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PubMed1988

Solubilization and partial characterization of phospholipase A from rat heart sarcoplasmic reticulum.

Phospholipase A has been solubilized from the sarcoplasmic reticulum of rat heart by treatment with Tris buffer, potassium chloride, taurodeoxycholate or octyl glucoside. On HPLC gel permeation, two phospholipases were identified at the void volume of a TSK 3000 column and at an apparent molecular mass of 60 kDa. The two activity peaks exhibited a predominance of phospholipase A1 activity (83-91%) and a lesser phospholipase C activity (4-9%) using sonicated 1-palmitoyl-2[1-14C]oleoylphosphatidylcholine liposomes as substrate. The voiding phospholipase A peak, which represented the bulk of the recovered activity, exhibited a requirement for calcium ions in the 0.3-3 microM range. The heat stability and response to mercuric ions was studied and some similarities were noted between the solubilized sarcoplasmic reticulum phospholipases A and the cytosolic phospholipases A of rat heart. It is speculated that the cytosolic phospholipase A which we reported earlier may represent in part phospholipase A released from sarcoplasmic reticulum during isolation of the subcellular membrane fractions.

Animals↗
PubMed1994

Cytosolic lipoprotein particles from milk-secreting cells contain fatty acid synthase and interact with endoplasmic reticulum.

Part of the fatty acid synthase in cytosol from mammary glands of lactating rats was in a complex with other proteins and with lipids. This complex eluted in the void volume from a gel filtration column with an exclusion limit of 5,000,000, and remained in a 3% polyacrylamide stacking gel during electrophoresis under nondenaturing conditions. Fatty acid synthase-containing lipoprotein particles ranged in density from 1.07 to 1.16 g/ml, and varied in protein to lipid ratios. Similar fatty acid synthase particles were present also in cytosol from cow mammary gland. Butyrophilin, xanthine oxidase, and a group of small GTP-binding proteins that included ADP-ribosylation factor, were identified as constituents of the lipoprotein complex. This complex interacted with endoplasmic reticulum and with lipid droplets in cell-free incubation mixtures. In ultrastructure fatty acid synthase-containing lipoprotein particles were homogeneous in appearance, but were heterogeneous in size, with apparent diameters of 40 to 170 nm. Immunocytochemically, antigen recognized by antibodies to fatty acid synthase were found to be present in these particles and on endoplasmic reticulum. Lipoprotein complexes bound to specific polypeptides of endoplasmic reticulum.

Animals↗
PubMed1980

Release of ribosomes from thylakoids and endoplasmic reticulum with trypsin.

Release by trypsin of chloroplast ribosomes from rough thylakoids of Chlamydomonas reinhardtii was compared with release by trypsin of ribosomes from endoplasmic reticulum of rat liver. Rough thylakoids could be stripped of ribosomes with trypsin. Ribosomes were released as a mixture of polysomes, monosomes, and subunits. Chymotrypsin was much less effective than trypsin. Released ribosomes behaved like intact particles on zone velocity centrifugation, but showed extensive modification of their polypeptides. Stripped thylakoids sedimented as membranes, but also showed extensive modification of their polypeptides. Some ribosomes could be released in polysomes from rough endoplasmic reticulum by trypsin, provided the incubation mixture contained rat liver extract as a source of RNAase inhibitor. In its absence, ribosomes were released predominantly as monosomes, as reported by others. The rat liver extract inhibited release of polysomes from rough thylakoids by trypsin. Thus, the difference in form in which ribosomes are released from thylakoids, and endoplasmic reticulum by trypsin may be due to the high RNAase content of the latter.

Animals↗
PubMed1980

Crosslinking of sarcoplasmic reticulum ATPase protein with 1,5-difluoro 2,4-dinitrobenzene.

The reacton of 1,5-difluoro-2,4-dinitrobenzene with the ATPase protein of rabbit skeletal sarcoplasmic reticulum caused a marked loss of the Ca2+-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) activity during an interval when 2 mol of the crosslinking reagent were incorporated/10(5) g of protein. The modified ATPase protein formed non-serial high molecular weight aggregates or oligomers during short (1--5 min) or long exposure (60 min) to the reagent at 25 degrees C or 4 degrees C. The same pattern was found when sarvoplasmic reticulum was treated similarly; only the ATPase protein formed oligomers (homopolymers). In all cases the ATPase protein monomer remained the predominant species present. During the appearance of the high molecular weight oligomers the Ca2+-ATPase activity was unaffected but Ca2+ uptake was inhibited. Major changes in the ATPase activity occurred when the monomeric ATPase protein was modified. Disubstituted dinitrophenylene derivatives of cysteine and tyrosine were found in modified ATPase protein and only a small amount of monosubstituted dinitrophenyl groups were identified. Thiolysis of the modified ATPase protein with 2-mercaptoethanol removed approx. 35% of the incorporated groups, but there was no restoration of the Ca2+-ATPase activity. Substrate MgATP2- protected the Ca2+-ATPase activity of the ATPase protein and sarcoplasmic reticulum but Ca2+ had no effect on the modificaton. Different conformational states of the ATPase protein could be ascertained from a comparison of the effects of Ca2+ and MgATP2- on the bifunctional reagent dinitrophenylation of the ATPOase protein with that of the monofunctional reagent 1-fluoro-2,4-dinitrobenzene (Bailin, G. (1980) Biochim. Biophys. Acta 623, 213-224). Intramolecular crosslinking of the ATPase protein predominated and oligomers which formed during the reaction were not essential for the maintenance of the ATPase activity.

Adenosine Triphosphatases↗
PubMed1984

Properties of the Ca-pumping ATPase of sarcoplasmic reticulum from vascular smooth muscle.

A microsomal fraction enriched in sarcoplasmic reticulum membranes has been isolated from bovine aorta smooth muscle. The properties of the Ca-pumping ATPase were compared to those of the enzymes of skeletal and cardiac sarcoplasmic reticulum. The kinetic (Km and turnover rate) and structural (tryptic digestion pattern) properties of the three ATPases were strikingly similar. The three enzymes, however, displayed (almost) no immunological cross-reactivity. Skeletal muscle sarcoplasmic reticulum and aorta microsomes did not contain phospholamban: their Ca-pumping activity was not regulated by either a cAMP-dependent or a calmodulin-dependent pathway.

Animals↗
PubMed1985

Stimulation of canine cardiac sarcoplasmic reticulum Ca2+ uptake by dihydropyridine Ca2+ antagonists.

We examined the effects of four Ca2+ antagonists that possess the ability to bind to calmodulin-felodipine, nitrendipine, prenylamine, and verapamil--as well as the effect of the calmodulin antagonist trifluoperazine on Ca2+ uptake and Ca2+ + Mg2+/ATPase activity in canine cardiac sarcoplasmic reticulum. In the presence of 20-30 microM felodipine and 100-200 microM nitrendipine, Ca2+ uptake increased from 69 nmoles X mg-1 X min-1 to 107 and 108 nmoles X mg-1 X min-1, respectively, with half-maximal stimulation occurring at 7.5 and 28 microM respectively. Ca2+ + Mg2+/ATPase activity was unchanged over the same concentration ranges. In contrast, both Ca2+ uptake and Ca2+ + Mg2+/ATPase activities were inhibited in the presence of 10-100 microM trifluoperazine (IC50 = 25 microM), 10-100 microM prenylamine (IC50 = 35 microM) and 100-200 microM verapamil (inhibition insufficient for IC50 determination). None of the drugs affected membrane permeability to Ca2+ as determined by passive 45Ca2+ efflux in the presence of ethyleneglycol bis(beta-amenoethyl ether)N,N,N1-tetraacetic acid (EGTA). Drug inhibition of calmodulin-dependent turkey gizzard myosin light chain kinase activation in a purified protein system was used as a direct measure of calmodulin antagonism, and felodipine, nitrendipine, trifluoperazine, prenylamine, and verapamil blocked this activation at IC50 values of 9.8, 55, 6.4, 31, and 93 microM respectively. None of the drugs studied, however, had any effect upon endogenous phospholamban phosphorylation in our cardiac sarcoplasmic reticulum preparations. These observations indicate that dihydropyridine Ca2+ antagonists stimulate cardiac sarcoplasmic reticulum Ca2+ uptake in vitro either by increasing the efficiency of the transport process or by inhibiting Ca2+-dependent Ca2+ release, and suggest that these effects do not result from interference with calmodulin-mediated processes.

Animals↗
PubMed1985

Effects of phenothiazine drugs on the active Ca2+ transport by sarcoplasmic reticulum.

The effect of phenothiazines (trifluoperazine and chlorpromazine) on the activity of the sarcoplasmic reticulum Ca2+ pump was investigated. These drugs have a biphasic action on the ATPase activity. They inhibit the enzyme at high concentrations, but below 150 microM, they promote a significant stimulation of the ATP hydrolysis, which is accompanied by a slight increase of the Ca2+ transport. Leaking of the membrane occurs only at drug concentrations above 150 microM. The phenothiazine stimulatory effect was not found in the isolated enzyme whose activity was inhibited at all concentrations studied. These results indicate that low concentrations of phenothiazines uncouple the sarcoplasmic reticulum Ca2+ pump without disrupting the membrane and that the drug stimulatory effect on the ATPase is not due to a direct interaction with the enzyme. It is suggested that a coupling factor or a specific microenvironment surrounding the enzyme regulates the association between the Ca2+ transport and the ATP hydrolysis by sarcoplasmic reticulum of the skeletal muscle cell.

Adenosine Triphosphate↗
PubMed1983

Abundant, uniquely oriented endoplasmic reticulum in capillaries of the CNS: demonstration using reduced-osmium and glucose-6-phosphatase cytochemistry.

In the endothelial cells of capillaries in the rat CNS, we have observed abundant, circumferentially oriented, smooth, membrane-bound profiles, found just beneath, and parallel to, the abluminal plasmalemma. These structures are seen particularly well in tissue exposed to potassium ferricyanide-reduced OsO4. We studied these structures cytochemically, using glucose-6-phosphatase as a marker for endoplasmic reticulum, and acid phosphatase as a marker for the Golgi-associated endoplasmic reticulum containing lysosomal enzymes (GERL). We found that they contained glucose-6-phosphate hydrolyzing activity but did not contain acid phosphatase activity. Comparable structures were not seen in the continuous capillaries of skeletal muscle. Based on their morphology and content of glucose-6-phosphate hydrolyzing activity, we conclude that these structures are uniquely oriented smooth endoplasmic reticulum, which is much more abundant in capillaries of the CNS than in other continuous capillaries. The function of this distinctive feature of the CNS capillary is not known.

Animals↗
PubMed1991

On the effect of lysophosphatidylcholine, platelet activating factor and other surfactants on calcium permeability in sarcoplasmic reticulum vesicles.

The effect of low concentrations of lysophosphatidylcholine (LPC), platelet-activating factor (PAF) and other surfactants (Triton X-100, C12E8, sodium dodecyl sulfate, sodium cholate and sodium deoxycholate) on membrane permeability of native sarcoplasmic reticulum vesicles and sarcoplasmic reticulum lipid vesicles, has been studied. Triton X-100, C12E8, sodium dodecyl sulfate, sodium cholate and sodium deoxycholate were all able to permeabilize membranes at concentrations of surfactants below their critical micellar concentration (CMC) in both lipid and native vesicles, being the K0.5 of calcium release from native vesicles lower than that from lipid vesicles. The values of these K0.5 were well correlated with the corresponding CMC values for each type of membrane. However, both LPC and PAF behaved in a different way since, although they induced permeabilization of the native vesicles at values of K0.5 close to their CMC, their K0.5 values for permeabilizing vesicles, prepared by using lipids extracted from sarcoplasmic reticulum, were much higher than their corresponding CMC.

Animals↗
PubMed1990

Inhibitory action of dantrolene on Ca-induced Ca2+ release from sarcoplasmic reticulum in guinea pig skeletal muscle.

The effects of dantrolene on the sarcoplasmic reticulum and contractile machinery were examined in skinned skeletal muscles of guinea pigs. Dantrolene inhibited Ca-induced Ca2+ release at 38 degrees C. The inhibitory effect of dantrolene on Ca2+ release was greater in the presence of caffeine or adenine nucleotide than in their absence at low Ca2+ concentrations. Dantrolene suppressed the maximum rate of Ca-induced Ca2+ release but did not affect the sensitivity to Ca2+. At 20 degrees C, however, dantrolene did not inhibit Ca2+ release induced by any Ca2+ concentration, regardless of the presence and absence of caffeine, adenine nucleotide or Mg2+. Ca2+ uptake into the sarcoplasmic reticulum and the Ca2+ sensitivity of the contractile machinery were not affected by dantrolene at 38 degrees C and 20 degrees C. These results suggest that dantrolene is a selective inhibitor of the Ca-induced Ca2+ release mechanism at physiological temperatures without having an effect on the Ca2+ pump of the sarcoplasmic reticulum or on the contractile machinery of skeletal muscle.

Adenine Nucleotides↗
PubMed1994

Potentiation of the twitch responses by inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase in frog atrial fibres.

In frog atrial fibres, cyclopiazonic acid as well as thapsigargin, which are inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase, induced a significant increase in the twitch amplitude without detectable changes in its kinetics. The measurements performed on chemically skinned fibres show that cyclopiazonic acid has no effect on the properties of contractile proteins. In the presence of a T-type Ca2+ channel blocker or L-type Ca2+ channel blocker, cyclopiazonic acid still induced a potentiation of the twitch while no effect was found in the presence of a Na(+)-Ca2+ exchange blocker. The effect of cyclopiazonic acid was not related to any modification in myofibrillar Ca2+ sensitivity or in Ca2+ influx through Ca2+ channels. It is proposed that the inhibition of the sarcoplasmic reticulum Ca(2+)-ATPase resulted in a potentiation of the effect of the Ca2+ influx and that the major role of the sarcoplasmic reticulum was to limit the intracellular Ca2+ concentration.

Animals↗
PubMed1983

Effect of tetrodotoxin relaxation of cultured skeletal muscle on the sarcoplasmic reticulum Ca2+-transport ATPase.

The spontaneous contractions of cultured chick skeletal muscle fibers were abolished by growth of cultures in the presence of tetrodotoxin (TTX). Inhibition of the contractile activity of cultured myofibers was associated with a marked reduction in the rate of azide-insensitive, ATP-dependent Ca2+ uptake by the total particulate fraction of cell homogenates and by purified sarcoplasmic reticulum. Myosin heavy chain (MHC) accumulation and azide-insensitive, ATP-dependent Ca2+ uptake into a total cell membrane fraction were measured simultaneously in the same culture dish. A decrease in the activity of the ATP-dependent Ca2+ uptake system preceded a significant reduction in MHC content of contraction-inhibited cultures. The reduced rate of Ca2+ uptake observed in the sarcoplasmic reticulum from TTX-treated cultures paralleled a decrease in the amount of enzymatically active Ca2+-transport ATPase. The cellular concentration of the ATPase was estimated from a measurement of the concentration of the Ca2+-dependent, hydroxylamine-sensitive, steady state level of phosphorylated intermediate formed in culture microsomes. In contrast to the changes observed in activity of the sarcoplasmic reticulum ATPase and MHC content of TTX-treated cultures, neither the specific activity of creatine kinase nor the accumulation of the MM isoenzyme were affected. It is therefore concluded that the contractile activity of muscle has a selective effect on the maintenance of the adult skeletal muscle phenotype.

Animals↗
PubMed1984

Electron-dense endoplasmic reticulum-like profiles closely associated with mitochondria in glomus cells of the carotid body after fixation with oxalate.

Fixation in the presence of oxalate was used to demonstrate the electron-dense Ca2+ precipitates in the endoplasmic reticulum in glomus cells of the carotid body. Glomus cells in intact carotid bodies or cells dissociated from the organ by treatment with collagenase were studied electron microscopically. In the intact organ as well as in dissociated glomus cells, electron-dense endoplasmic reticulum-like profiles were seen closely associated with mitochondria, while these lacked reaction product. The interspace between mitochondria was occupied by electron-dense, slightly distended ER, which appeared to contact the outer membrane of the mitochondria. Occasionally, a mitochondrion was in contact with several ER profiles or the ER formed an electron-dense 'cap' on the mitochondrion. The electron-dense precipitates could be removed from ultrathin sections with the calcium chelator ethyleneglycol-2(2-aminoethyl tetra-acetic acid) (EGTA). It is tentatively suggested that the endoplasmic reticulum could be involved in intracellular buffering of Ca2+ in the glomus cell, as has been previously suggested for neurons.

Animals↗
PubMed1986

Morphology of endoplasmic reticulum and Golgi elements following microinjection of rat liver microsomes into Xenopus laevis oocyte cytoplasm.

Fragments of rough endoplasmic reticulum and Golgi complex purified from rat liver homogenates were injected into Xenopus oocytes and the sites of microinjection analysed by electron microscopy at different times post-injection. The in vivo incubated fragments were located by their proximity to a microinjection vacuole, and identified by their association with specific morphological markers (peroxisomal cores associated with rough microsomes and lipoprotein particles with Golgi derivatives). Typical endoplasmic reticulum microsomes disappeared with time post-injection and seemed to be replaced by flattened cisternae of endoplasmic reticulum. Golgi fragments as defined by their content of lipoprotein particles became modified. Many were found associated with coated vesicles and some displayed membrane-coated regions. Furthermore lipoprotein particles were observed as integral components of Golgi stacks and were found within dilated rims in direct continuity with fenestrated Golgi saccules. The results suggest that the injected organelle fragments underwent transformation in vivo as a consequence of reconstitution.

Animals↗
PubMed1985

Tritrichomonas foetus: cytochemical visualization of the endoplasmic reticulum-Golgi complex and lipids.

The zinc iodide-osmium tetroxide technique was used to analyze the distribution of the endoplasmic reticulum-Golgi complex system of Tritrichomonas foetus. Interconnections between the cisternae of the endoplasmic reticulum as well as between cisternae of the Golgi complex were observed. The nuclear pores, as well as fenestrations in the Golgi complex, were evident. Three to four profiles of the endoplasmic reticulum were seen in the proximal marginal lamellae, related to the attachment of the recurrent flagellum to the protozoan body. No reaction product was seen in the costae, microtubules, glycogen particles, or hydrogenosomes. Imidazole-buffered osmium tetroxide solution was used to visualize lipids. Electron-dense materials were seen either free in the cytoplasm or within membrane-bounded cytoplasmic vesicles. A high contrast of some membranes, mainly of those which enclosed the hydrogenosomes, was observed in unstained sections.

Animals↗
PubMed1991

Modulation by phosphorylation of glycogen phosphorylase-sarcoplasmic reticulum interaction.

Glycogen phosphorylase b at concentrations close to those found in skeletal muscle interacts with sarcoplasmic reticulum membranes, but not with liposomes made of lipids extracted from these membranes, and is inhibited upon binding to the membrane. The interaction of glycogen phosphorylase with the sarcoplasmic reticulum membrane is modulated by phosphorylation, for the a form of this enzyme shows a K0.5 of interaction about 10-fold lower than the b form. Upon association to the membrane the fluorescence properties of the coenzyme of glycogen phosphorylase, pyridoxal-5'-phosphate, are strongly altered, for the fluorescence at 535 nm is partially quenched and the fluorescence at 415-420 nm increases. Using fluorescein labeled sarcoplasmic reticulum membranes we have found that the average conformation of the Ca2+ + Mg(2+)-ATPase is also altered on binding of phosphorylase b. In conclusion, the results reported in this paper suggest that glycogen phosphorylase and Ca2+ + Mg(2+)-ATPase directly interact under experimental conditions similar to those found in the sarcoplasm, and that this interaction is modulated by phosphorylation of the phosphorylase.

Animals↗
PubMed1992

BisG10, a K+ channel blocker, affects the calcium release channel from skeletal muscle sarcoplasmic reticulum.

The action of bisG10, a potent K+ channel inhibitor, was tested on the Ca2+ release from isolated sarcoplasmic reticulum vesicles of rabbit skeletal muscle. Using a rapid filtration technique, we found that the drug inhibited Ca(2+)-induced Ca2+ release elicited in the presence of extravesicular K+ as counter-ion. This inhibition was not reversed by the addition of valinomycin and still occurred when Cl- was used as co-ion, indicating that not only K+ channels are involved in the inhibiting effect. We found that bisG10 decreased the binding of ryanodine to sarcoplasmic reticulum vesicles, showing that bisG10 is able to block the sarcoplasmic reticulum Ca2+ release channel.

Animals↗
PubMed1986

Transbilayer distribution of phospholipid synthesizing enzymes in the sarcoplasmic reticulum membrane.

The topology of enzymes involved in phospholipid synthesis in the membrane of sarcoplasmic reticulum vesicles was investigated by means of limited proteolysis. The digestion of sarcoplasmic reticulum proteins with different proteases caused the considerable decrease of the activities of choline- and ethanolaminephosphotransferases without significant inactivation of glycerophosphate and lysophosphatidylcholine acyltransferases. On the basis of these results we concluded that the active sites of phosphotransferases are located on the cytoplasmic side, whereas the active sites of acyltransferase are present either on the extracytoplasmic side or burried into the sarcoplasmic reticulum membrane.

1-Acylglycerophosphocholine O-Acyltransferase↗