Isolation of phospholamban and a second proteolipid component from canine cardiac sarcoplasmic reticulum

Autonomic Regulation of the Goldfish Intact Heart

Autonomic regulation plays a central role in cardiac contractility and excitability in numerous vertebrate species. However, the role of autonomic regulation is less understood in fish physiology. Here, we used Goldfish as a model to explore the role of autonomic regulation. A transmural electrocardiogram recording showed perfusion of the Goldfish heart with isoproterenol increased the spontaneous heart rate, while perfusion with carbamylcholine decreased the spontaneous heart rate. Cardiac action potentials obtained via sharp microelectrodes exhibited the same modifications of the spontaneous heart rate in response to isoproterenol and carbamylcholine. Interestingly, the duration of the cardiac action potentials lengthened in the presence of both isoproterenol and carbamylcholine. To evaluate cardiac contractility, the Goldfish heart was perfused with the Ca²⁺ indicator Rhod-2 and ventricular epicardial Ca²⁺ transients were measured using Pulsed Local Field Fluorescence Microscopy. Following isoproterenol perfusion, the amplitude of the Ca²⁺ transient significantly increased, the half duration of the Ca²⁺ transient shortened, and there was an observable increase in the velocity of the rise time and fall time of the Ca²⁺ transient, all of which are compatible with the shortening of the action potential induced by isoproterenol perfusion. On the other hand, carbamylcholine perfusion significantly reduced the amplitude of the Ca²⁺ transient and increased the half duration of the Ca²⁺ transient. These results are interesting because the effect of carbamylcholine is opposite to what happens in classically used models, such as mouse hearts, and the autonomic regulation of the Goldfish heart is strikingly similar to what has been observed in larger mammalian models resembling humans.

Transmural Autonomic Regulation of Cardiac Contractility at the Intact Heart Level

The relationship between cardiac excitability and contractility depends on when Ca²⁺ influx occurs during the ventricular action potential (AP). In mammals, it is accepted that Ca²⁺ influx through the L-type Ca²⁺ channels occurs during AP phase 2. However, in murine models, experimental evidence shows Ca²⁺ influx takes place during phase 1. Interestingly, Ca²⁺ influx that activates contraction is highly regulated by the autonomic nervous system. Indeed, autonomic regulation exerts multiple effects on Ca²⁺ handling and cardiac electrophysiology. In this paper, we explore autonomic regulation in endocardial and epicardial layers of intact beating mice hearts to evaluate their role on cardiac excitability and contractility. We hypothesize that in mouse cardiac ventricles the influx of Ca²⁺ that triggers excitation–contraction coupling (ECC) does not occur during phase 2. Using pulsed local field fluorescence microscopy and loose patch photolysis, we show sympathetic stimulation by isoproterenol increased the amplitude of Ca²⁺ transients in both layers. This increase in contractility was driven by an increase in amplitude and duration of the L-type Ca²⁺ current during phase 1. Interestingly, the β-adrenergic increase of Ca²⁺ influx slowed the repolarization of phase 1, suggesting a competition between Ca²⁺ and K⁺ currents during this phase. In addition, cAMP activated L-type Ca²⁺ currents before SR Ca²⁺ release activated the Na⁺-Ca²⁺ exchanger currents, indicating Ca_v1.2 channels are the initial target of PKA phosphorylation. In contrast, parasympathetic stimulation by carbachol did not have a substantial effect on amplitude and kinetics of endocardial and epicardial Ca²⁺ transients. However, carbachol transiently decreased the duration of the AP late phase 2 repolarization. The carbachol-induced shortening of phase 2 did not have a considerable effect on ventricular pressure and systolic Ca²⁺ dynamics. Interestingly, blockade of muscarinic receptors by atropine prolonged the duration of phase 2 indicating that, in isolated hearts, there is an intrinsic release of acetylcholine. In addition, the acceleration of repolarization induced by carbachol was blocked by the acetylcholine-mediated K⁺ current inhibition. Our results reveal the transmural ramifications of autonomic regulation in intact mice hearts and support our hypothesis that Ca²⁺ influx that triggers ECC occurs in AP phase 1 and not in phase 2.

How Ca2+ influx is attenuated in the heart during a "fight or flight" response

Bazmi and Escobar highlight a recent investigation of the mechanisms that regulate Ca²⁺ influx during sympathetic stimulation.

Phospholamban and its phosphorylated form interact differently with lipid bilayers: a 31P, 2H, and 13C solid-state NMR spectroscopic study

Phospholamban (PLB) is a 52-amino acid integral membrane protein that helps to regulate the flow of Ca(2+) ions in cardiac muscle cells. Recent structural studies on the PLB pentamer and the functionally active monomer (AFA-PLB) debate whether its cytoplasmic domain, in either the phosphorylated or dephosphorylated states, is alpha-helical in structure as well as whether it associates with the lipid head groups (Oxenoid, K. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 10870-10875; Karim, C. B. (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 14437-14442; Andronesi, C.A. (2005) J. Am. Chem. Soc. 127, 12965-12974; Li, J. (2003) Biochemistry 42, 10674-10682; Metcalfe, E. E. (2005) Biochemistry 44, 4386-4396: Clayton, J. C. (2005) Biochemistry 44, 17016-17026). Comparing the secondary structure of the PLB pentamer and its phosphorylated form (P-PLB) as well as their interaction with the lipid bilayer is crucial in order to understand its regulatory function. Therefore, in this study, the full-length wild-type (WT) PLB and P-PLB were incorporated into 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC) phospholipid bilayers and studied utilizing solid-state NMR spectroscopy. The analysis of the (2)H and (31)P solid-state NMR data of PLB and P-PLB in POPC multilamellar vesicles (MLVs) indicates that a direct interaction takes place between both proteins and the phospholipid head groups. However, the interaction of P-PLB with POPC bilayers was less significant compared that with PLB. Moreover, the secondary structure using (13)C=O site-specific isotopically labeled Ala15-PLB and Ala15-P-PLB in POPC bilayers suggests that this residue, located in the cytoplasmic domain, is a part of an alpha-helical structure for both PLB and P-PLB.

Ca(2+) signaling in cardiac myocytes overexpressing the alpha(1) subunit of L-type Ca(2+) channel

Voltage-gated L-type Ca(2+) channels (LCCs) provide Ca(2+) ingress into cardiac myocytes and play a key role in intracellular Ca(2+) homeostasis and excitation-contraction coupling. We investigated the effects of a constitutive increase of LCC density on Ca(2+) signaling in ventricular myocytes from 4-month-old transgenic (Tg) mice overexpressing the alpha(1) subunit of LCC in the heart. At this age, cells were somewhat hypertrophic as reflected by a 20% increase in cell capacitance relative to those from nontransgenic (Ntg) littermates. Whole cell I(Ca) density in Tg myocytes was elevated by 48% at 0 mV compared with the Ntg group. Single-channel analysis detected an increase in LCC density with similar conductance and gating properties. Although the overexpressed LCCs triggered an augmented SR Ca(2+) release, the "gain" function of EC coupling was uncompromised, and SR Ca(2+) content, diastolic cytosolic Ca(2+), and unitary properties of Ca(2+) sparks were unchanged. Importantly, the enhanced I(Ca) entry and SR Ca(2+) release were associated with an upregulation of the Na(+)-Ca(2+) exchange activity (indexed by the half decay time of caffeine-elicited Ca(2+) transient) by 27% and SR Ca(2+) recycling by approximately 35%. Western analysis detected a 53% increase in the Na(+)-Ca(2+) exchanger expression but no change in the abundance of ryanodine receptor (RyR), SERCA2, and phospholamban. Analysis of I(Ca) kinetics suggested that SR Ca(2+) release-dependent inactivation of LCCs remains intact in Tg cells. Thus, in spite of the modest cardiac hypertrophy, the overexpressed LCCs form functional coupling with RyRs, preserving both orthograde and retrograde Ca(2+) signaling between LCCs and RyRs. These results also suggest that a modest but sustained increase in Ca(2+) influx triggers a coordinated remodeling of Ca(2+) handling to maintain Ca(2+) homeostasis.

Phospholamban: protein structure, mechanism of action, and role in cardiac function

A comprehensive discussion is presented of advances in understanding the structure and function of phospholamban (PLB), the principal regulator of the Ca2+-ATPase of cardiac sarcoplasmic reticulum. Extensive historical studies are reviewed to provide perspective on recent developments. Phospholamban gene structure, expression, and regulation are presented in addition to in vitro and in vivo studies of PLB protein structure and activity. Applications of breakthrough experimental technologies in identifying PLB structure-function relationships and in defining its interaction with the Ca2+-ATPase are also highlighted. The current leading viewpoint of PLB's mechanism of action emerges from a critical examination of alternative hypotheses and the most recent experimental evidence. The potential physiological relevance of PLB function in human heart failure is also covered. The interest in PLB across diverse biochemical disciplines portends its continued intense scrutiny and its potential exploitation as a therapeutic target.

Subunit requirements for expression of functional sodium pumps in yeast cells

Na+/K(+)-ATPase from animal cell membranes is known to consist of an alpha-subunit and a beta-subunit. Amino acids within the alpha-subunit have been shown to participate in the catalytic functions of the enzyme and in the binding of cardioactive steroids. Although the function of the beta-subunit is not known, expression of both alpha- and beta-subunits is required for the functional enzyme. A putative third subunit, the gamma-subunit, has been suggested to be a part of the functional Na+/K(+)-ATPase complex, based on experiments showing that both the catalytic alpha-subunit and a small peptide of M(r) = 11,000 can be labeled by a photoreactive ouabain analog. Although the primary structure for the putative gamma-subunit from rat and sheep was recently deduced from cDNA clones, participation of this small protein in the catalytic activity of the Na+/K(+)-ATPase has not been demonstrated. In experiments described here, the heterologous expression of Na+/K(+)-ATPase in yeast cells was used to investigate whether the gamma-subunit is an essential component of the Na+/K(+)-ATPase. Yeast cells do not contain an endogenous Na+/K(+)-ATPase. The alpha- and beta-subunits or the alpha-, beta- and the putative gamma-subunits of Na+/K(+)-ATPase were expressed in the yeast Saccharomyces cerevisiae and ouabain-sensitive ATPase, p-nitrophenylphosphatase, and 86Rb uptake activities were measured either in membranes prepared from transformed yeast cells, or in intact yeast cells. Nontransformed yeast cells or yeast cells transformed with the gamma-subunit alone served as controls. Northern analysis and Western blots demonstrated that yeast cells do not contain an endogenous peptide with significant sequence homology to the putative gamma-subunit. Yeast samples containing only Na+/K(+)-ATPase alpha and beta subunits were capable of ouabain-inhibitable enzymatic activity and 86Rb transport. No gamma-subunit-dependent differences in the measured enzymatic activities or transport properties were detected in the different samples. These observations establish that the alpha beta-subunit complex is the minimum structural unit required for all the ouabain-sensitive reactions of Na+/K(+)-ATPase.

Regulation of cardiac sarcoplasmic reticulum function by phospholamban

Calcium fluxes across the sarcoplasmic reticulum membrane are regulated by phosphorylation of a 27,000-dalton membrane-bound protein termed phospholamban. Phospholamban is phosphorylated by three different protein kinases (cAMP-dependent, Ca2+.CAM-dependent and Ca2+.phospholipid dependent) at apparently distinct sites. Phosphorylation by each of the protein kinases increases the rates of active calcium transport by sarcoplasmic reticulum vesicles. The stimulatory effects of protein kinases on the calcium pump may be reversed by an endogenous protein phosphatase activity. The phosphoprotein phosphatase can dephosphorylate both the cAMP-dependent and the Ca2+.CAM-dependent sites of phospholamban. Phosphorylation of phospholamban also occurs in situ, in perfused beating hearts, during the peak of the inotropic response to beta-adrenergic stimulation. Reversal of the stimulatory effects is associated with dephosphorylation of phospholamban. Thus, in vivo and in vitro studies suggest that phospholamban is a regulator for the calcium pump in cardiac sarcoplasmic reticulum. The degree of phospholamban phosphorylation determined by the interaction of specific protein kinases and phosphatases may represent an important control for sarcoplasmic reticulum function and, thus, for the contraction-relaxation cycle in the myocardium. In this review, we summarize recent evidence on physical and structural properties of phospholamban, the proposed structural molecular models for this protein, and the significance of its regulatory role both in vitro and in situ.

Ca2+-flux modulation by calciductin phosphorylation in cardiac sarcolemma

The epinephrine-induced inotropic effect of the myocardium can be attributed to phosphorylation of the sarcolemmal protein calciductin, as this event is accompanied by a 3.5-fold increase in ATP-independent, voltage-dependent CA2+ uptake by isolated sarcolemmal vesicles. This can be considered as the in vitro equivalent of the Ca2+ slow channel. Ca2+ uptake under these conditions is linear, with the degree of calciductin phosphorylation and inhibitor studies indicate the properties of the unphosphorylated channels are similar to those of the fully activated state. Calciductin has been purified and shows great similarities to phospholamban, a protein modulator of the sarcoplasmic reticulum Ca2+ pump. This raises the interesting possibility that calciductin and phospholamban are identical, although they serve different purposes in the sarcolemma and sarcoplasmic reticulum membranes.

Isolation of cardiac membrane proteolipids by high pressure liquid chromatography. A comparison of reticular and sarcolemmal proteolipids, phospholamban and calciductin

Membrane-bound phosphorylatable proteolipids were reported to play a role in the regulation of transmembrane Ca2+ fluxes by catecholamines. A generally applicable purification procedure is described by which such proteolipids as the cardiac sarcoplasmic reticulum phospholamban is purified by solvent extraction followed by high pressure liquid chromatography on microparticulate silica. Phospholamban is thereby purified with a yield of 3.37 mg from 100 mg of sarcoplasmic reticulum proteins, significantly higher than that obtained by any of the previously reported procedures. It appeared homogeneous upon dodecyl sulfate-polyacrylamide gel electrophoresis where it is stained by Coomassie blue and detected by autoradiography. The same procedure is applicable to cardiac sarcolemmal calciductin. Both proteolipids exhibit the same Mr 11 000 and pI 3.7 upon two-dimensional gel electrophoresis. Their amino acid compositions are very similar if not identical. This raises the intriguing possibility that phospholamban and calciductin are identical though they obviously belong to different membranes.

Chick heart plasma membranes. Isolation and analysis of autophosphorylation

Plasma membranes have been isolated from hearts of 10-day embryonic and newborn chicks. The membranes obtained were highly enriched in muscarinic acetylcholine receptors, K+ -stimulated, ouabain-sensitive p-nitrophenylphosphatase and 5'-nucleotidase. There was little contamination of the membrane fractions by the mitochondrial membranes or by contractile proteins. The autophosphorylation of the isolated membrane fractions was analyzed by measuring 32P incorporation from [gamma-32P]ATP into total membrane protein and into individual membrane components. Membranes obtained from embryonic hearts contained significantly more cAMP-dependent and -independent protein kinase activities than membranes from newborn chick hearts. Treatment of the membranes with Triton X-100 or the peptide ionophore alamethicin increased phosphorylation in membranes from either newborn or embryonic hearts. Membranes from embryonic hearts contained substrates for membrane-bound cAMP-dependent and -independent protein kinases either not observed or present in low amount in membranes from newborn hearts, and vice-versa. Notably, a 38 kDa protein was markedly phosphorylated by endogenous cAMP dependent protein kinase in plasma membrane enriched fractions from embryonic hearts. This phosphoprotein was not easily detected in any fraction obtained from newborn hearts. One cAMP-dependent phosphoprotein had an Mr of 27000 or 11000, depending on the conditions used to solubilize it. This protein was present in sarcolemma-enriched membranes as well as membrane fractions containing sarcoplasmic reticulum. There was more of this phosphoprotein in newborn heart membranes than in embryonic hearts. The phosphorylation of this protein was markedly enhanced by the peptide ionophore alamethicin. A second cAMP-dependent phosphoprotein with an Mr of 27000 was also detected in the sarcolemma-enriched membranes.

Purification and characterization of an (Na+ + K+)-ATPase proteolipid labeled with a photoaffinity derivative of ouabain

Highly purified lamb kidney (Na+ + K+)-ATPase was photoaffinity labeled with the tritiated 2-nitro-5-azidobenzoyl derivative of ouabain (NAB-ouabain). The labeled (Na+ + K+)-ATPase was mixed with unlabeled carrier enzyme. Two proteolipid (gamma 1 and gamma 2) fractions were then isolated by chromatography on columns of Sepharose CL-6B and Sephadex LH-60. The two fractions were interchangeable when rechromatographed on the LH-60 column, suggesting that gamma 1 is an aggregated form of gamma 2. The total yield was 0.8-1.5 mol of gamma component per mol of catalytic subunit recovered. This indicates that the gamma component is present in stoichiometric amounts in the Na+ + K+)-ATPase. The proteolipids that were labeled with NAB-ouabain copurified with the unlabeled proteolipids.

Isolation of two proteolipids from rabbit skeletal muscle sarcoplasmic reticulum

We have isolated two proteolipids from rabbit skeletal muscle sarcoplasmic reticulum by chromatography on columns of Sepharose CL-6B and Sephadex LH-60. One, PL-II, is identical to the proteolipid previously obtained by others using organic solvent extraction. The other, PL-I, has an amino acid composition very similar to those of proteolipids we previously isolated from canine cardiac SR and lamb kidney (Na,K)-ATPase.

Studies on phosphorylation of canine cardiac sarcoplasmic reticulum by calmodulin-dependent protein kinase

Two endogenous protein kinase activities, cAMP-dependent and calmodulin-Ca2+-dependent, are associated with isolated cardiac sarcoplasmic reticulum (SR) vesicles. Both kinases phosphorylate an endogenous substrate of approximately 22,000 daltons (phospholamban). The phosphorylation of phospholamban by either the intrinsic or by exogenous cAMP-dependent protein kinase is found to be Ca2+-independent between 0.05 and 100 microM free Ca2+. Calmodulin-dependent phosphorylation, on the other hand, does not require cAMP and is absolutely dependent on the presence of free Ca2+ over a concentration range that corresponds to physiological levels (10(-7) to 10(-5) M). Phosphorylation of SR vesicles by both kinases is additive and the extent of saturation of the cAMP-specific sites has no effect on the degree of stimulation by calmodulin or its Ca2+-dependence. Trifluoperazine, an inhibitor of calmodulin, inhibits calmodulin-dependent phosphorylation without affecting cAMP-dependent phosphorylation, indicating the presence of two types of kinases. This is made further evident by the selectivity of each kinase for exogenous substrates. Whereas cAMP-dependent protein kinase appears to phosphorylate histone ILA (a basic protein) preferentially, calmodulin-dependent protein kinase prefers phosvitin (an acidic protein).