ATPase Of Cardiac Sarcoplasmic Reticulum Research Articles

Involvement of electrostatic phenomena in phospholamban-induced stimulation of Ca uptake into cardiac sarcoplasmic reticulum

The activity of the Ca-pumping ATPase of cardiac sarcoplasmic reticulum (SR) is controlled by the phosphorylation of the intrinsic regulatory protein phospholamban (PLB), which affects both the apparent K m(Ca) and the V max of the transport process. We have investigated the correlation between phosphorylation of PLB and the surface potential of the SR membrane. This latter influences the local concentration of relevant ionic species near biological membranes and thus modulates the activity of ion pumps and channels. The partitioning of the anionic probe 8-anilino-1-naphthalenesulfonate (ANS −) into the SR membrane was found to be dependent on the phosphorylation level of PLB. Changes of the surface membrane potential up to 7 mV could be obtained by phosphorylation. The increase in the apparent affinity of the Ca pump for Ca 2+ induced by PLB phosphorylation was clearly reduced at high ionic strength, i.e. under conditions known to reduce the surface membrane potential and all processes dependent on it. The results show that electrostatic phenomena can account, in good part, for the modulation of the Ca pump by PLB in cardiac SR.

Heterogeneous distribution of calmodulin- and cAMP-dependent regulation of Ca2+ uptake in cardiac sarcoplasmic reticulum subfractions.

The activity of the Ca2+-pumping ATPase of cardiac sarcoplasmic reticulum is controlled by the phosphorylation level of the intrinsic membrane protein phospholamban. Phospholamban monomers contain two distinct phosphorylation sites for either the cAMP-dependent or the calmodulin-dependent kinase. The two kinases, however, preferentially phosphorylate different populations of phospholamban molecules and double phosphorylation of the same subunit by their concerted action is a phenomenon that occurs only under particular experimental conditions. This study investigates the phosphorylation pattern of phospholamban in various subfractions derived from dog cardiac sarcoplasmic reticulum. The results show that the endogenous calmodulin-dependent kinase preferentially phosphorylates the phospholamban population found in association with the cisternal compartments of the reticulum. The differential phosphorylation occurs despite the presence of sufficient amounts of the kinase in all sarcoplasmic reticulum subfractions. On the other hand, phospholamban molecules localized on the longitudinal system are preferential substrates for the cAMP-dependent kinase. Possibly, the different lipid and/or protein microenvironment of phospholamban in the various sarcoplasmic reticulum domains is responsible for the apparent heterogeneity of phosphorylation. The present findings are compatible with the concept of additive and independent action of the cAMP-dependent and calmodulin-dependent kinases on cardiac sarcoplasmic reticulum. The imply, however, that different regions of the sarcoplasmic reticulum network are controlled by the two regulatory mechanisms.

Localization of phospholamban in smooth muscle using immunogold electron microscopy.

Phospholamban, the putative regulator of the Ca2+-ATPase in cardiac sarcoplasmic reticulum, was immunolocalized in canine visceral and vascular smooth muscle. Gently disrupted tissues were labeled with an affinity-purified phospholamban polyclonal antibody and indirect immunogold, using preembedding techniques. The sarcoplasmic reticulum of smooth muscle cells was specifically labeled with patches of immunogold distributed in a nonuniform fashion, while the sarcolemma did not appear to contain any phospholamban. The outer nuclear envelopes were also observed to be heavily labeled with the affinity-purified phospholamban polyclonal antibody. These findings suggest that phospholamban may play a role in the regulation of cytoplasmic and intranuclear calcium levels in smooth muscle cells.

Oxygen radical-mediated lipid peroxidation and inhibition of Ca 2+-ATPase activity of cardiac sarcoplasmic reticulum

Oxygen radicals have been implicated as important mediators of myocardial ischemic and reperfusion injury. A major product of oxygen radical formation is the highly reactive hydroxyl radical via a biological Fenton reaction. The sarcoplasmic reticulum is one of the major target organelles injured by this process. Using a oxygen radical generating system consisting of dihydroxyfumarate and Fe 3+-ADP, we studied lipid peroxidation and Ca 2+-ATPase of cardiac sarcoplasmic reticulum. Incubation of sarcoplasmic reticulum with dihydroxyfumarate plus Fe 3+-ADP significantly inhibited enzyme activity. Addition of superoxide dismutase, superoxide dismutase plus catalase (15μg/ml) or iron chelator, deferoxamine (1.25–1000 μ m) protected Ca 2+-ATPase activity. Time course studies showed that this system inhibited enzyme activity in 7.5 to 10 min. Similar exposure of sarcoplasmic reticulum to dihydroxyfumarate plus Fe 3+-ADP stimulated malondialdehyde formation. This effect was inhibited by superoxide dismutase, catalase, singlet oxygen, and hydroxyl radical scavengers. EPR spin-trapping with 5,5-dimethyl-1-pyroline- N-oxide verified production of the hydroxyl radical. The combination of dihydroxyfumarate and Fe 3+-ADP resulted in a spectrum of hydroxyl radical spin trap adduct, which was abolished by ethanol, catalase, mannitol, and superoxide dismutase. The results demonstrate the role of oxygen radicals in causing inactivation of Ca 2+-ATPase and inhibition of lipid peroxidation of the sarcoplasmic reticulum which could possibly be one of the important mechanisms of oxygen radical-mediated myocardial injury.

S4-6 - Regulation of Ca pump ATPase of cardiac sarcoplasmic reticulum by the cAMP-phospholamban system

S4-6 - Regulation of Ca pump ATPase of cardiac sarcoplasmic reticulum by the cAMP-phospholamban system

Identification of membrane-bound calcium, calmodulin-dependent protein kinase II in canine heart

Phospholamban, the putative regulatory proteolipid of the Ca 2/Mg 2+ ATPase in cardiac sarcoplasmic reticulum, was selectively phosphorylated by a Ca 2+/calmodulin (CaM)-dependent protein kinase associated with a cardiac membrane preparation. This kinase also catalyzed the phosphorylation of two exogenous proteins known to be phosphorylated by the multifunctional Ca 2+/CaM-dependent protein kinase II Ca 2+/CaM-kinase II), i.e., smooth muscle myosin light chains and glycogen synthase a. The latter protein was phosphorylated at sites previously shown to be phosphorylated by the purified multifunctional Ca 2+/CaM-kinase II from liver and brain. The membrane-bound kinase did not phosphorylate phosphorylase b or cardiac myosin light chains, although these proteins were phosphorylated by appropriate, specific calmodulin-dependent protein kinases added exogenously. In addition to phospholamban, several other membrane-associated proteins were phosphorylated in a calmodulin-dependent manner. The principal one exhibited a M r of approximately 56,000, a value similar to that of the major protein (57,000) in a partially purified preparation of Ca 2+/CaM-kinase II from the soluble fraction of canine heart that was autophosphorylated in a calmodulin-dependent manner. These data indicate that the membrane-bound, calmodulin-dependent protein kinase that phosphorylates phospholamban in cardiac membranes is not a specific calmodulin-dependent kinase, but resembles the multifunctional Ca 2+/CaM-kinase II. Our data indicate that this kinase may be present in both the particulate and soluble fractions of canine heart.

Mechanism of the stimulation of cardiac sarcoplasmic reticulum calcium pump by calmodulin.

Calmodulin has been shown to stimulate the initial rates of Ca2+-uptake and Ca2+-ATPase in cardiac sarcoplasmic reticulum, when it is present in the reaction assay media for these activities. To determine whether the stimulatory effect of calmodulin is mediated directly through its interaction with the Ca2+-ATPase, or indirectly through phosphorylation of phospholamban by an endogenous protein kinase, two approaches were taken in the present study. In the first approach, the effects of calmodulin were studied on a Ca2+-ATPase preparation, isolated from cardiac sarcoplasmic reticulum, which was essentially free of phospholamban. The enzyme was preincubated with various concentrations of calmodulin at 0 degrees C and 37 degrees C, but there was no effect on the Ca2+-ATPase activity assayed over a wide range of [Ca2+] (0.1-10 microM). In the second approach, cardiac sarcoplasmic reticulum vesicles were prephosphorylated by an endogenous protein kinase in the presence of calmodulin. Phosphorylation occurred predominantly on phospholamban, an oligomeric proteolipid. The sarcoplasmic reticulum vesicles were washed prior to assaying for Ca2+ uptake and Ca2+-ATPase activity in order to remove the added calmodulin. Phosphorylation of phospholamban enhanced the initial rates of Ca2+-uptake and Ca2+-ATPase, and this stimulation was associated with an increase in the affinity of the Ca2+-pump for calcium. The EC50 values for calcium activation of Ca2+-uptake and Ca2+-ATPase were 0.96 +/- 0.03 microM and 0.96 +/- 0.1 microM calcium by control vesicles, respectively. Phosphorylation decreased these values to 0.64 +/- 0.12 microM calcium for Ca2+-uptake and 0.62 +/- 0.11 microM calcium for Ca2+-ATPase. The stimulatory effect was associated with increases in the apparent initial rates of formation and decomposition of the phosphorylated intermediate of the Ca2+-ATPase. These findings suggest that calmodulin regulates cardiac sarcoplasmic reticulum function by protein kinase-mediated phosphorylation of phospholamban.

Monoclonal antibodies (mAbs) to Ca 2+-ATPase of cardiac sarcoplasmic reticulum (SR) inhibiting Ca 2+-pump activity and differentiating fast and slow muscles

Monoclonal antibodies (mAbs) to Ca 2+-ATPase of cardiac sarcoplasmic reticulum (SR) inhibiting Ca 2+-pump activity and differentiating fast and slow muscles

Changes in the distances of Ca 2+−Ca 2+-ATP sites of the cardiac sarcoplasmic reticulum ATPase

Changes in the distances of Ca 2+−Ca 2+-ATP sites of the cardiac sarcoplasmic reticulum ATPase

Primary structure of the nucleotide binding domain of the Ca,Mg-ATPase from cardiac sarcoplasmic reticulum

The nucleotide binding domain of the active site of the Ca, Mg-ATPase of cardiac sarcoplasmic reticulum (SR) has been isolated using fluorescein isothiocyanate (FITC) as an active site label and sequenced. After removal of non-specifically incorporated FITC with hydroxylamine, the amount of label incorporated was stoichiometric with residual ATPase activity, demonstrating that the label was incorporated uniquely at the active site. The SR was succinylated before digestion by trypsin in order to obtain a peptide of sufficient length to determine if the cardiac SR ATPase is a candidate for the unidentified cDNA clone recently sequenced by MacLennan et al. ( Nature 316 : 696–700, 1985). The sequence of the labeled SR peptide, obtained by affinity chromatography on a FITC antibody column, was T S M S K M F K G P E V I D R. This sequence was identical with that predicted by the unidentified clone and is significantly different from the sequence reported by Kirley et al. ( Biochem. Biophys. Res. Commun. 130 : 732–738, 1985) for a FITC labeled peptide isolated from cardiac SR.

Structural characterization of phospholamban: A regulator of Ca pump ATPase of cardiac sarcoplasmic reticulum

Structural characterization of phospholamban: A regulator of Ca pump ATPase of cardiac sarcoplasmic reticulum

Purification and characterization of phospholamban from canine cardiac sarcoplasmic reticulum.

Phospholamban, a putative regulator of the Ca2+-dependent ATPase of cardiac sarcoplasmic reticulum (SR), was purified from canine cardiac SR membranes. Cardiac SR was extracted with deoxycholate and fractionated with ammonium sulfate followed by gel permeation high performance liquid chromatography in the presence of the nonionic detergent, octa-ethylene glycol mono-n-dodecyl ether (C12E8), and KI. Further purification was achieved with CM-Sepharose CL 6B column chromatography in the presence of C12E8. The purified phospholamban showed a single band of 22,000 daltons on neutral sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (Weber, K., and Osborn, M. (1969) J. Biol. Chem. 244, 4406-4412) and 27,000 daltons on alkaline SDS gels (Laemmli, U. K. (1970) Nature (Lond.) 227, 680-685). Boiling of phospholamban in 2% SDS produced total conversion into the lower molecular weight component on SDS gels (11,000 on Laemmli gel and 10,500 on Weber and Osborn gel). The apparent molecular weight of phospholamban on SDS gels was slightly increased by cAMP-dependent phosphorylation. The extent of phosphorylation catalyzed by cAMP-dependent protein kinase in the purified phospholamban preparations was about 42 nmol of phosphate/mg of protein when the protein concentration was determined by the method of Lowry et al. (Lowry, O. H., Rosebrough, N. J., Farr, A. L., and Randall, R. J. (1951) J. Biol. Chem. 193, 265-275), or 138 nmol/mg of protein based on the protein concentration estimated by the dye absorption method. Rabbit antisera were prepared against purified phospholamban. The obtained antisera were found to bind to purified phospholamban as well as that in cardiac SR. No reaction was detected in fast skeletal muscle SR by immunofluorescent staining of Western blots. The present preparation of purified phospholamban and the antisera should facilitate further understanding of the regulatory action of phospholamban on the calcium pump ATPase.

Effects of verapamil, diltiazem, nisoldipine and felodipine on sarcoplasmic reticulum

Verapamil, diltiazem, nisoldipine and felodipine, calcium antagonist drugs with different chemical structures, were studied for their effects on activities of sarcoplasmic reticulum (SR) isolated from dog cardiac and rabbit skeletal muscles. Nisoldipine and felodipine exerted biphasic actions on both cardiac and skeletal SR Ca2+-ATPase with maximum activation of 40–60% occurring at 20–40 μM for nisoldipine and 30–40% occurring at 15–30 μM for felodipine. At higher drug concentrations, Ca2+-ATPase was inhibited. In the presence of oxalate the maximum activation of the Ca2+ uptake rates at 5–20 μM nisoldipine were 30–50% for cardiac SR and 80–100 μM of the drug were 300–500% for skeletal SR. Felodipine inhibited the rate of Ca2+ uptake by dog cardiac SR, but activated Ca2+ uptake by rabbit skeletal SR with a maximum of 30–50% at 12–25 μM. At higher concentrations of the two drugs the rate of Ca2+ uptake was inhibited. In the absence of oxalate, i.e., limited tranport, nisoldipine shortened the duration of time that Ca2+ was bound to the cardiac and skeletal SR, while the rate of release of Ca2+ from skeletal SR was stimulated. Felodipine at low concentrations similarly caused a premature release of Ca2+ from skeletal SR at a rapid rate; at high concentrations both drugs did not alter Ca2+ binding but delayed Ca2+ release. Unlike nisoldipine and felodipine, verapamil and diltiazem inhibited the rates of Ca2+ transport both in cardiac and skeletal SR. The two drugs inhibited Ca2+-ATPase in cardiac SR but activated the enzyme in skeletal SR. Thus, these drugs caused complex and different effects on cardiac and skeletal SR, possibly resulting from perturbations of the lipid environment of the SR Ca2+-ATPase.

Alteration of acylphosphate formation of cardiac sarcoplasmic reticulum ATPase by calmodulin-dependent phosphorylation.

The calcium-dependent acylphosphate formed by the calcium transport ATPase of cardiac sarcoplasmic reticulum and the calcium-, calmodulin-dependent phosphoester(s) of sarcoplasmic reticulum fractions formed by a calcium-, calmodulin-dependent membrane-bound protein kinase can be distinguished by removal of calcium and/or magnesium by EDTA or hydroxylamine treatment of the acid denaturated membranes. Both procedures decompose the acylphosphate with little effect on the phosphoester(s). Calmodulin-dependent phosphorylation (2.44 nmol/mg SR protein) reduces the apparent K(Ca) of the acylphosphate steady state level of the calcium transport ATPase from 0.56 to 0.34 microM free calcium, without affecting the maximum phosphoenzyme level (0.93 versus 0.89 nmol/mg protein), and has little, if any, effect on the Hill-coefficient (1.32 versus 1.54).

Effects of phospholamban phosphorylation catalyzed by adenosine 3′:5′-monophosphate- and calmodulin-dependent protein kinases on calcium transport ATPase of cardiac sarcoplasmic reticulum

To elucidate the role of 22 000-dalton protein phospholamban, a putative regulator of Ca 2+-dependent ATPase of cardiac sarcoplasmic reticulum, we examined the relationship between cyclic AMP- and calmodulin-dependent phosphorylation of phospholamban and their effects on ATPase activity and calcium transport of cardiac sarcoplasmic reticulum. Cardiac microsomes were incubated with [γ- 32P]ATP or unlabeled ATP, catalytic subunit of cyclic AMP-dependent protein kinase and/or exogenous calmodulin, and subsequently assayed for ATPase activity and calcium uptake by cardiac sarcoplasmic reticulum. Cyclic AMP-dependent phosphorylation of phospholamban was independent of Ca 2+, whereas calmodulin-dependent phosphorylation of phospholamban was dependent on Ca 2+ within a range between 0.2 and 50 μ m. Cyclic AMP- and calmodulin-dependent phosphorylation of phospholamban occurred independently; when both kinases were operative, the amounts of phosphorylation were additive. Under these conditions, the phosphoproteins formed by cyclic AMP- and calmodulin-dependent protein kinases electrophoretically migrated as 11 000-dalton components when sodium dodecyl sulfate-solubilized phosphoproteins were boiled prior to polyacrylamide gel electrophoresis. The ATPase activity was stimulated by either cyclic AMP- or calmodulin-dependent phosphorylation of phospholamban at Ca 2+ concentrations up to 2 μ m. The extents of stimulation of ATPase activity were additive when both types of phosphorylation were functional. Calcium uptake was similarly augmented by cyclic AMP- and/or calmodulin-dependent phosphorylation of phospholamban. These results indicate that Ca 2+-dependent ATPase and calcium transport of cardiac sarcoplasmic reticulum are regulated by phospholamban phosphorylation catalyzed by cyclic AMP- and calmodulin-dependent protein kinases, thus suggesting a dual role of phospholamban in active calcium transport.

Calcium transport by cardiac sarcoplasmic reticulum and phosphorylation of phospholamban.

Active calcium transport by cardiac sarcoplasmic reticulum assumes a central role in the excitation-concentration coupling of the myocardium, in that Ca2+-dependent ATPase (mol.wt. 100 000) of cardiac sarcoplasmic reticulum serves as an energy transducer and a translocator of Ca2+ across the membrane. During the translocation of Ca2+, the ATPase undergoes a complex series of reactions during which the phosphorylated intermediate EP is formed. We documented how the elementary steps of the ATPase reaction are coupled with calcium translocation, and provided evidences to indicate that two key steps of ATPase correspond to the conformational change of the enzyme, and appear to alter the affinity of the enzyme for Ca2+. A line of evidence also indicated that Ca2+-dependent ATPase of cardiac sarcoplasmic reticulum is regulated by a specific protein named phospholamban (mol.wt. 22 000), which serves as a substrate for cyclic AMP-dependent protein kinase. Cyclic AMP-dependent phosphorylation of phospholamban resulted in a marked increase in the rate of turnover of the ATPase, by enhancing the rates of the key elementary steps, i.e. the steps at which the intermediate EP is formed and decomposed. Thus phospholamban is putatively thought to serve as a modulator of Cat2+-dependent ATPase of cardiac sarcoplasmic reticulum. A working model was proposed to interpret the mechanism. Also documented is a possibility that another protein kinase activatable by Ca2+ and calmodulin is functional in regulating the phospholamban-ATPase system, thus suggesting the existence of a dual control system, in which both cyclic AMP- and calmodulin-dependent phosphorylation are in control of the Cat2+-dependent ATPase. Such a control mechanism may provide the interpretation, at the cellular level, that catecholamines exert actions on myocardial contractility. Thus, catecholamine-mediated increases in intracellular cyclic AMP could enhance calcium fluxes across the membrane of sarcoplasmic reticulum, thus resulting in the increased rates of relaxation and, at the same time, the increased rate and extent of contraction. Such a mechanism could also be operational in the tissues, other than the myocardium, in which catecholamines and other hormones serve as the ‘first messenger’, producing intracellular cyclic AMP as the ‘second messenger’.

Effects of cyclic AMP- and calmodulin-dependent phosphorylation of phospholamban on calcium transport ATPase of cardiac sarcoplasmic reticulum

Effects of cyclic AMP- and calmodulin-dependent phosphorylation of phospholamban on calcium transport ATPase of cardiac sarcoplasmic reticulum

Mechanism of the stimulation of calcium ion dependent adenosine triphosphatase of cardiac sarcoplasmic reticulum by adenosine 3',5'-monophosphate dependent protein kinase.

Canine cardiac sarcoplasmic reticulum (SR) is known to be phosphorylated by adenosine 3',5'-monophosphate (cAMP) dependent protein kinase on a 22 000-dalton protein. Phosphorylation enhances the initial rate of Ca2+ uptake and Ca2+-ATPase activity. To determine the molecular mechanism by which phosphorylation regulates the calcium pump in SR, we examined the effect of cAMP-dependent protein kinase on the individual steps of the Ca2+-ATPase reaction sequence. Cardiac sarcoplasmic reticulum was preincubated with cAMP and cAMP-dependent protein kinse in the presence (phosphorylated SR) and absence (control) of adenosine 5'-triphosphate (ATP). Control and phosphorylated SR were subsequently assayed for formation (4-200 ms) and decomposition (0-73 ms) of the acid-stable phosphorylated enzyme (E approximately P) of Ca2+-ATPase in media containing 100 microM [ATP] and various free [Ca2+]. cAMP-dependent phosphorylation of SR resulted in pronounced stimulation of initial rates and levels of E approximately P formed at low free [Ca2+] (less than or equal to 7 microM), but the effect was less at high free Ca2+ (greater than or equal to 10 microM). This stimulation was associated with a decrease in the dissociation constant for Ca2+ binding and a possible increase in Ca2+ sites. The observed rate constant for E approximately P formation of calcium-preincubated SR was not significantly altered by phosphorylation. Phosphorylation also increased the initial rate of E approximately P decomposition. These findings indicate that phosphorylation of cardiac SR by cAMP-dependent protein kinase regulates several steps in the Ca2+-ATPase reaction sequence which result in an overall stimulation of the calcium pump observed at steady state.

Characterization of calmodulin effects on calcium transport in cardiac microsomes enriched in sarcoplasmic reticulum.

Calmodulin prepared from red cell hemolysates was found to significantly increase Ca2+ uptake into cardiac microsomal preparations enriched in sarcoplasic reticulum in a dose-dependent manner. The stimulation of calcium uptake by calmodulin was additive to that stimulation produced by maximal stimulatory concentrations of adenosine cyclic 3',5'-phosphate (cAMP) dependent protein kinase and cAMP, indicating separate mechanisms of action and potentially different modulatory roles for these two systems in the control of calcium transport. K+ significantly decreased calmodulin stimulation of calcium uptake, while in the absence of calmodulin, K+ increased Ca2+ uptake. In the absence of K+, calmodulin increased Ca2+ uptake to levels observed at maximal K+ concentrations without calmodulin present. Na+ produced effects similar to those of K+ in this preparation both in the presence and absence of calmodulin. The effect of calmodulin on the intermediate steps of the (Mg2+,Ca2+)ATPase in cardiac sarcoplasmic reticulum was also investigated. Calmodulin was found to reduce the steady-state level of the Ca2+-dependent phosphoprotein (ECaP) and increase the (Mg2+,Ca2+)ATPase activity of this preparation. Dephosphorylation of ECaP in the presence of Tris-ATP (0.5 mM) was significantly stimulated by calmodulin. These studies indicate that calmodulin stimulates Ca2+ transport in cardiac sarcoplasmic reticulum by increasing the turnover rate of the transport process.

Transient state kinetic studies of Ca 2+-dependent ATPase of cardiac sarcoplasmic reticulum. Effect of cyclic AMP-dependent protein kinase-catalyzed phosphorylation of phospholamban : M. Yamada, M. Tada, F. Ohmori, T. Kuzuya, M. Inui, and H. Abe. First Dept. of Medicine, Osaka Univ. School of Medicine, Osaka, Japan

Transient state kinetic studies of Ca 2+-dependent ATPase of cardiac sarcoplasmic reticulum. Effect of cyclic AMP-dependent protein kinase-catalyzed phosphorylation of phospholamban : M. Yamada, M. Tada, F. Ohmori, T. Kuzuya, M. Inui, and H. Abe. First Dept. of Medicine, Osaka Univ. School of Medicine, Osaka, Japan