SEARCH · PubMed Health

Results for “OSMOSIS AND PERMEABILITY”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22Linked to original sources

PubMed1991

Permeability-surface area product and reflection coefficient of the parietal pleura in dogs.

The parameters describing the permeability of the parietal pleura to liquid and total plasma proteins were measured in five anesthetized adult dogs. Small areas of parietal pleura (approximately 1 cm2) and the underlying endothoracic fascia were exposed through resection of the skin and the intercostal muscles. The portion of the thorax containing the pleural windows was removed from the chest and fixed over a bath of whole autologous plasma, the inner parietal pleural surface facing the bath. Small hemispheric Perspex capsules (surface area 0.28 cm2) connected to a pressure manometer were glued to the pleural windows; a subatmospheric pressure was set into the capsule chamber to create step hydraulic transpleural pressure gradients (delta P) ranging from 5 to 60 cmH2O. Transpleural liquid flows (Jv) and protein concentration of the capsular filtrate (Cfilt) and of the plasma bath were measured at each delta P. The transpleural protein flux (Js) at each delta P was calculated by multiplying Jv by the corresponding Cfilt. The hydraulic conductivity (Lp) of the parietal pleura was obtained from the slope of the Jv vs. delta P linear regression. The average Lp from 14 capsules was 9.06 +/- 4.06 (SD) microliters.h-1.cmH2O-1.cm-2. The mathematical treatment of the Js vs. Jv relationship allowed calculation of the unique Peclet number at the maximal diffusional protein flux and a corresponding osmotic permeability coefficient for plasma protein of 1 x 10(-5) +/- 0.97 x 10(-5) cm/s. The reflection coefficient calculated from the slope of the linear phase of the Js vs. Jv relationship was 0.11 +/- 0.05.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1984

Magnesium sulfate-induced water secretion in hamster small intestine.

We studied possible mechanisms of magnesium sulfate (MgSO4)-induced diarrhea. In vivo perfusion of hamster small intestine with an isotonic electrolyte solution containing 50 mM MgSO4 produced nearly three times as much fluid secretion as did a solution containing an equiosmotic amount of mannitol. We found that magnesium was absorbed at a faster rate than mannitol under these conditions, suggesting that differences in solute permeability do not explain the differences in secretory rates. Magnesium ion rather than sulfate appeared largely responsible for the effect as replacement of sulfate with chloride did not diminish the response. MgSO4 perfusion of a proximal intestinal segment did not affect water transport in an isolated distal segment suggesting that release of cholecystokinin or alterations in serum levels of other hormones were not responsible. Intestinal permeability, morphology, and cyclic nucleotide levels were normal after MgSO4 perfusion. Thus, MgSO4-induced diarrhea cannot be explained by the usual mechanisms, and additional processes responsible for intestinal secretion must exist.

Animals↗

PubMedDate not supplied

AVP-independent high osmotic water permeability of frog urinary bladder and autacoids.

In the isolated frog urinary bladder a 20- to 50-fold increase of the osmotic water permeability has been revealed in the absence of arginine vasopressin (AVP) as a result of several successive changes of the serosal Ringer solution. This increase of the osmotic water permeability was of the same magnitude as that of the effect of 1 nM AVP. Similarly to the effect of AVP, the amount of adenosine 3',5'-cyclic monophosphate (cAMP) in the cells rose, and aggregates of intramembraneous particles were formed in the apical plasma membrane of granular cells (as shown by the freeze-fracture method). Immunocytochemical studies using anti-actin monoclonal antibodies indicated depolymerization of F-actin following the AVP-independent change in water permeability. It was possible to decrease the high level of osmotic permeability to the initial level if 10 microl/ml of frog blood serum or a lipid extract of this blood serum, or 1 microM arachidonic acid or 1 nM prostaglandin E2 was added to the serosal Ringer solution. The rapid restoration of the osmotic water impermeability of the epithelium after the AVP- evoked effect was achieved by the addition to the serosal Ringer solution of Ringer solution in which intact frog urinary bladders had been previously incubated for 1 h. The data obtained indicate that the maintenance of the impermeability to water of the osmoregulating epithelium and the restoration of the initial low level of the osmotic permeability after the effect of AVP are due to participation of prostaglandin E2 and other autacoids as well as, probably, some physiologically active substances of a lipid nature that are present in the blood serum.

Animals↗

PubMed2005

Aquaporins from pathogenic protozoan parasites: structure, function and potential for chemotherapy.

Infectious diseases, caused by protozoa, such as malaria, sleeping sickness, Chagas' disease or leishmaniasis, are a global threat. The increase in the number of affected individuals and the rapid spread of drug-resistant strains call for specific novel strategies to combat human pathogenic parasites. In the search for novel drug targets, transport proteins for nutrients and metabolites of the parasite-host interface are getting into focus. The present review summarizes and discusses the currently available results on protozoan aquaporins. Various genes coding for aquaporin water and solute channels have been identified in the protozoan genomes and they are probable elements of the parasite's cell membrane. Phylogenetic analysis reveals that individual aquaporin genes are of bacterial or plant origin. So far, six protozoan aquaporins have been cloned and functionally characterized. Typically, these are bifunctional channels and pass water at intermediate to high rates as well as uncharged solutes. In the present review, amino acid compositions of the individual pore entries are compared and permeability properties are attributed to specific protein features. Furthermore, possible physiological roles in osmotic protection and metabolism are discussed. Finally, the potential of protozoan aquaporins for use as a target or entry pathway for chemotherapeutic compounds is reviewed.

Amino Acid Sequence↗

PubMed1963

TURBIDITY CHANGE DURING GLUCOSE PERMEATION IN ESCHERICHIA COLI.

Rogers, Dexter (Utah State University, Logan) and Shon-Hua Yu. Turbidity change during glucose permeation in Escherichia coli. J. Bacteriol. 85:1141-1149. 1963.-In contrast to the normal turbidity decrease, which was observed at pH 5.5, the turbidity of suspensions of Escherichia coli strain A (Weigle) increased during uptake of glucose permease substrates when uptake was studied at pH 6.5. Although the turbidity increase at pH 6.5 was the reverse of the expected change, it correlated with the uptake of substrate. During the later phase of permeation at pH 6.5, both uptake of substrate and turbidity change were reversed. We suggest that the observed turbidity response indicated the accumulation of substrate at the cell membrane when the mechanism for releasing substrate to the cytoplasm was inhibited at pH 6.5. Under this condition, uptake of substrate did not cause swelling because the substrate was osmotically inactive, since it may have been bound to the membrane. Permeability was observed to be susceptible to modification by prior treatment with substrate, by aging in the presence of chloramphenicol, and by treatment with dinitrophenol. Permeation and turbidity change are discussed in terms of a proposed phosphorylative mechanism of glucose transport.

Biological Transport↗

PubMed1995

Chorda-evoked opening of tight junctions in rat submandibular salivary acini demonstrated by microperoxidase.

The permeability of these junctions from the interstitium to the lumen was examined by using an ultrastructural tracer, microperoxidase, in conjunction with electron microscopy. In the resting gland, the reaction product of microperoxidase was seen in the interstitial and intercellular spaces, but not within acinar lumina; thus the tight junction was impermeable to microperoxidase (junction closed). Intraductal injection of hypertonic sucrose solution (1000 mOsm; 30 microliters) caused a sustained elevation of the luminal pressure, indicating osmotic water flow into the lumen due to the presence of a hypertonic solution. In this gland no opening of the tight junctions was observed. In the chorda-stimulated gland, microperoxidase entered the lumen through the tight junctions, that is, they became permeable to microperoxidase (junction open). These findings suggest that chorda stimulation opens the acinar tight junctions and that the paracellular secretory pathway may be involved in the secretion of small molecules and water from the submandibular acini.

Animals↗

PubMed2004

Osmotic blistering in enamel bonded with one-step self-etch adhesives.

One-step self-etch adhesives behave as permeable membranes after polymerization, permitting water to move through the cured adhesives. We hypothesize that osmotic blistering occurs in bonded enamel when these adhesives are used without composite coupling. Tooth surfaces from extracted human premolars were bonded with 5 one-step self-etch adhesives. They were immersed in distilled water or 4.8 M CaCl(2), and examined by stereomicroscopy, field-emission/environmental SEM, and TEM. Water blisters were observed in bonded enamel but not in bonded dentin when specimens were immersed in water. They collapsed when water was subsequently replaced with CaCl(2). Blisters were absent from enamel in specimens that were immersed in CaCl(2) only. Water trees were identified from adhesive-enamel interfaces. Osmotic blistering in enamel is probably caused by the low water permeability of enamel. This creates an osmotic gradient between the bonded enamel and the external environment, causing water sorption into the interface.

Acid Etching, Dental↗

PubMed1985

Effects of bilirubin on transepithelial transport of sodium, water, and urea.

Urinary concentrating defects and renal salt wasting have been described in the hyperbilirubinemic Gunn strain strain of rat. Homozygous animals demonstrate significant reductions in renal medullary urea and sodium ion concentrations. These observations are consistent with possible bilirubin associated disorders in the transepithelial transport of water and solute. To test this hypothesis, measurements of active sodium transport and passive water and urea fluxes were made in hemibladders isolated from the Dominican toad, Bufo marinus. Tissues were exposed to amphibian bicarbonate Ringer's solution containing 0.1 mM bilirubin with 0.05% bovine serum albumin (BSA) or BSA alone. Vasopressin-stimulated sodium transport, as reflected by short circuit current (SCC), was inhibited by 18 +/- 6% in the presence of bilirubin (N = 10; P less than 0.02). Cyclic AMP (p-Cl-phenylthio cAMP 10(-5) M) stimulated SCC was inhibited to a similar degree in the presence of bilirubin. The inhibition was noted only when bilirubin was in the serosal bath, and it could be abolished with BSA 0.5%. Bilirubin had no effect on the increase in SCC induced by higher concentrations of cyclic AMP (10(-4) M), aldosterone, or amphotericin B. Furthermore, bilirubin had no effect on the hydro-osmotic response to vasopressin and vasopressin-induced changes in urea permeability. These findings show that short-term exposure to bilirubin exerts a tissue-specific effect on the vasopressin-stimulated active transport of sodium but has no effect on the vasopressin-induced fluxes of water and urea.

Aldosterone↗

PubMed1995

A comparative study of diffusive and osmotic water permeation across bilayers composed of phospholipids with different head groups and fatty acyl chains.

Osmotic and diffusive water permeability coefficients Pf and Pd were measured for lipid vesicles of 100-250 nm diameter composed of a variety of phospholipids with different head groups and fatty acyl chains. Two different methods were applied: the H2O/D2O exchange technique for diffusive water flow, and the osmotic technique for water flux driven by an osmotic gradient. For phosphatidylcholines in the liquid-crystalline state at 70 degrees C, permeability constants Pd between 3.0 and 5.2.10(-4) cm/s and ratios Pf/Pd 7 and 23 were observed. The observation of a permeability maximum in the phase transition region and the fact that osmotically driven water flux is higher than diffusive water exchange suggest that water is diffusing through small transient pores arising from density fluctuations in the bilayers. The Pd values depend on the nature of the head group, on the chemical structure of the chains, and on the type of chain linkage. In the case of charged lipids, the ionic strength of the solution has a strong influence. For phosphatidylethanolamines, phosphatidic acids, and ether phosphatidylcholines, permeability constants Pd were considerably lower (2-4.10(-6) cm/s at 70 degrees C). For liquid-crystalline phosphatidylcholines, a strong reduction of Pd after addition of ethanol was observed (2-4.10(-6) cm/s at 70 degrees C). The experimental values are discussed in connection with different permeation models.

Biophysical Phenomena↗

PubMed2000

Inhibition of basolateral cAMP permeability in the toad urinary bladder.

1. The effect of sulphonylurea drugs on hydrosmotic flow across toad urinary bladder epithelium was re-evaluated in the present study. Glibenclamide, added to the basolateral medium, significantly enhanced the osmotic flow induced by low doses of antidiuretic hormone (ADH) or forskolin (FK), while it inhibited the effect of exogenous cyclic adenosine monophosphate (cAMP) or its non-hydrolysable bromo derivative, 8-Br-cAMP, added to the basolateral medium. These opposite effects of glibenclamide on the transepithelial osmotic flow can be explained by a reduction of cAMP permeability across the basolateral membrane of the epithelium. The decrease in cAMP permeability leads, according to the direction of the cAMP gradient, to firstly an enhanced osmotic flow when cAMP is generated intracellularly by addition of ADH and FK, glibenclamide reducing cAMP exit from the cell, and secondly a decreased osmotic flow in response to cAMP (and 8-Br-cAMP) added to the basolateral medium, glibenclamide inhibiting, in this case, their entry into the cell. 2. The demonstration that glibenclamide actually inhibits the basolateral cAMP permeability rests on the fact that firstly it decreases the release of cAMP into the basolateral medium by about 40 %, at each concentration of ADH or forskolin tested, secondly it increases the cAMP content of paired hemibladders incubated in the presence of ADH or FK, when intracellular degradation was prevented by phosphodiesterase inhibition, and thirdly it decreases also the uptake of basolateral 8-Br-[3H]cAMP into paired toad hemibladders. 3. Taken together, the present data demonstrate that glibenclamide inhibits the toad urinary bladder basolateral membrane permeability to cAMP, most probably by a direct interaction with a membrane protein not yet indentified but distinct from the sulphonylurea receptor.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1990

Water, proton, and urea transport in toad bladder endosomes that contain the vasopressin-sensitive water channel.

Vasopressin (VP) increases the water permeability of the toad urinary bladder epithelium by inducing the cycling of vesicles containing water channels to and from the apical membrane of granular cells. In this study, we have measured several functional characteristics of the endosomal vesicles that participate in this biological response to hormonal stimulation. The water, proton, and urea permeabilities of endosomes labeled in the intact bladder with fluorescent fluid-phase markers were measured. The diameter of isolated endosomes labeled with horse-radish peroxidase was 90-120 nm. Osmotic water permeability (Pf) was measured by a stopped-flow fluorescence quenching assay (Shi, L.-B., and A. S. Verkman. 1989. J. Gen. Physiol. 94:1101-1115). The number of endosomes formed when bladders were labeled in the absence of a transepithelial osmotic gradient increased with serosal [VP] (0-50 mU/ml), and endosome Pf was very high and constant (0.08-0.10 cm/s, 18 degrees C). When bladders were labeled in the presence of serosal-to-mucosal osmotic gradient, the number of functional water channels per endosome decreased (at [VP] = 0.5 mU/ml, Pf = 0.09 cm/s, 0 osmotic gradient; Pf = 0.02 cm/s, 180 mosmol gradient). Passive proton permeability was measured from the rate of pH decrease in voltage-clamped endosomes in response to a 1 pH unit gradient (pHin = 7.5, pHout = 6.5). The proton permeability coefficient (PH) was 0.051 cm/s at 18 degrees C in endosomes containing the VP-sensitive water channel; PH was not different from that measured in vesicles not containing water channels. Measurement of urea transport by the fluorescence quenching assay gave a urea reflection coefficient of 0.97 and a permeability coefficient of less than 10(-6) cm/s. These results demonstrate: (a) VP-induced endosomes from toad urinary bladder have extremely high Pf. (b) In states of submaximal bladder Pf, the density of functional water channels in endosomes in constant in the absence of an osmotic gradient, but decreases in the presence of a serosal-to-mucosal gradient, suggesting that the gradient has a direct effect on the efficiency of packaging of water channels into endosomes. (c) The VP-sensitive water channel does not have a high proton permeability. (d) Endosomes that cycle the water channel do not contain urea transporters. These results establish a labeling procedure in which greater than 85% of labeled vesicles from toad urinary bladder are endosomes that contain the VP-sensitive water channel in a functional form.

Animals↗

PubMed2005

Water- and cryoprotectant-permeability of mature and immature oocytes in the medaka (Oryzias latipes).

The permeability of the plasma membrane plays a crucial role in the successful cryopreservation of oocytes/embryos. To identify a stage feasible for the cryopreservation of teleost oocytes, we investigated the permeability to water and various cryoprotectants of medaka (Oryzias latipes) oocytes at the germinal vesicle (GV) and metaphase II (MII) stages. In sucrose solutions, the volume changes were greater in GV oocytes than MII oocytes. Estimated values for osmotically inactive volume were 0.41 for GV oocytes and 0.74 for MII oocytes. Water-permeability (microm/min/atm) at 25 degrees C was higher in GV oocytes (0.13+/-0.01) than MII oocytes (0.06+/-0.01). The permeability of MII oocytes to various cryoprotectants (glycerol, propylene glycol, ethylene glycol, and DMSO) was quite low because the oocytes remained shrunken during 2 h of exposure in the cryoprotectant solutions at 25 degrees C. When the chorion of MII oocytes was removed, the volume change was not affected, except in DMSO solution, where dechorionated oocytes shrunk and then regained their volume slowly; the P(DMSO) value was estimated to be 0.14+/-0.01x10(-3) cm/min. On the other hand, the permeability of GV oocytes to cryoprotectants were markedly high, the P(s) values (x10(-3) cm/min) for propylene glycol, ethylene glycol, and DMSO being 2.21+/-0.29, 1.36+/-0.18, and 1.19+/-0.01, respectively. However, the permeability to glycerol was too low to be estimated, because GV oocytes remained shrunken after 2 h of exposure in glycerol solution. These results suggest that, during maturation, medaka oocytes become less permeable to water and to small neutral solutes, probably by acquiring resistance to hypotonic conditions before being spawned in fresh water. Since such changes would make it difficult to cryopreserve mature oocytes, immature oocytes would be more suitable for the cryopreservation of teleosts.

Animals↗

PubMed1976

Osmotic behavior and permeability of osmotically lysed mitochondria.

Experiments were carried out with water-treated isolated rat liver mitochondria (mitochondria ghosts) previously studied by Caplan and Greenawalt (Caplan, A.I., and J.W. Greenawalt. 1966. J. Cell Biol. 31:455-472) and Vasington and Greenawalt (Vasington, F., and J. Greenawalt. 1968. J. Cell Biol. 39:661-675). The ghosts have permeability properties and osmotic behavior comparable to those of isolated mitochondria. Although they have lost most of their internal contents, they must have resealed. Four properties were found which have not been previously described in systems derived from biological membranes: (a) an osmotic behavior in the virtual absence of internal components. (b) a self-arranging property in the formation of invaginations corresponding in morphology to the cristae. The results suggest that the assembly of the molecular components of the inner membrane is sufficient to specify the morphology. Hence the surface area to volume ratio of the vesicles may specify the presence or absence of cristae-like folds. (c) an increase in the permeability of the membranes to sucrose in the presence of iso-osmotic concentrations of sucrose. (d) an independence of the light transmitted by suspensions of the vesicles from the refractive index of the external medium. This observations run counter to the general previous experience with either mitochondria or liposomes.

Animals↗

PubMed1987

The permeability of detachable latex rubber balloons. An in vitro study.

Previous studies have shown that a silicone rubber balloon filled with a hyperosmolar solution of contrast medium expands and may rupture because the wall acts as a semipermeable membrane. The deflation of latex rubber balloons in vivo has been attributed to the same cause. The authors investigated the behavior of latex rubber balloons filled with metrizamide solutions ranging from hyperosmolar to hypo-osmolar relative to plasma. When balloons were immersed in plasma, tritiated water passed bidirectionally through the wall at a rate unrelated to the concentration or volume of the contrast medium in the balloon. There was no exchange of sodium ions across the wall, and the osmolality of the metrizamide did not change over a period of up to six weeks. The authors conclude that deflation of a latex rubber balloon during the first few hours or days is not due to osmosis.

Animals↗

PubMed1983

Breeding condition, temperature, and the regulation of salt and water by pituitary hormones in the red-spotted newt, Notophthalmus viridescens.

In vivo integumental transepithelial potential (TEP) in the Eastern red-spotted newt (Notophthalmus viridescens) increases linearly with external [Na+] from 0.1 to 10 mM and is anion independent. Both integumental TEP and osmotic permeability are higher in laboratory-conditioned (LC, terrestrial) than in breeding-condition (BC, aquatic) newts at temperatures of 5-25 degrees. Prolactin (PRL) treatment of LC newts decreased both TEP and rate of water uptake. Arginine vasotocin (AVT) treatment resulted in a substantial increase in water uptake in LC newts, while little or no AVT response was seen in PRL-treated or BC newts. Hypophysectomy (HX) or ergocryptine treatment increased TEP in BC newts, whereas HX + PRL maintained TEP at control levels. Although ergocryptine and HX were without effect on water uptake in BC newts kept at 5 degrees for 9 days, HX + ACTH increased water uptake. HX produced a substantial fall in serum [Na+] in BC newts, while either PRL or ACTH replacement elevated serum [Na+]. Combined ACTH and PRL treatment returned serum [Na+] to control levels. These data suggest that high endogenous prolactin plays a significant role in maintaining serum [Na+] and integumental permeability and transport characteristics in breeding-condition N. viridescens. Although PRL and ACTH are both sodium retaining in the aquatic breeding stage, these two hormones promote opposite effects on the integument; PRL decreases both water uptake and integumental TEP, whereas ACTH (presumably acting through the adrenals) increases water uptake and possibly TEP.

Adrenocorticotropic Hormone↗

PubMed1979

Metabolic dependence of the offset of antidiuretic hormone-induced osmotic flow of water across the toad urinary bladder.

The elevated osmotic permeability to water induced by antidiuretic hormone (ADH) in the isolated urinary bladder of the toad is rapidly reversed by removal or washout of the ADH. This return to normal water permeability is delayed by the suppression of production of metabolic energy by any of three maneuvers: (i) low temperature (2 degrees C); (ii) inhibition of oxidative phosphorylation (10 mM azide or 0.5 mM 2,4 dinitrophenol); or (iii) inhibition of glycolysis (10 mM iodoacetate or 10 mM 2-deoxyglucose). Moreover exposure to cytochalasin B, 2.1 X 10(-5) M, either before or after initiation of the hormonal effect also delays the return of water permeability to normal following removal of ADH. When considered within constraints imposed by models which predict ADH's action on water permeability to be either via modulation of the fluidity of lipids in the membrane or via the figuration of proteins ("pores") in the lipid membrane, these observations on the inhibition of the reversal of ADH stimulation of water flow are more consistent with the protein (pore) theory and place limitations on the mechanisms by which proteins in such pores can return to the resting or impermeable state.

Animals↗

PubMed2001

The role of electroosmotic flow in transdermal iontophoresis.

Iontophoresis enhances transdermal drug delivery by three mechanisms: (a) the ion-electric field interaction provides an additional force which drives ions through the skin; (b) flow of electric current increases permeability of skin; and (c) electroosmosis produces bulk motion of the solvent itself that carries ions or neutral species, with the solvent 'stream'. The relative importance of electroosmotic flow is the subject of this review. Experimental observations and theoretical concepts are reviewed to clarify the nature of electroosmotic flow and to define the conditions under which electroosmotic flow is an important effect in transdermal iontophoresis. Electroosmotic flow is bulk fluid flow which occurs when a voltage difference is imposed across a charged membrane. Electroosmotic flow occurs in a wide variety of membranes, is always in the same direction as flow of counterions and may either assist or hinder drug transport. Since both human skin and hairless mouse skin are negatively charged above about pH 4, counterions are positive ions and electroosmotic flow occurs from anode to cathode. Thus, anodic delivery is assisted by electroosmosis, but cathodic delivery is retarded. Water carried by ions as 'hydration water' does not contribute significantly to electroosmotic flow. Rather electroosmotic flow is caused by an electrical volume force acting on the mobile counterions. The simple 'limiting law' theory commonly given in textbooks and some research articles is a very poor approximation for transdermal systems. However, several extensions of the limiting law are compatible with each other and with the available experimental data. One of these theories, the Manning theory, has been incorporated into a theory for the effect of electroosmotic flow on iontophoresis, the latter theory being in good agreement with experiment. Both theory and experimental data indicate that electroosmotic flow increases in importance as the size of the drug ion increases. The 'ionic' or Nernst-Planck effect is the largest contributor to flux enhancement for small ions. Increased skin permeability or the skin 'damage effect', is a significant factor for both large and small ions, particularly for experiments at high current density. For monovalent ions with Stokes radii larger than about 1 nm, electroosmotic flow is the dominant flow mechanism. Because of electroosmotic flow, transdermal delivery of a large anion (or negatively charged protein) from the anode compartment can be more effective than delivery from the cathode compartment.

Administration, Cutaneous↗

PubMed1993

Vasopressin activates collecting duct urea transporters and water channels by distinct physical processes.

The present studies were performed to investigate the kinetics of regulation of water channels and urea carriers in the rat terminal (IMCD) in response to vasopressin (AVP). The time courses of osmotic water permeability (Pf) and urea permeability (P(urea)) were measured in isolated perfused rat terminal IMCD segments following AVP stimulation and subsequently following AVP washout. Under control conditions, Pf and P(urea) kinetics were similar. Both transport processes exhibited complex patterns of activation with a period of rapid permeability increase followed by a period of slower increase. Both transporters also exhibited complex patterns of reversal following AVP washout, with a rapid permeability decrease (5 min) followed by a slower decrease toward the baseline value. The measurements were repeated in the presence of a lumen > bath osmotic gradient, a condition associated with a decreased rate of apical endocytosis in collecting ducts. The lumen > bath gradient did not alter the kinetics of Pf increase after AVP addition, but completely blocked the decrease in Pf normally seen with washout of AVP. In contrast, the lumen > bath osmotic gradient did not affect the decrease in urea permeability after AVP washout, but blocked the rapid phase of urea permeability increase following AVP addition. Thus imposition of a lumen > bath osmotic gradient resulted in separation of the time courses of P(urea) and Pf changes associated with AVP addition and washout. This finding indicates that the physical processes responsible for AVP-mediated alteration of urea transporter and water channel activity in the apical membrane are distinct.

Animals↗