Titles of related papers in other sections

The effect of hypoglycemic drugs chlorpropamide and phenformin has been tested on isolated liver plasma membranes as to their microviscosity parameters. Results reported suggest that both drugs are able to increase in vitro plasma membrane microviscosity in a dose-dependent way.     

Hypoglycemic drugs increase liver plasma membrane microviscosity in vitro.

The effect of hypoglycemic drugs chlorpropamide and phenformin has been tested on isolated liver plasma membranes as to their microviscosity parameters. Results reported suggest that both drugs are able to increase in vitro plasma membrane microviscosity in a dose-dependent way.     

Characterization of the plasma membrane Mg2+-ATPase from the yeast, Saccharomyces cerevisiae.

The plasma membrane of Saccharomyces cerevisiae has a Mg2+-dependent ATPase which is distinct from the mitochondrial Mg2+-ATPase and at the pH optimum of 5.5 has a Km for ATP of 1.7 mM and a Vmax of 0.42 mumol of ATP hydrolyzed/mg/min. At least three protein components of the crude membrane (Mr = 210,000, 160,000 and 115,000) are labeled with [gamma"32P]ATP at pH 5.5. These phosphoproteins form rapidly in the presence of Mg2+, rapidly turn over the bound phosphate when unlabeled ATP is added, and dephosphorylate after incubation in the presence of hydroxylamine. Vanadate, an inhibitor of the Mg2+-ATPase activity, blocks the phosphorylation of the 210,000- and 115,000-dalton proteins. At pH 7.0, only the 210,000- and 160,000-dalton proteins are phosphorylated. While these three phosphorylated intermediates have not been unambiguously identified as components of the Mg2+-ATPase, the finding of such phosphorylated components in association with that activity implies that this enzyme differs in mechanism from the mitochondrial proton pump and that it is similar in mechanism to the metal ion pumps ((Na+-K+)-ATPase and Ca2+-ATPase) of the mammalian plasma membrane.     

Regulation of fish gill Na(+)-K(+)-ATPase by selective sulfatide-enriched raft partitioning during seawater adaptation

Na(+)-K(+)-ATPase is arguably the most important enzyme in the animal cell plasma membrane, but the role of the membrane in its regulation is poorly understood. We investigated the relationship between Na(+)-K(+)-ATPase and membrane microdomains or "lipid rafts" enriched in sulfatide (sulfogalactosylceramide/SGC), a glycosphingolipid implicated as a cofactor for this enzyme, in the basolateral membrane of rainbow trout gill epithelium. Our studies demonstrated that when trout adapt to seawater (33 ppt), Na(+)-K(+)-ATPase relocates to these structures. Arylsulfatase-induced desulfation of basolateral membrane SGC prevented this relocation and significantly reduced Na(+)-K(+)-ATPase activity in seawater but not freshwater trout. We contend that Na(+)-K(+)-ATPase partitions into SGC-enriched rafts to help facilitate the up-regulation of its activity during seawater adaptation. We also suggest that differential partitioning of Na(+)-K(+)-ATPase between these novel SGC-enriched regulatory platforms results in two distinct, physiological Na(+) transport modes. In addition, we extend the working definition of cholesterol-dependent raft integrity to structural dependence on the sulfate moiety of SGC in this membrane.     

Orientation of synaptic plasma membrane vesicles containing calcium pump and sodium-calcium exchange activities

Sidedness of synaptic plasma membrane vesicles isolated from brain synaptosomes has been assessed by two distinct experimental approaches: first, analysis of (Na+ + K+)-ATPase, Mg2+-ATPase, and (Ca2+ + Mg2+)-ATPase activities before and after permeabilization of vesicles; second, analysis of Ca2+ fluxes via the Na+/Ca2+ exchanger, before and after modification of an imposed Na+ gradient by penetrating or nonpenetrating Na+ channel-modifying drugs. 0.05% saponin, which completely permeabilizes the vesicles, increases digitoxigenin-sensitive (Na+ + K+)-ATPase, basal Mg2+-ATPase, and (Ca2+ + Mg2+)-ATPase activities by 51.0, 47.4, and 83.6%, respectively. Saponin increases only the Vmax of the latter activity, the Km for Ca2+ (0.13 microM; the same as that for Ca2+-pumping) being unaltered by saponin. An increment of 20.5% in the Vmax of (Ca2+ + Mg2+)-ATPase activity with 10 microM A23187, reveals that the enzyme activity in nonpermeabilized vesicles is limited by the formation of a Ca2+ gradient. Thus, the saponin-induced increment in (Ca2+ + Mg2+)-ATPase due only to exposure of occluded sites (as opposed to Ca2+ gradient dissipation) is actually 52%, which is similar to values for both other ATPases, and suggests that 32-35% of plasma membranes exist in an inverted orientation. Vesicle orientation was independently assessed by the differential actions of tetrodotoxin (a membrane impermeant blocker) and veratridine (a membrane permeant agonist) on Na+-channel opening measured indirectly by dissipation of an imposed Na+ gradient utilized to drive a large 45Ca2+ accumulation via the Na+/Ca2+ exchanger. Tetrodotoxin reverses 35-44% of veratridine-mediated Na+ gradient-dissipation, the relative membrane-permeability of the two channel modifiers, suggesting that 56-65% of sealed vesicles are inverted. The concurrence of these two independent measurements of vesicle orientation reinforces their validity.     

Regulation of rat brain (Na+ +K+)-ATPase activity by cyclic AMP

The interaction between the (Na+ +K+)-ATPase and the adenylate cyclase enzyme systems was examined. Cyclic AMP, but not 5'-AMP, cyclic GMP or 5'-GMP, could inhibit the (Na+ +K+)-ATPase enzyme present in crude rat brain plasma membranes. On the other hand, the cyclic AMP inhibition could not be observed with purified preparations of (Na+ +K+)-ATPase enzyme. Rat brain synaptosomal membranes were prepared and treated with either NaCl or cyclic AMP plus NaCl as described by Corbin, J., Sugden, P., Lincoln, T. and Keely, S. ((1977) J. Biol. Chem. 252, 3854-3861). This resulted in the dissociation and removal of the catalytic subunit of a membrane-bound cyclic AMP-dependent protein kinase. The decrease in cyclic AMP-dependent protein kinase activity was accompanied by an increase in (Na+ +K+)-ATPase activity. Exposure of synaptosomal membranes containing the cyclic AMP-dependent protein kinase holoenzyme to a specific cyclic AMP-dependent protein kinase inhibitor resulted in an increase in (Na+ +K+)-ATPase enzyme activity. Synaptosomal membranes lacking the catalytic subunit of the cyclic-AMP-dependent protein kinase did not show this effect. Reconstitution of the solubilized membrane-bound cyclic AMP-dependent protein kinase, in the presence of a neuronal membrane substrate protein for the activated protein kinase, with a purified preparation of (Na+ +K+)-ATPase, resulted in a decrease in overall (Na+ +K+)-ATPase activity in the presence of cyclic AMP. Reconstitution of the protein kinase alone or the substrate protein alone, with the (Na+ +K+)-ATPase has no effect on (Na+ +K+)-ATPase activity in the absence or presence of cyclic AMP. Preliminary experiments indicate that, when the activated protein kinase and the substrate protein were reconstituted with the (Na+ +K+)-ATPase enzyme, there appeared to be a decrease in the Na+-dependent phosphorylation of the Na+-ATPase enzyme, while the K+-dependent dephosphorylation of the (Na+ +K+)-ATPase was unaffected.     

β-adrenergic receptors and enzymes in rat myocardial membranes: implications of fractionation procedures andβ-adrenoceptor antagonists

1. We performed an enzymatic characterization of two different fractionation procedures of ventricles from rat hearts. The enzymatic assays covered succinic dehydrogenase as a marker for inner mitochondrial membranes, monoamine oxidase as a marker for outer mitochondrial membranes, NADPH-cytochrome c reductase and RNA as endoplasmatic reticular markers, acid phosphatase as a lysosomal marker, and lactic dehydrogenase as a marker for the "soluble" compartment; DNA was estimated for nuclear contamination. 2. The plasma membrane markers 5'-nucleotidase, Ca2+-ATPase, Mg2+-ATPase, Na+-K+-ATPase, and adenylate cyclase were determined. 3. The roughly prepared membrane fractions showed increased yields of the membrane markers; the number of beta receptors, determined with (-)-[3H] dihydroalprenolol and DL-propranolol, amounted to 68 +/- 6 fmol/mg protein (KD = 3390 +/- 450 pmol, Hill coefficient = 1.5). 4. The membrane fraction prepared with a linear sucrose gradient showed an increased inner mitochondrial membrane marker; presumably the outer mitochondrial membrane was stripped off. The beta-receptor number was 39 +/- 3 fmol/mg protein (KD = 6250 +/- 300 pmol; Hill coefficient = 1.2).     

Gill Na+-K+-ATPase activity correlates with basolateral membrane lipid composition in seawater- but not freshwater-acclimated Arctic char (Salvelinus alpinus)

The successful migration of euryhaline teleost fish from freshwater to seawater requires the upregulation of gill Na+-K+-ATPase, an ion transport enzyme located in the basolateral membrane (BLM) of gill chloride cells. Following 39 days of seawater exposure, Arctic char had similar plasma sodium and chloride levels as individuals maintained in freshwater, indicating they had successfully acclimated to seawater. This acclimation was associated with an eightfold increase in gill Na+-K+-ATPase activity but only a threefold increase in gill Na+-K+-ATPase protein number, suggesting that other mechanisms may also modulate gill Na+-K+-ATPase activity. We therefore investigated the influence of membrane composition on Na+-K+-ATPase activity by examining the phospholipid, fatty acid, and cholesterol composition of the gill BLM from freshwater- and seawater-acclimated Arctic char. Mean gill BLM cholesterol content was significantly lower ( approximately 22%) in seawater-acclimated char. Gill Na+-K+-ATPase activity in individual seawater Arctic char was negatively correlated with BLM cholesterol content and positively correlated with %phosphatidylethanolamine and overall %18:2n6 (linoleic acid) content of the BLM, suggesting gill Na+-K+-ATPase activity of seawater-acclimated char may be modulated by the lipid composition of the BLM and may be especially sensitive to those parameters known to influence membrane fluidity. Na+-K+-ATPase activity of individual freshwater Arctic char was not correlated to any membrane lipid parameter measured, suggesting that different lipid-protein interactions may exist for char living in each environment.     

Studies on intestinal adenosine triphosphatases. II. Stabilitiies in different rat subcellular fractions.

Subcellular fraction (brush border, mitochondria, microsomes and plasma membranes) are isolated from the rat intestinal epithelial cells. A comparison was made between the effect of cold storage, freeze-thawing, heating and of some chemicals (DMSO, DTT, glycerol, sucrose) on the stability of Mg2+ and (Na+-K+) dependent ATPases in these fractions in order to determine possible difference linked to the localization in the enterocyte. Enzymatic activities were found more stable at -20 degrees C than at +4 degrees C. Microsomal (Na+-K+)-ATPase increased in activity until the 8th day, then declined. Brush border (Na+-K+)-ATPase was the least resistant of all fractions. For Mg2+-ATPase, that from mitochondria was that had lost much more activity (84%) in 15 days at +4 degrees C. With freeze-thawing there was a comparable decrease in all activities (20-35%). by heating between 35 and 60 degrees C, Mg2+-ATPase was shown to be more heat resistant than (Na+-K+)-ATPase. The addition of some stabilizing chemicals (DMSO, glycerol, sucrose) improved the heat stability of the two enzymes: better results were obtained with glycerol for Mg2+-ATPase and sucrose for (Na+-K+)-ATPase. These differences might be due to the compositon in membraine lipids or to the nature of the enzymes studied.     

Effects of rubratoxin B on the kinetics of cationic and substrate activation of (Na+-K+)-ATPase and p-nitrophenyl phosphatase.

Rubratoxin B, a lactone-containing bisanhydride metabolite of certain toxigenic molds, inhibited (Na+-K+)-stimulated ATPase activity of mouse brain microsomes in a dose-dependent manner with an estimated IC50 of 6 x 10(-6) M. Hydrolysis of ATP was linear with time and enzyme concentration, with or without rubratoxin in reaction mixtures. Altered pH and activity curves for (Na+-K+)-ATPase demonstrated comparable inhibition by rubratoxin in buffered acidic, neutral, and alkaline pH ranges. Kinetic studies of cationic-substrate activation of (Na+-K+)-ATPase indicated classical competitive inhibition for Na+ and K+. Results also showed competitive inhibition for K+ activated p-nitrophenyl phosphatase as demonstrated by altered binding site parameters without change in the catalytic velocity of dephosphorylation of the enzyme . phosphoryl complex. Noncompetitive inhibition with regards to activation by ATP and p-nitrophenyl phosphate was indicated by altered Vmax values with no change in Km values. Inhibition was partially restored by repeated washings. Preincubation with sulfhydryl agents protected the enzyme from inhibition. Cumulative inhibition studies with rubratoxin and ouabain indicated possible interaction between the two inhibitors of (Na+-K+)-ATPase. Rubratoxin appeared to exert its effects on (Na+-K+)-ATPase by interacting at Na+ and K+ sites.     

Characterisation of a high affinity Ca2+-stimulated, Mg2+-dependent ATPase in the rat parotid plasma membrane

Two Ca2+-stimulated ATPase activities have been identified in the plasma membrane of rat parotid: (a) a (Ca2+ + Mg2+)-ATPase with high affinity for free Ca2+ (apparent Km = 208 nM, Vmax = 188 nmol/min per mg) and requiring micromolar concentration of Mg2+ and (b) a (Ca2+ or Mg2+)-ATPase with relatively low affinity for free Ca2+ (K0.5 = 23 microM) or free Mg2+ (K0.5 = 26 microM). The low-affinity (Ca2+ or Mg2+)-ATPase can be maximally stimulated by Ca2+ alone or Mg2+ alone. The high-affinity (Ca2+ + Mg2+)-ATPase exhibits sigmoidal kinetics with respect to ATP concentration with K0.5 = 0.4 mM and a Hill coefficient of 1.91. It displays low substrate specificity with respect to nucleotide triphosphates. Although trifluoperazine inhibits the activity of the high affinity (Ca2+ + Mg2+)-ATPase only slightly, it inhibits the activity of the low-affinity (Ca2+ or Mg2+)-ATPase quite potently with 22 microM trifluoperazine inhibiting the enzymic activity by 50%. Vanadate, inositol 1,4,5-trisphosphate, phosphatidylinositol 4,5-bisphosphate, Na+,K+ and ouabain had no effect on the activities of both ATPases. Calmodulin added to the plasma membranes does not stimulate the activities of both ATPases. The properties of the high-affinity (Ca2+ + Mg2+)-ATPase are distinctly different from those of the previously reported Ca2+-pump activity of the rat parotid plasma membrane.     

Subcellular distribution of ATP-dependent calcium transport in rat duodenal epithelium

Subcellular fractionation studies were performed to delineate plasma membrane and intracellular membrane populations which might be involved in intracellular Ca2+-homeostasis of rat small intestinal epithelial cells. After a low-speed supernatant fraction had been suspended in 5% sorbitol and subjected to equilibrium centrifugation in a zonal rotor, the Golgi and endoplasmic reticulum markers, galactosyltransferase and NADPH-cytochrome -c reductase, were concentrated in a density region designated Window II. The basal-lateral membrane marker (Na+-K+)-ATPase was concentrated in a higher-density region designated Window III. ATP-dependent Ca2+ transport was equally distributed between the two windows. Several membrane populations could be resolved from each window with good recovery of Ca2+-transport activity by a second density gradient centrifugation step. Second density gradient fractions were subjected to counter-current partitioning in an aqueous polymer two-phase system. Basal-lateral membranes, characterized by an 11-fold enrichment of (Na+-K+)-ATPase, contained ATP-dependent Ca2+-transport activity with Vmax = 3.7 nmol/mg per min and Km = 0.5 microM. A major Golgi-derived population exhibited Ca2+-transport activity with Vmax and Km values similar to those of the basal-lateral membranes. One membrane population, presumed to have been derived from the endoplasmic reticulum, contained Ca2+-transport activity with Vmax = 4 nmol/mg per min and Km = 0.5 microM. In addition to demonstrating that ATP-dependent Ca2+-transport activity has a complex distribution within enterocytes, this study raises the possibility that the basolateral plasma membranes might account for a relatively minor portion of the cell's Ca2+-pumping ability.     

Parathyroid hormone-mediated regulation of Na+-K+-ATPase requires ERK-dependent translocation of protein kinase Calpha

Parathyroid hormone (PTH) inhibits Na+-K+-ATPase activity by serine phosphorylation of the alpha1 subunit through protein kinase C (PKC)- and extracellular signal-regulated kinase (ERK)-dependent pathways. Based on previous studies we postulated that PTH regulates sodium pump activity through isoform-specific PKC-dependent activation of ERK. In the present work utilizing opossum kidney cells, a model of renal proximal tubule, PTH stimulated membrane translocation of PKCalpha by 102 +/- 16% and PKCbetaI by 41 +/- 7% but had no effect on PKCbetaII and PKCzeta. Both PKCalpha and PKCbetaI phosphorylated the Na+-K+-ATPase alpha1 subunit in vitro. PTH increased the activity of PKCalpha but not PKCbetaI. Coimmunoprecipitation assays demonstrated that treatment with PTH enhanced the association between Na+-K+-ATPase alpha1 subunit and PKCalpha, whereas the association between Na+-K+-ATPase alpha1 subunit and PKCbetaI remained unchanged. A PKCalpha inhibitory peptide blocked PTH-stimulated serine phosphorylation of the Na+-K+-ATPase alpha1 subunit and inhibition of Na+-K+-ATPase activity. Pharmacologic inhibition of MEK-1 blocked PTH-stimulated translocation of PKCalpha, whereas transfection of constitutively active MEK-1 cDNA induced translocation of PKCalpha and increased phosphorylation of the Na+-K+-ATPase alpha1 subunit. In contrast, PTH-stimulated ERK activation was not inhibited by pretreatment with the PKCalpha inhibitory peptide. Inhibition of PKCalpha expression by siRNA did not inhibit PTH-mediated ERK activation but significantly reduced PTH-mediated phosphorylation of the Na+-K+-ATPase alpha1 subunit. Pharmacologic inhibition of phosphoinositide 3-kinase blocked PTH-stimulated ERK activation, translocation of PKCalpha, and phosphorylation of the Na+-K+-ATPase alpha1 subunit. We conclude that PTH stimulates Na+-K+-ATPase phosphorylation and decreases the activity of Na+-K+-ATPase by ERK-dependent activation of PKCalpha.     

Action of luliberine on the Na,K-ATPase and Ca-ATPase activity of the sarcolemma of the rat heart]

The effect of luliberine, one of the hypothalamic releasing-factors, upon the ATPase activity in the rat heart sarcolemma was investigated. A decrease in the (Na+-K+)-ATPase activity and stimulation of Ca-ATPase activity under the influence of luliberine were demonstrated. Inhibition of (Na+-K+)-ATPase by cAMP and noradrenaline was also revealed. A possibility of the direct and cAMP-mediated action of luliberine on the Na+-K+)-ATPase activity is suggested.     

Control of brain slice respiration by (Na+ + K+)-activated adenosine triphosphate and the effects of enzyme inhibitors.

The involvement of membrane (Na+ + K+)-ATPase (Mg2+-dependent, (Na+ + K+)-activated ATP phosphohydrolase, E.C. 3.6.1.3) in the oxygen consumption of rat brain cortical slices was studied in order to determine whether (Na+ + K+)-ATPase activity in intact cells can be estimated from oxygen consumption. The stimulation of brain slice respiration with K+ required the simultaneous presence of Na+. Ouabain, a specific inhibitor of (Na+ + K+)-ATPase, significantly inhibited the (Na+ + K+)-stimulation of respiration. These observations suggest that the (Na+ + K+)-stimulation of brain slice respiration is related to ADP production as a result of (Na+ + K+)-ATPase activity. However, ouabain also inhibited non-K+ -stimulated respiration. Additionally, ouabain markedly reduced the stimulation of respiration by 2,4-dinitrophenol in a high (Na+ + K+)-medium. Thus, ouabain depresses brain slice respiration by reducing the availability of ADP through (Na+ + K+)-ATPase inhibition and acts additionally by increasing the intracellular Na+ concentration. These studies indicate that the use of ouabain results in an over-estimation of the respiration related to (Na+ + K+)-ATPase activity. This fraction of the respiration can be estimated more precisely from the difference between slice respiration in high Na+ and K+ media and that in choline, K+ media. Studies were performed with two (Na+ + K+)-ATPase inhibitors to determine whether administration of these agents to intact rats would produce changes in brain respiration and (Na+ + K+)-ATPase activity. The intraperitoneal injection of digitoxin in rats caused an inhibition of brain (Na+ + K+)-ATPase and related respiration, but chlorpromazine failed to alter either (Na+ + K+)-ATPase activity or related respiration.     

Cytochemical localization of plasma membrane enzyme markers during interiorization of tachyzoites of Toxoplasma gondii by macrophages

The enzyme activity of Mg++-ATPase, Na+-K+-ATPase, 5'-nucleotidase and NAD(P)H-oxidase was cytochemically detected at the ultrastructural level in mouse peritoneal macrophages infected with untreated and with specific antibody-coated Toxoplasma gondii tachyzoites. The Mg++-ATPase and 5'-nucleotidase were distributed throughout the macrophages' plasma membrane but were not observed in the membrane lining endocytic vacuoles containing ingested parasites; however, Na+-K+-ATPase activity was detected in the macrophages' plasma membrane as well as in the parasitophorous vacuoles that contained untreated or specific antibody-coated parasites. Reaction product, indicative of NAD(P)H-oxidase, was detected in the parasitophorous vacuoles that contained only specific antibody-coated parasites.     

Cyclic stretch translocates the alpha2-subunit of the Na pump to plasma membrane in skeletal muscle cells in vitro

The Na+-K+-ATPase and its regulation is important for maintaining membrane potential and transmembrane Na(+) gradient in all skeletal muscle cells and thus is essential for cell survival and function. In our previous study, cyclic stretch activated the Na pump in cultured skeletal muscle cells. Presently, we investigated whether this stimulation was the result of translocation of Na+-K+-ATPase from endosomes to the plasma membrane, and also evaluated the role of phosphatidylinositol 3-kinase (PI 3-kinase), the activation of which initiated vesicular trafficking and targeting of proteins to specific cell compartments. Skeletal muscle cells were stretched at 25% elongation continuous for 24h using the Flexercell Strain Unit. The plasma membrane and endosome fractions were isolated and Western blotted to localize the Na+-K+-ATPase alpha1- and alpha2-subunit protein. The results showed stretch increased Na+-K+-ATPase alpha1- and alpha2-subunit protein expression in plasma membrane fractions and decreased it in endosomes. The alpha2-subunit had a more dynamic response to mechanical stretch. PI 3-kinase inhibitors (LY294002) blocked the stretch-induced translocation of the Na+-K+-ATPase alpha2-subunit, while LY294002 had no effect on the transfer of alpha1-subunit. We concluded that cyclic stretch mainly stimulated the translocation of the alpha2-subunit of Na+-K+-ATPase from endosomes to the plasma membrane via a PI 3-kinase-dependent mechanism in cultured skeletal muscle cells in vitro, which in turn increased the activity of the Na pump.     

The surface membrane of the small intestinal epithelial cell. I. Localization of adenyl cyclase.

The subcellular distribution of adenyl cyclase was investigated in small intestinal epithelial cells. Enterocytes were isolated, disrupted and the resulting membranes fractionated by differential and sucrose gradient centrifugation. Separation of luminal (brush border) and contra-luminal (basolateral) plasma membrane was achieved on a discontinuous sucrose gradient. The activity of adenyl cyclase was followed during fractionation in relation to other enzymes, notably those considered as markers for luminal and contraluminal plasma membrane. The luminal membrane was identified by the membrane-bound enzymes sucrase and alkaline phosphatase and the basolateral region by (Na+ + K+)-ATPase. Enrichment of the former two enzymes in purified luminal plasma membrane was 8-fold over cells and that of (Na+ + K+)-ATPase in purified bisolateral plasma membranes was 13-fold. F--activated adenyl cyclase co-purified with (Na+ + K+)-ATPase, suggesting a common localization on the plasma membrane. The distribution of K+-stimulated phosphatase and 5'-nucleotidase also followed (Na+ + K+)-ATPase during fractionation.     

Separation of hepatocyte plasma membrane domains by free flow electrophoresis

We have applied free flow electrophoresis to separate the canalicular and basolateral (sinusoidal and lateral) domains of rat hepatocyte plasma membranes. Hepatocyte plasma membranes were prepurified by rat zonal and discontinous sucrose gradient centrifugation. In electrophoretic separation, the canalicular membranes were more deflected toward the anode than the basolateral membranes. Na+-dependent taurocholate uptake could be measured in both membrane fractions, transport activity being highest in fractions containing the highest specific activity in the basolateral marker enzyme Na+-K+-ATPase. Thus, differences in electrophoretic mobility permit the separation of functional intact plasma membrane vesicles derived from basolateral and canalicular plasma membrane domains of rat hepatocyte.     

Studies on plasma membranes. XXIII. Hormone-like action of concanavalin A on liver plasma membranes: inhibition of (Na+-K+)ATPase.

Concanavalin A inhibits the (Na+-K+)-ATPase activity of isolated rat-liver plasma membranes, while leaving the Mg2+-ATPase unaffected. Glucagon and cyclic AMP act supplementary to the lectin in the inhibition. The lectin effect is counteracted by insulin and L-epinephrine, and is completely abolished by the beta-adrenergic blocking agent propranolol. Results are discussed on the basis of the known interactions of concanavalin A with plasma membrane components, including its hormone-like action.