Reduced hepatic aquaporin-9 and glycerol permeability are related to insulin resistance in non-alcoholic fatty liver disease.

BACKGROUND/OBJECTIVES: Glycerol represents an important metabolite for the control of lipid accumulation and hepatic gluconeogenesis. We investigated whether hepatic expression and functionality of aquaporin-9 (AQP9), a channel mediating glycerol influx into hepatocytes, is impaired in non-alcoholic fatty liver disease (NAFLD) and steatohepatitis (NASH) in the context of insulin resistance. SUBJECTS/METHODS: Liver biopsies were obtained from 66 morbid obese patients undergoing bariatric surgery (66% women, mean body mass index (BMI) 46.1±1.0 kg m(-2)) with available liver echography and pathology analysis of the biopsies in this cross-sectional study. Subjects were classified according to normoglycemia (NG), impaired glucose tolerance (IGT) or type 2 diabetes (T2D). Hepatic expression of AQP9 was analyzed by real-time PCR, western blotting and immunohistochemistry, while glycerol permeability (P(gly)) was measured by stopped-flow light scattering. RESULTS: AQP9 was the most abundantly (P<0.0001) expressed aquaglyceroporin in human liver (AQP9>>>AQP3>AQP7>AQP10). Obese patients with T2D showed increased plasma glycerol as well as lower P(gly) and hepatic AQP9 expression. The prevalence of NAFLD and NASH in T2D patients was 100 and 65%, respectively. Interestingly, AQP9 expression was decreased in patients with NAFLD and NASH as compared with those without hepatosteatosis, in direct relation to the degree of steatosis and lobular inflammation, being further reduced in insulin-resistant individuals. The association of AQP9 with insulin sensitivity was independent of BMI and age. Consistent with these data, fasting insulin and C-reactive protein contributed independently to 33.1% of the hepatic AQP9 mRNA expression variance after controlling for the effects of age and BMI. CONCLUSIONS: AQP9 downregulation together with the subsequent reduction in hepatic glycerol permeability in insulin-resistant states emerges as a compensatory mechanism whereby the liver counteracts further triacylglycerol accumulation within its parenchyma as well as reduces hepatic gluconeogenesis in patients with NAFLD.

An innovative methodology for the automated morphometric and quantitative estimation of liver steatosis.

The management of liver steatosis, due to its potential evolution towards severe diseases, requires accurate diagnosis. Fatty infiltration in liver diseases is commonly assessed histologically by semi-quantitative methods, which can be subjective. Automated computerized procedures using commercial software for image analysis have also been recently employed. The aim of the study was to develop an innovative automated computerized procedure to accurately evaluate both the morphometry and degree of lipid accumulation in liver. Fatty infiltration was assessed in paraffin- and resin-embedded samples of steatotic livers from rats undergoing 0, 3, 7, 14, and 30-day choline-deficient diet, and from liver biopsy of a morbidly obese patient undergoing bariatric surgery. Specific software was developed, which works with a morphological operator, in addition to a chromatic one to select lipid droplets. The choline-deficient diet induced steatosis with a gradual shift from micro- to macro-vesicular. In paraffin sections, the macrovesicles-to-microvesicles ratio and the degree of steatosis, when using only the chromatic operator, produced overestimates. Results were consistent in both rat and human samples. An improvement of topographic, morphometric and quantitative estimation of fatty liver infiltration is obtained with our software, working with a morphological operator and using semi-thin sections from resin-embedded samples. This innovative procedure may be applied to human liver samples, offering promising diagnostic and prognostic perspectives.

Severe liver steatosis correlates with nitrosative and oxidative stress in rats.

BACKGROUND: Little is known about nitric oxide (NO) metabolism and redox changes with hepatocyte adipocytic transformation. The aims of this study were to investigate the changes occurring in plasma and hepatic NO metabolites and redox balance in a rat experimental model of simple fatty liver, and to relate plasma with hepatic and mitochondrial changes at different degrees of steatosis. MATERIALS AND METHODS: Circulating and hepatic redox active and nitrogen regulating molecules thioredoxin, glutathione, protein thiols (PSH), mixed disulfides (PSSG), NO metabolites nitrosothiols, nitrite plus nitrate (NOx), and lipid peroxides (TBARs) were measured in rats fed a choline deprived (CD) diet for 30 days. RESULTS: At histology, the CD diet resulted in hepatocellular steatosis (75% of liver weight at day 30) with no signs of necro-inflammation. In plasma, thioredoxin, nitrosothiols and NOx were unchanged, while TBARs levels increased significantly and were positively related with hepatic TBARs (r = 0.87, P < 0.001) and lipid content (r = 0.90, P < 0.001). In the liver, glutathione initially increased (day 3) and then decreased. From day 14, PSH decreased and NO derivatives increased. Thioredoxin 1 had initially increased (days 7-14) and then decreased. In the mitochondria, on day 14, nitrosothiols were inversely related to thioredoxin 2 (r = 0.988, P < 0.05); on day 30, PSH were decreased by 70%, PSSG were doubled and related with nitrosothiols levels (r = 0.925, P < 0.001). CONCLUSION: Adipocytic transformation of hepatocytes is accompanied by major interrelated modifications of redox parameters and NO metabolism especially at mitochondrial level, suggesting an early adaptive protective response but also an increased predisposition towards pro-oxidant insults.

Aquaporins in the hepatobiliary tract. Which, where and what they do in health and disease.

The biological importance of the aquaporin family of water channels was recently acknowledged by the 2003 Nobel Prize for Chemistry awarded to the discovering scientist Peter Agre. Among the pleiotropic roles exerted by aquaporins in nature in both health and disease, the review addresses the latest acquisitions about the expression and regulation, as well as physiology and pathophysiology of aquaporins in the hepatobiliary tract. Of note, at least seven out of the thirteen mammalian aquaporins are expressed in the liver, bile ducts and gallbladder. Aquaporins are essential for bile water secretion and reabsorption, as well as for plasma glycerol uptake by the hepatocyte and its conversion to glucose during starvation. Novel data are emerging regarding the physio-pathological involvement of aquaporins in multiple diseases such as cholestases, liver cirrhosis, obesity and insulin resistance, fatty liver, gallstone formation and even microparasite invasion of intrahepatic bile ducts. This body of knowledge represents the mainstay of present and future research in a rapidly expanding field.

Ion transport across the gallbladder epithelium.

The function of the gallbladder is not only to store bile, but also to concentrate it during the interdigestive phase by means of salt-dependent water reabsorption. On the contrary, secretions of water and salt take place during the digestive phase. Dysregulation of ion absorption or secretion are common in many gallbladder diseases, such as colelithiasis. Transepithelial absorptions are determined by the Na+/K+ pump on the basolateral membrane, and by several apical membrane Na(+)-coupled transporters. Among these, some isoforms of Na+/H+ and Cl-/HCO3(-) exchangers have been studied. The presence of a Na(+)-Cl(-) simport has been molecularly and functionally characterized in some animal species. The ion transepithelial secretion is mainly dependent on an apical chloride transport attributable to a CFTR-like cAMP-activated channel with high permeability to HCO3(-). The apical membrane electrical potential is one of the factors influencing anion secretion and is maintained by the activity of cAMP-dependent K+ channels. The regulation of the activity of these channels is complex, because of their sensitivity to voltage, and to intracellular calcium and pH. The coordinated interplay underlying the regulation of transporters and channels needs to be clarified yet, as well as the interactions between transporters, channels and aquaporins.

Stimulation of Xenopus P2Y1 receptor activates CFTR in A6 cells.

Nucleotide binding to purinergic P2Y receptors contributes to the regulation of a variety of physiological functions in renal epithelial cells. Here, we investigate the regulatory mechanism of the P2Y1 receptor agonist 2-methylthioadenosine diphosphate (2-MeSADP) on Cl- transport in A6 cells, a commonly used model of the distal section of the Xenopus laevis nephron. Protein and mRNA expression analysis together with functional measurements demonstrated the basolateral location of the Xenopus P2Y1 receptor. 2-MeSADP increased intracellular [Ca2+] and cAMP and Cl- efflux, responses that were all inhibited by the specific P2Y1 receptor antagonist MRS 2179. Cl- efflux was also inhibited by the cystic fibrosis transmembrane conductance regulator (CFTR) blocker glibenclamide. Inhibition of either protein kinase A (PKA) or the binding between A-kinase-anchoring proteins (AKAPs) and the regulatory PKA RII subunit blocked the 2-MeSADP-induced activation of CFTR, suggesting that PKA mediates P2Y1 receptor regulation of CFTR through one or more AKAPs. Further, the truncation of the PDZ1 domain of the scaffolding protein Na+/H+ exchanger regulatory factor-2 (NHERF-2) inhibited 2-MeSADP-dependent stimulation of Cl- efflux, suggesting the involvement of this scaffolding protein. Activation or inhibition of PKC had no effect per se on basal Cl- efflux but potentiated or reduced the 2-MeSADP-dependent stimulation of Cl- efflux, respectively. These data suggest that the X laevis P2Y1 receptor in A6 cells can increase both cAMP/PKA and Ca2+/PKC intracellular levels and that the PKC pathway is involved in CFTR activation via potentiation of the PKA pathway.

Possible functional implications of aquaporin water channels in reproductive physiology and medically assisted procreation.

Spermatogenesis, the maturation of spermatozoa and their concentration and storage in the seminiferous vessels are associated with considerable fluid secretion or absorption in the male reproductive tract. These fluid movements are in total agreement with the presence of multiple aquaporin (AQP) water channel proteins in germ cells and other tissues within the male reproductive tract. A series of functions of prime importance have already been hypothesized for aquaporins in the physiology of male reproduction. Aquaporins could be involved in the early stages of spermatogenesis, in the secretion of tubular liquid and in the concentration and storage of spermatozoa in the epididymis. In the male reproductive tract, alterations in the expression and functionality and/or regulation of aquaporins have already been demonstrated to be at the basis of forms of male infertility. Indeed, rats with reduced reabsorption of seminiferous fluid in the efferent ducts have been shown to be sub-fertile or infertile. Functions have also been suggested in the fertilization process, where aquaporins may play a role in maintaining osmotic homeostasis in gametes during fertilization. Aquaporins have also been suggested to mediate water movement into antral follicles and to be the pathway for transtrophectodermal water movement during cavitation. Aquaporins are the subject of considerable technological interest for cryopreservation used in medically assisted procreation, as they could be the molecular pathway by which water and/or solutes move across the plasma membrane during the process of freezing/thawing gametes and embryos. Indeed, artificial expression ofAQP3 has been showed to improve the survival of mouse oocytes after cryopreservation.

Pathways of cholesterol crystallization in model bile and native bile.

Hypersecretion of hepatic cholesterol, chronic supersaturation of bile with cholesterol and rapid precipitation of cholesterol crystals in the gallbladder from cholesterol-enriched vesicles represent the primum movens in cholesterol gallstone formation. Physical-chemical factors and pathways leading to cholesterol crystallization can be investigated in artificial model biles and ex vivo in fresh human bile. Depending on modulatory factors (i.e., lipid concentration, bile salt or phospholipid species, humidity, mucins, etc.), cholesterol can precipitate in several forms (i.e., monohydrate, anhydrous) and habits (i.e., plate-like, needle-like, intermediate arcs, filaments, tubules, spirals). Careful analysis of biliary cholesterol crystals includes biochemical analysis of precipitated crystals, polarizing quantitative light microscopy, and turbidimetric methods. In this paper, recent concepts on cholesterol crystallization in artificial model biles as well as in human bile will be reviewed.

Possible involvement of aquaporin-7 and -8 in rat testis development and spermatogenesis.

Fluid secretion and reabsorption are of central importance in male reproductive (MR) physiology. However, the related molecular mechanisms are poorly known. Here, potential roles for AQP7 and AQP8, two aquaporin water channels abundantly expressed in the MR tract, were investigated by studying their expression and distribution in the developing testis of the Wistar rat. By semiquantitative RT-PCR and immunoblotting, first expression of AQP7 was noted at postnatal day 45 (P45), with levels increasing substantially at P90 and remaining at high levels thereafter. AQP8 began to be expressed at P15, rapidly increased until P20, and remained fairly stable thereafter. Immunohistochemical analyses demonstrated AQP7 in elongated spermatids, testicular spermatozoa, and residual bodies at P45 with increased signal intensity thereafter. AQP8 was observed in primary spermatocytes from P20 to P30 and, in elongated spermatids, residual bodies and Sertoli cells at P30 and thereafter. The ontogeny and distribution of AQP7 and AQP8 in rat testis suggest involvement in major physiologic changes in testis development and spermatogenesis.

Expression and localization of the aquaporin-8 water channel in rat testis.

Spermatogenesis and sperm maturation and storage are accompanied by significant movements of water, and multiple aquaporin transmembrane water channels (AQPs) have been recognized in the male reproductive tract. Nevertheless, the involvement of aquaporins in male reproductive physiology is mostly unknown. Here the expression and localization of AQP8 in rat spermatogenesis is defined and compared to that of AQP7, another aquaporin expressed in male germ cells. AQP8 mRNA was found in testis but not in epididymis, whereas the AQP7 transcript was present in both locations. By immunoblotting, the AQP8 protein was detected as a 25-kDa band and a 32- to 40-kDa diffuse component corresponding to the core and glycosylated protein, respectively. Membrane fractionation revealed AQP8 both in microsomal and plasma membrane-enriched fractions of rat testis while no apparent bands were detected in epididymis. AQP7 appeared as a 23- to 24-kDa band and was found both in testis and epididymis. By immunofluorescence, AQP8 labeling was found intracellularly as well as over the plasma membrane of germ cells throughout spermatogenesis. AQP7 was present in spermatids and spermatozoa and was predominant over the plasma membrane. AQP8 may be involved in the cytoplasmic condensation occurring during differentiation of spermatids into spermatozoa and in the generation of seminiferous tubule fluid.

Expression and immunolocalization of the aquaporin-8 water channel in rat gastrointestinal tract.

A remarkable amount, of water is transported in the gastrointestinal (GI) organs to fulfil the secretory and absorptive functions of the GI tract. However, the molecular basis of water movement in the GI epithelial barriers is still poorly known. Important clues about the mechanisms by which water is transported in the GI tract were provided by the recent identification of multiple aquaporin water channels expressed in GI tissues. Here we define the mRNA and protein expression and the cellular and subcellular distribution of aquaporin-8 (AQP8) in the rat GI tract. By semi-quantitative RT-PCR the AQP8 mRNA was detected in duodenum, proximal jejunum, proximal colon, rectum, pancreas and liver and, to a lesser extent, in stomach and distal colon. Immunohistochemistry using affinity-purified antibodies revealed AQP8 staining in the absorptive epithelial cells of duodenum, proximal jejunum, proximal colon and rectum where labeling was largely intracellular and confined to the subapical cytoplasm. Confirming previous results, AQP8 staining was seen at the apical pole of pancreatic acinar cells. Interestingly, both light and immunoelectron microscopy analyses showed AQP8 reactivity in liver where labeling was associated to hepatocyte intracellular vesicles and over the plasma membrane delimiting the bile canaliculi. A complex pattern was observed by immunoblotting with total membranes of the above GI organs incubated with affinity-purified anti-AQP8 antibodies which revealed multiple bands with molecular masses ranging between 28 and 45 kDa. This immunoblotting pattern was not modified after deglycosylation with N-glycosidase F except the 34-kDa band of liver that, as already reported, was partially down-shifted to 28 kDa. No bands were detected after preadsorption of the anti-AQP8 antibodies with the immunizing peptide. The cellular and subcellular distribution of AQP8 suggest physiological roles for this aquaporin in the absorption of water in the intestine and the secretion of bile and pancreatic juice in liver and pancreas, respectively. The large intracellular expression of AQP8 may indicate its recycling between the cytoplasmic compartment and the plasma membrane. The cytoplasmic localization observed may also relate to the involvement of AQP8 in processes of intracellular osmoregulation.

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.

Functional reconstitution and characterization of AqpZ, the E. coli water channel protein.

Understanding the selectivity of aquaporin water channels will require structural and functional studies of wild-type and modified proteins; however, expression systems have not previously yielded aquaporins in the necessary milligram quantities. Here we report expression of a histidine-tagged form of Escherichia coli aquaporin-Z (AqpZ) in its homologous expression system. 10-His-AqpZ is solubilized and purified to near homogeneity in a single step with a final yield of approximately 2.5 mg/l of culture. The histidine tag is removed by trypsin, yielding the native protein with the addition of three N-terminal residues, as confirmed by microsequencing. Sucrose gradient sedimentation analysis showed that the native, solubilized AqpZ protein is a trypsin-resistant tetramer. Unlike other known aquaporins, AqpZ tetramers are not readily dissociated by 1% SDS at neutral pH. Hydrophilic reducing agents have a limited effect on the stability of the tetramer in 1% SDS, whereas incubations for more than 24 hours, pH values below 5.6, or exposure to the hydrophobic reducing agent ethanedithiol cause dissociation into monomers. Cys20, but not Cys9, is necessary for the stability of the AqpZ tetramer in SDS. Upon reconstitution into proteoliposomes, AqpZ displays very high osmotic water permeability (pf > or = 10 x 10(-14) cm3 s-1 subunit-1) and low Arrhenius activation energy (Ea = 3.7 kcal/mol), similar to mammalian aquaporin-1 (AQP1). No permeation by glycerol, urea or sorbitol was detected. Expression of native and modified AqpZ in milligram quantities has permitted biophysical characterization of this remarkably stable aquaporin tetramer, which is being utilized for high-resolution structural studies.

Cloning, structural organization and chromosomal localization of the mouse aquaporin-8 water channel gene (Aqp8).

The gene encoding the mouse Aquaporin-8 water channel protein (Aqp8) was cloned and its genomic structure was defined. Aqp8 consists of six exons with boundaries at amino acids 1-4, 5-87, 88-129, 130-201, 202-246 and 247-261 which partially correspond to those of other known aquaporin genes. All splice sites conform to the GT-AG rule except the first one which is GT-GG. Primer extension and RNase protection analyses using mouse liver RNA demonstrated three initiation transcription sites located 385, 156 and 146 bp upstream from the translational start codon. No defined TATA box was found in the 5'-flanking region where numerous CAAT motifs and one GATA box were identified. Fluorescence in situ hybridization localized the Aqp8 locus to mouse chromosome 7F3. The 7F region is syntenic with human chromosomes 11, 16 and 10. These results (i) reveal marked structural distinction between the Aqp8 gene and the other known mammalian aquaporin genes, (ii) may now permit the molecular characterization of Aqp8 expression and (iii) represent a fundamental step for the construction of a target vector to generate transgenic Aqp8 knockout mice.

Visualization of AqpZ-mediated water permeability in Escherichia coli by cryoelectron microscopy.

Transport of water across the plasma membrane is a fundamental process occurring in all living organisms. In bacteria, osmotic movement of water across the cytoplasmic membrane is needed to maintain cellular turgor; however, the molecular mechanisms of this process are poorly defined. Involvement of aquaporin water channels in bacterial water permeability was suggested by the recent discovery of the aquaporin gene, aqpZ, in Escherichia coli. By employing cryoelectron microscopy to compare E. coli cells containing (AqpZ+) and lacking (AqpZ-) aquaporin, we show that the AqpZ water channel rapidly mediates large water fluxes in response to sudden changes in extracellular osmolarity. These findings (i) demonstrate for the first time functional expression of a prokaryotic water channel, (ii) evidence the bidirectional water channel feature of AqpZ, (iii) document a role for AqpZ in bacterial osmoregulation, and (iv) define a suitable model for studying the physiology of prokaryotic water transport.

Regulation of the Escherichia coli water channel gene aqpZ.

Osmotic movement of water across bacterial cell membranes is postulated to be a homeostatic mechanism for maintaining cell turgor. The molecular water transporter remained elusive until discovery of the Escherichia coli water channel, AqpZ, however the regulation of the aqpZ gene expression and physiological function of the AqpZ protein are unknown. Northern analysis revealed a transcript of 0.7 kb, confirming the monocistronic nature of aqpZ. Regulatory studies performed with an aqpZ::lacZ low copy plasmid demonstrate enhanced expression during mid-logarithmic growth, and expression of the gene is dependent upon the extracellular osmolality, which increased in hypoosmotic environments but strongly reduced in hyperosmolar NaCl or KCl. While disruption of the chromosomal aqpZ is not lethal for E. coli, the colonies of the aqpZ knockout mutant are smaller than those of the parental wild-type strain. When cocultured with parental wild-type E. coli, the aqpZ knockout mutant exhibits markedly reduced colony formation when grown at 39 degrees C. Similarly, the aqpZ knockout mutant also exhibits greatly reduced colony formation when grown at low osmolality, but this phenotype is reversed by overexpression of AqpZ protein. These results implicate AqpZ as a participant in the adaptive response of E. coli to hypoosmotic environments and indicate a requirement for AqpZ by rapidly growing cells.

The aquaporin-Z water channel gene of Escherichia coli: structure, organization and phylogeny.

Aquaporin water channel proteins are found throughout the plant and animal kingdoms, but the first prokaryotic water channel gene, aqpZ, was only recently identified in wild type Escherichia coli (Calamita G et al (1995) J Biol Chem 270, 29063-29066). Here we define the organization of aqpZ in E coli, produce the AqpZ protein and compare the AqpZ phylogeny to that of some known bacterial homologs. Physical mapping and sequence analyses confirmed the location of aqpZ at minute 19.7 on the E coli chromosome where it is transcribed counterclockwise. The monocistronic nature of aqpZ was clearly indicated by the structural organization of its surrounding genes, ybjD and ybjE' and by the presence of a typical Rho-independent transcriptional terminator following the aqpZ stop codon. Computer sequence analysis indicated the -35/-10 region located 72 bases upstream of the aqpZ start codon as the most likely aqpZ promoter. A series of potential cis-regulatory elements were found in the 400 bp region preceding the aqpZ ORF. The AqpZ protein, produced under T7 phi 10 control, showed a size of about 20 kDa by SDS-PAGE. Striking similarities were found between the E coli aqpZ and a gene included in the genome of the cyanobacterium Synechocystis sp PCC6803, a species permanently living a fresh water. These results may represent a fundamental step to characterize the regulation and the physiological features of the AqpZ water channel in prokaryotes.

Molecular cloning and characterization of AqpZ, a water channel from Escherichia coli.

The aquaporin family of molecular water channels is widely expressed throughout the plant and animal kingdoms. No bacterial aquaporins are known; however, sequence-related bacterial genes have been identified that encode glycerol facilitators (glpF). By homology cloning, a novel aquaporin-related DNA (aqpZ) was identified that contained no surface N-glycosylation consensus. The aqpZ RNA was not identified in mammalian mRNA by Northern analysis and exhibited bacterial codon usage preferences. Southern analysis failed to demonstrate aqpZ in mammalian genomic DNA, whereas a strongly reactive DNA was present in chromosomal DNA from Escherichia coli and other bacterial species and did not correspond to glpF. The aqpZ DNA isolated from E. coli contained a 693-base pair open reading frame encoding a polypeptide 28-38% identical to known aquaporins. When compared with other aquaporins, aqpZ encodes a 10-residue insert preceding exofacial loop C, truncated NH2 and COOH termini, and no cysteines at known mercury-sensitive sites. Expression of aqpZ cRNA conferred Xenopus oocytes with a 15-fold increase in osmotic water permeability, which was maximal after 5 days of expression, was not inhibited with HgCl2, exhibited a low activation energy (Ea = 3.8 kcal/mol), and failed to transport nonionic solutes such as urea and glycerol. In contrast, oocytes expressing glpF transported glycerol but exhibited limited osmotic water permeability. Phylogenetic comparison of aquaporins and homologs revealed a large separation between aqpZ and glpF, consistent with an ancient gene divergence.

Water pathways across a reconstituted epithelial barrier formed by Caco-2 cells: effects of medium hypertonicity.

Caco-2 cells, originated in a human colonic cancer, are currently used as model systems to study transepithelial transports. To further characterize their water permeability properties, clone P1 Caco-2 cells were cultured on permeable supports. At confluence, the transepithelial net water movement (Jw), mannitol permeability (Ps), and electrical resistance (R) were simultaneously measured. The observed results were correlated with transmission and freeze-fracture electron microscopy studies and compared with those obtained, in similar experimental conditions, in a typical mammalian epithelial barrier: the rabbit rectum. When the serosal solution was made hypertonic (50 mM polyethylene glycol-PEG), the spontaneously observed secretory Jw rapidly reversed, became absorptive and then stabilized. Simultaneously, the R values dropped and Ps went up. In the case of the rabbit rectal epithelium, a similar treatment did not elicit significant changes in the water permeability during the first 20 min following the osmotic challenge while there was a significant increase in the transepithelial resistance. After exposure to serosal hypertonicity, several morphological modifications developed in the Caco-2 cells: Localized dilations in the intercellular spaces and vacuoles in the cytoplasm appeared. Nevertheless, most cells remained in contact and no evidence of cell shrinking was observed. Simultaneously, the tight-junction structure was more or less disorganized. The filament network lost its sharpness and "omega" figures appeared, bordering the intercellular spaces. In some cases the tight-junction network was completely disrupted. In the case of the rabbit rectum the structural modifications were completely different: Serosal hypertonicity rapidly induced cell shrinking and the opening of the intercellular spaces, with no noticeable change in the tight-junction structure.(ABSTRACT TRUNCATED AT 250 WORDS)