into four subgroups, and each with similar gene structures. Two types of duplicate events in the HvOSCA genes family evolved under the effect of purification selection. By analysing the cis-acting elements in the HvOSCA promoter, it was speculated that the barley OSCA gene family is involved in the regulation of multiple signalling pathways. The expression of HvOSCA genes in barley tissues and their relative expression under drought stress revealed that the gene family plays a role in plant organ differentiation, growth and development and coping with abiotic stress. These results provide new valuable information for the functional analysis of HvOSCA genes and the genetic improvement of barley, and provide an important reference for follow-up research.

Vitamin D Regulates Natriuretic Peptide Receptor‐A Gene Expression via Modulation of Histone Deacetylases and DNA Methyltransferase Activity

Cardiac hormones atrial and brain natriuretic peptides (ANP and BNP) activate guanylyl cyclase/natriuretic peptide receptor‐A (GC‐A/NPRA) and produces the second messenger cGMP. cGMP activates downstream signaling and biological consequences of ANP/NPRA such as natriuresis, diuresis, vasorelaxation, antimitogenic responses, and cardiac and renal anti‐hypertrophic effects. The activity and expression of Npr1 gene (coding for GC‐A/NPRA) assessed primarily through ANP‐stimulated cGMP production, are mediated by agents, including autoregulation involving natriuretic peptides, osmotic sensors, and other external and internal stimuli, but the hormonal and epigenetic mechanisms mediating Npr1 regulation are not well understood. The objective of present study was to study the effect of Vitamin D (vitD) in the regulation of Npr1 gene transcription and expression via modulation of epigenetic factors. Our bioinformatics study with the murine Npr1promoter has shown presence of 4 vitD response elements (VDRE) in the region ‐583 to ‐495 from the transcription start site having perfect and VDRE‐like consensus sequence. Npr1promoter activity was determined by transient transfection of Npr1 promoter deletion constructs in cultured rat thoracic aortic smooth muscle cells (RTASMCs) and mouse mesangial cells (MMCs) treated with Vitamin D₃ (1α, 25‐dihydroxy; VD3) using dual luciferase assay kit. Luciferase assay demonstrated that VD3 enhances Npr1 promoter activity by more than 5‐6‐fold in RTASMCs and MMCs in a dose‐dependent manner. Western blot and densitometry analysis showed increasing concentrations of VD3 significantly induced NPRA protein levels by 3.5‐fold in MMCs and 4.5‐fold in RTASMCs and maximum effect was observed at 100 nM. There was 45%‐50% inhibition of histone deacetylase (HDAC) activity in presence of VD3 in both cell lines as measured by HDAC activity/inhibition ELISA kit. Moreover, treatment with VD3 reduced class I HDAC enzymes, HDAC1 and HDAC3 protein levels and dose‐dependently enhanced acetylation level of histones, H3 at lysine residues 9 and 14 (H3‐K9/14 ac) and H4 at lysine residue 12 (H4‐K14ac). Treatment of RTASMCs and MMCs with VD3 attenuated DNA methyltransferase (DNMT) activity by 40%‐55% as measured by DNMT activity/inhibition ELISA kit. The results demonstrate that VD3 regulate Npr1 gene expression by modulation of histone deacetylases and DNMT activity. The identification of epigenetic targets of vitamin D signaling as a regulator of Npr1gene transcription and protein expression will have important implications in hypertension and cardiovascular regulation.

Plant Salinity Sensors: Current Understanding and Future Directions.

Salt stress is a major limiting factor for plant growth and crop yield. High salinity causes osmotic stress followed by ionic stress, both of which disturb plant growth and metabolism. Understanding how plants perceive salt stress will help efforts to improve salt tolerance and ameliorate the effect of salt stress on crop growth. Various sensors and receptors in plants recognize osmotic and ionic stresses and initiate signal transduction and adaptation responses. In the past decade, much progress has been made in identifying the sensors involved in salt stress. Here, we review current knowledge of osmotic sensors and Na+ sensors and their signal transduction pathways, focusing on plant roots under salt stress. Based on bioinformatic analyses, we also discuss possible structures and mechanisms of the candidate sensors. With the rapid decline of arable land, studies on salt-stress sensors and receptors in plants are critical for the future of sustainable agriculture in saline soils. These studies also broadly inform our overall understanding of stress signaling in plants.

Differential response of cardiac aquaporins to hyperosmotic stress; salutary role of AQP1 against the induced apoptosis.

Multiple pathophysiological conditions are associated with disturbance of myocardial osmotic equilibrium, exerting detrimental effects on cardiac performance. Cardiac myocytes may encounter hyperosmotic stress during hyperglycemia, ischemia/reperfusion injury, myocardial infarction, diabetes mellitus, severe dehydration, hypoxia or heat stress. Aquaporins (AQPs) constitute transmembrane channels that facilitate water transport in response to osmotic gradients. Therefore, the present study aimed at probing into AQPs mode of response and potential role as effector molecules and sensors, under hyperosmotic stress. H9c2 cardiac myoblasts were left untreated (control) or were exposed to 0.5 M sorbitol so as to induce hyperosmotic stress conditions. After the experimental treatments, MTT assay was performed to assess cell viability. Endogenous mRNA levels of AQP1 and AQP7 were assessed by ratiometric RT-PCR. Their subcellular localization pattern was revealed by immunofluorescence microscopy. Protein levels of AQP1 and AQP7, as well as of apoptotic markers (cleaved caspase-3 and PARP), were detected by immunoblot analysis. Hyperosmotic stress (0.5 M sorbitol) induced a time-dependent upregulation of AQP7 (but not of AQP1) mRNA in H9c2 cells. Of note, biochemical and immunocytochemical analyses revealed the increased membrane-associated protein expression of AQP1, under these conditions, while AQP7 respective levels remained unchanged. Interestingly, inhibition of AQP1 by HgCl2, aggravated the sorbitol-induced apoptosis in H9c2 cells, as evidenced by chromatin condensation and fragmentation of caspase-3 and PARP. AQP1 and AQP7 are differentially regulated under hyperosmotic stress conditions in H9c2 cells. AQP1, acting as an osmotic stress sensor and response factor, exerts a beneficial effect against the sorbitol-induced apoptosis, potentially favoring preservation of cardiac function.

Living with Salt

Living with Salt

Structural basis for osmotic regulation of the DNA binding properties of H-NS proteins.

H-NS proteins act as osmotic sensors translating changes in osmolarity into altered DNA binding properties, thus, regulating enterobacterial genome organization and genes transcription. The molecular mechanism underlying the switching process and its conservation among H-NS family members remains elusive. Here, we focus on the H-NS family protein MvaT from Pseudomonas aeruginosa and demonstrate experimentally that its protomer exists in two different conformations, corresponding to two different functional states. In the half-opened state (dominant at low salt) the protein forms filaments along DNA, in the fully opened state (dominant at high salt) the protein bridges DNA. This switching is a direct effect of ionic strength on electrostatic interactions between the oppositely charged DNA binding and N-terminal domains of MvaT. The asymmetric charge distribution and intramolecular interactions are conserved among the H-NS family of proteins. Therefore, our study establishes a general paradigm for the molecular mechanistic basis of the osmosensitivity of H-NS proteins.

An Osmotic Pressure Sensor for Monitoring the Level of Hydration in Biological Fluids

A miniaturized hydration sensor that records the electrolyte balance in biological fluids and that translates this quantity into an osmotic pressure signal has been developed. The sensor measures 16 mm $\times \,\, 8$ mm and integrates a semi-permeable membrane, a piezoresistive pressure transducer, and an electronic readout platform. The dynamic measurement range of 220–340 mOsm $\text{L}^{-1}$ (280 ± 60 mOsm $\text{L}^{-1})$ covers a serious state of dehydration to a serious state of overhydration (represented by a $\Delta \text {hydration} = \pm 20$ %). Ion exchange membranes were found to be the best candidates for integration with the hydration sensor. Reverse osmosis membranes with physical pores (zero Da molecular cutoff) had high ionic leakage rates, in contrast to gas permeable membranes, which proved to be almost impermeable to water vapor. Consequently, the use of a Nafion NR211 membrane facilitated a sensor response with a time constant of 2.5 h, which conforms to the slow change in osmotic pressure associated with hydration. The low-power front-end circuit architecture is based on an analog pressure-to-frequency converter, consuming only 39.4 $\mu \text{W}$ , and capable of recording changes in hydration with a resolution of 5.71 bits (ENOB $_{\mathrm {SNR}})$ . The small size of the sensor components, combined with the interface low-power architecture, renders it suitable for further miniaturization required for implantation into the human body.

Quantifying Ligand Binding and Signal Transduction in Escherichia coli Using the Taz Chimera

The purpose of this project was to alter the EnvZ signal transduction process in Escherichia coli through replacement of its osmotic sensor with the aspartate binding region of the chemotaxis sensor protein Tar as in Utsumi et al.1 Aspartate binds Tar and signals nonrandom movement of the bacteria toward increasing aspartate concentration. EnvZ is normally an osmotic sensor that activates the protein OmpR. OmpR inactivates the OmpF gene promoter, which produces large membrane pores and it also activates the OmpC gene promoter which produces small membrane pores. Activity of the Tar / EnvZ hybrid, Taz will be determined by measuring light output from bacterial luciferase under control of the OmpC promoter. We hypothesize that light output will be proportional to aspartate concentration. A Tar knockout background will be used to prevent competition with the Taz chimera. This research was funded by NSF award #0966085 provided to Hartwick College and the Hartwick College Chemistry Department. 1Utsumi R, Brissette RE, Rampersaud A, Forst SA, Oosawa K, Inouye M. Activation of bacterial porin gene expression by a chimeric signal. Science. 1989.

Design and Characterization of an Osmotic Sensor for the Detection of Events Associated with Dehydration and Overhydration.

The level of hydration in the human body is carefully adjusted to control the electrolyte balance that governs the biochemical processes that sustain life. An electrolyte deficiency caused by de- or overhydration will not only limit human performance, but can also lead to serious health problems and death if left untreated. Because humans can withstand a change in hydration of only n}{}{pm}{20%}, frequent monitoring should be performed in risk groups. This paper presents an osmotic hydration sensor that can record the level of hydration as a function of osmotic pressure in phosphate buffered saline or sodium-chloride solutions that simulate the interstitial fluid in the body. The osmotic pressure is recorded with the aid of an ion-exchange membrane that facilitates the migration of water and cations, in favor of reverse osmosis or gas separation membranes. The hydration sensor is designed to be coupled to an inductively powered readout circuit designed for integration in a micro-implant that has previously been shown to consume only 76 n}{}mu{rm W} of power. The dynamic range spans a state of serious overhydration (220 n}{}{rm mOsm} {rm L}^{-1}) to a serious state of dehydration (340 n}{}{rm mOsm} {rm L}^{-1}) with a response time of n}{}{sim}{rm 7} {rm h} (for a variation of hydration of 20%).

ATHK1 acts downstream of hydrogen peroxide to mediate ABA signaling through regulation of calcium channel activity in Arabidopsis guard cells

Plants gradually develop their ability to tolerate environmental water deficit as part of the evolutionary process. Abscisic acid (ABA) plays a critical role during drought and osmotic stress. Several histidine protein kinases are regarded as osmotic sensors or regulators in the adaptive response of plants to water deficit. In this study, we report that ATHK1, which was previously shown to function as an osmotic regulator, is involved in ABA-induced stomatal signaling in Arabidopsis. Mutants null for ATHK1 expression were unable to transmit normal ABA responses in guard cells, including inducing stomatal closure, producing hydrogen peroxide and activating calcium influx. Moreover, patch clamp and confocal analysis demonstrated that ATHK1 may function downstream of hydrogen peroxide in ABA-induced stomatal closure, by regulating calcium channel activity and calcium oscillation in Arabidopsis guard cells.

The assessment of potentially interfering metabolites and dietary components in blood using an osmotic glucose sensor based on the concanavalin A–dextran affinity assay

Continuous surveillance of blood glucose is a prerogative of maintaining a tight glycaemic control in people suffering from diabetes mellitus. Implantable sensor technology offers the potential of conducting direct long term continuous glucose measurements, but current size restrictions and operational challenges have limited their applications. The osmotic sensor utilises diffusion to create a hydrostatic pressure that is independent of sensor operation and power consumption. This permits ultra-low power architectures to be realized with a minimal start-up time in a package suitable for miniaturization. In contrast, osmotic sensors suffer from the inability of their membranes to discriminate between different constituents in blood or the interstitial fluid that are of comparable size to glucose. By implementing an affinity assay based on the competitive bonding between concanavalin A and dextran, the selectivity of the membrane can be transferred to the glucose specific recognition of the affinity assay. The osmotic effect from the physiological levels of several key metabolites and nutritional components has been addressed identifying in particular ethanol, lactate and amino acids as potential interfering constituents. Both ascorbic acid and mannose would have a normal physiological concentration that is too low to be detected. The studies shows that an osmotic glucose sensor equipped with the con A–dextran affinity assay, is able to filter out potential interfering constituents present in blood, plasma and the interstitial fluid yet retaining a pressure that is proportional to glucose only.

Toward an Injectable Continuous Osmotic Glucose Sensor

The growing pandemic of diabetes mellitus places a stringent social and economic burden on the society. A tight glycemic control circumvents the detrimental effects, but the prerogative is the development of new more effective tools capable of longterm tracking of blood glucose (BG) in vivo. Such discontinuous sensor technologies will benefit from an unprecedented marked potential as well as reducing the current life expectancy gap of eight years as part of a therapeutic regime. A sensor technology based on osmotic pressure incorporates a reversible competitive affinity assay performing glucose-specific recognition. An absolute change in particles generates a pressure that is proportional to the glucose concentration. An integrated pressure transducer and components developed from the silicon micro- and nanofabrication industry translate this pressure into BG data. An in vitro model based on a 3.6 x 8.7 mm large pill-shaped implant is equipped with a nanoporous membrane holding 4-6 nm large pores. The affinity assay offers a dynamic range of 36-720 mg/dl with a resolution of +/-16 mg/dl. An integrated 1 x 1 mm(2) large control chip samples the sensor signals for data processing and transmission back to the reader at a total power consumption of 76 microW. Current studies have demonstrated the design, layout, and performance of a prototype osmotic sensor in vitro using an affinity assay solution for up to four weeks. The small physical size conforms to an injectable device, forming the basis of a conceptual monitor that offers a tight glycemic control of BG.

Autoimmunity to the Sodium-Level Sensor in the Brain Causes Essential Hypernatremia

Na(x) is the sodium-level sensor of body fluids in the brain involved in sodium homeostasis. Na(x)-knockout mice do not stop ingesting salt even when dehydrated. Here we report a case with clinical features of essential hypernatremia without demonstrable hypothalamic structural lesions, who was diagnosed as a paraneoplastic neurologic disorder. The patient had autoantibodies directed against Na(x), along with a ganglioneuroma composed of Schwann-like cells robustly expressing Na(x). The removal of the tumor did not reduce the autoantibody levels or relieve the symptoms. Intravenous injection of the immunoglobulin fraction of the patient's serum into mice induced abnormalities in water/salt intake and diuresis, which led to hypernatremia. In the brains of these mice, cell death was observed along with focal deposits of complement C3 and inflammatory infiltrates in circumventricular organs where Na(x) is specifically expressed. Our findings thus provide new insights into the pathogenesis of hypernatremia relevant to the sodium-level-sensing mechanism in humans.

Systemic osmotic signaling pathways function upstream of WNK and GCK‐VI kinases to regulate hypertonic stress resistance in C. elegans

The GCK‐VI subfamily of Ste20 kinases regulate volume‐sensitive ion channels and transporters. With no lysine (K) or WNK kinases interact with and activate the mammalian GCK‐VI kinases, PASK and OSR‐1. We recently reported that the C. elegans GCK‐VI kinase, GCK‐3, interacts and functions with the WNK kinase, WNK‐1, to regulate systemic volume and survival during hypertonic stress. The pathways that function upstream of WNK and GCK‐VI kinases are not defined. In C. elegans, cuticle collagens (e.g., DPY‐10) and novel proteins that are secreted from an epithelium near the cuticle (e.g., OSM‐11) have been postulated to be components of systemic mechanosensors that detect hypertonic shrinkage. Mutations in DPY or OSM proteins cause an osmotic stress resistant phenotype (Osr) characterized by elevated levels of the organic osmolyte glycerol and survival during exposure to hypertonic stress that is lethal to wild type worms. To determine if WNK‐1 or GCK‐3 function downstream from DPY or OSM proteins, we performed RNAi experiments. Knockdown of either kinase suppressed Osr in DPY and OSM mutants, but did not suppress glycerol accumulation or alter systemic volume. Our results demonstrate that WNK‐1 and GCK‐3 function downstream from systemic osmotic sensors to regulate hypertonic stress resistance by mechanisms that are independent from glycerol accumulation and volume regulation.

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.

E. coli squeezed into action

Escherichia coli chemoreceptors double as osmotic sensors by mechanically compressing in response to increased osmolarity, say Ady Vaknin and Howard Berg (Harvard University, Cambridge, MA). E. coli is always on the look-out for a better environment. As it swims, chemoattractant receptors talk to the flagellar motors, thus orienting the bacterium's travels. Using fluorescence polarization to image the receptors' position in living cells, Vaknin and Berg found that increased osmolarity caused receptors, joined in triplets like the legs of a tripod, to move closer together by about 10%. The squeeze stimulates kinase activity and the subsequent signaling pathway, prompting the bacterium to swim away from the potentially damaging environment. This compression can be explained by simple cell membrane dynamics. As osmotic stress increases, water leaves the cell. Reduced pressure from within causes a slackness in the membrane and an increase in its thickness—much as a rubber balloon acts as some air is let out. “We think that when the membrane thickens in response to osmotic stress, that changes the orientation of the receptors, making them move closer together,” says Berg. How changes in relative receptor position stimulate kinase activity is still unknown. The group is now investigating whether chemoattractant stimuli cause receptors to move further apart. They are also looking downstream at the effect of such mechanical perturbations on the flagellar motor's control of direction. Reference: Vaknin, M., and H.C. Berg. 2005. Proc. Natl. Acad. Sci. USA. doi:.10.1073/pnas.0510047103 [PubMed] [Cross Ref]

Abnormal osmotic regulation in trpv4-/- mice.

Osmotic homeostasis is one of the most aggressively defended physiological parameters in vertebrates. However, the molecular mechanisms underlying osmotic regulation are poorly understood. The transient receptor potential channel, vanilloid subfamily (TRPV4), is an osmotically activated ion channel that is expressed in circumventricular organs in the mammalian CNS, which is an important site of osmotic sensing. We have generated trpv4-null mice and observed abnormalities of their osmotic regulation. trpv4-/- mice drank less water and became more hyperosmolar than did wild-type littermates, a finding that was seen with and without administration of hypertonic saline. In addition, plasma levels of antidiuretic hormone were significantly lower in trpv4-/- mice than in wild-type littermates after a hyperosmotic challenge. Continuous s.c. infusion of the antidiuretic hormone analogue, dDAVP, resulted in systemic hypotonicity in trpv4-/- mice, despite the fact that their renal water reabsorption capacity was normal. Thus, the response to both hyper- and hypoosmolar stimuli is impaired in trpv4-/- mice. After a hyperosmolar challenge, there was markedly reduced expression of c-FOS in the circumventricular organ, the organum vasculosum of the lamina terminalis, of trpv4-/- mice compared with wild-type mice. This finding suggests that there is an impairment of osmotic sensing in the CNS of trpv4-/- mice. These data indicate that TRPV4 is necessary for the normal response to changes in osmotic pressure and functions as an osmotic sensor in the CNS.

The role of the G2 box, a conserved motif in the histidine kinase superfamily, in modulating the function of EnvZ.

Histidine kinase EnvZ, a transmembrane osmotic sensor for Escherichia coli, is a bifunctional enzyme having OmpR (its cognate response regulator) kinase and phosphorylated OmpR (OmpR-P) phosphatase activities. Its cytoplasmic domain consists of domain A responsible for dimerization of EnvZ, histidine phosphotransfer and phosphatase activities, and domain B responsible for ATP binding. Here, we have constructed a number of substitution mutations at the G2 box, one of the conserved motifs in domain B, and demonstrated that they influence the phosphatase activity of EnvZ over a wide range. The effects of ADP, a cofactor for the phosphatase activity, were found to be substantially different depending upon the mutations. The effects of these mutations were also examined in vivo using a chimeric Tar-EnvZ construct (Taz1-1), and the results agreed with the in vitro data for the phosphatase and kinase activities for all mutations. Using Taz1-1 carrying the T402A mutation, three independent intragenic suppressor mutations (T235M, S269L and E276K) were isolated, and all were found in domain A. Together, the present results demonstrate for the first time that domain A and domain B are functionally co-ordinated and topologically arranged in a specific manner. The G2 box may modulate the interaction between these two domains in response to extracellular osmolarity.

Selective response of human airway epithelia to luminal but not serosal solution hypertonicity. Possible role for proximal airway epithelia as an osmolality transducer.

The response of cultured human nasal epithelia to hypertonic bathing solutions was tested using ion-selective microelectrode and quantitative microscopy. Raised luminal, but not serosal, osmolality (+/- 150 mM mannitol) decreased Na+ absorption but did not induce Cl- secretion. Raised luminal osmolality increased cell Cl- activity, Na+ activity, and transepithelial resistance and decreased both apical and basolateral membrane potentials and the fractional resistance of the apical membrane; equivalent circuit analysis revealed increases in apical, basolateral, and shunt resistances. Prolonged exposure (10 min) to 430 mosM luminal solution elicited no regulation of any parameter. Optical measurements revealed a reduction in the thickness of preparations only in response to luminal hypertonic solutions. We conclude that (a) airway epithelial cells exhibit asymmetric water transport properties, with the apical membrane water permeability exceeding that of the basolateral membrane; (b) the cellular response to volume loss is a deactivation of the basolateral membrane K+ conductance and the apical membrane Cl- conductance; (c) luminal hypertonicity slows the rate of Na+ absorption but does not induce Cl- secretion; and (d) cell volume loss increases the resistance of the paracellular path. We speculate that these properties configure human nasal epithelium to behave as an osmotic sensor, transducing information about luminal solutions to the airway wall.