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PubMed1983

Inhibition of calmodulin-dependent and independent cardiac sarcoplasmic reticulum activities by R24571.

R24571 a derivative of the antimycotic miconazole, appears to be 5 to 8 times more potent than trifluoperazine in its ability to inhibit the calmodulin-dependent phosphorylation of cardiac sarcoplasmic reticulum vesicles. The cAMP-dependent protein kinase mediated phosphorylation of cardiac sarcoplasmic reticulum was not affected by R24571. Sarcoplasmic reticulum Ca-dependent ATPase phosphoprotein intermediate formation was inhibited by R24571 concentrations that were 20 to 30 times greater than those required to inhibit calmodulin-dependent phosphorylation. However, both Ca-dependent and independent ATPase activities, as well as calcium uptake, were inhibited by R24571 concentrations that were similar to, or less than, those concentrations required to inhibit calmodulin-dependent sarcoplasmic reticulum phosphorylation. These results indicate the caution that should be exercised in using this new compound in assessing the possible involvement of calmodulin in other membrane processes.

Adenosine Triphosphatases↗
PubMed1983

Phospholamban, the regulator of the cardiac sarcoplasmic reticulum calcium pump, does not copurify with the Ca2+-ATPase enzyme.

Canine cardiac sarcoplasmic reticulum is phosphorylated by adenosine 3',5'-monophosphate (cAMP)-dependent and by Ca2+-calmodulin-dependent protein kinases on an Mr 22 000 protein called phospholamban. Both types of phosphorylation are associated with an increase in the initial rate of Ca2+ transport. Thus, phospholamban appears to be a regulator for the calcium pump in cardiac sarcoplasmic reticulum. However, there is conflicting evidence as to the degree of association of the Ca2+-ATPase with its regulator, phospholamban. In this study, we report that phospholamban does not copurify with a Ca2+-ATPase preparation of high specific activity. Although 32P-labeled phospholamban is solubilized in the same fraction as the Ca2+-ATPase from cardiac sarcoplasmic reticulum, it dissociates from the Ca2+ pump during subsequent purification steps. Our isolation procedure results in an increase of over 4-fold in the specific activity of the Ca2+-ATPase, but a decrease of 2.5-fold in the specific activity of 32Pi-phosphoester bonds (pmol Pi/mg). Furthermore, the purified Ca2+-ATPase enzyme preparation is not a substrate for protein kinase in vitro to any significant extent. These data indicate that phospholamban does not copurify with the Ca2+-ATPase from cardiac sarcoplasmic reticulum. Isolation of a Ca2+-ATPase preparation essentially free of phospholamban will aid in future kinetic studies designed to elucidate similarities and differences in the Ca2+-ATPase parameters from cardiac and skeletal muscle (which is known not to contain phospholamban).

Animals↗
PubMed1985

Regulation of Ca2+ transport by cyclic 3',5'-AMP-dependent and calcium-calmodulin-dependent phosphorylation of cardiac sarcoplasmic reticulum.

Canine cardiac sarcoplasmic reticulum is phosphorylated by cyclic AMP-dependent and by Ca2+-calmodulin-dependent protein kinases on a 22 kDa protein, called phospholamban. Both types of phosphorylation have been shown to stimulate the initial rates of Ca2+ transport. To establish the interrelationship of the cAMP-dependent and Ca2+-calmodulin-dependent phosphorylation on Ca2+ transport, cardiac sarcoplasmic reticulum vesicles were preincubated under optimum conditions for: (a) cAMP-dependent phosphorylation, (b) Ca2+-calmodulin-dependent phosphorylation, and (c) combined cAMP-dependent and Ca2+-calmodulin-dependent phosphorylation. Control vesicles were treated under identical conditions, but in the absence of ATP, to avoid phosphorylation. Control and phosphorylated sarcoplasmic reticulum vesicles were subsequently centrifuged and assayed for Ca2+ transport in the presence of 2.5 mM Tris-oxalate. Our results indicate that cAMP-dependent and Ca2+-calmodulin-dependent phosphorylation can each stimulate calcium transport in an independent manner and when both are operating, they appear to have an additive effect. Stimulation of Ca2+ transport was associated with a statistically significant increase in the apparent affinity for calcium by each type of phosphorylation. The degree of stimulation of the calcium affinity was relatively proportional to the degree of phospholamban phosphorylation. These findings suggest the presence of a dual control system which may operate in independent and combined manners for regulating cardiac sarcoplasmic reticulum function.

Adenosine Triphosphate↗
PubMed1987

Failure of inositol 1,4,5-trisphosphate to elicit or potentiate Ca2+ release from isolated skeletal muscle sarcoplasmic reticulum.

This study was conducted to resolve the conflicting reports regarding the ability of inositol 1,4,5-trisphosphate (IP3) to elicit the release of Ca2+ from isolated sarcoplasmic reticulum derived from skeletal muscle. Three different conditions were employed, one of which has been reported to produce an IP3 induced release of Ca2+. Sarcoplasmic reticulum vesicles with and without intact feet structures failed to respond to added IP3. In addition, IP3 had no effect on the Ca2+-induced release of Ca2+. These results suggest that, unlike other tissue, IP3 does not mobilize the release of Ca2+ from skeletal muscle sarcoplasmic reticulum. IP3 is therefore unlikely to be the physiological signal linking transverse-tubule depolarization with Ca2+ release from the sarcoplasmic reticulum.

Animals↗
PubMed1994

Immunolabelling for VDAC, the mitochondrial voltage-dependent anion channel, on sarcoplasmic reticulum from amphibian skeletal muscle.

Patch-clamp studies of ion channels in the sarcoball membrane, a relatively pure preparation of sarcoplasmic reticulum, had earlier revealed a high-conductance anion channel with some properties similar to the mitochondrial voltage-dependent anion channel (VDAC). Using post-embedding immunolabelling, the presence of VDAC was investigated in sarcoball preparations from the semitendinosus muscle of the cane toad Bufo marinus. As expected, the outer membrane of mitochondria found within the interior of skinned fibres was decorated with gold label. Surprisingly, sarcoplasmic reticulum membrane was also labelled. The sarcoball membranes, which could arise from either the sarcoplasmic reticulum or from mitochondria, were also labelled. These results indicate the presence of a VDAC-like protein in the sarcoplasmic reticulum.

Animals↗
PubMed1980

Phosphorylation of a 100 000 dalton component and its relationship to calcium transport in sarcoplasmic reticulum from rabbit skeletal muscle.

Sarcomplasmic reticulum from rabbit fast skeletal muscle contains intrinsic protein kinase activity (ATP:protein phosphotransferase, EC 2.7.1.37) and a substrate. The protein kinase activity was Mg2+ dependent and could also phosphorylate exogenous protein substrates. Autophosphorylation of sarcoplasmic reticulum vesicles was not stimulated by cyclic AMP, neither was it inhibited by the heat-stable protein kinase inhibitor protein. The phosphorylated membranes had the characteristics of a protein with a phosphoester bond. An average of 73 pmol Pi/mg protein were incorporated in 10 min at 30 degrees C. Addition of exogenous cyclic AMP-dependent protein kinase increased the endogenous level of phosphorylation by 25-100%. Sarcoplasmic reticulum membrane phosphorylation, mediated by either endogenous cyclic AMP-independent or exogenous cyclic AMP-dependent protein kinase, occurred on a 100 000 dalton protein and both enzyme activities resulted in enhanced calcium uptake and Ca2+-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3), in a manner similar to cardiac microsomal preparations. Regulation of Ca2+ transport in skeletal sarcoplasmic reticulum may be mediated by phosphorylation of a 100 000 dalton component of these membranes.

Animals↗
PubMed1984

Dantrolene and calcium uptake by the sarcoplasmic reticulum of malignant hyperpyrexia-susceptible pigs.

Sarcoplasmic reticulum membranes were isolated from the skeletal muscle of pigs susceptible or resistant to the anaesthetic complication malignant hyperpyrexia. The skeletal muscle relaxant dantrolene was shown to be without effect on the Ca2+ -dependent ATPase activity of either sarcoplasmic reticulum preparation. The active transport of calcium by sarcoplasmic reticulum from both sources was also unaffected by the presence of dantrolene. These findings indicate that dantrolene does not exert its pharmacological action in skeletal muscle by stimulating calcium uptake by the sarcoplasmic reticulum.

Adenosine Triphosphatases↗
PubMed1988

Postmortem changes in the level of calcium pumping adenosine triphosphatase in rat heart sarcoplasmic reticulum.

The activity of calcium pumping adenosine triphosphatase (Ca2+-ATPase) in cardiac sarcoplasmic reticulum plays a pivotal role in myocardiac contraction-relaxation. The Ca2+-ATPase activity is controlled by phosphorylation and dephosphorylation of a sarcoplasmic reticulum protein "phospholamban" in response to neurotransmitters and drugs. To clarify the role of Ca2+-ATPase in the development of cardiac rigor mortis, we examined the changes of cardiac rigidity and cardiac sarcoplasmic reticulum Ca2+-ATPase activity up to 5 h after the decapitation of rats. Fifteen minutes after decapitation, the rats showed a cardiac rigidity on left ventricles. After 30 min, rigidity was obvious over the whole heart. After 1 h, the rigidity reached a high degree which was maintained for the rest of the observation period. On the other hand, the Ca2+-ATPase activity controlled by phosphorylation and dephosphorylation of phospholamban did not change for the whole observation period (5 h). Another Ca2+-ATPase activity representing the total amount of Ca2+-ATPase in sarcoplasmic reticulum gradually decreased. The data suggest that no significant phosphorylation or dephosphorylation of phospholamban occurs for a short time, at least for 5 h, after death and that the Ca2+-ATPase tends to relax the myocardium against the development of cardiac rigor mortis.

Animals↗
PubMed1995

Structure-activity relationship of bromoeudistomin D, a powerful Ca2+ releaser in skeletal muscle sarcoplasmic reticulum.

Bromoeudistomin D and 9-methyl-7-bromoeudistomin D which have a beta-carboline skeleton are powerful Ca2+ releasers from skeletal muscle sarcoplasmic reticulum exhibiting caffeine-like properties. We examined the effects of bromoeudistomin D analogues on Ca(2+)-induced Ca2+ release from skeletal muscle sarcoplasmic reticulum. Among bromoeudistomin D analogues, the Ca(2+)-releasing activities of carboline derivatives were higher than those of carbazole derivatives, suggesting that a carboline skeleton is significantly important for the manifestation of Ca(2+)-releasing activity and Ca2+ sensitivity of Ca(2+)-induced Ca2+ release. On the contrary, the analogues which have a carbazole skeleton and bromine at C-6 inhibit both Ca(2+)- and caffeine-induced Ca2+ release. 9-Methyl-substitution of the analogue elevated its Ca(2+)-releasing activity. Moreover, there is a close correlation between the enhancement of [3H]ryanodine binding to sarcoplasmic reticulum by the analogues and the activation of Ca2+ release by them. Bromoudistomin D analogues may provide valuable information about the structure-function relationship of the ryanodine receptor/Ca2+ release channels in skeletal muscle sarcoplasmic reticulum.

Animals↗
PubMed1991

Degradation of proteins within the endoplasmic reticulum.

Certain newly synthesized proteins within the endoplasmic reticulum undergo rapid turnover by a non-lysosomal proteolytic pathway. Biochemical and morphological evidence has suggested that these proteins never leave the endoplasmic reticulum before they are degraded. The mechanism(s) for the selective targeting of proteins for degradation within the endoplasmic reticulum is still not understood, but appears to rely on specific structural determinants on the protein substrates. Important cellular functions are likely to be served by this endoplasmic reticulum degradative system, including disposal of abnormal proteins and the selective turnover of metabolically regulated proteins.

Animals↗
PubMed2004

There is more to life and death than mitochondria: Bcl-2 proteins at the endoplasmic reticulum.

Proteins of the Bcl-2 family are important regulators of cell fate. The role of these proteins in controlling mitochondrial apoptotic processes has been extensively investigated, although exact molecular mechanisms are incompletely understood. However, mounting evidence indicates that these proteins also function at the endoplasmic reticulum and other locations within the cell. Both pro- and anti-apoptotic Bcl-2 family members can regulate endoplasmic reticulum calcium, cellular pH and endoplasmic reticulum resident proteins. In this review, we discuss the activities and potential targets of Bcl-2 family members at the endoplasmic reticulum and other cellular locations.

Animals↗
PubMed2004

Calcium release from the Golgi apparatus and the endoplasmic reticulum in HeLa cells stably expressing targeted aequorin to these compartments.

Extracellular agonists mobilize Ca2+ from SERCA-comprising intracellular Ca2+ stores located in both the Golgi apparatus and the endoplasmic reticulum. Ca2+ release from both these compartments was studied in HeLa cells stably expressing the luminescent Ca2+ indicator aequorin specifically targeted to these compartments. Changes in lumenal [Ca2+] as detected by the aequorin measurements were correlated with parallel changes in total Ca2+ content of the stores. The latencies and initial rates of Ca2+ release from the Golgi apparatus and the endoplasmic reticulum were quite similar. However, maximal Ca2+ release measured with Golgi-targeted aequorin terminated faster than that from the endoplasmic reticulum. The rate and extent of Ca2+ depletion from both compartments correlated well with the peak amplitude of the cytosolic [Ca2+] rise. Time-course experiments further revealed that the peak of the cytosolic Ca2+ response occurred before the lumenal [Ca2+] reached its lowest level. We conclude that both the Golgi apparatus and the endoplasmic reticulum contribute to the rise in cytosolic [Ca2+] upon agonist stimulation, but the kinetics of the Ca2+ release are different.

Aequorin↗
PubMed2005

Calcium release from ryanodine receptors in the nucleoplasmic reticulum.

Ca(2+) signals control DNA synthesis and repair, gene transcription, and other cell functions that occur within the nucleus. The nuclear envelope can store Ca(2+) and release it into the nucleus via either the inositol 1,4,5-trisphosphate receptor (InsP3R) or the ryanodine receptor (RyR). Furthermore, many cell types have a reticular network within their nuclei and InsP3Rs on this nucleoplasmic reticulum permit local subnuclear control of Ca(2+) signals and Ca(2+)-dependent intranuclear events. However, it is unknown whether RyR similarly is expressed on the nucleoplasmic reticulum and can control subnuclear Ca(2+) signals. Here we report that the type 1 RyR is expressed on intranuclear extensions of the sarcoplasmic reticulum of C2C12 cells, a skeletal muscle derived cell line. In addition, two-photon photorelease of caged Ca(2+) in the region of the nucleoplasmic reticulum evoked Ca(2+)-induced Ca(2+) release (CICR) within the nucleus, which could be suppressed by the RyR inhibitor dantrolene. These results show that intranuclear extensions of the nuclear envelope have functional RyR and provide a possible mechanism whereby cells expressing RyR can regulate Ca(2+) signals in discrete regions within the nucleus.

Animals↗
PubMed2004

The calcium pump of the endoplasmic reticulum plays a role in midline signaling during early zebrafish development.

During early vertebrate development, a signaling network is activated along the midline of the embryo. This signaling network induces the neural tube floor plate and ventral brain regions. In turn, induction of the ventral brain region is important for bilateral division of the forebrain and bilateral separation of the eyes. The present study provides direct evidence for a role of the endoplasmic reticulum Ca(2+) pump in zebrafish midline signaling. The endoplasmic reticulum Ca(2+) pump was inhibited in zebrafish embryos using thapsigargin or cyclopiazonic acid. Inhibition of the endoplasmic reticulum Ca(2+) pump during early gastrulation induces cyclopia, mimicking defects observed in cyclops, squint, one-eyed pinhead, and silberblick mutant embryos. In contrast, inhibition of the endoplasmic reticulum Ca(2+) pump during mid-gastrulation does not induce cyclopia, but does induce tail defects, mimicking defects observed in no-tail mutant embryos. This study is the first to relate thapsigargin and cyclopiazonic acid with induction of cyclopia. In addition, obtained results provide new information on the roles of Ca(2+) in embryonic development and may lead to new insights on the mechanisms underlying holoprosencephaly, a relatively common brain defect in human development.

Animals↗
PubMed2000

Chronic valproate treatment increases expression of endoplasmic reticulum stress proteins in the rat cerebral cortex and hippocampus.

BACKGROUND: Sodium valproate is a highly effective treatment for bipolar disorder, but its mechanism of action remains poorly understood. We recently found with differential display polymerase chain reaction that valproate regulates the expression of the endoplasmic reticulum stress protein GRP78 in the rat cerebral cortex. In our study, we investigated the effect of this drug on the other members of the endoplasmic reticulum stress protein family, GRP94 and calreticulin, and we studied the brain regional distribution of GRP78, GRP94, and calreticulin. METHODS: Immunohistochemistry was used to measure protein levels of GRP78, GRP94, and calreticulin after treatment with sodium valproate (300 mg/kg, intraperitoneal) in specific rat brain regions. RESULTS: We report here that chronic treatment with valproate also increased expression of other members of the endoplasmic reticulum stress protein family, such as GRP94 and calreticulin. The brain regional distribution of these changes was similar for all three proteins, with marked increase detected in the frontal cortex, parietal cortex, and CA1 region of the hippocampus. CONCLUSIONS: Because GRP78, GRP94, and calreticulin possess molecular chaperone activity and bind Ca(2+) in the endoplasmic reticulum, the pharmacologic action of valproate may involve one or more of these processes.

Animals↗
PubMed1986

The reaction of N-(1-pyrene)maleimide with sarcoplasmic reticulum.

The excimer fluorescence of the adduct of N-(1-pyrene)maleimide (PMI) with the Ca2+-ATPase was proposed as a probe of ATPase-ATPase interactions in sarcoplasmic reticulum (Lüdi and Hasselbach, Eur. J. Biochem., 1983, 130:5-8). We tested this proposition by analyzing the spectral properties and stoichiometry of the adducts of pyrenemaleimide with sarcoplasmic reticulum and with dithiothreitol and by comparing the effects of various detergents on the excimer fluorescence of the two adducts, with their influence on the sedimentation characteristics, ATPase activity, and light scattering of the pyrenemaleimide-labeled sarcoplasmic reticulum. These studies indicate that pyrenemaleimide reacts nearly randomly with several SH groups on the Ca2+-ATPase, and suggest that the observed excimer fluorescence of pyrenemaleimide-labeled sarcoplasmic reticulum may reflect intramolecular phenomena rather than ATPase-ATPase interactions. Further work is required to establish the relative contribution of intra- and intermolecular mechanisms to the excimer fluorescence.

Animals↗
PubMed1986

Sarcolemmal Na-Ca exchange and sarcoplasmic reticulum calcium uptake in developing chick heart.

Ontogenetic changes in calcium transport mediated by the sarcolemmal Na-Ca exchanger and by the sarcoplasmic reticulum calcium pump were studied in crude membranes from chick heart. Transport activities were evaluated per mass of membrane protein and heart tissue. Relative to unit heart mass Na-Ca exchange activity increases linearly from embryonic day 4 to day 10 of newborn stage. The overall increase is about 20-fold. An excellent correlation exists between activity of sodium gradient-induced calcium uptake and ouabain-sensitive (Na,K)-ATPase in crude membranes of embryonic, newborn and adult hearts. In the same membrane preparations active calcium uptake into vesicles of sarcoplasmic reticulum increases about 3-fold from embryonic day 4 to embryonic day 7, and then increases continuously until day 20. This is followed by a 3-fold elevation in reticular calcium accumulation at hatching on day 21. Maximal sarcoplasmic reticulum calcium transport activity reached at day 10 after hatching is 40- to 50-fold greater than activity values at embryonic day 4. In adult hearts the activities of both Na-Ca exchange and reticular calcium uptake drop to levels characteristic for the late embryonic period. This comparative study of sarcolemmal sodium gradient-dependent calcium flux and reticular calcium sequestration demonstrates that during chick heart differentiation the two calcium transport systems do not develop in parallel. Na-Ca exchange appears to play a greater role in calcium control of embryonic as compared to newborn and adult hearts. By contrast, the contribution of sarcoplasmic reticulum calcium transport to cardiac calcium movements becomes more predominant during and after hatching.

Animals↗
PubMed1995

Decreased inotropic response to beta-adrenergic stimulation and normal sarcoplasmic reticulum calcium stores in the spontaneously hypertensive rat heart.

Cardiac hypertrophy in the spontaneously hypertensive rat has been shown to be accompanied by a diminished inotropic response to beta-adrenergic stimulation. This diminished response has been attributed to abnormalities in various components of the beta-adrenergic signaling system. There is also evidence that regulation of intracellular Ca2+ cycling may be altered in the hypertrophied heart of the spontaneously hypertensive rat. We proposed that the diminished response to beta-adrenergic stimulation may reflect abnormalities in Ca2+ cycling, specifically alterations in the ability of the sarcoplasmic reticulum to effectively release and resequester Ca2+. We have used the unique combination of functional measurements on isolated, isometrically contracting papillary muscles from hearts of 26-week-old spontaneously hypertensive rats and their Wistar-Kyoto controls, together with electron probe microanalysis measurements of sarcoplasmic reticulum Ca2+ content in the same muscles after rapid freezing, to determine the availability of Ca2+ for activation of contraction, following beta-adrenergic stimulation. We observed a significant decrease in the inotropic response to beta-adrenergic stimulation in papillary muscles from the spontaneously hypertensive rats, as compared with Wistar-Kyoto controls, however in these same muscles, frozen during relaxation, there was no evidence of an accompanying decrease in the size of the sarcoplasmic reticulum Ca2+ store. In an additional group of muscles which were frozen during contraction, the amount of Ca2+ remaining in the sarcoplasmic reticulum after stimulated release was also not different in the two strains. These results indicate that the decreased inotropic response to beta-adrenergic stimulation in hypertrophied hearts of the spontaneously hypertensive rat is unlikely to be due to decreased availability of Ca2+ for activation of contraction. Additionally, to determine whether there is intracellular Ca2+ overload in the cardiac muscle cells of hearts of spontaneously hypertensive rats, we measured the amount of Ca2+ in mitochondrial and at the level of the myofilaments by electron probe microanalysis. These results indicate that intracellular Ca2+ overload does not accompany cardiac hypertrophy in the spontaneously hypertensive rat. This study therefore shows no correlation between altered intracellular Ca2+ cycling and the decreased inotropic response to isoproterenol in the spontaneously hypertensive rat at 26 weeks of age.

Actin Cytoskeleton↗