Passive Transport Processes Research Articles

Physiologically based pharmacokinetics of cyclosporine A: reevaluation of dose-nonlinear kinetics in rats.

The disposition kinetics of Cyclosporine A (CyA) in rat, based on measurement in arterial blood, appeared dose-linear over a wide i.v. dose range (1.2-30 mg/kg). Physiologically based pharmacokinetic (PBPK) analysis, however, demonstrated that this was an apparent observation resulting from counterbalancing nonlinear factors, such as saturable blood and tissue distribution, as well as clearance (CLb). A PBPK model was successfully developed taking into account these multiple nonlinear factors. Tissue distribution was distinctly different among various organs, being best described by either a linear model (muscle, fat; Model 1), one involving instantaneous saturation (lung, heart, bone, skin, thymus; Model 2), noninstantaneous saturation (kidney, spleen, liver, gut; Model 3), or one with saturable efflux (brain; Model 4). Overall, the whole body volume of distribution at steady state for unbound CyA (Vuss) decreased with increasing dose, due at least in part to saturation of tissue-cellular cyclophilin binding. Clearance, essentially hepatic, and described by the well-stirred model, was also adequately characterized by Michaelis-Menten kinetics, Km 0.60 microgram/ml. In model-based simulations, both volume of distribution at steady state (Vss,b) and CLb varied in a similar manner with dose, such that terminal t1/2 remained apparently unchanged; these dose responses were attenuated by saturable blood binding. CyA concentration measured in arterial blood was not always directly proportional to the true exposure, i.e., unbound or target tissue concentrations. The PBPK model not only described comprehensively such complicated PK relationships but also permitted assessment of the sensitivity of individual parameters to variation in local nonlinear kinetics. Using this approach, dose-dependent CyA uptake into brain was shown to be sensitive to both active and passive transport processes, and not merely the affinity of the active (efflux) transporter at the level of the blood-brain barrier.

Bedload and water-column transport and colonization processes by post-settlement benthic macrofauna: Does infaunal density matter?

In this study, we document dispersal and colonization by post-settlement benthic macrofauna on a large (250×500 m) area of intertidal sandflat at Wiroa Island in the Manukau Harbour, New Zealand, over a 2-week period in February 1994. We examine the effects of variation in the natural density of large (>15 mm) Macomona liliana Iredale (a tellinid bivalve) on these processes. Post-settlement transport was measured using cylindrical bedload and water-column traps positioned at 22 experimental sites within the study site. Macrofaunal colonization was documented using small pans of defaunated sediment deployed at each of the experimental sites. Greatest macrofaunal colonization of pans occurred in areas of greatest sediment reworking and deposition, suggesting transport and deposition of post-settlement stages may be passive processes. For some species [e.g. Austrovenus stutchburyi (Wood)] there was a significant positive relationship between their abundance and the weight of sediment in bedload traps, as well as wind conditions over the study period. This is consistent with passive transport processes, but does not exclude the possibility of active dispersal processes also being important. Differences in the hydrodynamic conditions (e.g. increased wind generated wave activity) over the 2-week study may have contributed to changes in abundance on the 2 sampling dates. For other species (e.g. Macomona liliana) the absence of significant positive relationships between abundance in either bedload or water-column traps and the weight of sediment retained in traps, as well as between ambient densities and the net flux of individuals across sites, is consistent with active dispersal and colonization. There was a negative relationship between background density of large Macomona liliana and numbers of smaller conspecifics (≤4 mm) and other bivalves [ Austrovenus stutchburyi, Cyclomactra ovata (Gray) and Nucula hartvigiana Pfeiffer] colonizing pans of defaunated sediment. The results suggest that 10 to 100s m scale variation in the natural densities of an abundant siphonal surface deposit feeder may have a significant influence on dispersal and colonization and, thus, the spatial patterns of macrofauna on the tidal flat.

Correlation between cell killing effect and cell membrane potential after heat treatment: analysis using fluorescent dye and flow cytometry

Exposure of mammalian cells to a temperature between 42 and 44°C produces multiple effects on cellular metabolism. However, none of these effects have been demonstrated to be casually related to cell death. The plasma membrane has also been suggested to be one of the targets for hyperthermia. Hyperthermia influences the membrane fluidity and membrane fluidity affects both passive diffusion and active transport process. Here, we examined the effect of hyperthermia on the transmembrane potential V-79 cells using a lopophilic probe and flow cytometry. The underlying principle on which this method is based is that a freely diffusible cation distributes itself across the membrane according with the transmembrane potential and with the concentration gradient. By these methods, we tried to find the relation between the change of surviving fraction and membrane potential. We revealed that the membrane potential becomes to be depolarized by heat treatment between 41 and 44°C immediately after heat treatment. In the relationship between the change in membrane potential and cell surviving fraction, the difference between 43 and 44°C was not statistically significant but between 42 and 43°C was statistically significant. These results imply that the cell membrane (potassium ion channel) is one of the targets of heat treatment and that duration of depolarized condition leads to cell death.

Characterization of bile salt uptake by Giardia lamblia

We have shown previously that Giardia lamblia takes up conjugated bile salt in vitro, and have now investigated the mechanism by which this occurs. Uptake of sodium taurocholate (TC) and glycocholate (GC) with respect to time had an initial exponential component followed by a linear component, consistent with a combination of both active and passive transport processes. The presence of an active transport process was further supperted by experiments which showed that bile salt uptake: (i) was concentration dependent (apparent K m's for TC and GC were 0.21 and 0.63 mM, respectively); (ii) was competitively inhibitable; (iii) was reduced by the metabolic inhibitor sodium fluoride (50 mM) and low temperature (4°C). Bile salt was not taken up by glutaraldehyde-fixed parasites, indicating that bile salt was not merely being adsorbed on to the parasite surface. Differential centrifugation of lyzed parasites following exposure to radiolbelled GC, showed that the majority of bile salt was located in the cytosol fraction (76%) with a relatively minor component associated with cell membrane, indicating that bile salt had been internalized. Bile salt analysis of extracts of parasites and culture medium indicated that GC had not been metabolized by Giardia. Thus, like the mammalian ileum, Giardia appears to take up conjugated bile salts by active and passive transport processes. Conjugated bile salts are known to promote encystation and thus these uptake mechanisms may constitute an important survival mechanism for the parasite enabling it to complete its life cycle.

Cellular aspects of transvascular exchange: a 40-year perspective.

Study of the microcirculation began with microscopic anatomy in the 17th century, but it was not until the mid-19th century that the concept of microcirculatory exchange was developed, in association with the great expansion in knowledge of physical chemistry and cell biology. In the first half of the 20th century, the roles of diffusion and osmosis (ultrafiltration) were put on a quantitative foundation and the physiological basis for regulation of the microcirculation was established. During the past 40 years, our understanding of transvascular exchange has been deepened and widened through two major developments: 1) the application of electron microscopy to the study of microvascular structure and 2) the use of sophisticated physical and mathematical models of passive transport processes to analyze and interpret experimental data. In the past few years, the focus of microvascular research has been returning to cell biology and the role of endothelial cells in controlling transvascular exchange. The 21st century holds promise of many exciting new developments.

Studies on the gastrointestinal absorption of phosphocitrate, a powerful controller of hydroxyapatite formation

In situ gastrointestinal perfusion and a chemical induced subcutaneous calcergy model were used to examine the oral bioavailability of the potent calcification inhibitor, phosphocitrate (PC). Formation of calcergic plaques was observed to decrease by approx. 34% when 450 mg PC/kg body weight per day was given to rats by gavage. An equivalent response was observed when only 10 mg PC/kg per day was given intraperitoneally thus indicating reduced bioavailability of the compound by the oral route. Luminal metabolism of PC did not contribute significantly to its overall poor oral bioavailability. Data derived from perfusion of in situ isolated intestinal segments with PC concentrations from 5 to 20 mM indicated luminal absorption by a passive transport process. Studies with radiolabeled PC confirmed the presence of small amounts of [ 14C]PC and [ 14C]citrate in portal blood. It was concluded that the membrane transport characteristics for PC were limiting transfer and that a possible future improvement might arise through the incorporation of a lipophilic moiety into the molecule.

Passage of aerosolized BSA and the nona-peptide dDAVP via the respiratory tract in young and adult rats.

The passage of the protein marker, bovine serum albumin (BSA, MW = 67,000), and the nona-peptide, 1-deaminocysteine-8-D-arginine vasopressin (dDAVP, MW = 1067), from the respiratory tract into the blood when applied as an aerosol with a MMAD of 1.7 microns was studied in 14-, 30-, and 100-120-day-old (adult) healthy rats and in adult rats with lung injury. In blood serum of adult rats the levels of immunoreactive BSA reached its maximum 16-24 h after a 1-h aerosol exposure with a calculated total passage of 6.4 +/- 1.8% of the given dose. dDAVP serum levels measured by RIA peaked after 0.5-1 h, giving a total passage of 84.3 +/- 12.9%. With increasing exposure periods from 0.5 to 3 h, which thereby increased the lung burden, the serum levels of BSA and dDAVP increased linearly indicating passive transepithelial transport processes for both molecules. For the young rats, similar serum level-time curves were obtained like those of the adult, with similar total passages of BSA, 4.6 +/- 0.8% for the 14-day-old rats and 5.2 +/- 1.6% for the 30-day-old rats. For dDAVP the total passage was significantly lower in both the 14-day-old rats, 40.9 +/- 12.1%, and the 30-day-old rats, 16.7 +/- 6.1% (p less than .05), as compared to the adult rats. Acute lung inflammation induced in rats by intratracheal instillation of 5 mg ferritin/kg body wt prior to a 1-h marker aerosol exposure increased the passage of BSA (58.7 +/- 18.8%, p less than .05), while the dDAVP passage was less affected (99.2 +/- 25.2%, p greater than .05) as compared to the healthy adult rats. The results indicate that after aerosol exposure the total passage of dDAVP over the respiratory tract was higher than that of the macromolecule BSA, the passage appeared to increase with the maturity of the rats and by inflammatory changes in the lung tissue.

Serial scintigraphic imaging for visualization of passive transport processes in the human Fallopian tube

Spontaneous migration of a radionuclide tracer from the vagina to the peritoneum may be visualized by scintigraphic imaging (hysterosalpingoscintigraphy, HSS). A prospective study was designed to evaluate diagnostic criteria for normal tubal passage of a control group (n = 7) and to establish the predictive value of the HSS technique in defining functional deficiency in anatomically patent tubes. In 56 patients with tubal and unexplained infertility, a comparison between the results of the tracer migration study and of contrast hysterosalpingography and laparoscopy was made. The overall correlation was 65%. Clearly discrepant results (i.e. an abnormal migration pattern in anatomically patent tubes) were recorded in 18% and were positively, yet not significantly associated with tubal adhesive disease and with a history of tubal microsurgery. Interpretation of scans was equivocal in another 18% of patients due to undetectable ascension of the tracer to the uterus. It is suggested that the radionuclide is moved forward by the same passive transport processes which are concerned with support of the migration of spermatozoa to the ovum, and that failure of tubal migration of the tracer may render patients eligible for in-vitro fertilization/embryo transfer treatment.

Initial contact, exploration and attachment of barnacle (Balanus amphitrite) cyprids settling in flow

Settlement responses of barnacle (Balanus amphitrite) cyprids to boundary-layer flows were examined in laboratory flume-experiments. The leading-edge configuration of flat plates was altered in order to manipulate flows without changing surface topography or freestream velocity. Settlement along the plates correlated strongly with downstream gradients in shear stress. Analyses of video images taken during the experiments indicate that cyprids first contact plates in regions where plate-ward advection is high, and subsequent exploratory movement along the plate is oriented with flow direction at the plate surface. After exploration, cyprids reject a surface more frequently in a fast flow (10 cm s−1 freestream velocity) than in a slow flow (5 cm s−1), but rejection occurs in shear stresses well below the threshold that would prevent attachment and exploration. A higher rejection rate does not result in lower settlement, however, since contact rate is higher in fast than slow flows. The movement of cyprids in flow thus appears to be a passive transport process during the initial contact stage of settlement, but an active behavioral response to flow direction and shear stress during later stages of exploration and attachment.

Metabolic Reduction in Hypothermia: Pathophysiological Problems and Natural Examples - Part 1

Following a description of the protective effects of hypothermia, the limitations resulting from "cold swelling" of the brain, caused by a progressive discrepancy between active and passive cellular transport processes, are discussed. The observation that the tolerable limits of metabolic reduction lie well below the normal turnover rates leads to a short survey of the evolution of energy metabolism. The gradual development of the latter from anaerobiosis to aerobiosis and from poikilothermy to homeothermy apparently provides the background for transient reductions to lower rates which spontaneously occur even in higher vertebrates. As an impressive example of such a natural strategy, the anaerobic survival of diving turtles is outlined which essentially depends on an efficient buffering. This dependence bears analogy to cardioplegia, where an increase in buffering capacity can greatly retard the fall of energy turnover, thus leading, as is shown by microcalorimetry, to an intermediate plateau of anaerobic metabolism.

Digital Imaging Fluorescence Microscopy: Statistical Analysis of Photobleaching and Passive Cellular Uptake Processes

This chapter discusses the fluorescence photobleaching phenomenon and the errors that arise when it is not controlled. Objective criteria are developed to eliminate photobleaching as a variable in kinetic experiments. The photobleaching of a single fluorescent monomolecular film is used as an experimental tool to address a major assumption in fluorescence microscopy: that the specimen in the focal plane, when considered on a pixel-by-pixel basis, is uniformly illuminated. Because of their sensitivity, fluorescent techniques are potentially quite useful, particularly for active and passive mass transport processes. The photobleaching kinetics of only one kind of fluorophore in a uniform microenvironment is easily and accurately described by a simple mathematical model for a first-order reaction: a monoexponential decay and a background term. In poorly defined systems, such as the complex milieu of the cell, the evaluation of kinetic processes is much more complicated, particularly with living cells where both active and passive transport processes occur simultaneously and the cell is continually changing shape and position. The chapter emphasizes the utilization of statistical criteria to evaluate the performance of image processing algorithms.

Lack of protective effect of oral enprostil, a synthetic prostaglandin E2, on intestinal transport and morphology following abdominal irradiation in the rat

Previous studies have demonstrated that abdominal irradiation alters intestinal uptake of nutrients. The purpose of this study was to determine the effect of an orally administered synthetic prostaglandin E2, enprostil, given on three occasions shortly prior to a single exposure to 600 cGy external abdominal irradiation, on intestinal active and passive transport processes and villus morphology measured 7 days later. Animals were sham-irradiated (CONT) or were exposed to a single dose of 600 cGy external abdominal irradiation (RAD); two and one mornings before the day of irradiation or sham irradiation, and 1 h before irradiation or sham irradiation enprostil was administered. One half of CONT and RAD groups were dosed orally with enprostil, 5 micrograms/kg body weight, and the other half of the CONT and RAD groups were dosed with placebo. Seven days later the in vitro uptake of glucose, galactose, long-chain fatty acids, and cholesterol was determined in the four groups (CONT with and without enprostil, and RAD with and without enprostil). In CONT, enprostil was associated with increased jejunal uptake of glucose and ileal uptake of galactose. In RAD given enprostil, there was increased jejunal uptake of galactose but reduced ileal uptake of glucose and galactose. The expected radiation-associated decline in jejunal galactose uptake was prevented with enprostil. In CONT given enprostil, there was increased jejunal uptake of fatty acid (FA) 14:0 and 16:0 but reduced uptake of FA 18:0, 18:1, and 18:2; enprostil had no effect on lipid uptake in the ileum in CONT.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for critical-period programming of intestinal transport function: variations in the dietary ratio of polyunsaturated to saturated fatty acids alters ontogeny of the rat intestine

2-week isocaloric modifications in the dietary ratio of polyunsaturated/saturated fatty acids (P/S) alters intestinal transport in rats. This study was undertaken to test the hypotheses that (1) the fatty acid composition of a nutritionally adequate diet in early life has lasting consequences for active and passive intestinal transport processes; and (2) early life feeding experiences with diets of varying fatty acid composition influence the intestines' ability to adaptively up- or down-regulate intestinal transport in later life. Female Sprague-Dawley rats were weaned onto S or P and were maintained on these diets for 2, 10 or 12 weeks. An in vitro uptake technique was used in which the bulk phase was vigorously stirred to reduce the effective resistance of the intestinal unstirred water layer. P decreased and S increased the uptake of glucose, and this effect was progressive from 2 to 12 weeks. Switching from a P to an S diet decreased jejunal but increased ileal uptake of glucose, whereas switching from an S to a P diet was associated with a decline in both the jejunal and the ileal uptake of glucose. The ileal uptake of galactose increased as the animals grew on either P or S. Switching from P to S resulted in a decline in ileal uptake of galactose, whereas the opposite effect was observed when switching from S to P. The effect of feeding P or S on hexose uptake was influenced by the animals' dietary history: ileal glucose and galactose uptake was lower in animals fed P at an early age (PSP) than in animals fed P for the first time in later life (SSP). Jejunal glucose and galactose uptake was also lower in animals fed S at an early age (SPS) than in those fed S for the first time in later life (PPS). The alterations in the uptake of long-chain saturated and unsaturated fatty acids and cholesterol did not progress with longer periods of feeding, and in the jejunum, lipid uptake did not change when switching from P to S or S to P. Early feeding with P (PSP vs. SSP) was associated with lower jejunal uptake of 18:3 and lower ileal uptake of 12:0, whereas previous feeding with S (SPS vs. PPS) was associated with lower ileal uptake of cholesterol. The changes in uptake of hexoses and lipids was not explained by differences in the animals' food consumption, body or intestinal weight or mucosal surface area. Thus, (1) the effect of changes in the dietary P/S ratio on hexose uptake are fast, progressive and irreversible, whereas the general effect on lipid uptake is nonprogressive and irreversible; and (2) the intestinal uptake of hexoses and some lipids is subject to critical-period programming. This late effect of early nutrition signifies that the adaptability of intestinal transport is subject to dietary variations shortly after weaning.

Effect of physical activity on calcium and phosphorus metabolism in the rat

Changes in skeletal mass, nutritional calcium and phosphorus balance, and intestinal calcium absorption were studied in four groups of rats: control, exercise allowing free access to food, exercise with pair-feeding to control levels, and immobilization. The exercise regimen consisted of treadmill running 25 m/min, 60 min/day, 5 days/wk for 13 wk; rats were immobilized by bilateral sciatic denervation. The total body Ca (TBCa) was measured by neutron activation analysis as an index of skeletal mass. Standard metabolic balance techniques were used to determine calcium and phosphorus balance, and an in situ duodenal loop ligation preparation was used to study the active and passive intestinal Ca transport processes. Exercise promoted a positive Ca and P balance and increased the skeletal mass, largely as a result of an increase in 1,25-dihydroxyvitamin D and an enhancement of the intestinal Ca absorption efficiency. Urinary excretion of Ca and P did not differ from control levels and food intake was not a factor because pair-fed rats responded to exercise almost identically to those fed ad libitum. Conversely, immobilization caused a decrease in TBCa and a lower Ca and P balance. These effects are the result of an increased urinary mineral excretion, greater endogenous fecal excretion, and decreased mineral absorption efficiency in the intestine.

Renal Calcium Transport: Sites and Insights

In the kidney the absorption of calcium in the proximal tubule is mediated largely through passive transport processes, in the distal tubule by hormone-sensitive transcellular movement, whereas the loop on Henle represents a hybrid situation. The ability of calcium-transporting cells to accommodate wide ranges of transcellular movement without disturbing cytosolic calcium-dependent processes suggests the presence of mechanisms to coordinate apical entry and basolateral efflux of calcium.

Effect of nonprotein thiols in the intestinal tissue on the transport of salicylate.

Rat intestinal salicylate transport showed concentration dependency, especially up to 22.5 mM in the mucosal medium, in an experiment using in vitro everted rat intestinal sac prepared from jejunum, ileum and colon. Thus, it is suggested that both saturatable and unsaturatable transport processes are involved in the intestinal salicylate transport. However, it was also estimated that intestinal salicylate absorption at high concentration in the clinical therapeutic context occurred predominantly through the unsaturatable (passive) transport process. Only the saturatable transport process seems to be influenced by nonprotein thiol levels in the intestinal tissue; there was a decrease of salicylate transport with decrease of nonprotein thiols. Further, since the saturatable transport process was restored even by the exogenous nonprotein thiols such as cysteamine, a process of nonspecific binding to the mucosal membrane rather than a specific carrier system may be involved in the saturatable transport of salicylate.

Dietary fat selectively alters transport properties of rat jejunum.

The influence of dietary fatty acid composition on intestinal active and passive transport function, brush border membrane composition, and morphology was examined in rats. Animals fed a semisynthetic diet high in saturated fatty acids demonstrated enhanced in vitro jejunal uptake of decanoic, dodecanoic, palmitic, stearic, and linoleic acid, as well as cholesterol and chenodeoxycholic and taurochenodeoxycholic acid, as compared with uptake in animals fed a semisynthetic diet high in polyunsaturated fatty acids but equivalent in total content of fat and other nutrients, or as compared with Purina chow. Feeding the saturated fatty acid diet was also associated with reduced jejunal uptake of a range of concentrations of glucose, enhanced ileal uptake of leucine, unchanged uptake of galactose, and lower uptake of decanol. The semisynthetic diets did not alter brush border membrane protein, sucrase or alkaline phosphatase activities, cholesterol, or total phospholipids, although the percentage of jejunal amine phospholipids was higher than in rats fed chow. The morphologic differences between the jejunum and ileum were abolished in animals fed the high polyunsaturated fatty acid diet; in rats fed the high saturated fatty acid diet, there was reduced mean ileal villus height, width, thickness, surface area, cell size, and villus density, as well as reduced mucosal surface area. The changes in jejunal transport were not correlated with the alterations in morphology, unstirred layer resistance, food intake, or body weight gain. It is proposed that small changes in the percentage of total dietary lipids composed of essential and nonessential fatty acids (without concurrent alterations in dietary total fat, carbohydrate, or protein) influence active and passive intestinal transport processes in the rat.

Ion transport mechanisms in cultured bovine corneal endothelial cells.

Intracellular potential measurements of confluent monolayers of cultured bovine corneal endothelial cells were used to define passive ion transport processes in these cells. Previous studies (Jentsch et al., J. Membr. Biol. 78:103 (1984); Jentsch et al., J. Membr. Biol. 81:189 (1984] have provided the experimental basis for a cellular model, in which bicarbonate entry across the basolateral membrane is indirectly driven by a Na+/H+-exchanger, which is inhibitable by amiloride (1mM). Bicarbonate and sodium should leave the cell via an electrogenic bicarbonate sodium cotransport, which is inhibitable by the disulfonic stilbene derivates SITS or DIDS. This model is also consistent with results from transendothelial studies. In this paper, we briefly review the evidence we have obtained for this model and demonstrate, that the electrical response to sodium (depolarization upon Na+-removal) is neither due to an inhibition of Na+/K+-ATPase nor explainable in terms of changes in K+-conductance. This is concluded from the observation of these responses in the presence of ouabain (10(-4)M) or barium (1mM).

Electrogenic sodium-bicarbonate symport in cultured corneal endothelial cells

Intracellular potential measurements on confluent monolayers of cultured bovine corneal endothelial cells were used to define passive ion transport processes in these cells. Previous studies [11, 12] have provided the experimental basis for a cellular model, is which bicarbonate entry across the basolateral membrane in indirectly driven by a Na+/H+-exchanger, which is inhibitable by amiloride (1 mmol/l). Na+ and HCO3- leave the cell via an electrogenic bicarbonate sodium cotransport, which is inhibitable by the disulfonic stilbene derivates SITS or DIDS. This model is also compatible with transepithelial work from other groups. In this paper, we briefly review the evidence we have obtained for this model.

Phospholipids, free sterols and adenosine triphosphatase of plasma membrane‐enriched preparations from roots of citrus genotypes differing in chloride exclusion ability

Plasma membrane‐enriched preparations from fibrous roots of three citrus genotypes differing in their abilities for chloride exclusion, and grown in the presence of 0,50 or 100 mM NaCl for 4 weeks, were analysed for phospholipid and free sterol content and vanadate‐sensitive adenosine triphosphatase (ATPase) activity over a range of temperatures. The best chloride excluder, Rangpur lime (Citrus reticulaia Blanco var. austera hyb.?), had significantly higher maximal ATPase activity than both the moderate chloride excluder. Kharna khatta (Citrus kharna Raf.), and the worst chloride excluder, Etrog citron (Citrus medico L.), at all assay temperatures below 28°C. Salt treatment had no effect on maximal ATPase activity of either Rangpur lime or Etrog citron but resulted in increased activity of the enzyme in Kharna khatta at temperatures below 28°C.Arrhenius plots of ATPase activity from the three citrus genotypes showed that, in controls, the activation energy (E.,) of Rangpur lime ATPase was significantly lower than that of both Kharna khatta and Etrog citron. The thermotropic phase transition temperature (Tf) for Rangpur lime (27°C) was also lower than for the other citrus genotypes (31°C). Salt treatment resulted in increases in both Ea and T, for Rangpur lime, decreases in both parameters for Kharna khatta and no change of either parameter for Etrog citron.An inverse relationship between Ea and the phospholipid to free sterol ratio was evident for plasma membrane preparations from all three citrus genotypes in the presence and absence of salt treatment suggesting that changes in membrane fluidity, particularly those induced by free sterols, have the potential to influence active as well as passive ion transport processes and thus may play a significant role in the chloride exclusion mechanism.