Separation of cardiac plasmalemma into cell surface and T-tubular components. Distribution of saxitoxin- and nitrendipine-binding sites

To compare surface sarcolemmal with T-tubular distributions of [3H]saxitoxin (STX)- and [3H]nitrendipine (NTD)-binding sites, we centrifuged membrane vesicles from sheep and bovine ventricles on a 10-40% linear sucrose gradient from which fractions were assayed for STX and NTD binding; for markers of surface sarcolemma (ouabain-sensitive Na,K-ATPase activity, [3H]quinuclidinyl benzilate binding); and for markers of junctional sarcoplasmic reticulum known to be preferentially associated with T-tubules (ryanodine-sensitive Ca2+ uptake, calsequestrin, an Mr 300,000 putative phosphorylatable "foot" protein, and electron microscopically visible junctional sarcoplasmic reticulum-plasmalemma complexes). We identified three distinct peaks in the sucrose gradient, each characterized by significant high and low affinity STX- and high affinity NTD-binding: Peak I (approximately 19% sucrose), highly enriched in surface sarcolemma; Peak III (approximately 36% sucrose), enriched in junctional sarcoplasmic reticulum markers and hence in junctional sarcoplasmic reticulum complexes with T-tubule; and Peak II (approximately 27% sucrose), showing greatest specific STX binding and only moderate NTD binding, enriched in T-tubular membrane, unassociated with junctional sarcoplasmic reticulum. For ventricular myocytes, the ratio NTD sites/STX sites was 2.5 for surface sarcolemma, but only approximately 1.0 for T-tubules. Unlike data published for mammalian skeletal muscle, sheep and beef cardiac NTD receptors were not significantly more concentrated in T-tubular than in surface plasmalemma.     

(Ca2+ + Mg2+)-ATPase in enriched sarcolemma from dog heart

An enriched fraction of plasma membranes was prepared from canine ventricle by a process which involved thorough disruption of membranes by vigorous homogenization in dilute suspension, sedimentation of contractile proteins and mitochondria at 3000 X g followed by sedimentation of a microsomal fraction at 200 000 X g. The microsomal suspension was then fractionated on a discontinuous sucrose gradient. Particles migrating in the density range 1.0591--1.1083 were characterized by (Na+ + K+)-ATPase activity and [3H]ouabain binding as being enriched in sarcolemma and were comprised of nonaggregated vesicles of diameter approx. 0.1 micron. These fractions contained (Ca2+ + Mg2+)-ATPase which appreared endogenous to the sarcolemma. The enzyme was solubilized using Triton X-100 and 1 M KCl and partially purified. Optimal Ca2+ concentration for enzyme activity was 5--10 microM. Both Na+ and K+ stimulated enzyme activity. It is suggested that the enzyme may be involved in the outward pumping of Ca2+ from the cardiac cell.     

Na+-H+ exchange in cardiac sarcolemmal vesicles

The transport of Na+ by a purified sarcolemmal vesicular preparation from canine ventricular tissue was studied as a function of both internal and external pH. The uptake of Na+ into sarcolemmal vesicles increased upon raising the extravesicular pH of the reaction medium. Half-maximal uptake of Na+ was observed at a pHo of about 8.1 and maximal uptake occurred at pH 8.6. The uptake of Na+ by sarcolemma was also dependent upon the intravesicular pH. Na+ uptake into sarcolemmal vesicles was greatly attenuated in the absence of a H+ gradient across the membrane. Transport of Na+ was potently inhibited by amiloride, a known blocker of Na+-H+ exchange. LiCl was also an effective inhibitor of Na+ transport. In the presence of optimal H+ gradients, Na+ uptake was linear for the first 5 seconds of the reaction and exhibited a Vmax of 290 nmol Na+/mg per min and a KNa of 3.5 mM. These experiments strongly indicate the presence of a Na+-H+ exchange system in cardiac sarcolemma. This activity appeared to be relatively specific for this membrane fraction. The identification of Na+-H+ exchange activity in a sarcolemmal vesicular fraction from the heart will permit extensive characterization of the regulation and kinetics of this antiporter in future investigations.     

Calcium/sodium exchange in purified secretory vesicles from bovine neurohypophyses

Purified secretory vesicles isolated from bovine neurohypophyses take up Na+ under the same circumstances where an efflux of Ca2+ takes place, suggesting a Na+/Ca2+ exchange. Potassium cannot substitute for Na+ in this process. Also, a Ca2+/Ca2+ exchange can occur. Inhibiting the latter process by Mg2+ allowed to estimate an apparent KM of 0.7 microM free Ca2+ and a maximal uptake of 1.5 nmol X mg protein-1 X min-1 Ca2+ in exchange for Na+. The vesicles did not contain plasma membrane marker (Na+/K+ ATPase) as shown by distribution analyses on the density gradients on which they were purified. Similarly, distribution studies also showed that no other ATPase activity could be detected in the purified vesicle fraction. It is concluded that a Na+/Ca2+ exchange is operating across the secretory vesicle membrane and that it is not directly dependent on ATP hydrolysis.     

Subcellular distribution of leukotriene C4 binding units in cultured bovine aortic endothelial cells

The subcellular distribution of specific binding sites for [3H]leukotriene C4 ([3H]LTC4) was analyzed after sedimentation of organelles from disrupted bovine aortic endothelial cells on sucrose density gradients and was shown to be in membrane fractions I (20% sucrose) and IV (35% sucrose). Saturation binding studies of [3H]LTC4 on endothelial cell monolayers at 4 degrees C demonstrated high-affinity binding sites with a dissociation constant (Kd) of 6.8 +/- 2.2 nM (mean +/- SD) and a density of 0.12 +/- 0.02 pmol/10(6) cells. At 4 degrees C, the specific binding of [3H]LTC4 by each of the subcellular fractions reached equilibrium at 30 min and remained stable for an additional 60 min. After 30 min of incubation with [3H]LTC4, the addition of excess unlabeled LTC4 to each subcellular fraction reversed more than 70% of [3H]LTC4 binding in 10 min. The [3H]LTC4 binding activities of subcellular fractions were enhanced approximately twofold to fourfold in the presence of Ca2+, Mg2+, and Mn2+, whereas Na+, K+, and Li+ were without effect. As measured by saturation experiments, the Kd and density of LTC4 binding sites in fraction I were 4.8 +/- 1.6 nM and 16.5 +/- 1.9 pmol/mg of protein, respectively, and in fraction IV were 4.7 +/- 1.5 nM and 81.4 +/- 19 pmol/mg of protein, respectively. Inhibition of [3H]LTC4 binding in membrane-enriched subcellular fractions I and IV by LTC4 occurred with molar inhibition constant (Ki) values of 4.5 +/- 0.1 nM and 4.7 +/- 1.2 nM, respectively, whereas Ki values for LTD4 were 570 +/- 330 nM and 62.5 +/- 32.8 nM, respectively, and for LTE4 were greater than 1000 nM for each fraction; LTB4 and reduced glutathione were even less active. FPL55712, a putative antagonist of the sulfidopeptide LT components of slow reacting substance of anaphylaxis, had Ki values of 1520 +/- 800 nM and 1180 +/- 720 nM for [3H]LTC4 binding sites on membrane-enriched subcellular fractions I and IV, respectively. Thus as defined by Kd, Ki, and specificity, the LTC4 binding units that are distributed to the plasma membrane and the binding units in the subcellular fraction of greater density were similar to each other. Pretreatment of the isolated subcellular membrane fractions with trypsin abolished [3H]LTC4 binding by fraction I, enriched for the plasma membrane marker 5' nucleotidase, and that by fraction IV, enriched for the mitochondrial membrane marker succinate-cytochrome C reductase.(ABSTRACT TRUNCATED AT 400 WORDS)     

Vasopressin, insulin and peroxide(s) of vanadate (pervanadate) influence Na+ transport mediated by (Na+, K+)ATPase or Na+/H+ exchanger of rat liver plasma membrane vesicles

Uptake of 22Na+ by liver plasma membrane vesicles, reflecting Na+ transport by (Na+, K+)ATPase or Na+/H+ exchange was studied. Membrane vesicles were isolated from rat liver homogenates or from freshly prepared rat hepatocytes incubated in the presence of [Arg8]vasopressin or pervanadate and insulin. The ATP dependence of (Na+, K+)ATPase-mediated transport was determined from initial velocities of vanadate-sensitive uptake of 22Na+, the Na(+)-dependence of Na+/H+ exchange from initial velocities of amiloride-sensitive uptake. By studying vanadate-sensitive Na+ transport, high-affinity binding sites for ATP with an apparent Km(ATP) of 15 +/- 1 microM were observed at low concentrations of Na+ (1 mM) and K+ (1mM). At 90 mM Na+ and 60 mM K+ the apparent Km(ATP) was 103 +/- 25 microM. Vesiculation of membranes and loading of the vesicles prepared from liver homogenates in the presence of vasopressin increased the maximal velocities of vanadate-sensitive transport by 3.8-fold and 1.9-fold in the presence of low and high concentrations of Na+ and K+, respectively. The apparent Km(ATP) was shifted to 62 +/- 7 microM and 76 +/- 10 microM by vasopressin at low and high ion concentrations, respectively, indicating that the hormone reduced the influence of Na+ and K+ on ATP binding. In vesicles isolated from hepatocytes preincubated with 10 nM vasopression the hormone effect was conserved. Initial velocities of Na+ uptake (at high ion concentrations and 1 mM ATP) were increased 1.6-1.7-fold above control, after incubation of the cells with vasopressin or by affinity labelling of the cells with a photoreactive analogue of the hormone. The velocity of amiloride-sensitive Na+ transport was enhanced by incubating hepatocytes in the presence of 10 nM insulin (1.6-fold) or 0.3 mM pervanadate generated by mixing vanadate plus H2O2 (13-fold). The apparent Km(Na+) of Na+/H+ exchange was increased by pervanadate from 5.9 mM to 17.2 mM. Vesiculation and incubation of isolated membranes in the presence of pervanadate had no effect on the velocity of amiloride-sensitive Na+ transport. The results show that hormone receptor-mediated effects on (Na+, K+)ATPase and Na+/H+ exchange are conserved during the isolation of liver plasma membrane vesicles. Stable modifications of the transport systems or their membrane environment rather than ionic or metabolic responses requiring cell integrity appear to be involved in this regulation.     

The sinusoidal domain of the plasma membrane of rat hepatocytes contains an amiloride-sensitive Na+/H+ antiport

ATP-dependent trapping of [14C]methylamine was demonstrated in vesicles selectively derived from the sinusoidal plasma membrane of rat hepatocytes; activity was lacking in vesicles prepared from the canalicular domain of the plasma membrane of rat hepatocytes. The proton movement was inhibited by carbonyl cyanide p-trifluoromethoxyphenylhydrazone, strophanthidin, vanadate, amiloride, and absence of sodium. 22Na efflux from sinusoidal membrane vesicles increased inversely to extravesicular pH. The results indicate that the sinusoidal plasma membrane of rat hepatocytes contains a Na+/H+ antiport.     

Isolation of plasma membrane vesicles, derived from transverse tubules, by selective homogenization of subcellular fractions of frog skeletal muscle in isotonic media.

A new technique for isolating fragmented plasma membranes from skeletal muscle has been developed that is based on gentle mechanical disruption of selected homogenate fractions. (Na+ + K+)-stimulated, Mg2+-dependent ATPase was used as an enzymatic marker for the plasma membrane, Ca2+-stimulated, Mg2+-dependent ATPase as a marker for sarcoplasmic reticulum, and succinate dehydrogenase for mitochondria. Cell segments in an amber low-speed (800 x g) pellet of a frog muscle homogenate were disrupted by repeated gentle shearing with a Polytron homogenizer. Sarcoplasmic reticulum was released into the low-speed supernatant, whereas most of the plasma membrane marker remained in a white, fluffy layer of the sediment, which contained sarcolemma and myofibrils. Additional gentle shearing of the white low-speed sediment extracted plasma membranes in a form that required centrifugation at 100,000 x g for pelleting. This pellet, the fragmented plasma membrane fraction, had a relatively high specific activity of (Na+ + K+)-stimulated ATPase compared with the other fractions, but it had essentially no Ca2+-stimulated ATPase activity and only a small percentage of the succinate dehydrogenase activity of the homogenate. Experimental evidence suggests that the fragmented plasma membrane fraction is derived from delicate transverse tubules rather than from the thicker, basement membrane-coated sarcolemmal sheath of muscle cells. Electron microscopy showed small vesicles lined bu a single thin membrane. Hydroxyproline, a characteristic constituent of collagen and basememt membrane, could not be detected in this fraction.     

Na+/H+ exchange activity in the plasma membrane of Arabidopsis

In plants, Na+/H+ exchangers in the plasma membrane are critical for growth in high levels of salt, removing toxic Na+ from the cytoplasm by transport out of the cell. The molecular identity of a plasma membrane Na+/H+ exchanger in Arabidopsis (SOS1) has recently been determined. In this study, immunological analysis provided evidence that SOS1 localizes to the plasma membrane of leaves and roots. To characterize the transport activity of this protein, purified plasma membrane vesicles were isolated from leaves of Arabidopsis. Na+/H+ exchange activity, monitored as the ability of Na to dissipate an established pH gradient, was absent in plants grown without salt. However, exchange activity was induced when plants were grown in 250 mm NaCl and increased with prolonged salt exposure up to 8 d. H+-coupled exchange was specific for Na, because chloride salts of other monovalent cations did not dissipate the pH gradient. Na+/H+ exchange activity was dependent on Na (substrate) concentration, and kinetic analysis indicated that the affinity (apparent Km) of the transporter for Na+ is 22.8 mm. Data from two experimental approaches supports electroneutral exchange (one Na+ exchanged for one proton): (a) no change in membrane potential was measured during the exchange reaction, and (b) Na+/H+ exchange was unaffected by the presence or absence of a membrane potential. Results from this research provide a framework for future studies into the regulation of the plant plasma membrane Na+/H+ exchanger and its relative contribution to the maintenance of cellular Na+ homeostasis during plant growth in salt.     

Isolation and characterization of Sertoli cell plasma membranes and associated plasminogen activator activity

Plasma membranes were isolated from the cultured Sertoli cells of 20-day-old rat testes by differential centrifugation and sucrose density fractionation. The distribution and purity of subcellular components was determined by marker enzyme analysis of gradient fractions. The plasma membrane fraction showed an enrichment in two plasma membrane marker enzymes, 5'-nucleotidase and ouabain-sensitive Na+/K+-ATPase-specific activities, of 9- and 23-fold, respectively. Forty-two percent and 52% of the total cellular 5'-nucleotidase and ouabain-sensitive Na+/K+-ATPase activities, respectively, were found in the membrane fraction. The protein yield of plasma membrane was approximately 6% of the total cellular protein. Two-dimensional polyacrylamide gel electrophoresis was used to compare [35S] methionine- and [3H] glucosamine-labeled membrane proteins. The incorporation of [35S] methionine and [3H] glucosamine was increased in several proteins when the cultured Sertoli cells were treated with follicle-stimulating hormone, insulin, retinol, and testosterone. Isolated Sertoli cell membranes contained a membrane-associated form of plasminogen activator. Analysis of this plasminogen activator demonstrated that the membrane-associated enzyme existed primarily as a single 38,000-40,000-Mr form.     

[3H]Nimodipine specific binding to cardiac myocytes and subcellular fractions

[³H]Nimodipine binding was studied in isolated myocytes from rat heart and in partially purified sarcolemma, sarcoplasmic reticulum and mitochondrial fractions from dog heart. In isolated myocytes, the density of [³H]nimodipine specific sites (10⁶ per cell) was close to the density of [³H]QNB sites (0.8 × 10⁶ per cell) and higher than that of [³H]DHA sites (0.2 × 10⁶ per cell). During subcellular fractionation, [³H]nimodipine binding did not copurify with plasma membrane markers. The highest densities were found in fractions enriched in sarcolemma or in sarcoplasmic reticulum. No specific binding was found in mitochondria. These results indicate that the localization of [³H]nimodipine sites is not restricted to areas of the plasma membrane rich in β-adrenoceptors, muscarinic receptors and sodium pump sites.     

Subfractionation of rat liver plasma membrane. Uneven distribution of plasma membrane-bound enzymes on the liver cell surface.

Plasma membranes were isolated from rat liver mainly under isotonic conditions. As marker enzymes for the plasma membrane, 5'-nucleotidase and (Na+ + K+)-ATPase were used. The yield of plasma membrane was 0.6-0.9 mg protein per g wet weight of liver. The recovery of 5'-nucleotidase and (Na+ +K+)-ATPase activity was 18 and 48% of the total activity of the whole-liver homogenate, respectively. Judged from the activity of glucose-6-phosphatase and succinate dehydrogenase in the plasma membrane, and from the electron microscopic observation of it, the contamination by microsomes and mitochondria was very low. A further homogenization of the plasma membrane yielded two fractions, the light and heavy fractions, in a discontinuous sucrose gradient centrifugation. The light fraction showed higher specific activities of 5'-nucleotidase, alkaline phosphatase, (Na+ +K+)-ATPase and Mg2+-ATPase, whereas the heavy one showed a higher specific activity of adenylate cyclase. Ligation of the bile duct for 48 h decreased the specific activities of (Na2+ +K+)-ATPase and Mg2+-ATPase in the light fraction, whereas it had no significant influence on the activities of these enzymes in the heavy fraction. The specific activity of alkaline phosphate was elevated in both fractions by the obstruction of the bile flow. Electron microscopy on sections of the plasma membrane subfractions showed that the light fraction consisted of vesicles of various sizes and that the heavy fractions contained membrane sheets and paired membrane strips connected by junctional complexes, as well as vesicles. The origin of these two fractions is discussed and it is suggested that the light fraction was derived from the bile front of the liver cell surface and the heavy one contained the blood front and the lateral surface of it.     

Synthesis of plasmalemmal glycoproteins in intestinal epithelial cells. Separation of Golgi membranes from villus and crypt cell surface membranes; glycosyltransferase activity of surface membrane

The relationship between Golgi and cell surface membranes of intestinal cells was studied. These membranes were isolated from intestinal crypt cells and villus cells. The villus cell membranes consisted of microvillus membrane, a Golgi-rich fraction, and two membrane fractions interpreted as representing lateral-basal membranes. The villus cell microvillus membrane was purified by previously published techniques while the other membranes were obtained from isolated cells by differential centrifugation and density gradient velocity sedimentation. The two membrane fractions obtained from villus cells and considered to be lateral-basal membranes were enriched for Na+,K+-ATPase activity, but one also showed enrichment in glycosyltransferase activity. The Golgi membrane fraction was enriched for glycosyltransferase activity and had low to absent Na+,K+-ATPase activity. Adenylate cyclase activity was present in all membrane fractions except the microvillus membrane but co-purified with Golgi rather than lateral-basal membranes. Electron microscopy showed that the Golgi fraction consisted of variably sized vesicles and cisternalike structures. The two lateral-basal membrane fractions showed only vesicles of smaller, more uniform size. After 125I labeling of isolated intact cells, radioactivity was found associated with the lateral-basal and microvillus membrane fractions and not with the Golgi fraction. Antibody prepared against lateral-basal membrane fractions reacted with the surface membrane of isolated villus cells. The membrane fractions from isolated crypt cells demonstrated that all had high glycosyltransferase activity. The data show that glycosyltransferase activity, in addition to its Golgi location, may be a significant property of the lateral-basal portion of the intestinal villus cell plasma membrane. Data obtained with crypt cells support earlier data and show that the crypt cell surface membrane possesses glycosyltransferase activity.     

Regulation of calcium content in bovine spermatozoa

Plasma membrane vesicles isolated from bovine epididymal and ejaculated spermatozoa have widely different capabilities for transporting Ca2+. Spermatozoa were ruptured by nitrogen cavitation, and the plasma membrane fraction was harvested after low speed and sucrose gradient centrifugation; purity was assessed by marker enzyme analyses, electron microscopy, and sedimentation properties. Plasma membrane vesicles isolated from epididymal sperm accumulate Ca2+ passively at a faster rate and to a greater extent than vesicles prepared from ejaculated sperm. Ca2+ transport across bovine sperm plasma membranes is an ATP-independent, Na+-dependent process that obligatorily exchanges intravesicular Na+ for external Ca2+. The rate of Na+/Ca2+ exchange is significantly lower in ejaculated sperm vesicles than in those of epididymal sperm. Bovine plasma membranes contain little or no Ca2+-dependent ATPase activity. It is suggested that, at the time of ejaculation, calcium flux into bovine sperm is prevented by the interaction of the plasma membrane with putative factors in seminal fluid that specifically interfere with Na+/Ca2+ exchange. We have isolated a protein from seminal plasma that prevents calcium accumulation by bovine epididymal sperm (Rufo, G. A., Jr., Singh, J. P., Babcock, D. F., and Lardy, H. A. (1982) J. Biol. Chem. 257, 4627-4632). A protein with properties resembling those of the seminal calcium transport inhibitor is found on the membrane vesicles from ejaculated sperm but not on membranes from epididymal sperm. We conclude that this protein binds strongly to the plasma membrane of bovine sperm and is responsible for preventing calcium uptake by ejaculated sperm.     

Orientation and integrity of plasma membrane vesicles obtained from carrot protoplasts

Two fractions enriched in plasma membrane derived from suspension-cultured carrot (Daucus carota L.) cells were examined to determine if they differed from each other either in physical nature or in orientation. Parameters studied included the protein composition of purified membranes derived from trypsinized and nontrypsinized protoplasts as well as from trypsinized purified plasma membranes, the effect of inhibitors and membrane perturbants on ATPase activity, the binding of [acetyl-¹⁴C]concanavalin A to purified membrane fractions, and the competitive removal of [acetyl-¹⁴C]concanavalin A from purified membranes derived from [acetyl-¹⁴C]concanavalin A-labeled protoplasts. One fraction (at density of 1.102 grams per cubic centimeter on Renografin gradients) appears to be a mixed population of `tightly' sealed vesicles with the majority being rightside-out vesicles of plasma membrane, and the other fraction (density 1.128 grams per cubic centimeter) apparently is a population of predominantly `leaky' vesicles and/or nonvesicular fragments of plasma membrane, a large portion of which appear to be `leaky' inside-out vesicles. In addition, it is shown that plasma membrane-enriched fractions can be distinguished from cellular endomembranes on the basis of protein and glycoprotein composition.     

Dystrophin-glycoprotein complex is highly enriched in isolated skeletal muscle sarcolemma

mAbs specific for protein components of the surface membrane of rabbit skeletal muscle have been used as markers in the isolation and characterization of skeletal muscle sarcolemma membranes. Highly purified sarcolemma membranes from rabbit skeletal muscle were isolated from a crude surface membrane preparation by wheat germ agglutination. Immunoblot analysis of subcellular fractions from skeletal muscle revealed that dystrophin and its associated glycoproteins of 156 and 50 kD are greatly enriched in purified sarcolemma vesicles. The purified sarcolemma was also enriched in novel sarcolemma markers (SL45, SL/TS230) and Na+/K(+)-ATPase, whereas t-tubule markers (alpha 1 and alpha 2 subunits of dihydropyridine receptor, TS28) and sarcoplasmic reticulum markers (Ca2(+)-ATPase, ryanodine receptor) were greatly diminished in this preparation. Analysis of isolated sarcolemma by SDS-PAGE and densitometric scanning demonstrated that dystrophin made up 2% of the total protein in the rabbit sarcolemma preparation. Therefore, our results demonstrate that although dystrophin is a minor muscle protein it is a major constituent of the sarcolemma membrane in skeletal muscle. Thus the absence of dystrophin in Duchenne muscular dystrophy may result in a major disruption of the cytoskeletal network underlying the sarcolemma in dystrophic muscle.     

Arabidopsis fatty acid desaturase FAD2 is required for salt tolerance during seed germination and early seedling growth

Fatty acid desaturases play important role in plant responses to abiotic stresses. However, their exact function in plant resistance to salt stress is unknown. In this work, we provide the evidence that FAD2, an endoplasmic reticulum localized ω-6 desaturase, is required for salt tolerance in Arabidopsis. Using vacuolar and plasma membrane vesicles prepared from the leaves of wild-type (Col-0) and the loss-of-function Arabidopsis mutant, fad2, which lacks the functional FAD2, we examined the fatty acid composition and Na+-dependent H+ movements of the isolated vesicles. We observed that, when compared to Col-0, the level of vacuolar and plasma membrane polyunsaturation was lower, and the Na+/H+ exchange activity was reduced in vacuolar and plasma membrane vesicles isolated from fad2 mutant. Consistent with the reduced Na+/H+ exchange activity, fad2 accumulated more Na+ in the cytoplasm of root cells, and was more sensitive to salt stress during seed germination and early seedling growth, as indicated by CoroNa-Green staining, net Na+ efflux and salt tolerance analyses. Our results suggest that FAD2 mediated high-level vacuolar and plasma membrane fatty acid desaturation is essential for the proper function of membrane attached Na+/H+ exchangers, and thereby to maintain a low cytosolic Na+ concentration for salt tolerance during seed germination and early seedling growth in Arabidopsis.     

Solubilization and purification of the prostaglandin E2 receptor from cardiac sarcolemma.

A prostaglandin E2 (PGE2) receptor was solubilized and isolated from cardiac sarcolemma membranes. Its binding characteristics are almost identical to those of the membrane bound receptor. [3H]PGE2 binding to solubilized and membrane bound receptor was sensitive to elevated temperature and no binding was observed in the absence of NaCl. No significant effects of DTT, ATP, Mg2+, Ca2+ or of changes in buffer pH were observed on [3H]PGE2 binding to either solubilized or membrane-bound receptor. Unlabelled PGE1 displaced over 90% of [3H]PGE2 from the CHAPS-solubilized receptor. PGD2, PGI2, PGF2 alpha and 6-keto-PGF1 alpha were not effective in displacing [3H]PGE2 from the receptor. Scatchard analysis of [3H]PGE2 binding to CHAPS-solubilized receptor revealed the presence of two types of PGE2 binding sites with Kd of 0.33 +/- 0.05 nM and 3.00 +/- 0.27 nM and Bmax of 0.5 +/- 0.04 and 2.0 +/- 0.1 pmol/mg of protein. The functional PGE2 receptor was isolated from CHAPS-solubilized SL membrane using two independent methods: first by a WGA-Sepharose chromatography and second by sucrose gradient density centrifugation. Receptor isolated by these two methods bound [3H]PGE2. Unlabelled PGE1 and PGE2 displaced [3H]PGE2 from the purified receptor. Scatchard analysis of [3H]PGE2 binding to purified receptor revealed the presence of the two binding sites as observed for the membrane bound and CHAPS-solubilized receptor. SDS-polyacrylamide gel electrophoresis of the purified receptor fractions revealed the presence of a protein band of M(r) of approx. 100,000. This 100-kDa was photolabelled with [3H]azido-PGE2, a photoactive derivative of PGE2. We propose that this 100-kDa protein is a cardiac PGE2 receptor.     

A two sodium ion/D-glucose symport mechanism: membrane potential effects on phlorizin binding

Apical membrane vesicles isolated from a continuous renal cell line, LLC-PK1, catalyze electrogenic Na+-stimulated hexose transport and Na+-dependent binding of 3H-labeled 1-[2-(beta-D-glucopyranosyloxy)-4, 6-dihydroxyphenyl]-3-(4-hydroxyphenyl)-1-propanone [( 3H]phlorizin), a competitive ligand of this transport system. Phlorizin was not itself transported across the membrane and thus can serve as a probe of the binding step. The stoichiometry of Na+-dependent phlorizin binding in vesicles was 1:1, whereas Na+/hexose cotransport in vesicles exhibited a 2:1 stoichiometry. Na+ increased the affinity of phlorizin binding without affecting the total number of binding sites. An increased number of Na+-dependent phlorizin binding sites was observed under conditions of interior-negative membrane potential. These results are consistent with a model of the Na+/glucose cotransport cycle in which the unloaded transporter is negatively charged and its orientation influenced by membrane potential. Glucose and one sodium ion interact with the transporter, resulting in an uncharged complex. Binding of a second sodium ion triggers translocation of glucose and both sodium ions via formation of a loaded carrier complex bearing a single positive charge.     

Apical plasma membrane vesicles formed from organ donor colon demonstrate Na+ and H+ conductances and Na+/H+ exchange

Apical plasma membrane vesicles were prepared from human organ donor colon mucosal scrapings. These vesicles were enriched 10-fold in cysteine-sensitive alkaline phosphatase activity compared to starting homogenates, and showed minimal contamination of microsomal, mitochondrial or basolateral membranes. Transport studies using [22Na] uptake into proximal colonic vesicles demonstrated Na+ and H+ conductances, Na+/H+ exchange and amiloride inhibition of Na+ uptake. The isolation of these apical vesicles will permit detailed study of human colonic transport processes.