Expression Of Water Channel Protein Research Articles

Bifidobacterium longum S3 alleviates loperamide-induced constipation by modulating intestinal acetic acid and stearic acid levels in mice.

Constipation is a major gastrointestinal (GI) symptom worldwide, with diverse causes of formation, and requires effective and safe therapeutic measures. In the present study, we used loperamide hydrochloride to establish a constipation model and assessed the effect of Bifidobacterium on constipation and its possible mechanism of relief. The results showed that B. longum S3 exerted a constipation-relieving effect primarily by improving the gut microbiota, enriching genera including Lactobacillus, Alistipes, and Ruminococcaceae UCG-007, and decreasing the bacteria Lachnospiraceae NK4B4 group. These changes may thereby increase acetic acid and stearic acid (C18:0) levels, which significantly increase the expression levels of ZO-1 and MUC-2, repair intestinal barrier damage and reduce inflammation (IL-6). Furthermore, it also inhibited oxidative stress levels (SOD and CAT), decreased the expression of water channel proteins (AQP4 and AQP8), significantly elevated the Gas, 5-HT, PGE2, and Ach levels, and reduced nNOS and VIP levels to improve the intestinal luminal transit time and fecal water content. Collectively, these changes resulted in the alleviation of constipation.

Exogenous Easily Extractable Glomalin-Related Soil Protein Induces Differential Response in Plant Growth of Tea Plants via Regulating Water Channel Protein Expression

Glomalin, a glycoprotein secreted by arbuscular mycorrhizal fungi (AMFs), exhibits multiple beneficial functions in regard to plant growth. However, the roles and regulatory mechanisms of exogenous easily extractable glomalin-related soil protein (EE-GRSP) in water and their effects on the quality of tea plants (Camellia sinensis (L.) O. Ktze.) remain unclear. The present study aimed to investigate the effects of a quarter-strength exogenous EE-GRSP solution (1/4 EE-GRSP), half-strength exogenous EE-GRSP solution (1/2 EE-GRSP), three-quarter-strength exogenous EE-GRSP solution (3/4 EE-GRSP), and full-strength exogenous EE-GRSP solution (full EE-GRSP) on plant growth, the root system architecture, leaf water status, and the tea quality of tea seedlings, along with examining the changes in the relative expression of water channel proteins (AQPs) in tea plants. The results indicated that exogenous EE-GRSP of different strengths had different effects on both the growth performance (height, leaf numbers, and biomass) and root architecture parameters of tea seedlings, and the best positive effects on plant growth and the root architecture appeared under the three-quarter-strength exogenous EE-GRSP treatment. Similarly, the exogenous EE-GRSP application also differently affected tea quality indicators, in which only the quarter- and half-strength exogenous EE-GRSP solutions significantly increased most of the indicators, including carbohydrates, tea polyphenols, total amino acids, catechins, and flavonoids. Moreover, the half- and three-quarter-strength exogenous EE-GRSP treatments significantly increased the leaf relative water content (LRWC), but all of the exogenous EE-GRSP treatments significantly decreased the leaf water potential (LWP). Furthermore, the expression of AQP genes in the root system of tea plants was related to the strength of the exogenous EE-GRSP treatments, and different genes were significantly up-regulated or down-regulated under the treatment of exogenous EE-GRSP at different strengths. Moreover, the correlation analysis showed that most of the relative expression of AQPs was significantly and positively correlated with tea plant growth, the root architecture, and the leaf relative water content, but negatively correlated with tea quality indicators; however, the expression of CsNIPs and CsSIPs was markedly and negatively correlated with plant growth performance. Therefore, we speculated that the application of exogenous EE-GRSP could facilitate plant growth and improve the quality indirectly by regulating the expression of root AQPs, thus ameliorating the water uptake and nutrient accumulation in tea plants.

Effects of Betaine Supplementation on Live Performance, Selected Blood Parameters, and Expression of Water Channel and Stress-Related mRNA Transcripts of Delayed Placement Broiler Chicks.

This study examined the effect of supplemental betaine on live performance, selected blood parameters, and gene expression of water channel proteins (Aquaporins, AQP) of broiler chicks delayed in placement for 48 h post-hatch. In total, 540 newly-hatched male broiler chicks were obtained from a local hatchery and were randomly allotted to one of five treatments with nine replicates per treatment (12 chicks per replicate). Chicks were either placed immediately, control; held for 48 h post-hatch with no access to feed or water, Holdnull; held for 48 h with free access to drinking water only, HoldW; held for 48 h with free access to drinking water supplemented with 1 ml per L of betaine solution (40% betaine), HoldB1; or held for 48 h with free access to drinking water supplemented with 2 ml per L of betaine solution (40% betaine), HoldB2 group. The results showed that post-hatch holding for 48 h depressed feed intake and body weight gain during the entire 15 d study period with no beneficial effect of supplemental betaine. Chicks in the HoldB2 group had elevated serum glucose, triglycerides, and aspartate aminotransferase 48 h post-hatch. Early water deprivation directly affected the brain proopiomelanocortin (POMC) and hepatic glucocorticoid receptors (GR) expression and induced significant changes in various aquaporins (AQP1, AQP2, AQP4, and AQP9). In conclusion, betaine supplementation to chicks held for 48 h post-hatch resulted in some changes in blood biochemical indices with no effects on performance during the first 15 days of life. The results suggest that betaine supplementation could ameliorate the stressful effects of water deprivation on POMC and GR expression and maintain cellular osmosis through interactions with variable aquaporins expression, particularly the AQP1 and AQP2. Further investigations are required to investigate the molecular mechanisms underlying the selective regulatory expression of different aquaporins in relation to betaine supplementation.

Wnt and inflammatory pathway mediated differential expression of water channel protein in glaucomatous trabecular meshwork

Wnt and inflammatory pathway mediated differential expression of water channel protein in glaucomatous trabecular meshwork

The expression of water channel proteins during human salivary gland development: a topographic study of aquaporins 1, 3 and 5

Some members of aquaporin family (AQP) plays crucial functions in salivary synthesis and secretion. These proteins expression has already been reported during salivary gland formation, however no previous studies in human developing glands have been performed. We evaluated AQP1, 3 and 5 expression through the stages of human salivary gland morphogenesis and discuss the possible role of AQP for glandular maturation. Human salivary glands derived from foetuses aged between 14 and 25 weeks were submitted to immunohistochemistry. At the bud stage, membrane expression of AQP1, 3 and 5 were observed within the epithelial bud cells presenting a similar apicolateral pattern, also found at the pseudoglandular stage, present within the terminal portions of future acini, while AQP5 was also particularly strong at the apical membrane of pre-acinar and pre-ductal cells. AQP5 was co-localised with Cytokeratin 7. Similar AQP1, 3 and 5 expression were observed at the following canalicular stage, where distinct and strongly luminal and acinar AQP5 expression is present. During the final terminal bud stage, AQP1 was only identified in serous acini, myoepithelial and endothelial cells, while differentiated mucous acinar cells and ducts were negative. AQP3 was detected at apicolateral membranes of both mucous and serous acini. AQP5 also showed a diffuse expression in mucous and serous acini, in addition to strong apical membrane expression within lumen of intercalated ductal cells. This topographic analysis of AQP1, 3 and 5 revealed differences in the expression pattern throughout salivary gland developmental stages, suggesting different roles for each protein in human glandular maturation.

Aquaporin 11, a regulator of water efflux at retinal Müller glial cell surface decreases concomitant with immune-mediated gliosis.

BackgroundMüller glial cells are important regulators of physiological function of retina. In a model disease of retinal inflammation and spontaneous recurrent uveitis in horses (ERU), we could show that retinal Müller glial cells significantly change potassium and water channel protein expression during autoimmune pathogenesis. The most significantly changed channel protein in neuroinflammatory ERU was aquaporin 11 (AQP11). Aquaporins (AQP, 13 members) are important regulators of water and small solute transport through membranes. AQP11 is an unorthodox member of this family and was assigned to a third group of AQPs because of its difference in amino acid sequence (conserved sequence is only 11 %) and especially its largely unknown function.MethodsIn order to gain insight into the distribution, localization, and function of AQP11 in the retina, we first developed a novel monoclonal antibody for AQP11 enabling quantification, localization, and functional studies.ResultsIn the horse retina, AQP11 was exclusively expressed at Müller glial cell membranes. In uveitic condition, AQP11 disappeared from gliotic Müller cells concomitant with glutamine synthase. Since function of AQP11 is still under debate, we assessed the impact of AQP11 channel on cell volume regulation of primary Müller glial cells under different osmotic conditions. We conclude a concomitant role for AQP11 with AQP4 in water efflux from these glial cells, which is disturbed in ERU. This could probably contribute to swelling and subsequent severe complication of retinal edema through impaired intracellular fluid regulation.ConclusionsTherefore, AQP11 is important for physiological Müller glia function and the expression pattern and function of this water channel seems to have distinct functions in central nervous system. The significant reduction in neuroinflammation points to a crucial role in pathogenesis of autoimmune uveitis.

The expression of water channel protein 2 and Na+-K+-ATPase in acute kidney injury

Objective To investigate the role of AQP2 and Na +-K +-ATPase in the pathogenesis of kidney injury with multiple organ dysfunction syndrome,and try to find the express characteristics of them.Methods A total of 72 healthy rats were randomly (random number) divided into two groups:control group (n =24) and Lipopolysaccharide group (n =48).The Lipopolysaccharide group rats were injected with 5 mg/kg lipopolysaccharide at the beginning while the control group was 0.9％ sodium chloride.After the model was succeeded,the rats were put to death at 6 h,12 h,24 h,2 days,3 days and 5 days equally.The urine and blood were collected.Blood were used biochemical tests to check.kidney AQP2 protein and mRNA expression level in the organization were applied the immune organized and RT-PCR technique to detect.Applied kit for determining the content and activity of sodium/potassium-atpase.Results The volume of urine in LPS group decrease quickly at 12 h and 24 h,but increased after 2 days.Urea nitrogen and creatinine increased gradually,and peaked at 48 h,after then gradually decline.AQP2 mRNA and protein expression decreased,and minimize at 48 h.The content of Na+-K+-ATP ase has no obvious difference,but the activity significantly decreased at the beginning,then increased gradually,but it was still lower than the control group.Conclusions In renal injury rats model with multiple organ dysfunction syndrome,AQP2 is the structure of renal reabsorption function,while Na +-K +-ATPase directly involved in or indirectly reflected the state of kidney energy metabolism.Recovery of AQP2 protein and energy metabolism,before the rat kidney function improved. Key words: Multiple organ dysfunction syndrome ; Kidney injury ; Water channel protein; Na+-K+-ATPase ; Endotoxin ; Inflammation; Lipopolysaccharide ; Energy metabolism

Analysis of the Expression of Water Channel Proteins in Human Colon Cancer Cell Line SW620

Analysis of the Expression of Water Channel Proteins in Human Colon Cancer Cell Line SW620

Atractylodes macrocephala regulates water channel protein expression by hypertonic stress in renal collecting duct cells

Edema is a symptom that results from the abnormal accumulation of fluid in the body. The cause of edema is related to the level of aquaporin (AQP)2 protein expression, which regulates the reabsorption of water in the kidney. Edema is caused by overexpression of the AQP2 protein when the concentration of Na+ in the blood increases. The rhizome of Atractylodes macrocephala has been used in traditional Oriental medicine as a diuretic drug; however, the mechanism responsible for the diuretic effect of the aqueous extract from A. macrocephala rhizomes (AAM) has not yet been identified. We examined the effect of the AAM on the regulation of water channels in the mouse inner medullary collecting duct (mIMCD)‐3 cells under hypertonic stress. In the mIMCD3 cells, the AAM attenuates a hypertonicity‐induced increase in AQP2 expression as well as the trafficking of AQP2 to the apical plasma membrane. Tonicity‐responsive enhancer binding protein (TonEBP) is a transcription factor known to play a central role in cellular homeostasis by regulating the expression of some proteins, including AQP2. Western immunoblot analysis demonstrated that the protein and mRNA expression levels of TonEBP also decrease after AAM treatment. These results suggest that the AAM has a diuretic effect by suppressing water reabsorption via the downregulation of the TonEBP‐AQP2 signaling pathway

Effects of curcumin on levels of nitric oxide synthase and AQP-4 in a rat model of hypoxia–ischemic brain damage

This study examines the preventive and therapeutic effects of curcumin on brain edema after hypoxic–ischemic brain damage (HIBD) in a rat model. Male Sprague-Dawley rats were divided into four groups: a sham group (SH), a hypoxic–ischemic group (HI) without drug treatment, a hypoxic–ischemic group (CU) with curcumin injection, and a hypoxic–ischemic group with DMSO injection (solvent control, SC). HIBD treatment led to edema and ultrastructural changes in the hippocampus, increased the activity levels of nitric oxide synthase (NOS) in the brain (P<0.05), and raised the expression of water channel protein 4 (Aquaporin-4, AQP-4) in the blood–brain barrier (BBB) (P<0.05). Curcumin injection, but not the control DMSO injection, partially reversed HIBD-induced brain edema and morphological changes, as well as HIBD-induced increase in NOS activities and AQP-4 expression (P<0.05). In conclusion, our results showed that BBB ultrastructural changes may play an important role in the formation and development of brain edema after HIBD. Curcumin may protect the BBB ultrastructure and thus decrease brain edema following HIBD by down-regulating HIBD-induced increase in NOS activities and AQP-4 protein expression.

Bioresponse to Nanotubes: Surface Chemistry of Carbon Nanotubes Impacts the Growth and Expression of Water Channel Protein in Tomato Plants (Small 15/2012)

Bioresponse to Nanotubes: Surface Chemistry of Carbon Nanotubes Impacts the Growth and Expression of Water Channel Protein in Tomato Plants (Small 15/2012)

Surface Chemistry of Carbon Nanotubes Impacts the Growth and Expression of Water Channel Protein in Tomato Plants

Specific properties of carbon nanotubes, such as their level of agglomeration in the medium and their surface characteristics, can be critical for the physiological response of plants upon application of carbon nanotubes. The correlations among the level of aggregation, the type of functional group on the surface of the carbon nanotubes, and the growth performance of tomato plants are documented.

Aquaporin 2 expression increased by glucagon in normal rat inner medullary collecting ducts

It is well known that Glucagon (Gl) is released after a high protein diet and participates in water excretion by the kidney, principally after a protein meal. To study this effect in in vitro perfused inner medullary collecting ducts (IMCD), the osmotic water permeability (Pf; mum/s) at 37 degrees C and pH 7.4 in normal rat IMCDs (n = 36) perfused with Ringer/HCO(3) was determined. Gl (10(-7) M) in absence of Vasopressin (AVP) enhanced the Pf from 4.38 +/- 1.40 to 11.16 +/- 1.44 microm/s (P < 0.01). Adding 10(-8), 10(-7), and 10(-6) M Gl, the Pf responded in a dose-dependent manner. The protein kinase A inhibitor H8 blocked the Gl effect. The specific Gl inhibitor, des-His(1)-[Glu(9)] glucagon (10(-7) M), blocked the Gl-stimulated Pf but not the AVP-stimulated Pf. There occurred a partial additional effect between Gl and AVP. The cAMP level was enhanced from the control 1.24 +/- 0.39 to 59.70 +/- 15.18 fm/mg prot after Gl 10(-7) M in an IMCD cell suspension. The immunoblotting studies indicated an increase in AQP2 protein abundance of 27% (cont 100.0 +/- 3.9 vs. Gl 127.53; P = 0.0035) in membrane fractions extracted from IMCD tubule suspension, incubated with 10(-6) M Gl. Our data showed that 1) Gl increased water absorption in a dose-dependent manner; 2) the anti-Gl blocked the action of Gl but not the action of AVP; 3) Gl stimulated the cAMP generation; 4) Gl increased the AQP2 water channel protein expression, leading us to conclude that Gl controls water absorption by utilizing a Gl receptor, rather than a AVP receptor, increasing the AQP2 protein expression.

Effects of Acetazolamide Combined with or without NaHCO3 on Suppressing Neoplasm Growth, Metastasis and Aquaporin-1 (AQP1) Protein Expression

This study was made to explore the effects of acetazolamide on tumor growth,metastasis and the possible mechanisms. The mice bearing with Lewis lung carcinomaswere taken as the animal model. The effects of acetazolamide were compared with thecombination treatment of NaHCO3 on tumor growth, metastasis and carbonic anhydraseactivity in lung and tumor tissues using imidazole-Tris technique. And also the possible roleof AQP1 in tumor tissues was investigated by Western blot and immuno-histochemicalanalysis. Results showed that acetazolamide alone could sharply reduce the number of lungmetastasis and primary tumor growth, and appeared in a dose-dependent manner.Acetazolamide significantly inhibited carbonic anhydrase activity in tumor tissue. After theaddition of NaHCO3, the suppression of acetazolamide on tumor growth, number ofmetastasis and carbonic anhydrase activity in primary tumor tissue could not be alteredsignificantly, but the inhibitory rate of metastasis in lung and carbonic anhydrase activity inlung tissue appeared to show a reversal trend in the dose dependency from the acetazolamidetreatment alone. The exactly mechanisms need to be clarified in future. Western blot andimmunohistochemical analysis demonstrated that AQP1 expression in the tumor tissue washigher than both tissue of normal and treated with acetazolamide treatment alone.Combination with NaHCO3 could not synergistically inhibit the expression of AQP1 withacetazolamide. The results suggested that the mechanism of acetazolamide on anti-tumor especially on its anti-metastasis actions might partly involve either inhibiting the carbonic anhydrase activity or reducing AQP1 water channel protein expression, whatever if treated with or without NaHCO3.

Expression of water channel proteins (aquaporins) in the rat Eustachian tube and middle ear mucosa

Conclusion. Diverse expression of the different subtypes of aquaporins in different parts of the Eustachian tube and middle ear suggests region-specific functions of the aquaporins in the normal physiology of the tubotympanum and also suggests that they may play roles in the pathophysiology of otitis media. Objectives. The epithelial cells of the middle ear and Eustachian tube must maintain adequate water balance for normal function of the mucociliary system. Since aquaporins (AQPs) are known to play critical roles in water homeostasis, we investigated their expression in the tubotympanum of the rat. Methods. The expression of AQP subtypes 1, 2, 4, 5, and 7 were examined in the rat Eustachian tube and middle ear using RT-PCR, Western blotting, and immunohistochemistry. Results. Transcripts for AQP 1, 4, and 5 were detected in the Eustachian tube and middle ear. Expression of these molecules at the protein level was confirmed by Western blot analysis. Immunohistochemical analysis demonstrated that AQP 4 was localized to the basolateral membranes of ciliated epithelial cells while AQP 5 was localized to the apical surface of serous gland cells, but not goblet cells, in the rat Eustachian tube. AQP 1 was found to be expressed by the subepithelial fibroblasts.

Msx2 Promotes Vaginal Epithelial Differentiation and Wolffian Duct Regression and Dampens the Vaginal Response to Diethylstilbestrol

In utero exposure to diethylstilbestrol (DES) leads to patterning defects in the female reproductive tract (FRT) and a propensity to the development of vaginal adenocarcinomas in humans. In the mouse, DES treatment similarly induces a plethora of FRT developmental defects, including stratification of uterine epithelium and presence of glandular tissue in cervix and vagina. Uterine abnormalities are associated with repression of the homeobox gene Msx2, and DES leads to an altered uterine response in Msx2 mutants including a dilated uterine lumen. Here we investigate the role of Msx2 in normal vaginal development and in FRT response to DES. During vaginal development, Msx2 is required for Tgfbeta2 and Tgfbeta3 expression and for proper vaginal epithelial differentiation. Moreover, Msx2 is involved in caudal Wolffian duct regression by promoting apoptosis. Consistently, neonatal DES exposure represses Msx2 expression in the Wolffian duct epithelium and inhibits its apoptosis and subsequent regression. Intriguingly, although DES treatment also represses Msx2 expression in the vaginal epithelium, a much more severe DES-induced vaginal phenotype was observed in Msx2 mutant mice, including a complete failure of Müllerian vaginal epithelial stratification and a severely dilated vaginal lumen, accompanied by loss of p63 and water channel protein expression. These results demonstrate a critical role for Msx2 in counteracting the effect of DES on FRT patterning and suggest that the response to DES may be highly variable depending on the genotype of an individual.

From plant tissue culture to biotechnology: Scientific revolutions, abiotic stress tolerance, and forestry

Plant biotechnology—especially in vitro regeneration and cell biology, DNA manipulation and biochemical engineering—is already changing the agricultural scene in three major areas: control of plant growth, protecting plants against biotic stress, and production of specialty foods, biochemicals and pharmaceuticals. Plant biotechnology faces several major challenges in the coming decades: alleviating the hazards of abiotic stress (especially salinity, drought, and extreme temperatures), improving pest control, maintenance and improvement of the environment, improvement of food quality and design of ‘specialty food’ using biochemical engineering, and production of biomaterials. Two parallel research approaches will most likely exist simultaneously in the near future: the transgenic approach (expression of unique genes and specific promoters and transcription factors), and the non-transgenic approach (genomics-assisted gene discovery, marker-assisted selection, efficient mutations, and clonal agriculture). Drought and salinity are the most serious threats to agriculture and to the environment in many parts of the world. Key molecular factors that are being used for genetic engineering of stress-tolerant plants include: over-expression of specific transcription factors, characterization of dehydrin proteins, over-production of osmoprotectants, expression of water channel proteins and ion transporters, expression and characterization of molecular chaperones, including a novel boiling-stable homo-oligomeric SP1 protein. Although molecular breeding is routine in agriculture, forest-tree species have been left far behind. However, the increasing demand for wood and its products and the reduction of available harvestable forests has recently led to the introduction of several molecular and biotechnological tools into forest-tree research and improvement. Among these are in vitro propagation, the identification of molecular markers, and genetic engineering for specific traits. Achievements today in plant biotechnology have already surpassed all previous expectations. The full realization and impact of the new developments depend not only on continued successful and innovative research and development activities, but also on a favorable regulatory climate and public acceptance. Plant scientists now have a central role in society.

Altered ubiquitination and stability of aquaporin-1 in hypertonic stress.

Aquaporin-1 (AQP1) water channel protein expression is increased by hypertonic stress. The contribution of changes in protein stability to hypertonic induction of AQP1 have not been described. Incubation of BALB/c fibroblasts spontaneously expressing AQP1 with proteasome inhibitors increased AQP1 expression, suggesting basal proteasome-dependent degradation of the protein. Degradation by the proteasome is thought to be triggered by polyubiquitination of a target protein. To determine whether AQP1 is ubiquitinated, immunoprecipitation with anti-AQP1 antibodies was performed, and the resultant samples were probed by protein immunoblot for the presence of ubiquitin. Immunoblots demonstrated ubiquitination of AQP1 under control conditions that increased after treatment with proteasome inhibitors (MG132, lactacystin). Exposure of cells to hypertonic medium for as little as 4 h decreased ubiquitination of AQP1, an effect that persisted through 24 h in hypertonic medium. Using metabolic labeling with [(35)S]methionine, the half-life of AQP1 protein under isotonic conditions was found to be <4 h. AQP1 protein half-life was markedly increased by exposure of cells to hypertonic medium. These observations provide evidence that aquaporins are a target for ubiquitination and proteasome-dependent degradation. Additionally, these studies demonstrate that reduced protein ubiquitination and increased protein stability lead to increased levels of AQP1 expression during hypertonic stress.

A novel method to determine the diffusional water permeability of oocyte plasma membranes

Measurements of the cell membrane diffusional water permeability (Pd) are important to characterize water passage across water channels and across the lipid bilayer component of the membrane. Existing methods for those measurements are involved; however, we report here a simple procedure to estimate Pd in Xenopus laevis oocytes and similar large cells. Due to the different densities of H2O and D2O (heavy water), an oocyte transferred from normal medium to a D2O-based medium floats initially, but subsequently sinks when a certain amount of the water originally in them is replaced by the D2O that diffuses in. We describe how the 'flotation time' (time that oocytes float in a heavy water solution before they start sinking) yields the Pd of the plasma membrane. Determination of Pd by this procedure and by the rate of tritiated water (T2O) efflux give for Pd results which are very close: 2.2 +/- 0.2 (n = 8) and 2.0 +/- 0.1 (n = 6) microns/s, respectively (T = 10 degrees C). Furthermore, our method detects the increase in Pd elicited in oocytes by either expression of water channel proteins, or by treating them with the pore-forming antibiotic amphotericin B. This method appears useful to gauge the expression and function of pore-forming, water-permeable membrane proteins.

A novel method to determine the diffusional water permeability of oocyte plasma membranes

Measurements of the cell membrane diffusional water permeability (P d) are important to characterize water passage across water channels and across the lipid bilayer component of the membrane. Existing methods for those measurements are involved; however, we report here a simple procedure to estimate P d in Xenopus laevis oocytes and similar large cells. Due to the different densities of H 2O and D 2O (heavy water), an oocyte transferred from normal medium to a D 2O-based medium floats initially, but subsequently sinks when a certain amount of the water originally in them is replaced by the D 2O that diffuses in. We describe how the ‘flotation time’ (time that oocytes float in a heavy water solution before they start sinking) yields the P d of the plasma membrane. Determination of P d by this procedure and by the rate of tritiated water (T 2O) efflux give for P d results which are very close: 2.2 ± 0.2 ( n = 8) and 2.0 ± 0.1 ( n = 6) μm/s, respectively (T = 10°C). Further-more, our method detects the increase in P d elicited in oocytes by either expression of water channel proteins, or by treating them with the pore-forming antibiotic amphotericin B. This method appears useful to gauge the expression and function of pore-forming, water-permeable membrane proteins.