Static and dynamic components of renal cortical brush border and basolateral membrane fluidity: role of cholesterol

The effect of methanol on the structural parameters of neuronal membrane lipid bilayers

The structures of the intact synaptosomal plasma membrane vesicles (SPMVs) isolated from bovine cerebral cortexs, and the outer and the inner monolayer separately, were evaluated with 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1,3-di(1-pyrenyl)propane (Py-3-Py) as fluorescent reporters and trinitrophenyl groups as quenching agents. The methanol increased bulk rotational and lateral mobilities of SPMVs lipid bilayers. The methanol increased the rotational and lateral mobilities of the outer monolayers more than of the inner monolayers. n-(9-Anthroyloxy)stearic acid (n-AS) were used to evaluate the effect of the methanol on the rotational mobility at the 16, 12, 9, 6, and 2 position of aliphatic chains present in phospholipids of the SPMVs outer monolayers. The methanol decreased the anisotropy of the 16-(9-anthroyloxy)palmitic acid (16-AP), 12-(9-anthroyloxy)stearic acid (12-AS), 9-(9-anthroyloxy)stearic acid (9-AS), and 6-(9-anthroyloxy)stearic acid (6-AS) in the SPMVs outer monolayer but it increased the anisotropy of 2-(9-anthroyloxy)stearic acid (2-AS) in the monolayers. The magnitude of the increased rotational mobility by the methanol was in the order at the position of 16, 12, 9, and 6 of aliphatic chains in phospholipids of the outer monolayers. Furthermore, the methanol increased annular lipid fluidity and also caused membrane proteins to cluster. The important finding is that was far greater increase by methanol in annular lipid fluidity than increase in lateral and rotational mobilities by the methanol. Methanol alters the stereo or dynamics of the proteins in the lipid bilayers by combining with lipids, especially with the annular lipids. In conclusion, the present data suggest that methanol, in additions to its direct interaction with proteins, concurrently interacts with membrane lipids, fluidizing the membrane, and thus inducing conformational changes of proteins known to be intimately associated with membranes lipids.

Amphiphilic effects of dibucaine·HCl on rotational mobility of n-(9-anthroyloxy)stearic acid in neuronal and model membranes

We studied dibucaine's effects on specific locations of n-(9-anthroyloxy)palmitic acid or stearic acid (n-AS) within phospholipids of synaptosomal plasma membrane vesicles isolated from bovine cerebral cortex (SPMV) and model membranes. Giant unilamellar vesicles (GUVs) were prepared with total lipids (SPMVTL) and mixture of several phospholipids (SPMVPL) extracted from SPMV. Dibucaine.HCl increased rotational mobility (increased disordering) of hydrocarbon interior, but it decreased mobility (increased ordering) of membrane interface, in both native and model membranes. The degree of rotational mobility in accordance with the carbon atom numbers of phospholipids comprising neuronal and model membranes was in the order at the 16, 12, 9, 6 and 2 position of aliphatic chain present in phospholipids. The sensitivity of increasing or decreasing effect of rotational mobility of hydrocarbon interior or surface region by dibucaine.HCl differed depending on the neuronal and model membranes in the descending order of SPMV, SPMVPL and SPMVTL.

Amphiphilic effects of local anesthetics on rotational mobility in neuronal and model membranes

To provide a basis for studying the molecular mechanism of pharmacological action of local anesthetics, we carried out a study of the membrane actions of tetracaine, bupivacaine, lidocaine, prilocaine and procaine. Fluorescence polarization of 12-(9-anthroyloxy)stearic acid (12-AS) and 2-(9-anthroyloxy)stearic acid (2-AS) were used to examine the effects of local anesthetics on differential rotational mobility between polar region and hydrocarbon interior of synaptosomal plasma membrane vesicles (SPMV) isolated from bovine cerebral cortex, and liposomes of total lipids (SPMVTL) and phospholipids (SPMVPL) extracted from the SPMV. The two membrane components differed with respect to 2 and 12 anthroyloxy stearate (2-AS, 12-AS) probes, indicating that a difference in the membrane fluidity may be present. In a dose-dependent manner, tetracaine, bupivacaine, lidocaine, prilocaine and procaine decreased anisotropy of 12-AS in the hydrocarbon interior of the SPMV, SPMVTL and SPMVPL, but tetracaine, bupivacaine, lidocaine and prilocaine increased anisotropy of 2-AS in the membrane interface. These results indicate that local anesthetics have significant disordering effects on hydrocarbon interior of the SPMV, SPMVTL and SPMVPL, but have significant ordering effects on the membrane interface, and thus they could affect the transport of Na(+) and K(+) in nerve membranes, leading to anesthetic action.

Influence of the physical states of membrane surface area and center area on lysosomal proton permeability

The physical state of the lysosomal membrane was modulated with the membrane fluidizers n-propanol and n-octanol and with the membrane rigidifiers cholesteryl hemisuccinate and cholesterol. Membrane fluidity was examined by the steady-state fluorescence anisotropy of 2-(9-anthroyloxy) palmitic acid and 16-(9-anthroyloxy) palmitic acid. Fluidizing the membranes at the surface and center areas increased the proton permeability coefficient by 92.8 and 18.0%, respectively. Rigidifying the membranes at the surface and center areas decreased the coefficient by 68.2 and 40.2%, respectively. Proton leakage of the lysosomes increased and decreased similar to the coefficient changes with the treatments. The results indicate that lysosomal proton permeability is affected by its membrane's physical state, and the physical state of the membrane surface area affects the proton permeability more markedly. The proton permeability coefficient of liposomes was similar to that of lysosomes, suggesting that efflux of lysosomal protons might occur through the lipid part of the bilayer but not transmembrane proteins.

Amelioration of renal lesions associated with diabetes by dietary curcumin in streptozotocin diabetic rats

Curcumin, the coloring principle of the commonly used spice turmeric (Curcuma longa) was fed at 0.5% in the diet to streptozotocin-induced diabetic Wistar rats for 8 weeks. Renal damage was assessed by the amount of proteins excreted in the urine and the extent of leaching of renal tubular enzymes: NAG, LDH, AsAT, AlAT, alkaline and acid phosphatases. The integrity of kidney was assessed by measuring the activities of several key enzymes of the renal tissue: glucose-6-phosphate dehydrogenase, glucose-6-phosphatase, and LDH (Carbohydrate metabolism), aldose reductase and sorbitol dehydrogenase (polyol pathway), transaminases, ATPases and membrane PUFA/SFA ratio (membrane integrity). Data on enzymuria, albuminuria, activity of kidney ATPases and fatty acid composition of renal membranes in diabetic condition suggested that dietary curcumin brought about significant beneficial modulation of the progression of renal lesions in diabetes. These findings were also corroborated by histological examination of kidney sections. It is inferred that this beneficial ameliorating influence of dietary curcumin on diabetic nephropathy is possibly mediated through its ability to lower blood cholesterol levels.

Calcium and phosphorous deficiencies alter the lipid composition and fluidity of intestinal basolateral membranes

Steady-state fluorescence polarization and lipid composition studies were undertaken on intestinal basolateral membranes (BLM) from chicks adapted to a calcium deficient (low Ca) or a phosphorous deficient diet (low P). The fluorescence anisotropy showed that fluidity of intestinal BLM was increased by the mineral deprivations, but the response of the membranes varied with the specific fluorophore used. The "static" component of fluidity, assessed by 1,6-diphenyl-1,3,5-hexatriene (DPH), was increased whereas the "dynamic" component, monitored with DL-12-(9-anthroyloxy)-stearic acid (12-AS), was not modified. Low P diet produced significant changes in lipid composition such as a decrease in the cholesterol content and in the sphyngomyelin (Sph) and phosphatidylserine plus phosphatidylinositol fractions (PS + PI) and increment in the phosphatidylcholine (PC) proportion. The percent of monounsaturated fatty acids was increased by the low P diet due mainly to an increase in the oleic acid fraction. Minor changes such as a decrease in the palmitic acid and increases in the 22:5n3 and 22:6n3 fatty acids were caused by Ca deficiency. The alteration of the biochemical and biophysical membrane properties of the BLM of the mineral deficient groups might play a role in the enhanced intestinal Ca and P absorption.

Effects of ethanol on lateral and rotational mobility of plasma membrane vesicles isolated from cultured mouse myeloma cell line Sp2/0-Ag14

Intramolecular excimerization of Py-3-Py and fluorescence polarization of DPH were used to evaluate effects of ethanol on the rate and range of the lateral mobility and the range of the rotational mobility of bulk bilayer structures of the Sp2/0-PMV. In a concentration-dependent manner, ethanol increased the rate and range of the lateral mobility and the range of the rotational mobility of bulk bilayer structures of Sp2/0-PMV. Selective quenching of DPH by trinitrophenyl groups was utilized to examine the range of transbilayer asymmetric rotational mobility of the Sp2/0-PVM. The anisotropy (r), limiting anisotropy (r(infinity)) and order parameter (S) of DPH in the inner monolayer were 0.022, 0.029 and 0.063, respectively, greater than calculated for the outer monolayer of the Sp2/0-PMV. Selective quenching of DPH by trinitrophenyl groups was also used to examine the transbilayer asymmetric effects of ethanol on the range of the rotational mobility of the Sp2/0-PMV. Ethanol had a greater increasing effect on the range of the rotational mobility of the outer monolayer as compared to the inner monolayer of the Sp2/0-PMV. It has been proven that ethanol exhibits a selective rather than nonselective fluidizing effect within the transbilayer domains of the Sp2/0-PMV.

Surface membrane polarity of proximal tubular cells: alterations as a basis for malfunction

The surface membrane of proximal tubular cells is organized into distinct apical and basolateral membrane domains. The establishment and maintenance of these biochemically, structurally and physiologically distinct domains involves a multi-stage process involving cell-cell, cell-ECM interactions, and polarized targeting mechanisms. Ischemia, via cellular ATP depletion, results in a series of structural, biochemical and functional alterations that lead to loss of proximal tubular cell surface membrane polarity. Of central importance is the rapidly-occurring, duration-dependent disruption and dissociation of the actin cytoskeleton and associated surface membrane structures. This results in numerous cellular alterations including loss of cell-cell contact, cell-extracellular matrix adhesion and surface membrane polarity. Redistribution of surface membrane proteins and lipids into the alternate domain results in the cells inability to function properly. Repair of these disorders involves re-establishment of the actin cytoskeleton and apical and basolateral surface membrane domains. Recent information indicates growth factors may play a role in hastening this repair process.

A unique feature of lipid dynamics in small intestinal brush border membrane

The lipid composition of the brush border membrane (BBM) or apical plasma membrane of enterocytes is characterized by a remarkably high glycosphingolipid content (glycosphingolipid: phospholipid:neutral lipid mole ratio of about 1:1:1). A manifestation of the high glycolipid content of the BBM is the lipid fluidity which is low compared to other mammalian plasma membranes and related to it a steep flexibility gradient: hydrocarbon chain segments close to the lipid-water interface have quasi-crystalline packing while hydrocarbon chain segments close to the centre of the lipid bilayer behave like a fluid. An important function of the BBM is the absorption of dietary lipids. The absorption of cholesterol from bile salt micelles has been shown to be protein-mediated. The integral membrane protein responsible for this activity has features similar to non-specific lipid transfer proteins. Another remarkable property of the BBM is described here: phospholipids are exchanged between the lipid bilayer of the BBM and the lipid bilayers of small unilamellar egg phosphatidylcholine (PC) vesicles. In the course of this probably 1:1 exchange, endogenous BBM phospholipids move out of the BBM and the lipid loss is compensated by the insertion of exogenous PC from the small unilamellar vesicles. This exchange activity is probably due to the same protein(s) responsible for lipid absorption in this membrane or at least related to the absorptive capacity of the BBM. The unique feature of small intestinal BBM is that the on- and off-rate of certain lipids is remarkably high: the underlying structure of this activity is still unknown.

Developmental changes of renal brushborder membrane ionic permeability

Renal brush-border membrane vesicles (BBMV) show an age-dependent increase in height of the Na(+)-gradient driven overshoot for glucose and proline uptake. Conversely, early uptake of 22Na+ is more rapid in BBMV from kidney of 7-day-old vs. adult rats. To understand the mechanisms responsible for these observations, ionic permeability characteristics of BBMV from different aged animals were determined using an electrical potential sensitive fluorescent dye, diS-C3(5). Absolute and relative ionic permeabilities were determined after a 3-h incubation in 100 mM KCl. Intravesicular K+ ([K+]in), a measure of absolute K+ permeability, was calculated from the extravesicular K+ at which valinomycin produced no potential difference (PD). [K+]in was significantly lower in vesicles from 7-day, compared to adult (P < 0.01). While Cl- permeability, relative to that of K+ was similar, PNa+/PK+ decreased significantly with age (P < 0.05, 7 day vs. adult). In the presence of an inwardly directed NaCl gradient, the lower PNa+ relative to PCl- of the adult vesicles would result in a less positive intravesicular charge, which would therefore augment Na(+)-solute co-transport. Fluorescence polarization studies also show that lipids from BBM vesicles of 7-day-old rats are more fluid than those from adult. These differences are likely due to developmental lipid compositional changes, which influence membrane transport and permeability characteristics. These findings would explain, in part, the age-dependent alterations of renal BBMV solute transport.

Disease processes in epithelia: the role of the actin cytoskeleton and altered surface membrane polarity

The establishment and maintenance of cell polarity is essential for normal epithelial function. Disruption of the underlying processes, either as a primary inborn defect or as a secondary result of other pathologic processes, can lead to loss of epithelial polarity and further cellular and organ-level dysfunction. Continued elucidation of the processes involved may prove fruitful both in the understanding of basic cell biology and in the understanding and treatment of a variety of disease states.

Effect of cholesterol oxidase treatment on physical state of renal brush border membranes: evidence for a cholesterol pool interacting weakly with membrane lipids

Treatment with cholesterol oxidases has shown that cholesterol is heterogeneously distributed in brush borders isolated from the apical membrane domain of the renal and intestinal epithelial cells [Bloj, B., & Zilversmit, D. B. (1982) J. Biol. Chem. 257, 7608-7614; El Yandouzi, E. H., & Le Grimellec, C. (1992) Biochemistry 31, 547-551]. To better understand the origin of cholesterol heterogeneity, the effects of cholesterol oxidation by Brevibacterium sp. cholesterol oxidase on the physical state of renal brush border membrane vesicles were determined using steady-state fluorescence polarization and differential phase fluorescence of 1,6-diphenyl-1,3,5-hexatriene (DPH). Selective quenching by trinitrobenzenesulfonate indicated that DPH distributes equally between outer and inner membrane leaflets. Oxidation of 90% of the cholesterol decreased the steady-state anisotropy of DPH, determined at 37 degrees C, by 14%. This modification corresponded to a change in the lipid order, the rotational rate of the probe being unaffected. Oxidation of the cholesterol corresponding to the readily accessible pool (30% of the total cholesterol), on the other hand, had a very limited effect on the membrane physical state. This contrasted with the linear decrease in both anisotropy and fluorescence lifetime of DPH obtained when cholesterol was replaced by cholestenone in liposomes made of phosphatidylcholine/sterol (1/1 molar ratio). These results indicate that, in brush border membranes, the cholesterol readily accessible to cholesterol oxidase interacts only weakly with the other membrane lipids.

Biophysical and biochemical alterations of renal cortical membranes in diabetic rat

The objective of this study was to determine whether streptozotocin-induced diabetes mellitus in the rat causes alterations in the lipid composition and fluidity of renal brush border membranes (BBM) and basolateral membranes (BLM). Compared to membranes of non-diabetic rats, BBM and BLM of diabetic rats contained 31% and 26%, respectively, less arachidonic acid and 36% and 46%, respectively, more linoleic acid esterfied in phospholipids. These changes were accompanied by a decrease in the average number of double bonds per mole of fatty acid, a measure of fatty acid unsaturation. In diabetic rats BLM had a higher total phospholipid/protein ratio (567 +/- 20 vs. 482 +/- 15 nmol/mg protein, P < 0.01), less cholesterol (369 +/- 30 vs. 512 +/- 34 nmol/mg protein, P < 0.01), more phosphatidylcholine (+72%) and less sphingomyelin (-22%) than did BBM. These differences were identical to those observed between BLM and BBM of non-diabetic rats. In control rats BLM was more fluid than BBM as assessed by the steady state fluorescence anisotrophy of diphenylhexatriene and by glycerol permeability. In diabetic rats the fluidity of BLM was not different from that of BBM as assessed by the steady state fluorescence anisotrophy of diphenylhexatriene whereas BLM was slightly more fluid than BBM as assessed by glycerol permeability. By both measures BLM and BBM from diabetic rats were significantly less fluid than BLM and BBM from control rats. Removal of proteins and cholesterol in sequence was accompanied by an increase in membrane fluidity in both groups. However, in no instance did the removal of proteins or cholesterol abolish the difference between the fluidity of diabetic membranes and that of control membranes. From these data we conclude that the reduction in fluidity of renal BLM and BBM in the diabetic rat is due to the change in the composition of fatty acids esterified in membrane phospholipids.

Actin cytoskeleton, tubular sodium and the renal synthesis of dopamine

The present study has examined the effect of colchicine and cytochalasin B, two cytoskeleton disrupter compounds, on the formation of dopamine in slices of rat renal cortex loaded with exogenous L-3,4-dihydroxyphenylalanine (L-DOPA); the deamination of newly formed dopamine into 3,4-dihydroxyphenylacetic acid (DOPAC) was also examined. The accumulation of newly formed dopamine and DOPAC in kidney slices loaded with L-DOPA (10-100 microM) was found to be dependent on the concentration of L-DOPA, being similar in control conditions and in preparations treated with increasing concentrations of colchicine (5, 10 and 50 microM). By contrast, cytochalasin B (5, 10 and 50 microM) was found to produce a concentration-dependent reduction in the formation of dopamine and of its deaminated metabolite DOPAC in kidney slices loaded with L-DOPA (10-100 microM). The inhibitory effect of cytochalasin B on the formation of dopamine was found to be completely abolished in kidney slices pretreated with ouabain (500 microM) or when sodium concentration in the incubation was reduced from 120 to 20 mM. On its own, ouabain (500 microM) was found to reduce the formation of dopamine by 55%; the effect of reducing sodium concentration in the incubation medium to 20 mM was also a significant reduction (53% decrease) in the formation of dopamine. The accumulation of DOPAC did always parallel that of its parent amine. It is concluded that the renal formation of dopamine is dependent on the concentration of sodium in the medium and the integrity of the tubular transport of sodium, namely on the association between actin cytoskeleton and Na+,K(+)-ATPase, appears to be determinant.

Ischemic acute renal failure

Underperfusion of the kidneys often results in the development of ischemic acute renal failure. This review summarizes the recent developments in the understanding of the pathophysiology, diagnosis, and treatment of this serious and costly disorder that affects almost 5% of hospitalized patients.

Membrane fluidity and sodium transport by renal membranes from dogs with spontaneous idiopathic Fanconi syndrome

To comprehend the renal defect underlying the idiopathic Fanconi syndrome in the Basenji dog, we have used isolated renal brush border membrane vesicles to examine two factors that influence membrane nonelectrolyte transport processes, sodium flux and membrane fluidity. We have found that there is no significant difference in the rate of uptake of 100 mmol/L 22Na+ and conclude that the previously observed defects in the sodium gradient-stimulated overshoot of glucose and of proline are not related to an alteration in the flux of sodium at physiological concentrations. Since carrier proteins exist in a lipid milieu, alteration in the physical state of the lipid membrane can determine transport function. Renal brush border preparations from normal and affected animals were studied by measuring fluorescence polarization to assess differences in the physical state of the membranes using the fluorescent probe, DPH, which quantitates inner core membrane fluidity. Membranes from affected dogs consistently showed a higher fluidity as measured by eta, a parameter of DPH fluorescence polarization. Since membrane fluidity is related to lipid composition, the data suggest that there may be an important alteration in the lipids in renal membranes of affected animals.

Effects of lindane on fluidity and lipid composition in rat renal cortex membranes

The influence of lindane upon dynamic properties of plasma membranes from rat renal cortex has been investigated using a fluorescence polarization technique. Preincubation with lindane increased membrane fluidity in a manner that is dose-dependent. This increase was higher in brush border membranes than in basolateral membranes. However, a significant decrease of the membrane fluidity was found in brush border membranes when rats were injected with lindane for 12 days. A possible solution to this difference could involve a resistance to membrane disordering by lindane through a regulatory mechanism that would balance the amount of cholesterol and phospholipid classes in the renal cortex membranes of lindane-injected rats.

Membrane fractions display different lipid and enzyme content in three cell types in 16-cell stage embryos of sea urchins

Three cell types were isolated from dissociated 16-cell sea urchin embryos. Four membrane density fractions from discontinuous gradients have different proportions of lipids, surfacer markers and enzymes for the three cell types. Assays of lipid content, CH/PLIPID and SPH/PC ratios, acyl chain length, level of unsaturation by proton NMR and assays of enzyme activity revealed variation at the same density between the three cell types and among different densities from one cell type. There were also differences between whole embryos and dissociated embryo cells. There was no typical membrane domain at a particular density common to the cell types. Cell surface characteristics and polarity of adult cells rely on which lipid domains and enzymes are present, their association with cytoskeleton and how they are localized. At the 16-cell stage these characteristics are still very dynamic as revealed by cytochemical localization of Na+/K(+)-ATPase which varied with cell type and suggests endocytosis at set times in the division cycle. Polarity has not been permanently set for Na+/K(+)-ATPase yet. Membrane enzyme and lipid distributions unique to the three cell types seen in this study suggest parcelling out or insertion of new membrane domains occurs during early sea urchin cleavage. Perturbation of membrane density distribution and lipid content occurs after treatment of embryos with animalizing and vegetalizing teratogens which alter development.

Cell membrane fluidity in the intact kidney proximal tubule measured by orientation-independent fluorescence anisotropy imaging

Membrane fluidity was measured in the isolated perfused proximal tubule from rabbit kidney. The apical and basolateral plasma membranes of tubule cells were stained separately with the fluidity-sensitive fluorophore trimethylammonium-diphenyl-hexatriene (TMA-DPH) by luminal or bath perfusion. Fluorescence anisotropy (r) of TMA-DPH was mapped with spatial resolution using an epifluorescence microscope (excitation 380 nm, emission greater than 410 nm) equipped with rotatable polarizers and a quantitative imaging system. To measure r without the confounding effects of fluorophore orientation, images were recorded with emission polarizer parallel and perpendicular to a continuum of orientations of the excitation polarizer. The theoretical basis of this approach was developed and its limitations were evaluated by mathematical modeling. The tubule inner surface (brush border) was brightly stained when the lumen was perfused with 1 microM TMA-DPH for 5 min; apical membrane r was 0.281 +/- 0.006 (23 degrees C). Staining of the tubule basolateral membrane by addition of TMA-DPH to the bath gave a significantly lower r of 0.242 +/- 0.010 (P less than 0.005); there was no staining of the brush border membrane. To interpret anisotropy images quantitatively, effects of tubule geometry, TMA-DPH lifetime, fluorescence anisotropy decay, and objective-depolarization were evaluated. Steady-state and time-resolved r and lifetimes in the intact tubule, measured by a nanosecond pulsed microscopy method, were compared with results in isolated apical and basolateral membrane vesicles from rabbit proximal tubule measured by cuvette fluorometry; r was 0.281 (apical membrane) and 0.276 (basolateral membrane) (23 degrees C). These results establish a methodology to quantitate membrane fluidity in the intact proximal tubule, and demonstrate a significantly higher fluidity in the basolateral membrane than in the apical membrane.

Alterations in the establishment and maintenance of epithelial cell polarity as a basis for disease processes

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Characterization of ischemia-induced loss of epithelial polarity

Total renal ischemia for various time intervals (0-50 min) resulted in the rapid and duration-dependent redistribution of polarized membrane lipids and proteins in renal proximal tubule cells. Following only 15 min of ischemia, apical membrane enrichment of NaK-ATPase, normally a basolateral membrane (BLM) enzyme, had increased (1.6 +/- 0.6 vs. 2.9 +/- 1.2, P less than 0.01). In vivo histochemical localization of NaK-ATPase showed reaction product throughout the apical microvillar region. PTH-stimulatable adenylate cyclase, another BLM protein, was also found in ischemic but not control apical membrane fractions. One dimensional SDS-PAGE showed four bands, present in control BLM and ischemic apical membranes, which could not be found in control apical membrane fractions. Immunohistochemical localization of leucine aminopeptidase (LAP) showed the enzyme was limited to the apical domain in control cells. Following ischemic injury (50 min), LAP staining could be seen within the cell and along the BLM. Following 24 hr of reperfusion, the BLM distribution of LAP was further enhanced. With cellular recovery from ischemic injury (5 days), LAP was again only visualized in the apical membrane. Duration-dependent alterations in apical and BLM lipids were also observed. Apical sphingomyelin and phosphatidylserine and the cholesterol-to-phospholipid ratio decreased rapidly while apical phosphatidylcholine and phosphatidylinositol increased. Taken together, these results indicate renal ischemia causes rapid duration-dependent reversible loss of surface membrane polarity in proximal tubule cells.

Toluene-induced alterations in rat synaptosomal membrane composition and function

Toluene is a widely used organic solvent that can produce acute central nervous system (CNS) effects. Since toluene reaches relatively high concentrations in the CNS and is extremely lipophilic, we investigated its effects on rat brain membrane composition and function. Toluene (1 g/kg, lh) did not alter total brain microsomal phospholipid (PL) or cholesterol (CL) content. However, synaptosomal PL was decreased (24%), while synaptosomal CL was unaltered. The PL/CL ratio, an indirect index of membrane fluidity, did not change, suggesting that toluene did not affect membrane fluidity. Fluorescence polarization studies employing 1,6-diphenyl-1,3,5-hexatriene (DPH) showed that toluene did not alter synaptosomal membrane fluidity after administration in vivo (1 g/kg) or in vitro (0.5 to 5.0 mM). Dose-response and time-course studies showed that toluene maximally decreased synaptosomal PL after 1 g/kg, 1 h. The dose-response and time-course studies also showed that the toluene-induced decreases in PL were a result of specific decreases in phosphatidylethanolamine (PE). Since PE was decreased, we assessed whether toluene altered synaptosomal membrane function by investigating phospholipid methylation, a reaction which uses PE as its initial substrate. Toluene decreased the incorporation of methyl groups into lipid when [3H]-methionine was used as the methyl donor, but did not affect methylation when [3H]-adenosylmethionine was the methyl donor. These data suggest that toluene-induced specific decreases in synaptosomal PE and inhibition of phospholipid methylation may alter normal synaptic function and play a critical role in the mechanism(s) of action of toluene's CNS effects.