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PubMedDate not supplied

Interactions between the rough endoplasmic reticulum and the nuclear membrane during increased proliferation of the endoplasmic reticulum.

Flickinger (1978) has shown conclusively that when there is demand for membrane by the nucleus, such as during repair of a damaged nuclear envelope, several membrane continuities develop between the RER and the nuclear envelope. This experiment was set up to determine if reversal of flow of membrane from the nuclear envelope to the RER would occur during phenobarbital-stimulated increased synthesis of RER. During the increased synthesis of RER following the stimulation of liver cells, it was observed that there was increase in the number of membrane continuities between the RER and the nuclear envelope. Increased blebbing and vesicle formation from the nuclear envelope was also seen. These vesicles were liberated as RER into the cytoplasm. It is therefore suggested that the nuclear envelope and the RER are in a state of equilibrium (RER in equilibrium NE) and membrane can be transferred either way depending on demand through the many membrane continuities that develop between the two organelles.

Animals↗
PubMed2006

Interferon-gamma potentiates endoplasmic reticulum stress-induced death by reducing pancreatic beta cell defence mechanisms.

AIMS/HYPOTHESIS: A tight control of endoplasmic reticulum homeostasis is crucial for beta cell function and survival. We recently described that IL-1beta plus IFN-gamma deplete endoplasmic reticulum Ca2+ stores in beta cells, leading to endoplasmic reticulum stress and apoptosis. IL-1beta alone induced endoplasmic reticulum stress but failed to induce beta cell death, while IFN-gamma alone neither caused endoplasmic reticulum stress nor induced beta cell death. This suggests that IFN-gamma aggravates endoplasmic reticulum stress induced by IL-1beta, eventually triggering apoptosis. Here we tested this hypothesis and the mechanisms involved, by investigating the effects of IFN-gamma on endoplasmic reticulum-stress-induced beta cell apoptosis caused by a specific blocker of the sarcoendoplasmic-reticulum pump Ca2+-ATPase (SERCA). MATERIALS AND METHODS: INS-1E cells or beta cells were pretreated with IFN-gamma and then exposed to the SERCA blocker cyclopiazonic acid (CPA) for induction of endoplasmic reticulum stress. Cell death was evaluated by Hoechst 342 and propidium iodide staining. Expression of genes related to endoplasmic reticulum stress was determined by real-time RT-PCR, while activation of the endoplasmic reticulum stress response was determined by analysing X-box binding protein-1 (Xbp1) splicing and using a reporter construct containing five copies of the unfolded protein response element (UPRE). RESULTS: CPA induces endoplasmic reticulum stress and apoptosis in insulin-producing cells. Pretreatment with IFN-gamma decreased the basal level of spliced Xbp1 mRNA, the basal and CPA-induced activity of the UPRE reporter, and the mRNA expression of several endoplasmic reticulum chaperones (Bip, Grp94 and Orp 150) and Sec61a. Furthermore, CPA-induced Chop mRNA expression and beta cell apoptosis were potentiated in cells that had been pretreated with IFN-gamma. CONCLUSIONS/INTERPRETATION: CPA-induced endoplasmic reticulum stress and apoptosis is enhanced in IFN-gamma-treated beta cells. These effects are mediated via downregulation of the expression of genes involved in beta cell defence against endoplasmic reticulum stress.

Animals↗
PubMed1986

Enhanced Ca2+-induced calcium release by isolated sarcoplasmic reticulum vesicles from malignant hyperthermia susceptible pig muscle.

To further define the possible involvement of sarcoplasmic reticulum calcium accumulation and release in the skeletal muscle disorder malignant hyperthermia (MH), we have examined various properties of sarcoplasmic reticulum fractions isolated from normal and MH-susceptible pig muscle. A sarcoplasmic reticulum preparation enriched in vesicles derived from the terminal cisternae, was further fractionated on discontinuous sucrose density gradients (Meissner, G. (1984) J. Biol. Chem. 259, 2365-2374). The resultant MH-susceptible and normal sarcoplasmic reticulum fractions, designated F0-F4, did not differ in yield, cholesterol and phospholipid content, or nitrendipine binding capacity. Calcium accumulation (0.27 mumol Ca/mg per min at 22 degrees C), Ca2+-ATPase activity (0.98 mumol Pi/mg per min at 22 degrees C), and calsequestrin content were also similar for MH-susceptible and normal sarcoplasmic reticulum fraction F3. To examine sarcoplasmic reticulum calcium release, fraction F3 vesicles were passively loaded with 45Ca (approx. 40 nmol Ca/mg), and rapidly diluted into a medium of defined Ca2+ concentration. Upon dilution into 1 microM Ca2+, the extent of Ca2+-dependent calcium release measured after 5 s was significantly greater for MH-susceptible than for normal sarcoplasmic reticulum, 65.9 +/- 2.8% vs. 47.7 +/- 3.9% of the loaded calcium, respectively. The C1/2 for Ca2+ stimulation of this calcium release (5 s value) from MH-susceptible sarcoplasmic reticulum also appeared to be shifted towards a higher Ca2+-sensitivity when compared to normal sarcoplasmic reticulum. Dantrolene had no effect on calcium release from fraction F3, however, halothane (0.1-0.5 mM) increased the extent of calcium release (5 s) similarly in both MH-susceptible and normal sarcoplasmic reticulum. Furthermore, Mg2+ was less effective at inhibiting, while ATP and caffeine were more effective in stimulating, this Ca2+-dependent release of calcium from MH-susceptible, when compared to normal sarcoplasmic reticulum. Our results demonstrate that while sarcoplasmic reticulum calcium-accumulation appears unaffected in MH, aspect(s) of the sarcoplasmic reticulum Ca2+-induced calcium release mechanism are altered. Although the role of the Ca2+-induced calcium release mechanism of sarcoplasmic reticulum in situ is not yet clear, our results suggest that an abnormality in the regulation of sarcoplasmic reticulum calcium release may play an important role in the MH syndrome.

Animals↗
PubMedDate not supplied

Local anaesthetic bupivacaine alters function of sarcoplasmic reticulum and sarcolemmal vesicles from rabbit masseter muscle.

The effects of local anaesthetics, bupivacaine and lidocaine, on Ca2+ flux behaviour of sarcoplasmic reticulum and on sarcolemmal functions were studied in the rabbit masseter muscle. The experiments were performed on sarcoplasmic reticulum and sarcolemmal vesicles prepared at 1 to 10 days after injection of local anaesthetics or saline into masseter muscle as well as on sarcoplasmic reticulum vesicles prepared from non-treated rabbits (for assessment of the effect on in vitro incubation with local anaesthetics). Bupivacaine potently reduced the efficiency of active sarcoplasmic reticulum Ca2+ transport as evaluated by coupling ratio (Ca2+ transported/ATP hydrolyzed, in the presence of oxalate) at 3 days after the injection; there was only a slight degree of uncoupling of Ca2+ transport from ATP hydrolysis with lidocaine injection. Bupivacaine but not lidocaine, at 3 days after injection, decreased both the apparent permeability of sarcoplasmic reticulum vesicles to Ca2+, determined by measuring net efflux of Ca2+ after stopping pump-mediated fluxes, and the steady-state Ca2+ load in sarcoplasmic reticulum, but had no effect on overall turnover of the Ca2+ATPase. The effects of bupivacaine on apparent sarcoplasmic reticulum Ca2+ permeability and steady-state Ca2+ load were inhibited by a Ca2+ antagonist verapamil. The reduction of Ca2+ uptake of sarcoplasmic reticulum and the protective effect of verapamil were reproduced in unfractionated homogenates prepared at 3 days after bupivacaine injection. In vitro exposure of sarcoplasmic reticulum vesicles to bupivacaine (0.5 to 50 mM) reduced steady-state Ca2+ load in a dose-dependent manner. The observed effect elicited by bupivacaine (25 mM) was partially protected by procaine, an inhibitor of Ca2(+)-induced Ca2+ release from sarcoplasmic reticulum, or by specific closure of the sarcoplasmic reticulum Ca2+ release channel by ryanodine, suggesting the possibility that in vitro exposure of sarcoplasmic reticulum vesicles to bupivacaine may produce an increase in apparent permeability of sarcoplasmic reticulum to Ca2+. In sarcolemma, bupivacaine reduced Na+,K(+)-ATPase and Na(+)-Ca2+ exchange activities at 3 days after injection; the effects on sarcolemmal vesicles were prevented by verapamil. These results suggest that although the effects elicited by bupivacaine injection and the in vitro exposure to bupivacaine on steady-state Ca2+ load of sarcoplasmic reticulum vesicles were similar, the membrane properties of the vesicles from bupivacaine-treated masseter muscles and those from normal untreated muscles may not be the same, which indicates that pure bupivacaine effect is due partly by an effect on ryanodine- and procaine-sensitive Ca2+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Anesthetics, Local↗
PubMed1983

Mechanism of the stimulation of Ca2+-dependent ATPase of skeletal muscle sarcoplasmic reticulum by protein kinase.

Sarcoplasmic reticulum isolated from moderately fast rabbit skeletal muscle contains intrinsic adenosine 3',5'-monophosphate (cAMP)-independent protein kinase activity and a substrate of 100 000 Mr. Phosphorylation of skeletal sarcoplasmic reticulum by either endogenous membrane bound or exogenous cAMP-dependent protein kinase results in stimulation of the initial rates of Ca2+ transport and Ca2+-ATPase activity. To determine the molecular mechanism by which protein kinase-dependent phosphorylation regulates the calcium pump in skeletal sarcoplasmic reticulum, we examined the effects of protein kinase on the individual steps of the Ca2+-ATPase reaction sequence. Skeletal sarcoplasmic reticulum vesicles were preincubated with cAMP and cAMP-dependent protein kinase in the presence (phosphorylated sarcoplasmic reticulum) and absence (control sarcoplasmic reticulum) of adenosine 5'-triphosphate (ATP). Control and phosphorylated sarcoplasmic reticulum were subsequently assayed for formation (5-100 ms) and decomposition (0-73 ms) of the acid-stable phosphorylated enzyme (E approximately P) of Ca2+-ATPase. Protein kinase mediated phosphorylation of skeletal sarcoplasmic reticulum resulted in pronounced stimulation of initial rates and levels of E approximately P in sarcoplasmic reticulum preincubated with either ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) prior to assay (Ca2+-free sarcoplasmic reticulum), or with calcium/EGTA buffer (Ca2+-bound sarcoplasmic reticulum). These effects were evident within a wide range of ionized Ca2+. Phosphorylation of skeletal sarcoplasmic reticulum by protein kinase also increased the initial rate of E approximately P decomposition. These findings suggest that protein kinase-dependent phosphorylation of skeletal sarcoplasmic reticulum regulates several steps in the Ca2+-ATPase reaction sequence which result in an overall stimulation of the active calcium transport observed at steady state.

Animals↗
PubMed1976

Reconstitution of an active calcium pump in sarcoplasmic reticulum.

Recovery of calcium transport and calcium-activated ATPase activity was studied in relation to the retention of protein components in sarcoplasmic reticulum reconstituted after solubilization with deoxycholate and centrifugation, followed by removal of the detergent from the supernatant by dialysis. Control sarcoplasmic reticulum was similarly treated except for omission of deoxycholate. Maximum capacity for oxalate- and phosphate-supported calcium uptake was increased 2- to 3-fold in reconstituted sarcoplasmic reticulum compared to original and control. Calcium uptake velocity of the reconstituted sarcoplasmic reticulum was approximately 80% that of original and 90% of control sarcoplasmic reticulum. Calcium uptake/ATP hydrolysis ratio was approximately 2 in the original sarcoplasmic reticulum and decreased to approximately 1 in the control and reconstituted sarcoplasmic reticulum. Calcium storage in the absence of calcium-precipitating anion was approximately 85% in control and 70% in reconstituted sarcoplasmic reticulum, compared to the original sarcoplasmic reticulum. Ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid-induced calcium release after phosphate-supported calcium uptake was slower in reconstituted sarcoplasmic reticulum than in original or control sarcoplasmic reticulum. Polyacrylamide gel electrophoresis of original and control sarcoplasmic reticulum showed similar amounts of protein components of approximately 93,000, 59,000, 50,000, 30,000 to 37,000, and 20,000 to 26,000 daltons. Reconstituted sarcoplasmic reticulum, however, lost over 85% of the 50,000- and 20,000- to 26,000-dalton proteins while retaining most of its calcium transport functions.

Adenosine Triphosphatases↗
PubMedDate not supplied

Sarcoplasmic reticulum membrane and heart development.

Intracellular Ca2+ concentrations in cardiac cells are dependent on trans-sarcolemmal Ca2+ fluxes and the ability of sarcoplasmic reticulum to release and take up Ca2+. Ca2+ accumulation by sarcoplasmic reticulum membranes causes muscle to relax, whereas Ca2+ release from sarcoplasmic reticulum initiates contraction. Ca2+ transport by the sarcoplasmic is mediated by a Ca2+-dependent ATPase enzyme. Ca2+ release from sarcoplasmic reticulum may be mediated by a ligant-gated Ca2+ channel. The physiological role of sarcoplasmic reticulum in developing muscle is not well established. In this report we investigated the composition and function of sarcoplasmic reticulum membranes during cardiac myogenesis. Phospholamban, a major phosphoprotein in mature sarcoplasmic reticulum membranes was present during early stages of cardiac myogenesis. The embryonic form of phospholamban was phosphorylated by cAMP-dependent protein kinase but not in the presence of Ca2+ and calmodulin. Ca2+ uptake and Ca2+-dependent ATPase activity were low in fetal sarcoplasmic reticulum compared to adult control membranes, although the apparent affinities of the enzyme for Ca2+ were similar. Sarcoplasmic reticulum vesicles used in these studies had very low levels of plasma membrane and mitochondrial contamination. The amounts of both 110-kDa Ca2+-ATPase and 55-kDa calsequestrin in the sarcoplasmic reticulum membrane were lower in fetal sarcoplasmic reticulum vesicles compared to mature membranes. Ca2+-ATPase and calsequestrin were identified in the isolated sarcoplasmic reticulum vesicles using specific antibodies produced against these membrane proteins. Age-related differences in Ca2+ transport properties of cardiac sarcoplasmic reticulum and in the amount of Ca2+-ATPase and calsequestrin may explain alterations in the regulation of intracellular Ca2+ concentrations in fetal heart muscle. This may relate to the developmental changes observed in myocardial function.

Adenosine Triphosphatases↗
PubMed1989

Changes in endoplasmic reticulum during spermiogenesis in the mouse.

Changes in the endoplasmic reticulum of mouse spermatids during spermiogenesis were examined by scanning electron microscopy, applying the OsO4-DMSO-OsO4 method, which permits 3-dimensional observation of cell organelles. At the same time, the endoplasmic reticulum was stained selectively by the Ur-Pb-Cu method, and 0.5 micron-thick sections were prepared for observation by transmission electron microscopy. The results demonstrated stereoscopically the mode of disappearance of the endoplasmic reticulum. In spermatids of the early maturation phase, the endoplasmic reticulum was of uniform diameter, branched and anastomosed, forming a complicated three-dimensional network throughout the cytoplasm. A two-dimensional net was also noted to have formed just beneath the plasma membrane and about Sertoli cell processes invaginating the spermatid cytoplasm. As spermiogenesis progressed, the spread-out endoplasmic reticulum gradually aggregated to form a condensed, glomerulus-like structure consisting of a very thin endoplasmic reticulum connected to the surrounding endoplasmic reticulum. This structure corresponds to the so-called "radial body". Thus, the endoplasmic reticulum may aggregate, condense, be transformed into a radial body, and be removed from the cytoplasm. The two-dimensional endoplasmic reticulum-net, just beneath the plasma membrane and surrounding processes of Sertoli cells, disappeared in spaces where the three-dimensional endoplasmic reticulum network was scarce. Both the two-dimensional endoplasmic reticulum-net structure and the three-dimensional endoplasmic reticulum network disappeared at the same time, indicating that they may be closely related.

Animals↗
PubMed1997

Phosphate transport into the sarcoplasmic reticulum of skinned fibres from rat skeletal muscle.

The rate, magnitude and pharmacology of inorganic phosphate (Pi) transport into the sarcoplasmic reticulum were estimated in single, mechanically skinned skeletal muscle fibres of the rat. This was done, indirectly, by using a technique that measured the total Ca2+ content of the sarcoplasmic reticulum and by taking advantage of the 1:1 stoichiometry of Ca2+ and Pi transport into the sarcoplasmic reticulum lumen during Ca-Pi precipitation-induced Ca2+ loading. The apparent rate of Pi entry into the sarcoplasmic reticulum increased with increasing myoplasmic [Pi] in the 10 mM-50 mM range at a fixed, resting myoplasmic pCa of 7.15, as judged by the increase in the rate of Ca-Pi precipitation-induced sarcoplasmic reticulum Ca2+ uptake. At 20 mM myoplasmic [Pi] the rate of Pi entry was calculated to be at least 51 microM s-1 while the amount of Pi loaded appeared to saturate at around 3.5 mM (per fibre volume). These values are approximations due to the complex kinetics of formation of different species of Ca-Pi precipitate formed under physiological conditions. Phenylphosphonic acid (PhPA, 2.5 mM) inhibited Pi transport by 37% at myoplasmic pCa 6.5 and also had a small, direct inhibitory effect on the sarcoplasmic reticulum Ca2+ pump (16%). In contrast, phosphonoformic acid (PFA, 1 mM) appeared to enhance both the degree of Pi entry and the activity of the sarcoplasmic reticulum Ca2+ pump, results that were attributed to transport of PFA into the sarcoplasmic reticulum lumen and its subsequent complexation with Ca2+. Thus, results from these studies indicate the presence of a Pi transporter in the sarcoplasmic reticulum membrane of mammalian skeletal muscle fibres that is (1) active at physiological concentrations of myoplasmic Pi and Ca2+ and (2) partially inhibited by PhPA. This Pi transporter represents a link between changes in myoplasmic [Pi] and subsequent changes in sarcoplasmic reticulum luminal [Pi]. It might therefore play a role in the delayed metabolic impairment of sarcoplasmic reticulum Ca2+ release seen during muscle fatigue, which should occur abruptly once the Ca-Pi solubility product is exceeded in the sarcoplasmic reticulum lumen.

Animals↗
PubMed1996

Movement of Ca(2+)-ATPase molecules within the sarcoplasmic/endoplasmic reticulum in skeletal muscle.

The endoplasmic reticulum undergoes rapid, microscopic changes in its structure, including extension and anastomosis of tubular elements. Such dynamism is expected to manifest itself also as rapid intermixing of membrane components, at least within subdomains of the endoplasmic reticulum. Here we present evidence of a similar dynamism in the sarcoplasmic reticulum of developing skeletal muscle. The sarcoplasmic reticulum is sometimes considered a specialized type of endoplasmic reticulum, but it appears to be a rather static set of membrane-bound elements, repetitively arranged to enwrap each sarcomere of each myofibril. Both endoplasmic reticulum and sarcoplasmic reticulum contain P-type Ca(2+)-ATPases that transport calcium from the cytosol into their lumen. In the experiments reported here, chicken and mouse cells were fused by polyethylene glycol, natural myogenic cell fusion, or Sendai virus. The redistribution of Ca(2+)-ATPase molecules between chick and mouse endoplasmic reticulum/sarcoplasmic reticulum was followed by immunofluorescence microscopy in which species-specific monoclonal antibodies to chick and mouse Ca(2+)-ATPases were used. Redistribution was time- and temperature-dependent but independent of protein synthesis as well as the method of cell fusion. Intermixing occurred on a time scale of tens of minutes at 37 degrees C. These results verify the dynamic nature of the sarcoplasmic reticulum and illustrate an aspect of the special relationship between endoplasmic reticulum and sarcoplasmic reticulum.

Animals↗
PubMed1982

Role of the reticulum in the stability and shape of the isolated human erythrocyte membrane.

In order to examine the widely held hypothesis that the reticulum of proteins which covers the cytoplamsic surface of the human erythrocyte membrane controls cell stability and shape, we have assessed some of its properties. The reticulum, freed of the bilayer by extraction with Triton X-100, was found to be mechanically stable at physiological ionic strength but physically unstable at low ionic strength. The reticulum broke down after a characteristic lag period which decreased 500-fold between 0 degrees and 37 degrees C. The release of polypeptide band 4.1 from the reticulum preceded that of spectrin and actin, suggesting that band 4.1 might stabilize the ensemble but is not essential to its integrity. The time-course of breakdown was similar for ghosts, the reticulum inside of ghosts, and the isolated reticulum. However, at very low ionic strength, the reticulum was less stable within the ghost than when free; at higher ionic strength, the reverse was true. Over a wide range of conditions the membrane broke down to vesicles just as the reticulum disintegrated, presumably because the bilayer was mechanically stabilized by this network. The volume of both ghosts and naked reticula varied inversely and reversibly with ionic strength. The volume of the naked reticulum varied far more widely than the ghost, suggesting that its deformation was normally limited by the less extensible bilayer. The contour of the isolated reticulum was discoid and often dimpled or indented, as visualized in the fluorescence microscope after labeling of the ghosts with fluoroscein isothiocyanate. Reticula derived from ghosts which had lost the ability to crenate in isotonic saline were shriveled, even though the bilayer was smooth and expanded. Conversly, ghosts crenated by dinitrophenol yielded smooth, expanded reticula. We conclude that the reticulum is a durable, flexible, and elastic network which assumes and stabilizes the contour of the membrane but is not responsible for its crenation.

Actins↗
PubMed1988

A monoclonal antibody to the Ca2+-ATPase of cardiac sarcoplasmic reticulum cross-reacts with slow type I but not with fast type II canine skeletal muscle fibers: an immunocytochemical and immunochemical study.

Ca2+-ATPase of the sarcoplasmic reticulum was localized in cryostat sections from three different adult canine skeletal muscles (gracilis, extensor carpi radialis, and superficial digitalis flexor) by immunofluorescence labeling with monoclonal antibodies to the Ca2+-ATPase. Type I (slow) myofibers were strongly labeled for the Ca2+-ATPase with a monoclonal antibody (II D8) to the Ca2+-ATPase of canine cardiac sarcoplasmic reticulum; the type II (fast) myofibers were labeled at the level of the background with monoclonal antibody II D8. By contrast, type II (fast) myofibers were strongly labeled for Ca2+-ATPase of rabbit skeletal sarcoplasmic reticulum. The subcellular distribution of the immunolabeling in type I (slow) myofibers with monoclonal antibody II D8 corresponded to that of the sarcoplasmic reticulum as previously determined by electron microscopy. The structural similarity between the canine cardiac Ca2+-ATPase present in the sarcoplasmic reticulum of the canine slow skeletal muscle fibers was demonstrated by immunoblotting. Monoclonal antibody (II D8) to the cardiac Ca2+-ATPase binds to only one protein band present in the extract from either cardiac or type I (slow) skeletal muscle tissue. By contrast, monoclonal antibody (II H11) to the skeletal type II (fast) Ca2+-ATPase binds only one protein band in the extract from type II (fast) skeletal muscle tissue. These immunopositive proteins coelectrophoresed with the Ca2+-ATPase of the canine cardiac sarcoplasmic reticulum and showed an apparent Mr of 115,000. It is concluded that the Ca2+-ATPase of cardiac and type I (slow) skeletal sarcoplasmic reticulum have at least one epitope in common, which is not present on the Ca2+-ATPase of sarcoplasmic reticulum in type II (fast) skeletal myofibers. It is possible that this site is related to the assumed necessity of the Ca2+-ATPase of the sarcoplasmic reticulum in cardiac and type I (slow) skeletal myofibers to interact with phosphorylated phospholamban and thereby enhance the accumulation of Ca2+ in the lumen of the sarcoplasmic reticulum following beta-adrenergic stimulation.

Animals↗
PubMed1981

Influence of monovalent cations on the Ca2+-ATPase of sarcoplasmic reticulum isolated from rabbit skeletal and dog cardiac muscles. An interpretation of transient-state kinetic data.

Transient-state kinetics of phosphorylation and dephosphorylation of the Ca2+-ATPase of sarcoplasmic reticulum vesicles from rabbit skeletal and dog cardiac muscles were studied in the presence of varying concentrations of monovalent and divalent cations. Monovalent cations affect the two types of sarcoplasmic reticulum differently. When the rabbit skeletal sarcoplasmic reticulum was Ca2+ deficient, preincubation with K+ (as compared with preincubation with choline chloride) did not affect initial phosphorylation at various concentrations of Ca2+, added with ATP to phosphorylate the enzyme. This is in contrast to preincubation with K+ of the Ca2+-deficient dog cardiac sarcoplasmic reticulum, which resulted in an increase in the phosphoenzyme level. When Ca2+ was bound to the rabbit skeletal sarcoplasmic reticulum, K+ inhibited E - P formation; but under the same conditions, E - P formation of dog cardiac sarcoplasmic reticulum was activated by K+ at 12 microM Ca2+ and inhibited at 0.33 and 1.3 microM Ca2+. Li+, Na+ and K+ also have different effects on E - P decomposition of skeletal and cardiac sarcoplasmic reticulum. The latter responded less to these cations than the former. Studies with ADP revealed differences between the two types of sarcoplasmic reticulum. For rabbit skeletal sarcoplasmic reticulum, 40% of the phosphoenzyme formed was 'ADP sensitive', and the decay of the remaining E - P was enhanced by K+ and ADP. Dog cardiac sarcoplasmic reticulum yielded about 40--48% ADP-sensitive E - P, but the decomposition rate of the remaining E - P was close to the rate measured in the absence of ADP. Thus, these studies showed certain qualitative differences in the transformation and decomposition of phosphoenzymes between skeletal and cardiac muscle which may have bearing on physiological differences between the two muscle types.

Animals↗