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1.
Summary The hydraulic conductivity of rabbit gallbladder epithelium has been studied using a continuous volumetric method based on capacitance measurements. The time resolution for measuring osmotic flows is in the range of seconds. Volume flows have been induced by osmotic gradients between 0 and 100 mosmol. In this range the flow-force relation is linear and theP
f value is 9.3×10–3 cm/sec. After correction for solute polarization effects, theP
f value amounts to 0.05 cm/sec. The observed flow is constant between 5 sec up to 20 min after a sudden increase in the osmolarity of the mucosal solution. The wet weight of the gallbladder tissue decreases by 22% and increases by 30% during osmotic flows from serosa to mucosa and from mucosa to serosa, respectively. Volume flows induced by hydrostatic pressure gradients on the mucosal surface are linearly related to the driving forces between 0 and 40 mbar. TheP
f value is 0.15 cm/sec. The volume flows are constant between 2 sec and 15 min after pressure application. The flow-force relation for pressure gradients on the serosal surface is markedly nonlinear for gradients greater than 5 mbar. Below 5 mbar theP
f value is 4.5 cm/sec. From electrical measurements, e.g., resistance and streaming potentials, and from flux studies with inulin and polyethylene glycol 4000, it is concluded that hydrostatic and osmotic gradients are not comparable when they are applied to gallbladder epithelium. They induce volume flows across different pathways, e.g., osmosis predominantly across the cellular route and pressure filtration predominantly across paracellular routes. 相似文献
2.
Summary Literature data suggest that water accumulation by the human fetus is driven by osmotic gradients of small solutes. However, the existence of such gradients has not been supported by prior measurements. Attempts to estimate the size of the gradient necessary to drive net water movement have been seriously hampered by the lack of permeability data for the syncytiotrophoblast membranes. Stopped-flow light scattering techniques were employed to measure the osmotic water permeability (P
f
)of microvillous (MVM) and basal membrane (BM) vesicles isolated from human term placenta. At 37°C, the P
f
was determined to be 1.9±0.06 × 10+–3 cm/sec for MVM and 3.1±0.20 × 10+–3 cm/sec for BM (mean ±SD, n = 6). At 23°C, P
f
was reduced to 0.7±0.04 × 10+–3 cm/sec in MVM and 1.6±0.05 × 10+–3 cm/sec in BM. These P
f
values are comparable to those observed in membranes where water has been shown to permeate via a lipid diffusive mechanism. Arrhenius plots of P
f
over the range 20–40°C were linear, with activation energies of 13.6 ± 0.6 kcal/mol for MVM and 12.9±1.0 kcal/mol for BM. Water permeation was not affected by mercurial sulfhydryl agents and glucose transport inhibitors. These data clearly suggest that water movement across human syncytiotrophoblast membranes occurs by a lipid diffusion pathway. As noted in several other epithelial tissues, the basal membrane has a higher water permeability than the microvillous membrane. It is speculated that water accumulation by the human fetus could be driven by a solute gradient small enough to be within the error of osmolarity measurements.We thank the staff of the labor and delivery ward at University of San Francisco Medical Center for help in obtaining placental tissue. This work was supported by NIH grant HD 26392. Dr. Jansson was supported by the Sweden-America Foundation, The Swedish Society of Medicine, The Swedish Society for Medical Research, and the Swedish Medical Research Council. 相似文献
3.
Summary Water transport across the mammalian collecting tubule is regulated by vasopressin-dependent water channel insertion into and retrieval from the cell apical membrane. The time course of osmotic water permeability (P
f
) following addition and removal of vasopressin (VP) and 8-Br-cAMP was measured continuously by quantitative fluorescence microscopy using an impermeant fluorophore perfused in the lumen. Cortical collecting tubules were subjected to a 120 mOsm bath-to-lumen osmotic gradient at 37°C with 10–15 nl/min lumen perfusion and 10–20 ml/min bath exchange rate. With addition of VP (250 U/ml), there was a 23±3 sec (sem,n=16) lag in whichP
f
did not change, followed by a rise inP
f
(initial rate 1.4±0.2×10–4 cm/sec2) to a maximum of 265±10×10–4 cm/sec. With addition of 8-Br-cAMP (0.01–1mm) there was an 11±2 sec lag. For [8-Br-cAMP]=0.01, 0.1 and 1mm, the initial rate ofP
f
increase following the lag was (units 10–4 cm/sec2): 1.1±0.1, 1.2±0.1 and 1.7±0.3. MaximumP
f
was (units 10–4 cm/sec): 64±4, 199±9 and 285±11. With removal of VP,P
f
decreased to baseline (12×10–4 cm/sec) with aT
1/2 of 18 min; removal of 0.1 and 1mm 8-Br-cAMP gaveT
1/2 of 4 and 8.5 min. These results demonstrate (i) a brief lag in theP
f
response, longer for stimulation by VP than by 8-Br-cAMP, representing the transient build-up of biochemical intermediates proximal to the water channel insertion step, (ii) similar initialdP
f
/dt (water channel insertion) over a wide range of [8-Br-cAMP] and steady-stateP
f
values, and (iii) more rapidP
f
decrease with removal of 8-Br-cAMP than with VP. These pre-steady-state results define the detailed kinetics of the turn-on and turn-off of tubuleP
f
and provide kinetic evidence that the rate-limiting step for turn-on ofP
f
is not the step at which VP regulates steady-stateP
f
. If water channel insertion is assumed to be the rate-limiting step in the turn-on ofP
f
, these results raise the possibility that water channels must be activated following insertion into the apical membrane. 相似文献
4.
Summary Osmotic water permeability of the apical membrane of toad urinary epithelium is increased greatly by vasopressin (VP) and is associated with exocytic addition of granules and aggrephores at the apical surface. To determine the physiological role of granule exocytosis, we measured the osmotic water permeability and membrane fluidity of isolated granules, surface membranes and microsomes prepared from toad bladder in the presence and absence of VP.P
f
was measured by stopped-flow light scattering and membrane fluidity was examined by diphenylhexatriene (DPH) fluorescence anisotropy. In response to a 75mm inward sucrose gradient, granule size decreased with a single exponential time constant of 2.3±0.1 sec (sem, seven preparations, 23°C), corresponding to aP
f
of 5×10–4 cm/sec; the activation energy (E
a
) forP
f
was 17.6±0.8 kcal/mole. Under the same conditions, the volume of surface membrane vesicles decreased biexponentially with time constants of 0.13 and 1.9 sec; the fast component comprised 70% of the signal. Granule, surface membrane and microsome time constants were unaffected by VP. However, in surface membranes, there was a small decrease (6±2%) in the fraction of surface membranes with fast time constant. DPH anisotropies were 0.253 (granules), 0.224 (surface membrane fluidity is remarkably lower than that of surface and microsomal membranes, and (4) rapid water transport occurs in surface membrane vesicles. The unique physical properties of the granule suggests that apical exocytic addition of granule membrane may be responsible for the low water permeability of the unstimulated apical membrane. 相似文献
5.
Summary Microvillous vesicles were prepared from term human placenta by shearing, differential centrifugation and Mg2+ precipitation. Vesicles were purified further on a sucrose density gradient producing two bands with densities of 1.16 to 1.18 g/ml (C1) and 1.13 to 1.15 g/ml (C2). The C2 fraction, which had a 24-fold enrichment of alkaline phosphatase and a three-fold reduction in Na+, K+-ATPase activity compared to homogenates, was used to measure osmotic water (P
f
) permeability.P
f
was measured from the time course of scattered light intensity following exposure of vesicles to specified gradients of impermeant solutes.P
f
decreased from 3.0×10–3 to 0.6×10–3 cm/sec with increasing gradient size (65 to 730mm; 23°C). Four possible causes of this behavior were examined theoretically and experimentally: an unstirred layer, saturation of water transport, large changes in the vesicle surface area with changes in volume and a structural restriction to vesicle volume change. The measured dependence ofP
f
on gradient size and the effect of the channel-forming ionophore gramicidin onP
f
fit best to the theoretical dependences predicted by a structural restriction mechanism. This finding was supported by experiments involving the effects onP
f
of increased solution viscosity, initial vesicle volume, the magnitude of transmembrane volume flow, and the effects of gradient size on activation energy (E
a) forP
f
. The decreasedP
f
resulting from a structural restriction limiting vesicle volume change was modeled mathematically as a second barrier in series with the vesicle membrane.E
a
measured using a 250-mm inwardly directed sucrose gradient was 5.4±0.6 kcal/mol (T>27°C) and 10.0±0.6 kcal/mol (T<27°C).E
a above 27°C is in the range normally associated with transmembrane passage of water via aqueous channels. Water transport was not inhibited byp-chloromercuribenzenesulfonate. 相似文献
6.
Brush border membrane vesicles (BBMV) maintain an initial hydrostatic pressure difference between the intra- and extravesicular
medium, which causes membrane strain and surface area expansion (Soveral, Macey & Moura, 1997). This has not been taken into
account in prior osmotic water permeability P
f
evaluations. In this paper, we find further evidence for the pressure in the variation of stopped-flow light scattering traces
with different vesicle preparations. Response to osmotic shock is used to estimate water permeability in BBMV prepared with
buffers of different osmolarities (18 and 85 mosM). Data analysis includes the dissipation of both osmotic and hydrostatic
pressure gradients. P
f
values were of the order of 4 × 10−3 cm sec−1 independent of the osmolarity of the preparation buffer. Arrhenius plots of P
f
vs. 1/T were linear, showing a single activation energy of 4.6 kcal mol−1. The initial osmotic response which is significantly retarded is correlated with the period of elevated hydrostatic pressure.
We interpret this as an inhibition of P
f
caused by membrane strain and suggest how this inhibition may play a role in cell volume regulation in the proximal tubule.
Received: 8 August 1996/Revised: 4 March 1997 相似文献
7.
Water permeability of the plasma membrane (PM) and the vacuolar membrane (VM) is important for intracellular and transcellular
water movement in plants, because mature plant cells have large central vacuoles. We have developed a new method for measuring
the osmotic water permeability of the PM and VM (P
f1 and P
f2, respectively) in individual plant cells. Here, the theoretical basis and procedure of the method are discussed. Protoplasts
isolated from higher plant tissues are used to measure P
f1 and P
f2. Because of the semi-permeability (selective permeability) of cellular membranes, protoplasts swell or shrink under hypotonic
or hypertonic conditions. A theoretical three-compartment model is presented for simulating time-dependent volume changes in the vacuolar and cytoplasmic spaces in a protoplast during osmotic
excursions. The model describes the theoretical relationships between P
f1, P
f2 and the bulk osmotic water permeability of protoplasts (P
f(bulk)). The procedure for measuring the osmotic water permeability is: (1) P
f(bulk) is calculated from the time when half of the total change in protoplast volume is completed, by assuming that the protoplast
has a single barrier to water movement across it (two-compartment model); (2) P
f2 of vacuoles isolated from protoplasts is obtained in the same manner; and (3) P
f1 is determined from P
f(bulk) and P
f2 according to the three-compartment model. The theoretical relationship between P
fl
(m s−1) and L
Pl
(hydraulic conductivity, l=1, 2) (m s−1 Pa−1) is also discussed.
Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorised users.
Tsuneo Kuwagata and Mari Murai-Hatano contributed equally to the paper. 相似文献
8.
Intra- and transcellular water movements in plants are regulated by the water permeability of the plasma membrane (PM) and
vacuolar membrane (VM) in plant cells. In the present study, we investigated the osmotic water permeability of both PM (P
f1) and VM (P
f2), as well as the bulk osmotic water permeability of a protoplast (P
f(bulk)) isolated from radish (Raphanus sativus) roots. The values of P
f(bulk) and P
f2 were determined from the swelling/shrinking rate of protoplasts and isolated vacuoles under hypo- or hypertonic conditions.
In order to minimize the effect of unstirred layer, we monitored dropping or rising protoplasts (vacuoles) in sorbitol solutions
as they swelled or shrunk. P
f1 was calculated from P
f(bulk) and P
f2 by using the ‘three-compartment model’, which describes the theoretical relationship between P
f1, P
f2 and P
f(bulk) (Kuwagata and Murai-Hatano in J Plant Res, 2007). The time-dependent changes in the volume of protoplasts and isolated vacuoles fitted well to the theoretical curves, and
solute permeation of PM and VM was able to be neglected for measuring the osmotic water permeability. High osmotic water permeability
of more than 500 μm s−1, indicating high activity of aquaporins (water channels), was observed in both PM and VM in radish root cells. This method
has the advantage that P
f1 and P
f2 can be measured accurately in individual higher plant cells.
Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. It includes four appendices, four tables and two figures.
Mari Murai-Hatano and Tsuneo Kuwagata contributed equally to the paper.
An erratum to this article is available at . 相似文献
9.
Solenov EI Nesterov VV Baturina GS Khodus GR Ivanova LN 《European biophysics journal : EBJ》2003,32(7):614-619
We report a novel approach for assessing the volume of living cells which allows quantitative, high-resolution characterization
of dynamic changes in cell volume while retaining the cell functionality. The aim of this study was to evaluate the short-term
effect of vasopressin on basolateral cell surface water permeability in the outer medullary collecting duct (OMCD). The permeability
of the basolateral cell membrane was determined in the tubules where the apical membrane was blocked with oil injected into
the lumen. The apparent coefficient of water permeability (P
f) was evaluated by measuring the cell swelling after the step from hypertonic to isotonic medium (600 mosm to 300 mosm). Desmopressin
(dDAVP) induced an increase of the basolateral P
f from 113.7±8.5 μm/s in control cells to 186.6±11.4 μm/s in micro-dissected fragments of the OMCD incubated in vitro (10−7 M dDAVP, 30 min at 37 °C) (P<0.05). Mercury caused pronounced inhibition of basolateral water permeability (26.0±6.9 μm/s; P<0.05). The effect of mercury (1.0 mM HgCl2) was reversible: after washing the fragments with PBS for 20 min, P
f values were restored to the control levels (125.0±9.5 μm/s). The results of the study indicate the existence of a mechanism
controlling the osmotic water permeability of the basolateral cell membrane in the OMCD epithelium. 相似文献
10.
NEVIS AH 《The Journal of general physiology》1958,41(5):927-958
Osmotic and diffusion permeabilities (Pf and Pd) of invertebrate nerve fibers to tritiated water were measured to determine what water flux studies could reveal about "the nerve membrane" and to directly test the possibility of active transport of water into or out of invertebrate nerve fibers. Pf/Pd ratios for lobster walking leg nerve fibers were found to be about 20 ± 7 at 14°C. Pd measurements were made for squid giant axons at 25°C. and found to yield a value of 4 x 10–4 cm.–1 sec.–1. When combined with the data of D. K. Hill for Pf, a Pf/Pd ratio of 21 ± 5 is obtained. These Pf/Pd ratios correspond to "effective pore radii" of about 16 ± 4 angstrom units, according to theories developed by Koefoed-Johnsen and Ussing and independently by Pappenheimer and his colleagues. Variations of water flux ratios with temperatures were studied and apparent activation energies calculated for both diffusion experiments and osmotic filtration experiments using the Arrhenius equation, and found to be close to 3 to 5 cal. per mole of water transferred. Cyanide (5 x 10–3 molar) and iodoacetate (1 x 10–3 molar) poisoned lobster leg nerve fibers showed no appreciable change in diffusion or osmotic filtration water effluxes. Caution in interpreting these proposed channels as simple pores was emphasized, but the possibility that such channels exist and are related to ionic flow is not incompatible with electrophysiological data. 相似文献
11.
C. Capurro P. Ford C. Ibarra P. Ripoche M. Parisi 《The Journal of membrane biology》1994,138(2):151-157
The water permeability properties of ovarian oocytes from Xenopus laevis and Bufo arenarum, a toad species found in the Buenos Aires region, were studied. We report that: (i) the water osmotic permeability (P
f, cm/sec × 10–4) was significantly higher in Bufo (6°C=12.3±2.4; 18°C = 20.8±4.8) than in Xenopus oocytes (6°C=5.3±0.3; 18°C=6.2±1.6). The corresponding water diffusion permeability values (P
d, cm/sec × 10–4) were: Xenopus = 2.3±0.3 (6°C) and 4.8±0.7 (18°C); Bufo=2.7±0.4 (6°C) and 6.0 ±0.5 (18°C). (ii) Amphotericin B increased the P
f and P
d values. The observed P
fP
d ratio was not significantly different from the expected results (n=3), after amphotericin B incorporation in both species. This means that the influence of unstirred layers and other potential artifactual compounds did not significantly affect our experimental results, (iii) Preincubation with gramicidin during 12 hr induced a clear increase in the oocyte volume. After that, a hypotonic shock only slightly increased the oocyte volume. Conversely, a hypertonic challenge induced a volume change significantly higher than the one observed in control conditions, (iv) Mercury ions did not affect the osmotic permeability in Xenopus oocytes but clearly inhibited, in a reversible way, the osmotic permeability in oocytes from B. arenarum. (v) Mercury ions did not reduce P
d values in either species, (vi) The P
fP
d values calculated from the differences observed in these parameters between both species were 11.9±5.1 at 18°C and 15.5±2.4 at 6°C. These numbers are similar to those previously reported in the case of membranes having water channels. From these results, we propose that water channels are present in the ovarian oocyte from B. arenarum but not in the ovarian oocyte from X. laevis.This work was supported by Fundación Antorchas, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET, Argentina) and Universidad de Buenos Aires (UBA). It was developed in the frame of an INSERM (France)-CONICET cooperative program. 相似文献
12.
We have recently shown that the osmotic water permeability (P
f
) of proximal tubules from neonatal rabbits is higher than that of adults (AJP 271:F871-F876, 1996). The developmental change
in P
f
could be due to differences in one or more of the components in the path for transepithelial water transport. The present
study examined developmental changes in water transport characteristics of the proximal tubule apical membrane by determining
P
f
and aquaporin 1 (AQP1) expression in neonatal (10–14 days old) and adult rabbit renal brush border membrane vesicles (BBMV).
AQP1 abundance in the adult BBMV was higher than the neonatal BBMV. At 25°C the P
f
of neonatal BBMV was found to be significantly lower than the adult BBMV at osmotic gradients from 50 to 250 mOsm/kg water.
The activation energy for osmotic water movement was higher in the neonatal BBMV than the adult BBMV (9.19 ± 0.37 vs. 5.09 ± 0.57 kcal · deg−1· mol−1, P < 0.005). Osmotic water movement in neonatal BBMV was inhibited 17.9 ± 1.3% by 1 mm HgCl2 compared to 34.3 ± 3.8% in the adult BBMV (P < 0.005). These data are consistent with a significantly greater fraction of water traversing the apical membrane lipid bilayer
in proximal tubules of neonates than adults. The lower P
f
of the neonatal BBMV indicates that the apical membrane is not responsible for the higher transepithelial P
f
in the neonatal proximal tubule.
Received: 18 December 1997/Revised: 3 April 1998 相似文献
13.
Hg compounds block membrane transport units behaving as water channels. Here we show that Hg induces an apical water pathway in toad skins pretreated with 10–3 M CH3ClHg or HgCl2, added to the outer bathing medium. Washing with SO4-Ringer caused a several-fold increase in net water flow (J
w
) and osmotic permeability coefficient (P
f
) that was reversed by re-exposure to Cl- or NO3-Ringer and mimicked by gluconate-Ringer. These P
f
changes could be elicited repeatedly and were present if, and only if, anion replacements took place in the inner bathing solution. Such inner polarity was related to the anion permeability of the epidermal basolateral membrane: impermeant anions (SO4, gluconate) increased P
f
; permeant anions (Cl, NO3) did not change basal P
f
but reversed the high P
f
induced by impermeant anions. Hg induced the appearance of aggregates that persisted despite repeated washings of the skins during 4–5 h, and whether P
f
was high (SO4-Ringer) or low (Cl-Ringer) before skin fixation.The Hg-induced apical water pathway in toad skin appears to be a unique model for studying the interplay between cell volume, cell ionic composition and water permeability.We thank P. Brawand and P. Fruleux for technical assistance. This work has been supported by grants from the Swiss National Science Foundation (Nos. 31-30030.90 to A.G. and R.C.dS., and 32-34090.92 to P.M.) 相似文献
14.
We have recently demonstrated that while the osmotic water permeability (P
f
) of neonatal proximal tubules is higher than that of adult tubules, the P
f
of brush-border membrane vesicles from neonatal rabbits is lower than that of adults. The present study examined developmental
changes in the water transport characteristics of proximal tubule basolateral membranes by determining aquaporin 1 (AQP1)
protein abundance and the P
f
in neonatal (10–14 days old) and adult rabbit renal basolateral membrane vesicles (BLMV). At 25°C the P
f
of neonatal BLMV was significantly lower than the adult BLMV at osmotic gradients ranging from 40 to 160 mOsm/kg water. The
activation energies for osmotic water movement were identical in the neonatal and adult BLMV (8.65 ± 0.47 vs. 8.86 ± 1.35 kcal · deg−1· mol−1). Reflection coefficients for sodium chloride and sodium bicarbonate were identical in both the neonatal and adult BLMV and
were not different from one. Mercury chloride (0.5 mm) reduced osmotic water movement by 31.3 ± 5.5% in the adult BLMV, but by only 4.0 ± 4.0% in neonatal vesicles (P < 0.01). Adult BLMV AQP1 abundance was higher than that in the neonate. These data demonstrate that neonatal BLMV have a
lower P
f
and AQP1 protein abundance than adults and that a significantly greater fraction of water traverses the basolateral membrane
lipid bilayer and not water channels in neonates compared to adults. The lower P
f
of the neonatal BLMV indicates that the basolateral membrane is not responsible for the higher transepithelial P
f
in the neonatal proximal tubule.
Received: 8 July 1999/Revised: 9 November 1999 相似文献
15.
Brush border membrane vesicles, BBMV, from eel intestinal cells or kidney proximal tubule cells were prepared in a low osmolarity
cellobiose buffer. The osmotic water permeability coefficient P
f
for eel vesicles was not affected by pCMBS and was measured at 1.6 × 10−3 cm sec−1 at 23°C, a value lower than 3.6 × 10−3 cm sec−1 exhibited by the kidney vesicles and similar to published values for lipid bilayers. An activation energy E
a
of 14.7 Kcal mol−1 for water transport was obtained for eel intestine, contrasting with 4.8 Kcal mol−1 determined for rabbit kidney proximal tubule vesicles using the same method of analysis. The high value of E
a
, as well as the low P
f
for the eel intestine is compatible with the absence of water channels in these membrane vesicles and is consistent with
the view that water permeates by dissolution and diffusion in the membrane. Further, the initial transient observed in the
osmotic response of kidney vesicles, which is presumed to reflect the inhibition of water channels by membrane stress, could
not be observed in the eel intestinal vesicles. The P
f
dependence on the tonicity of the osmotic shock, described for kidney vesicles and related to the dissipation of pressure
and stress at low tonicity shocks, was not seen with eel vesicles. These results indicate that the membranes from two volume
transporter epithelia have different mechanisms of water permeation. Presumably the functional water channels observed in
kidney vesicles are not present in eel intestine vesicles. The elastic modulus of the membrane was estimated by analysis of
swelling kinetics of eel vesicles following hypotonic shock. The value obtained, 0.79 × 10−3 N cm−1, compares favorably with the corresponding value, 0.87 × 10−3 N cm−1, estimated from measurements at osmotic equilibrium.
Received: 28 January 1999/Revised: 15 June 1999 相似文献
16.
17.
Osmosis in Cortical Collecting Tubules : A Theoretical and Experimental Analysis of the Osmotic Transient Phenomenon 总被引:3,自引:0,他引:3
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James A. Schafer Clifford S. Patlak Thomas E. Andreoli 《The Journal of general physiology》1974,64(2):201-228
This paper reports a theoretical analysis of osmotic transients and an experimental evaluation both of rapid time resolution of lumen to bath osmosis and of bidirectional steady-state osmosis in isolated rabbit cortical collecting tubules exposed to antidiuretic hormone (ADH). For the case of a membrane in series with unstirred layers, there may be considerable differences between initial and steady-state osmotic flows (i.e., the osmotic transient phenomenon), because the solute concentrations at the interfaces between membrane and unstirred layers may vary with time. A numerical solution of the equation of continuity provided a means for computing these time-dependent values, and, accordingly, the variation of osmotic flow with time for a given set of parameters including: Pf (cm s–1), the osmotic water permeability coefficient, the bulk phase solute concentrations, the unstirred layer thickness on either side of the membrane, and the fractional areas available for volume flow in the unstirred layers. The analyses provide a quantitative frame of reference for evaluating osmotic transients observed in epithelia in series with asymmetrical unstirred layers and indicate that, for such epithelia, Pf determinations from steady-state osmotic flows may result in gross underestimates of osmotic water permeability. In earlier studies, we suggested that the discrepancy between the ADH-dependent values of Pf and PDDw (cm s–1, diffusional water permeability coefficient) was the consequence of cellular constraints to diffusion. In the present experiments, no transients were detectable 20–30 s after initiating ADH-dependent lumen to bath osmosis; and steady-state ADH-dependent osmotic flows from bath to lumen and lumen to bath were linear and symmetrical. An evaluation of these data in terms of the analytical model indicates: First, cellular constraints to diffusion in cortical collecting tubules could be rationalized in terms of a 25-fold reduction in the area of the cell layer available for water transport, possibly due in part to transcellular shunting of osmotic flow; and second, such cellular constraints resulted in relatively small, approximately 15%, underestimates of Pf. 相似文献
18.
Water permeability of thin lipid membranes 总被引:18,自引:11,他引:7
The osmotic permeability coefficient, Pf, and the tagged water permeability coefficient, Pd, were determined for thin (<100 A) lipid membranes formed from ox brain lipids plus DL-α-tocopherol; their value of approximately 1 x 10-3 cm/sec is within the range reported for plasma membranes. It was established that Pf = Pd. Other reports that Pf > Pd can be attributed to the presence of unstirred layers in the experimental determination of Pd. Thus, there is no evidence for the existence of aqueous pores in these thin phospholipid membranes. The adsorption onto the membrane of a protein that lowers its electrical resistance by a factor of 103 was found not to affect its water permeability; however, glucose and sucrose were found to interact with the membrane to modify Pf. Possible mechanisms of water transport across these films are discussed, together with the implications of data obtained on these structures for plasma membranes. 相似文献
19.
Diecke FP Cacace VI Montalbetti N Ma L Kuang K Iserovich P Fischbarg J 《The Journal of membrane biology》2011,242(1):41-51
Layers of rabbit corneal endothelial cells were cultured on permeable inserts. We characterized the diffusional permeability
of the cell layer to nonelectrolyte and charged molecules and compared the diffusional and filtration permeabilities of the
paracellular and transcellular pathways. We determined the rates of diffusion of 3H- and 14C-labeled nonelectrolyte test molecules and estimated the equivalent pore radius of the tight junction. Negatively charged
molecules permeate slower than neutral molecules, while positively charged molecules permeate faster. Palmitoyl-dl-carnitine, which opens tight junctions, caused an increase of permeability and equivalent pore radius. Diffusional water
permeability was determined with 3H-labeled water; the permeabilities of the tight junction and lateral intercellular space were calculated using tissue geometry
and the Renkin equation. The diffusional permeability (P
d
) of the paracellular pathway to water is 0.57 μm s−1 and that of the transcellular path is 2.52 μm s−1. From the P
d
data we calculated the filtration permeabilities (P
f
) for the paracellular and transcellular pathways as 41.3 and 30.2 μm s−1, respectively. In conclusion, the movement of hydrophilic molecules through tight junctions corresponds to diffusion through
negatively charged pores (r = 2.1 ± 0.35 nm). The paracellular water permeability represents 58% of the filtration permeability
of the layer, which points to that route as the site of sizable water transport. In addition, we calculated for NaCl a reflection
coefficient of 0.16 ≤ σNaCl ≤ 0.33, which militates against osmosis through the junctions and, hence, indirectly supports the electro-osmosis hypothesis. 相似文献
20.
Paola Carpi-Medina Vladimir León Joussef Espidel Guillermo Whittembury 《The Journal of membrane biology》1988,104(1):35-43
Summary The diffusive water permeability (P
d
) of the plasma membrane of proximal kidney tubule cells was measured using a1H-NMR technique. The values obtained for the exchange time (T
ex) across the membrane were independent of the cytocrit and of the Mn2+ concentration (in the range 2.5 to 5mm). At 25°C the calculatedP
d
value was (per cm2 of outer surface area without taking into account membrane invaginations) 197±17 m/sec. This value equals 22.3±1.9 m/sec when the invaginations are taken into account. Cell exposure to 2.5mm parachloromercuribenzenesulfonic acid,pCMBS, (for 20 to 35 min) reducedP
d
to 45% of its control value. Fivemm dithiothreitol, DTT, reverted this effect. The activation energy for the diffusive water flux was 5.2±1.0 kcal/mol under control conditions. It increased to 9.1±2.2 kcal/mol in the presence of 2.5mm
pCMBS. Using our previous values for the osmotic water permeability (P
os) in proximal straight tubular cells theP
os/P
d
ratio equals 18±1, under control conditions, and 3.2±0.3 in the presence ofpCMBS. These experimental results indicate the presence of pathways for water, formed by proteins, crossing these membranes, which are closed bypCMBS. Assuming laminar flow (within the pore), fromP
os/P
d
of 13 to 18 an unreasonably large pore radius of 12 to 15 Å is calculated which would not hinder cell entry of known extracellular markers. Alternatively, for a single-file pore, 11 to 20 would be the number of water molecules which would be in tandem inside the pore. The water permeability remaining in the presence ofpCMBS indicates water permeation through the lipid bilayer. There are similarities between these results and those obtained in human red blood cells and in the apical cell membrane of the toad urinary bladder. 相似文献