Alpha-adrenergic stimulation of sarcolemmal protein phosphorylation and slow responses in intact myocardium

Hormonal regulation of Na+-K+-ATPase from the evolutionary perspective

Na⁺-K⁺-ATPase, an α/β heterodimer, is an ancient enzyme that maintains Na⁺ and K⁺ gradients, thus preserving cellular ion homeostasis. In multicellular organisms, this basic housekeeping function is integrated to fulfill the needs of specialized organs and preserve whole-body homeostasis. In vertebrates, Na⁺-K⁺-ATPase is essential for many fundamental physiological processes, such as nerve conduction, muscle contraction, nutrient absorption, and urine excretion. During vertebrate evolution, three key developments contributed to diversification and integration of Na⁺-K⁺-ATPase functions. Generation of novel α- and β-subunits led to formation of multiple Na⁺-K⁺-ATPase isoenyzmes with distinct functional characteristics. Development of a complex endocrine system enabled efficient coordination of diverse Na⁺-K⁺-ATPase functions. Emergence of FXYDs, small transmembrane proteins that regulate Na⁺-K⁺-ATPase, opened new ways to modulate its function. FXYDs are a vertebrate innovation and an important site of hormonal action, suggesting they played an especially prominent role in evolving interaction between Na⁺-K⁺-ATPase and the endocrine system in vertebrates.

Phospholemman is not required for the acute stimulation of Na⁺-K⁺-ATPase α₂-activity during skeletal muscle fatigue

The Na(+)-K(+)-ATPase α2-isoform in skeletal muscle is rapidly stimulated during muscle use and plays a critical role in fatigue resistance. The acute mechanisms that stimulate α2-activity are not completely known. This study examines whether phosphorylation of phospholemman (PLM/FXYD1), a regulatory subunit of Na(+)-K(+)-ATPase, plays a role in the acute stimulation of α2 in working muscles. Mice lacking PLM (PLM KO) have a normal content of the α2-subunit and show normal exercise capacity, in contrast to the greatly reduced exercise capacity of mice that lack α2 in the skeletal muscles. Nerve-evoked contractions in vivo did not induce a change in total PLM or PLM phosphorylated at Ser63 or Ser68, in either WT or PLM KO. Isolated muscles of PLM KO mice maintain contraction and resist fatigue as well as wild type (WT). Rb(+) transport by the α2-Na(+)-K(+)-ATPase is stimulated to the same extent in contracting WT and contracting PLM KO muscles. Phosphorylation of sarcolemmal membranes prepared from WT but not PLM KO skeletal muscles stimulates the activity of both α1 and α2 in a PLM-dependent manner. The stimulation occurs by an increase in Na(+) affinity without significant change in Vmax and is more effective for α1 than α2. These results demonstrate that phosphorylation of PLM is capable of stimulating the activity of both isozymes in skeletal muscle; however, contractile activity alone is not sufficient to induce PLM phosphorylation. Importantly, acute stimulation of α2, sufficient to support exercise and oppose fatigue, does not require PLM or its phosphorylation.

Downregulation of miR-151-5p contributes to increased susceptibility to arrhythmogenesis during myocardial infarction with estrogen deprivation

Estrogen deficiency is associated with increased incidence of cardiovascular diseases. But merely estrogen supplementary treatment can induce many severe complications such as breast cancer. The present study was designed to elucidate molecular mechanisms underlying increased susceptibility of arrhythmogenesis during myocardial infarction with estrogen deprivation, which provides us a new target to cure cardiac disease accompanied with estrogen deprivation. We successfully established a rat model of myocardial ischemia (MI) accompanied with estrogen deprivation by coronary artery ligation and ovariectomy (OVX). Vulnerability and mortality of ventricular arrhythmias increased in estrogen deficiency rats compared to non estrogen deficiency rats when suffered MI, which was associated with down-regulation of microRNA-151-5p (miR-151-5p). Luciferase Reporter Assay demonstrated that miR-151-5p can bind to the 3′-UTR of FXYD1 (coding gene of phospholemman, PLM) and inhibit its expression. We found that the expression of PLM was increased in (OVX+MI) group compared with MI group. More changes such as down-regulation of Kir2.1/I_K1, calcium overload had emerged in (OVX+MI) group compared to MI group merely. Transfection of miR-151-5p into primary cultured myocytes decreased PLM levels and [Ca²⁺]_i, however, increased Kir2.1 levels. These effects were abolished by the antisense oligonucleotides against miR-151-5p. Co-immunoprecipitation and immunofluorescent experiments confirmed the co-localization between Kir2.1 and PLM in rat ventricular tissue. We conclude that the increased ventricular arrhythmias vulnerability in response to acute myocardial ischemia in rat is critically dependent upon down-regulation of miR-151-5p. These findings support the proposal that miR-151-5p could be a potential therapeutic target for the prevention of ischemic arrhythmias in the subjects with estrogen deficiency.

Coordinated regulation of cardiac Na(+)/Ca (2+) exchanger and Na (+)-K (+)-ATPase by phospholemman (FXYD1)

Phospholemman (PLM) is the founding member of the FXYD family of regulators of ion transport. PLM is a 72-amino acid protein consisting of the signature PFXYD motif in the extracellular N terminus, a single transmembrane (TM) domain, and a C-terminal cytoplasmic tail containing three phosphorylation sites. In the heart, PLM co-localizes and co-immunoprecipitates with Na(+)-K(+)-ATPase, Na(+)/Ca(2+) exchanger, and L-type Ca(2+) channel. The TM domain of PLM interacts with TM9 of the α-subunit of Na(+)-K(+)-ATPase, while its cytoplasmic tail interacts with two small regions (spanning residues 248-252 and 300-304) of the proximal intracellular loop of Na(+)/Ca(2+) exchanger. Under stress, catecholamine stimulation phosphorylates PLM at serine(68), resulting in relief of inhibition of Na(+)-K(+)-ATPase by decreasing K(m) for Na(+) and increasing V(max), and simultaneous inhibition of Na(+)/Ca(2+) exchanger. Enhanced Na(+)-K(+)-ATPase activity lowers intracellular Na(+), thereby minimizing Ca(2+) overload and risks of arrhythmias. Inhibition of Na(+)/Ca(2+) exchanger reduces Ca(2+) efflux, thereby preserving contractility. Thus, the coordinated actions of PLM during stress serve to minimize arrhythmogenesis and maintain inotropy. In acute cardiac ischemia and chronic heart failure, either expression or phosphorylation of PLM or both are altered. PLM regulates important ion transporters in the heart and offers a tempting target for development of drugs to treat heart failure.

Role of conformational sampling of Ser16 and Thr17-phosphorylated phospholamban in interactions with SERCA

Phosphorylation of phospholamban (PLB) at Ser16 and/ or Thr17 is believed to release its inhibitory effect on sarcoplasmic reticulum calcium ATPase. Ser16 phosphorylation of PLB has been suggested to cause a conformational change that alters the interaction between the enzyme and protein. Using computer simulations, the conformational sampling of Ser16 phosphorylated PLB in implicit membrane environment is compared here with the unphosphorylated PLB system to investigate these conformational changes. The results suggest that conformational changes in the cytoplasmic domain of PLB upon phosphorylation at Ser16 increase the likelihood of unfavorable interactions with SERCA in the E2 state prompting a conformational switch of SERCA from E2 to E1. Phosphorylation of PLB at Thr17 on the other hand does not appear to affect interactions with SERCA significantly suggesting that the mechanism of releasing the inhibitory effect is different between Thr17 phosphorylated and Ser16 phosphorylated PLB.

Phospholemman: a novel cardiac stress protein

Phospholemman (PLM), a member of the FXYD family of regulators of ion transport, is a major sarcolemmal substrate for protein kinases A and C in cardiac and skeletal muscle. In the heart, PLM co-localizes and co-immunoprecipitates with Na(+)-K(+)-ATPase, Na(+)/Ca(2+) exchanger, and L-type Ca(2+) channel. Functionally, when phosphorylated at serine(68), PLM stimulates Na(+)-K(+)-ATPase but inhibits Na(+)/Ca(2+) exchanger in cardiac myocytes. In heterologous expression systems, PLM modulates the gating of cardiac L-type Ca(2+) channel. Therefore, PLM occupies a key modulatory role in intracellular Na(+) and Ca(2+) homeostasis and is intimately involved in regulation of excitation-contraction (EC) coupling. Genetic ablation of PLM results in a slight increase in baseline cardiac contractility and prolongation of action potential duration. When hearts are subjected to catecholamine stress, PLM minimizes the risks of arrhythmogenesis by reducing Na(+) overload and simultaneously preserves inotropy by inhibiting Na(+)/Ca(2+) exchanger. In heart failure, both expression and phosphorylation state of PLM are altered and may partly account for abnormalities in EC coupling. The unique role of PLM in regulation of Na(+)-K(+)-ATPase, Na(+)/Ca(2+) exchanger, and potentially L-type Ca(2+) channel in the heart, together with the changes in its expression and phosphorylation in heart failure, make PLM a rational and novel target for development of drugs in our armamentarium against heart failure. Clin Trans Sci 2010; Volume 3: 189-196.

Role of phospholemman phosphorylation sites in mediating kinase-dependent regulation of the Na+-K+-ATPase

Phospholemman (PLM) is a major target for phosphorylation mediated by both PKA (at Ser68) and PKC (at both Ser63 and Ser68) in the heart. In intact cardiac myocytes, PLM associates with and inhibits Na(+)-K(+)-ATPase (NKA), mainly by reducing its affinity for internal Na(+). The inhibition is relieved upon PLM phosphorylation by PKA or PKC. The aim here was to distinguish the role of the Ser63 and Ser68 PLM phosphorylation sites in mediating kinase-induced modulation of NKA function. We expressed wild-type (WT) PLM and S63A, S68A, and AA (Ser63 and Ser68 to alanine double mutant) PLM mutants in HeLa cells that stably express rat NKA-α(1) and we measured the effect of PKA and PKC activation on NKA-mediated intracellular Na(+) concentration decline. PLM expression (WT or mutant) significantly decreased the apparent NKA affinity for internal Na(+) and had no significant effect on the maximum pump rate (V(max)). PKA activation with forskolin (20 μM) restored NKA Na(+) affinity in cells expressing WT but not AA PLM and did not affect V(max) in either case. Similarly, PKC activation with 300 nM phorbol 12,13-dibutyrate increased NKA Na(+) affinity in cells expressing WT, S63A, and S68A PLM and had no effect in cells expressing AA PLM. Neither forskolin nor phorbol 12,13-dibutyrate affected NKA function in the absence of PLM. We conclude that PLM phosphorylation at either Ser63 or Ser68 is both necessary and sufficient for completely relieving the PLM-induced NKA inhibition.

AFos inhibits phenylephrine-mediated contractile dysfunction by altering phospholamban phosphorylation

Using neonatal rat ventricular myocytes, we previously reported that the expression of a dominant negative form of the c-Fos proto-oncogene (AFos) inhibited activator protein 1 activity and blocked the induction of the pathological gene profile stimulated by phenylephrine (PE) while leaving growth unaffected. We now extend these observations to the adult rat ventricular myocyte (ARVM) to understand the relationship between gene expression, growth, and function. Ventricular myocytes were isolated from adult rats and infected with adenovirus expressing beta-galactosidase (control) or AFos. The cells were subsequently treated with PE, and protein synthesis, gene program, calcium transients, and contractility were evaluated. As seen with the neonatal rat ventricular myocytes, in control cells PE stimulated an increase in protein synthesis, induced the pathological gene profile, and exhibited both depressed contractility and calcium transients. Although ARVMs expressing AFos still had PE-induced growth, pathological gene expression as well as contractility and calcium handling abnormalities were inhibited. To determine a possible mechanism of the preserved myocyte function in AFos-expressing cells, we examined phospholamban (PLB) and sarco(endo)plasmic reticulum calcium-ATPase proteins. Although there was no change in total PLB or sarco(endo)plasmic reticulum calcium-ATPase expression in response to PE treatment, PE decreased the phosphorylation of PLB at serine-16, an observation that was prevented in AFos-expressing cells. In conclusion, although PE-induced growth was unaffected in AFos-expressing ARVMs, the expression of the pathological gene profile was inhibited and both contractile function and calcium cycling were preserved. The inhibition of functional deterioration was, in part, due to the preservation of PLB phosphorylation.

Agonistic antibody to the alpha1-adrenergic receptor mobilizes intracellular calcium and induces phosphorylation of a cardiac 15-kDa protein

Hypertension is a major cause for hypertrophic remodelling of the myocardium. Agonistic autoantibodies to extracellular loops of the alpha(1)-adrenergic receptor (alpha(1)-AR) have been identified in patients with arterial hypertension. However, intracellular reactions elicited by these agonistic antibodies remain elusive. An anti-peptide antibody (anti-alpha(1)) was generated against the second extracellular loop of the alpha(1)-AR that bound to its peptide epitope with high affinity (K (D) approximately 50 nM). We studied anti-alpha(1) effects on intracellular calcium (Ca(i)), a key factor in cellular remodelling, and receptor-mediated cardiac protein phosphorylation. Anti-alpha(1) induced pronounced but transient increases in Ca(i) in CHO cells expressing the human alpha(1)-AR (CHO-alpha(1)) and in neonatal cardiomyocytes. Preincubation experiments failed to demonstrate a tonic effect of anti-alpha(1) on Ca(i). However, preincubation with the antibody attenuated the effect of the alpha(1)-AR antagonist prazosin. In neonatal cardiomyocytes anti-alpha(1) induced a robust phosphorylation of a 15-kDa protein that is involved in alpha(1)-AR signalling. Our data support the notion that elevation of Ca(i) is a general feature of agonistic antibodies' action and constitute an important pathogenic component of hypertension-associated autoantibodies. Furthermore, we suggest that agonistic antibodies to the alpha(1)-AR contribute to hypertrophic remodelling of cardiac myocytes, and that the cardiac 15-kDa protein is a relevant downstream target of their action.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Regulation of Ca2+ and Na+ in normal and failing cardiac myocytes

Ca(2+) in cardiac myocytes regulates contractility and relaxation, and Ca(2+) and Na (+)regulation are linked via Na(+)/Ca(2+) exchange (NCX). Heart failure (HF) is accompanied by contractile dysfunction and arrhythmias, both of which may be due to altered cellular Ca(2+) handling. Smaller Ca(2+) transient and sarcoplasmic reticulum (SR) Ca(2+) content cause systolic dysfunction in HF. The reduced SR Ca(2+) content is due to: (a) reduced SR Ca(2+)-ATPase function (which also contributes to diastolic dysfunction), (b) increased expression and function of NCX (which competes with SR Ca(2+)-ATPase during relaxation, but preserves diastolic function), and (c) enhanced diastolic SR Ca(2+) leak. Relative contributions of these may vary with HF etiology and stage. Triggered arrhythmias (e.g., delayed afterdepolarizations [DADs]) are prominent in HF. DADs are due to spontaneous SR Ca(2+) release and consequent activation of transient inward NCX current, which in HF allows DADs to more readily trigger arrhythmogenic action potentials. Thus NCX and Na(+) are critical in systolic and diastolic function and arrhythmias. [Na(+)](i) is elevated in HF, which may limit SR unloading and provide some Ca(2+) influx during the HF action potential, thus limiting the depression of systolic function. High [Na(+)](i) in HF is due to enhanced Na(+) influx. Cellular Na(+)/K(+)-ATPase (NKA) function appears unaltered, despite reduced NKA expression. This dichotomy led us to test NKA regulation by phospholemman (PLM). We find that PLM regulates NKA in a manner analogous to phospholamban regulation of SR Ca(2+)-ATPase (i.e., inhibition that is relieved by PLM phosphorylation). We measured intermolecular FRET between PLM and NKA, which is reduced upon PLM phosphorylation. The lower expression level of more phosphorylated PLM in HF may explain the above dichotomy. Thus, altered Ca(2+) and Na(+) handling contributes to altered contractile function and arrhythmogenesis in HF.

Phospholemman phosphorylation alters its fluorescence resonance energy transfer with the Na/K-ATPase pump

Phospholemman (PLM) or FXYD1 is a major cardiac myocyte phosphorylation target upon adrenergic stimulation. Prior immunoprecipitation and functional studies suggest that phospholemman associates with the Na/K-pump (NKA) and mediates adrenergic Na/K-pump regulation. Here, we tested whether the NKA-PLM interaction is close enough to allow fluorescence resonance energy transfer (FRET) between cyan and yellow fluorescent (CFP/YFP) fusion proteins of Na/K pump and phospholemman and whether phospholemman phosphorylation alters such FRET. Co-expressed NKA-CFP and PLM-YFP in HEK293 cells co-localized in the plasma membrane and exhibited robust FRET. Selective acceptor photobleach increased donor fluorescence (F(CFP)) by 21.5 +/- 4.1% (n = 13), an effect nearly abolished when co-expressing excess phospholemman lacking YFP. Activation of protein kinase C or A progressively and reversibly decreased FRET assessed by either the fluorescence ratio (F(YFP)/F(CFP)) or the enhancement of donor fluorescence after acceptor bleach. After protein kinase C activation, forskolin did not further reduce FRET, but after forskolin pretreatment, protein kinase C could still reduce FRET. This agreed with phospholemman phosphorylation measurements: by protein kinase C at both Ser-63 and Ser-68, but by protein kinase A only at Ser-68. Expression of PLM-YFP and PLM-CFP resulted in even stronger FRET than for NKA-PLM (F(CFP) increased by 37 +/- 1% upon YFP photobleach), and this FRET was enhanced by phospholemman phosphorylation, consistent with phospholemman multimerization. Co-expressed PLM-CFP and Na/Ca exchange-YFP were highly membrane co-localized, but FRET was undetectable. We conclude that phospholemman and Na/K-pump are in very close proximity (FRET occurs) and that phospholemman phosphorylation alters the interaction of Na/K-pump and phospholemman.

Cytoplasmic tail of phospholemman interacts with the intracellular loop of the cardiac Na+/Ca2+ exchanger

Phospholemman (PLM), a member of the FXYD family of small ion transport regulators, inhibits cardiac Na+/Ca2+ exchanger (NCX1). NCX1 is made up of N-terminal domain consisting of the first five transmembrane segments (residues 1-217), a large intracellular loop (residues 218-764), and a C-terminal domain comprising the last four transmembrane segments (residues 765-938). Using glutathione S-transferase (GST) pull-down assay, we demonstrated that the intracellular loop, but not the N- or C-terminal transmembrane domains of NCX1, was associated with PLM. Further analysis using protein constructs of GST fused to various segments of the intracellular loop of NCX1 suggest that PLM bound to residues 218-371 and 508-764 but not 371-508. Split Na+/Ca2+ exchangers consisting of N- or C-terminal domains with different lengths of the intracellular loop were co-expressed with PLM in HEK293 cells that are devoid of endogenous PLM and NCX1. Although expression of N-terminal but not C-terminal domain alone resulted in correct membrane targeting, co-expression of both N- and C-terminal domains was required for correct membrane targeting and functional exchange activity. NCX1 current measurements indicate that PLM decreased NCX1 current only when the split exchangers contained residues 218-358 of the intracellular loop. Co-immunoprecipitation experiments with PLM and split exchangers suggest that PLM associated with the N-terminal domain of NCX1 when it contained intracellular loop residues 218-358. TM43, a PLM mutant with its cytoplasmic tail truncated, did not co-immunoprecipitate with wild-type NCX1 when co-expressed in HEK293 cells, confirming little to no interaction between the transmembrane domains of PLM and NCX1. We conclude that PLM interacted with the intracellular loop of NCX1, most likely at residues 218-358.

Cardiac myocytes Ca2+ and Na+ regulation in normal and failing hearts

Ca(2+) is a central player in the excitation-contraction coupling of cardiac myocytes, the process that enables the heart to contract and relax. Mishandling of Ca(2+) is a central cause of both contractile dysfunction and arrhythmias in pathophysiological conditions such as heart failure (HF). Upon electrical excitation, Ca(2+) enters the myocytes via voltage-gated Ca(2+) channels and induces further Ca(2+) release from the sarcoplasmic reticulum (SR). This raises the free intracellular Ca(2+) concentration ([Ca(2+)](i)), activating contraction. Relaxation is driven by [Ca(2+)](i) decline, mainly due to re-uptake into the SR via SR Ca(2+)-ATPase and extrusion via the sarcolemmal Na(+)/Ca(2+) exchange, NCX. Intracellular Na(+) concentration ([Na(+)](i)) is a main regulator of NCX, and thus [Na(+)](i) plays an important role in controlling the cytosolic and SR [Ca(2+)]. [Na(+)](i) may have an even more important role in HF because NCX is generally upregulated. There are several pathways for Na(+) entry into the cells, whereas the Na(+)/K(+) pump (NKA) is the main Na(+) extrusion pathway and therefore is essential in maintaining the transmembrane Na(+) gradient. Phospholemman is an important regulator of NKA function (decreasing [Na(+)](i) affinity unless it is phosphorylated). Here we discuss the interplay between Ca(2+) and Na(+) in myocytes from normal and failing hearts.

Phospholemman overexpression inhibits Na+-K+-ATPase in adult rat cardiac myocytes: relevance to decreased Na+ pump activity in postinfarction myocytes

Messenger RNA levels of phospholemman (PLM), a member of the FXYD family of small single-span membrane proteins with putative ion-transport regulatory properties, were increased in postmyocardial infarction (MI) rat myocytes. We tested the hypothesis that the previously observed reduction in Na+-K+-ATPase activity in MI rat myocytes was due to PLM overexpression. In rat hearts harvested 3 and 7 days post-MI, PLM protein expression was increased by two- and fourfold, respectively. To simulate increased PLM expression post-MI, PLM was overexpressed in normal adult rat myocytes by adenovirus-mediated gene transfer. PLM overexpression did not affect the relative level of phosphorylation on serine68 of PLM. Na+-K+-ATPase activity was measured as ouabain-sensitive Na+-K+ pump current (Ip). Compared with control myocytes overexpressing green fluorescent protein alone, Ip measured in myocytes overexpressing PLM was significantly (P < 0.0001) lower at similar membrane voltages, pipette Na+ ([Na+]pip) and extracellular K+ ([K+]o) concentrations. From -70 to +60 mV, neither [Na+]pip nor [K+]o required to attain half-maximal Ip was significantly different between control and PLM myocytes. This phenotype of decreased V(max) without appreciable changes in K(m) for Na+ and K+ in PLM-overexpressed myocytes was similar to that observed in MI rat myocytes. Inhibition of Ip by PLM overexpression was not due to decreased Na+-K+-ATPase expression because there were no changes in either protein or messenger RNA levels of either alpha1- or alpha2-isoforms of Na+-K+-ATPase. In native rat cardiac myocytes, PLM coimmunoprecipitated with alpha-subunits of Na+-K+-ATPase. Inhibition of Na+-K+-ATPase by PLM overexpression, in addition to previously reported decrease in Na+-K+-ATPase expression, may explain altered V(max) but not K(m) of Na+-K+-ATPase in postinfarction rat myocytes.

Expression and phosphorylation of the na-pump regulatory subunit phospholemman in heart failure

Intracellular [Na] is approximately 3 mmol/L higher in heart failure (HF; in our arrhythmogenic rabbit model), and this can profoundly affect cardiac Ca and contractile function via Na/Ca exchange and Na/H exchange. Na/K-ATPase is the primary mechanism of Na extrusion. We examine here in HF rabbits (and human hearts) expression of Na/K-ATPase isoforms and phospholemman (PLM), a putative Na/K-ATPase regulatory subunit that inhibits pump function and is a major cardiac phosphorylation target. Na/K-ATPase alpha1- and alpha2-isoforms were reduced in HF in rabbit ventricular homogenates (by 24%) and isolated myocytes (by 30% and 17%), whereas alpha3 was increased (50%) in homogenates and decreased (52%) in myocytes (P<0.05). Homogenate Na/K-ATPase activity in left ventricle was also decreased in HF. However, we showed previously that Na/K-ATPase characteristics in intact ventricular myocytes were unaltered in HF. To reconcile these findings, we assessed PLM expression, phosphorylation, and association with Na/K-ATPase. PLM coimmunoprecipitated with Na/K-ATPase alpha1 and alpha2 in control and HF rabbit myocytes. PLM expression was reduced in HF by 42% in isolated rabbit left ventricular (LV) myocytes, by 48% in rabbit LV homogenates, and by 24% in human LV homogenate. The fraction of PLM phosphorylated at Ser-68 was increased dramatically in HF. Our results are consistent with a role for PLM analogous to that of phospholamban for SR Ca-ATPase (SERCA): inhibition of Na/K-ATPase function that is relieved on PLM phosphorylation. So reduced Na/K-ATPase expression in HF may be functionally offset by lower inhibition by PLM (because of reduced PLM expression and higher PLM phosphorylation).

Phospholemman-Phosphorylation Mediates the β-Adrenergic Effects on Na/K Pump Function in Cardiac Myocytes

Cardiac sympathetic stimulation activates beta-adrenergic (beta-AR) receptors and protein kinase A (PKA) phosphorylation of proteins involved in myocyte Ca regulation. The Na/K-ATPase (NKA) is essential in regulating intracellular [Na] ([Na]i), which in turn affects [Ca]i via Na/Ca exchange. However, how PKA modifies NKA function is unknown. Phospholemman (PLM), a member of the FXYD family of proteins that interact with NKA in various tissues, is a major PKA substrate in heart. Here we tested the hypothesis that PLM phosphorylation is responsible for the PKA effects on cardiac NKA function using wild-type (WT) and PLM knockout (PLM-KO) mice. We measured NKA-mediated [Na]i decline and current (IPump) to assess beta-AR effects on NKA function in isolated myocytes. In WT myocytes, 1 micromol/L isoproterenol (ISO) increased PLM phosphorylation and stimulated NKA activity mainly by increasing its affinity for internal Na (Km decreased from 18.8+/-1.4 to 13.6+/-1.5 mmol/L), with no significant effect on the maximum pump rate. This led to a significant decrease in resting [Na]i (from 12.5+/-1.8 to 10.5+/-1.4 mmol/L). In PLM-KO mice under control conditions Km (14.2+/-1.5 mmol/L) was lower than in WT, but comparable to that for WT in the presence of ISO. Furthermore, ISO had no significant effect on NKA function in PLM-KO mice. ATPase activity in sarcolemmal vesicles also showed a lower Km(Na) in PLM-KO versus WT (12.9+/-0.9 versus 16.2+/-1.5). Thus, PLM inhibits NKA activity by decreasing its [Na]i affinity, and this inhibitory effect is relieved by PKA activation. We conclude that PLM modulates the NKA function in a manner similar to the way phospholamban affects the related SR Ca-ATPase (inhibition of transport substrate affinity, that is relieved by phosphorylation).

Identification of an endogenous inhibitor of the cardiac Na+/Ca2+ exchanger, phospholemman

Rapid and precise control of Na(+)/Ca(2+) exchanger (NCX1) activity is essential in the maintenance of beat-to-beat Ca(2+) homeostasis in cardiac myocytes. Here, we show that phospholemman (PLM), a 15-kDa integral sarcolemmal phosphoprotein, is a novel endogenous protein inhibitor of cardiac NCX1. Using a heterologous expression system that is devoid of both endogenous PLM and NCX1, we first demonstrated by confocal immunofluorescence studies that both exogenous PLM and NCX1 co-localized at the plasma membrane. Reciprocal co-immunoprecipitation studies revealed specific protein-protein interaction between PLM and NCX1. The functional consequences of direct association of PLM with NCX1 was the inhibition of NCX1 activity, as demonstrated by whole-cell patch clamp studies to measure NCX1 current density and radiotracer flux assays to assess Na(+)-dependent (45)Ca(2+) uptake. Inhibition of NCX1 by PLM was specific, because a single mutation of serine 68 to alanine in PLM resulted in a complete loss of inhibition of NCX1 current, although association of the PLM mutant with NCX1 was unaltered. In native adult cardiac myocytes, PLM co-immunoprecipitated with NCX1. We conclude that PLM, a member of the FXYD family of small ion transport regulators known to modulate Na(+)-K(+)-ATPase, also regulates Na(+)/Ca(2+) exchange in the heart.

Serine 68 of phospholemman is critical in modulation of contractility, [Ca2+]i transients, and Na+/Ca2+ exchange in adult rat cardiac myocytes

Overexpression of phospholemman (PLM) in normal adult rat cardiac myocytes altered contractile function and cytosolic Ca2+ concentration ([Ca2+]i) homeostasis and inhibited Na+/Ca2+ exchanger (NCX1). In addition, PLM coimmunoprecipitated and colocalized with NCX1 in cardiac myocyte lysates. In this study, we evaluated whether the cytoplasmic domain of PLM is crucial in mediating its effects on contractility, [Ca2+]i transients, and NCX1 activity. Canine PLM or its derived mutants were overexpressed in adult rat myocytes by adenovirus-mediated gene transfer. Confocal immunofluorescence images using canine-specific PLM antibodies demonstrated that the exogenous PLM or its mutants were correctly targeted to sarcolemma, t-tubules, and intercalated discs, with little to none detected in intracellular compartments. Overexpression of canine PLM or its mutants did not affect expression of NCX1, sarco(endo)plasmic reticulum Ca(2+)-ATPase, Na(+)-K(+)-ATPase, and calsequestrin in adult rat myocytes. A COOH-terminal deletion mutant in which all four potential phosphorylation sites (Ser62, Ser63, Ser68, and Thr69) were deleted, a partial COOH-terminal deletion mutant in which Ser68 and Thr69 were deleted, and a mutant in which all four potential phosphorylation sites were changed to alanine all lost wild-type PLM's ability to modulate cardiac myocyte contractility. These observations suggest the importance of Ser68 or Thr69 in mediating PLM's effect on cardiac contractility. Focusing on Ser68, the Ser68 to Glu mutant was fully effective, the Ser63 to Ala (leaving Ser68 intact) mutant was partially effective, and the Ser68 to Ala mutant was completely ineffective in modulating cardiac contractility, [Ca2+]i transients, and NCX1 currents. Both the Ser63 to Ala and Ser68 to Ala mutants, as well as PLM, were able to coimmunoprecipitate NCX1. It is known that Ser68 in PLM is phosphorylated by both protein kinases A and C. We conclude that regulation of cardiac contractility, [Ca2+]i transients, and NCX1 activity by PLM is critically dependent on Ser68. We suggest that PLM phosphorylation at Ser68 may be involved in cAMP- and/or protein kinase C-dependent regulation of cardiac contractility.

Hypertrophy, increased ejection fraction, and reduced Na-K-ATPase activity in phospholemman-deficient mice

Phospholemman (FXYD1), a 72-amino acid transmembrane protein abundantly expressed in the heart and skeletal muscle, is a major substrate for phosphorylation in the cardiomyocyte sarcolemma. Biochemical, cellular, and electrophysiological studies have suggested a number of possible roles for this protein, including ion channel modulator, taurine-release channel, Na(+)/Ca(2+) exchanger modulator, and Na-K-ATPase-associated subunit. We have generated a phospholemman-deficient mouse. The adult null mice exhibited increased cardiac mass, larger cardiomyocytes, and ejection fractions that were 9% higher by magnetic resonance imaging compared with wild-type animals. Notably, this occurred in the absence of hypertension. Total Na-K-ATPase activity was 50% lower in the phospholemman-deficient hearts. Expression (per unit of membrane protein) of total Na-K-ATPase was only slightly diminished, but expression of the minor alpha(2)-isoform, which has been specifically implicated in the control of contractility, was reduced by 60%. The absence of phospholemman thus results in a complex response, including a surprisingly large reduction in intrinsic Na-K-ATPase activity, changes in Na-K-ATPase isoform expression, increase in ejection fraction, and increase in cardiac mass. We hypothesize that a primary effect of phospholemman is to modulate the Na-K-ATPase and that its reduced activity initiates compensatory responses.

Effects of phospholemman downregulation on contractility and [Ca(2+)]i transients in adult rat cardiac myocytes

Phospholemman (PLM) expression was increased in rat hearts after myocardial infarction (MI). Overexpression of PLM in normal adult rat cardiac myocytes altered contractile function and cytosolic Ca(2+) concentration ([Ca(2+)](i)) homeostasis in a manner similar to that observed in post-MI myocytes. In this study, we tested whether PLM downregulation in normal adult rat myocytes resulted in contractility and [Ca(2+)](i) transient changes opposite to those observed in post-MI myocytes. Compared with control myocytes infected with adenovirus (Adv) expressing green fluorescent protein (GFP) alone, myocytes infected with Adv expressing both GFP and rat antisense PLM (rASPLM) had 23% less PLM protein (P < 0.012) at 3 days, but no differences were found in sarcoplasmic reticulum (SR) Ca(2+)-ATPase, Na(+)/Ca(2+) exchanger (NCX1), Na(+)-K(+)-ATPase, and calsequestrin levels. SR Ca(2+) uptake and whole cell capacitance were not affected by rASPLM treatment. Relaxation from caffeine-induced contracture was faster, and NCX1 current amplitudes were higher in rASPLM myocytes, indicating that PLM downregulation enhanced NCX1 activity. In native rat cardiac myocytes, coimmunoprecipitation experiments indicated an association of PLM with NCX1. At 0.6 mM [Ca(2+)](o), rASPLM myocytes had significantly (P < 0.003) lower contraction and [Ca(2+)](i) transient amplitudes than control GFP myocytes. At 5 mM [Ca(2+)](o), both contraction and [Ca(2+)](i) transient amplitudes were higher in rASPLM myocytes. This pattern of contractile and [Ca(2+)](i) transient behavior in rASPLM myocytes was opposite to that observed in post-MI rat myocytes. We conclude that downregulation of PLM in normal rat cardiac myocytes enhanced NCX1 function and affected [Ca(2+)](i) transient and contraction amplitudes. We suggest that PLM downregulation offers a potential therapeutic strategy for ameliorating contractile abnormalities in MI myocytes.

Phospholemman modulates Na+/Ca2+ exchange in adult rat cardiac myocytes

Previous studies have shown that overexpression of phospholemman (PLM) affected contractile function and Ca(2+) homeostasis in adult rat myocytes. We tested the hypothesis that PLM modulated Na(+)/Ca(2+) exchanger (NCX1) activity. PLM was overexpressed in adult rat myocytes by adenovirus-mediated gene transfer. After 72 h, the half-time of relaxation from caffeine-induced contracture, an estimate of forward NCX1 activity, was prolonged 1.8-fold (P < 0.003) in myocytes overexpressing PLM compared with control myocytes overexpressing green fluorescent protein alone. Reverse NCX1 current (3 Na(+) out:1 Ca(2+) in) was significantly (P < 0.0001) lower in PLM myocytes, especially at more positive voltages. Immunofluorescence demonstrated colocalization of PLM and NCX1 to the plasma membrane and t-tubules. Resting membrane potential, action potential amplitude and duration, myocyte size, and NCX1 and calsequestrin protein levels were not affected by PLM overexpression. At 5 mM extracellular [Ca(2+)] ([Ca(2+)](o)), the depressed contraction amplitudes in PLM myocytes were increased towards normal by cooverexpression with NCX1. At 0.6 mM [Ca(2+)](o), the supranormal contraction amplitudes in PLM myocytes were reduced by cooverexpression with NCX1. We conclude that PLM modulated myocyte contractility partly by inhibiting Na(+)/Ca(2+) exchange.

Overexpression of phospholemman alters contractility and [Ca(2+)](i) transients in adult rat myocytes

Previous studies showed increased phospholemman (PLM) mRNA after myocardial infarction (MI) in rats (Sehl PD, Tai JTN, Hillan KJ, Brown LA, Goddard A, Yang R, Jin H, and Lowe DG. Circulation 101: 1990-1999, 2000). We tested the hypothesis that, in normal adult rat cardiac myocytes, PLM overexpression alters contractile function and cytosolic Ca(2+) concentration ([Ca(2+)](i)) homeostasis in a manner similar to that observed in post-MI myocytes. Compared with myocytes infected by control adenovirus expressing green fluorescent protein (GFP) alone, Western blots indicated a 41% increase in PLM expression after 72 h (P < 0.001) but no changes in Na(+)/Ca(2+) exchanger, SERCA2, and calsequestrin levels in myocytes infected by adenovirus expressing GFP and PLM. At 5 mM extracellular [Ca(2+)] ([Ca(2+)](o)), maximal contraction amplitudes in PLM-overexpressed myocytes were 24% (P < 0.005) and [Ca(2+)](i) transient amplitudes were 18% (P < 0.05) lower than control myocytes. At 0.6 mM [Ca(2+)](o), however, contraction and [Ca(2+)](i) transient amplitudes were significantly (P < 0.05) higher in PLM-overexpressed than control myocytes (18% and 42%, respectively); at 1.8 mM [Ca(2+)](o), the differences in contraction and [Ca(2+)](i) transient amplitudes were narrowed. This pattern of contractile and [Ca(2+)](i) transient abnormalities in PLM-overexpressed myocytes mimics that observed in post-MI rat myocytes. We suggest that PLM overexpression observed in post-MI myocytes may partly account for contractile abnormalities by perturbing Ca(2+) fluxes during excitation-contraction.

Gene structure and expression of phospholemman in mouse

Phospholemman (PLM) is a small transmembrane cardiac protein that is the major sarcolemmal substrate for phosphorylation in response to adrenergic stimulation. PLM likely plays a role in muscle contractility and cell volume regulation through its function as a channel or a channel regulator. We are the first to describe the structure of the PLM gene and to demonstrate PLM cDNA splice variants. We cloned the murine PLM cDNA and used it as a probe to isolate the gene from a 129/SvJ genomic library. The gene contains seven introns and eight exons. The coding sequence is interrupted by five introns; the 5' untranslated region by two. Using rapid amplification of 5' cDNA ends we identified transcription start sites and four splice variants of the 5' untranslated domain. There was no TATA box or CAAT box in the putative promoter regions. The gene has several stretches of dinucleotide repeats. The 3' untranslated domains of mouse PLM cDNA clones show sequence differences not accounted for by alternative splicing. Mouse PLM shares 93, 83 and 80% amino acid identity with rat, dog, and human PLMs, respectively. Tissue expression of murine PLM parallels that in other species, being highest in heart, skeletal muscle, and liver.

Reduction of phospholemman expression decreases osmosensitive taurine efflux in astrocytes

The role of phospholemman (PLM) in taurine and Cl(-) efflux elicited by 30% hyposmotic solution was studied in cultured cerebellar astrocytes with reduced PLM expression by antisense oligonucleotide (AO) treatment. PLM, a substrate for protein kinases (PK) C and A, is a protein that increases an anion current in Xenopus oocytes and forms taurine-selective channels in lipid bilayers. Taurine contributes as an osmolyte to regulatory volume decrease (RVD) and is highly permeable through PLM channels in bilayers. Two antisense oligonucleotides (AO1 and AO2) effectively decreased the expression of the PLM protein by 40% and 30%, respectively, and markedly reduced [(3)H]taurine efflux by 67% and 62%. AO treatment also decreased the osmosensitive release of Cl(-), followed as (125)I. The inhibition of Cl(-) efflux (23% for AO1 and 13% for AO2) was notably lower than for [(3)H]taurine. The contribution of PKC and PKA in the function of PLM was also evaluated in astrocytes. Pharmacological activation or inhibition of PKC and PKA revealed that the osmosensitive taurine efflux is essentially PKC-independent while (125)I efflux is reduced by the PKC blockers H-7 (21%) and Gö6983 (41%). The PKA activator forskolin and dbcAMP increased taurine efflux by 66-70% and (125)I efflux by 21-45%. Norepinephrine increased the osmosensitive taurine efflux at about the same extent as dbcAMP and forskolin, and this was reduced by PKA blockers. These results suggest that PLM plays a role in RVD in astrocytes by predominantly influencing taurine fluxes, which are modulated by PKA but not PKC.

Volume sensitive efflux of taurine in HEK293 cells overexpressing phospholemman

The role of the phospholemman (PLM) on the efflux of taurine and chloride induced by swelling was studied in HEK293 cells overexpressing stable transfected PLM. PLM, a substrate for protein kinases C and A, is a protein that induces an anion current in Xenopus oocytes and forms taurine-selective channels in lipid bilayers. Taurine contributes as an osmolyte to regulatory volume decrease (RVD) and is highly permeable through PLM channels in bilayers. In PLM-overexpressing cells the process of RVD was more rapid and efficient (75%) than in control cells (44%). Also, [(3)H]taurine and (125)I efflux induced by hyposmolarity were markedly increased (30-100%) in two subclones of cells overexpressing PLM. This increased efflux was sensitive to the Cl channel blockers DDF, NPPB and DIDS. Acute treatment of control cells with isoproterenol and norepinephrine induced a significant potentiation (50-60%) of [(3)H]taurine release induced by hyposmolarity. In PLM-overexpressing cells the potentiation by these drugs was higher (100%). Insulin induced also an increase in [(3)H]taurine release, but only in PLM-overexpressing cells (50%). These results indicate that PLM may play a role in the RVD and that its phosphorylation may have a physiological significance during this process. The mechanisms involved in this process could include the activation of PLM itself as channel or the modulation of other preexisting channels.

Modulation of Xenopus oocyte-expressed phospholemman-induced ion currents by co-expression of protein kinases

Phospholemman (PLM), the major sarcolemmal substrate for phosphorylation by cAMP-dependent kinase (PKA) protein kinase C (PKC) and NIMA kinase in muscle, induces hyperpolarization-activated anion currents in Xenopus oocytes, most probably by enhancing endogenous oocyte currents. PLM peptides from the cytoplasmic tail are phosphorylated by PKA at S68, by NIMA kinase at S63, and by PKC at both S63 and S68. We have confirmed the phosphorylation sites in the intact protein, and we have investigated the role of phosphorylation in the regulatory activity of PLM using oocyte expression experiments. We found: (1) the cytoplasmic domain is not essential for inducing currents in oocytes; (2) co-expression of PKA increased the amplitude of oocyte currents and the amount of PLM in the oocyte membrane largely, but not exclusively, through phosphorylation of S68; (3) co-expression of PKA had no effect on a PLM mutant in which all putative phosphorylation sites had been inactivated by serine to alanine mutation (SSST 62, 63, 68, 69 AAAA); (4) co-expression of PKC had no effect in this system; (5) co-expression of NIMA kinase increased current amplitude and membrane protein level, but did not require PLM phosphorylation. These findings point to a role for phosphorylation in the function of PLM.

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.

Cardiac protein phosphorylation: functional and pathophysiological correlates

Protein phosphorylation acts a pivotal mechanism in regulating the contractile state of the heart by modulating particular levels of autonomic control on cardiac force/length relationships. Early studies of changes in cardiac protein phosphorylation focused on key components of the excitation-coupling process, namely phospholamban of the sarcoplasmic reticulum and myofibrillar troponin I. In more recent years the emphasis has shifted towards the identification of other phosphoproteins, and more importantly, the delineation of the mechanistic and signaling pathways regulating the various known phosphoproteins. In addition to cAMP- and Ca(2+)-calmodulin-dependent kinase processes, these have included regulation by protein kinase C and the ever-emerging family of growth factor-related kinases such as the tyrosine-, mitogen- and stress-activated protein kinases. Similarly, the role of protein dephosphorylation by protein phosphatases has been recognized as integral in modulating normal cardiac cellular function. Recent studies involving a variety of cardiovascular pathologies have demonstrated that changes in the phosphorylation states of key cardiac regulatory proteins may underlie cardiac dysfunction in disease states. The emphasis of this comprehensive review will be on discussing the role of cardiac phosphoproteins in regulating myocardial function and pathophysiology based not only on in vitro data, but more importantly, from ex vivo experiments with corroborative physiological and biochemical evidence.

Structural domains in phospholemman: a possible role for the carboxyl terminus in channel inactivation

Phospholemman (PLM) is a small (72-amino acid) transmembrane protein found in cardiac sarcolemma that is a major substrate for several protein kinases in vivo. Detailed structural data for PLM is lacking, but several studies have described an ion conductance that results from PLM expression in oocytes. Moreover, addition of purified PLM to lipid bilayers generates similar ion currents, suggesting that the PLM molecule itself might be sufficient for channel formation. To provide a framework for understanding the function of PLM, we investigated PLM topology and structure in sarcolemmal membrane vesicles and analyzed purified recombinant PLM. Immunoblot analyses with site-specific antibodies revealed that the extracellular segment (residues 1 to 17) exists in a protected configuration highly resistant to proteases, even in detergent solutions. The intracellular portion of the molecule (residues 38 to 72), in contrast, was highly susceptible to proteases. Trypsin treatment produced a limit peptide (residues 1 to 43), which showed little change in electrophoretic mobility in SDS gels and retained the ion-channel activity in lipid bilayers that is characteristic of the full-length protein. In addition, we found that conductance through PLM channels exhibited rapid inactivation during depolarizing ramps at voltages greater than +/- 50 mV, Channels formed by trypsinized PLM or recombinant PLM 1-43 exhibited dramatic reductions in voltage-dependent inactivations. Our data point to distinct domains within the PLM molecule that may correlate with functional properties of channel activity observed in oocytes and lipid bilayers.

Characterization of the human and rat phospholemman (PLM) cDNAs and localization of the human PLM gene to chromosome 19q13.1

Previous reports have demonstrated that the phospholemman (PLM), a 72-residue plasma-membrane protein enriched in skeletal muscle and heart, is a major substrate phosphorylated in response to insulin and adrenergic stimulation. Here we describe the isolation and characterization of human and rat PLM cDNA from the heart. Both PLM proteins share significant nucleotide and amino acid sequence and structural similarities with the previously published canine PLM and, to a lesser degree, with Na+/K(+)-ATPase gamma subunit, Mat-8 protein, and CHIF protein. Despite the functional diversity, all these proteins are quite small and possess a single transmembrane domain. Human PLM appears to be a unique gene localized on chromosome 19q13.1. The PLM mRNA is widely distributed in human tissues, with the highest expression in skeletal muscle and heart, suggesting a functional role in muscle contraction. Like canine PLM, both human and rat PLM induce a hyperpolarization-activated chloride current when expressed in Xenopus oocytes. The high degree of sequence and functional conservation among the mammalian PLM proteins indicates that this gene is conserved throughout evolution.

alpha 1-Adrenoceptor stimulation inhibits the isoproterenol-induced effects on myocardial contractility and protein phosphorylation

In the present study the influence of alpha 1-adrenoceptor stimulation on the beta-adrenoceptor agonist-induced increases in contractile parameters and protein phosphorylation was determined in isolated perfused hearts and isolated cardiac myocytes, respectively. Methoxamine inhibited the isoproterenol-induced increases in left ventricular pressure and heart rate dose dependently up to 90% and 75%, respectively; the EC50 of this antiadrenergic effect was 4.4 microM. The alpha 1-adrenoceptor antagonist, prazosin (1 microM), greatly diminished methoxamine's inhibitory action, confirming the alpha 1-adrenoceptor-mediated mechanism. The inotropic effect of glucagon was inhibited by methoxamine in a similar manner. Radioligand binding assays with [3H]dihydroalprenolol demonstrated that the antiadrenergic action of methoxamine is not due to an unspecific beta-adrenoceptor blocking property. In an additional experimental series the effects of methoxamine and isoproterenol on the protein phosphorylation pattern of isolated cardiac myocytes were investigated. Isoproterenol increased the phosphorylation state of five proteins (6-kDa, phospholamban; 15-kDa; 28-kDa, troponin I; 97-kDa; 140-kDa) while in the experiments with methoxamine the 15-kDa protein was the only phosphorylated substrate. In the presence of methoxamine the isoproterenol-induced phosphorylation of phospholamban, troponin I and the 97-kDa and 140-kDa protein was markedly inhibited while the phosphorylation state of the 15-kDa protein remained unaltered. The present study clearly demonstrated that alpha 1-adrenoceptor stimulation potently inhibits the beta-adrenoceptor-mediated changes in contractile force and phosphorylation of key regulatory proteins, most likely through modulation of cAMP metabolism.

Unitary anion currents through phospholemman channel molecules

Phospholemman (PLM) is a 72-amino-acid peptide with a single transmembrane domain, the expression of which induces chloride currents in Xenopus oocytes. It has remained unknown whether PLM is an ion channel or acts as a channel regulator. Here we show, by measuring unitary anion currents across planar phospholipid bilayers to which immunoaffinity-purified recombinant PLM was added, that it does indeed form ion channels. Excised patches of oocytes expressing PLM had similar currents. Of the ions tested, the sulphonic amino acid taurine was the most permeant, and expression of PLM increased fluxes of radiolabelled taurine in oocytes. Phospholemman is the smallest protein in cell membranes known to form an ion channel and the taurine selectivity suggests that it is involved in cell volume regulation.

Regulation of phospholamban and troponin-I phosphorylation in the intact rat cardiomyocytes by adrenergic and cholinergic stimuli: roles of cyclic nucleotides, calcium, protein kinases and phosphatases and depolarization

Protein phosphorylation was investigated in [32P]-labeled cardiomyocytes isolated from adult rat heart ventricles. The beta-adrenergic stimulation (by isoproterenol, ISO) increased the phosphorylation of inhibitory subunit of troponin (TN-I), C-protein and phospholamban (PLN). Such stimulation was largely mediated by increased adenylyl cyclase (AC) activity, increased myoplasmic cyclic AMP and increased cyclic AMP dependent protein kinase (A-kinase)-catalyzed phosphorylation of these proteins in view of the following observations: (a) dibutyryl-and bromo-derivatives of cyclic AMP mimicked the stimulatory effect of ISO on protein phosphorylation while (b) Rp-cyclic AMP was found to attenuate ISO-dependent stimulation. Unexpectedly, 8-bromo cyclic GMP was found to markedly increase TN-I and PLN phosphorylation. Both beta 1- and beta 2-adrenoceptors were present and ISO binding to either receptor was found to stimulate myocyte AC. However, the stimulation of the beta 2-AR only marginally increased while the stimulation of beta 1-AR markedly increased PLN phosphorylation. Other stimuli that increase tissue cyclic AMP levels also increased PLN and TN-I phosphorylation and these included isobutylmethylxanthine (non-specific phosphodiesterase inhibitor), milrinone (inhibits cardiotonic inhibitable phosphodiesterase, sometimes called type III or IV) and forskolin (which directly stimulates adenylyl cyclase). Cholinergic agonists acting on cardiomyocyte M2-muscarinic receptors that are coupled to AC via pertussis toxin(PT)-sensitive G proteins inhibited AC and attenuated ISO-dependent increases in PLN and TN-I phosphorylation. The in vivo PT treatment, which ADP-ribosylated Gi-like protein(s) in the myocytes, markedly attenuated muscarinic inhibitory effect on PLN and TN-I phosphorylation on one hand and, increased the beta-adrenergic stimulation, on the other. Controlled exposure of isolated myocytes to N-ethyl maleimide, also led to the findings similar to those seen following the PT treatment. Exposure of myocytes to phorbol, 12-myristate, 13-acetate (PMA) increased the protein phosphorylation, augmenting the stimulation by ISO, and such augmentation was antagonized by propranolol suggesting modulation of the beta-adrenoceptor coupled AC pathway by PMA. Okadaic acid (OA) exposure of myocytes also increased protein phosphorylation with the results supporting the roles for type 1 and 2A protein phosphatases in the dephosphorylation of PLN and TN-I. Interestingly OA treatment attenuated the muscarinic inhibitory effect which was restored by subsequent brief exposure of myocytes to PMA. While the stimulation of alpha adrenoceptors exerted little effect on the phosphorylation of PLN and TN-I, inactivation of alpha adrenoceptors by chloroethylclonidine (CEC), augmented beta-adrenergically stimulated phosphorylation. KCl-dependent depolarization of myocytes was observed to potentiate ISO-dependent increase in phosphorylation (incubation period 15 sec to 1 min) as well as to accelerate the time-dependent decline in this phosphorylation seen upon longer incubation. Verapamil decreased ISO-stimulated protein phosphorylation in the depolarized myocytes. Depolarization was found to have little effect on the muscarinic inhibitory action on phosphorylation. Prior treatment of myocytes with PMA, was found to augment ISO-stimulated protein phosphorylation in the depolarized myocytes. Such augmented increases were completely blocked by propranolol. Forskolin also stimulated PLN and TN-I phosphorylation. Prior exposure of myocytes to forskolin followed by incubation in the depolarized and polarized media showed that PLN was dephosphorylated more rapidly in the depolarized myocytes. The results support the view that both cyclic AMP and calcium signals cooperatively increase the rates of phosphorylation of TN-I and PLN in the depolarized cardiomyocytes during beta-adrenergic stimulation. (ABSTRACT TRUNCATED)

Endothelin-1 does not phosphorylate phospholamban and troponin I in intact beating rat hearts

To determine a role of phosphorylation of specific cardiac regulatory proteins in the positive inotropic effect of endothelin-1, we examined phosphorylation of sarcoplasmic reticulum and myofibrillar proteins in perfused beating rat hearts treated with endothelin-1. In parallel experiments, the effects of isoprenaline and phorbol-12,13-dibutyrate (PDB) on protein phosphorylation were also tested. In 32Pi-labeled hearts, perfusion with isoprenaline (100 nM) caused 4.4- and 10.4-fold increases in the degree of phosphorylation of phospholamban in sarcoplasmic reticulum and of troponin I in myofibrils, respectively. In contrast, neither endothelin-1 (100 nM) nor PDB (1 microM) significantly changed the phosphorylation state of these proteins. These findings provide evidence that phosphorylation of major cardiac regulatory proteins is not responsible for the positive inotropic action of endothelin-1.

Targeted ablation of the phospholamban gene is associated with markedly enhanced myocardial contractility and loss of beta-agonist stimulation

Phospholamban is the regulator of the Ca(2+)-ATPase in cardiac sarcoplasmic reticulum (SR), and it has been suggested to be an important determinant in the inotropic responses of the heart to beta-adrenergic stimulation. To determine the role of phospholamban in vivo, the gene coding for this protein was targeted in murine embryonic stem cells, and mice deficient in phospholamban were generated. The phospholamban-deficient mice showed no gross developmental abnormalities but exhibited enhanced myocardial performance without changes in heart rate. The time to peak pressure and the time to half-relaxation were significantly shorter in phospholamban-deficient mice compared with their wild-type homozygous littermates as assessed in work-performing mouse heart preparations under identical venous returns, afterloads, and heart rates. The first derivatives of intraventricular pressure (+/- dP/dt) were also significantly elevated, and this was associated with an increase in the affinity of the SR Ca(2+)-ATPase for Ca2+ in the phospholamban-deficient hearts. Baseline levels of these parameters in the phospholamban-deficient hearts were equal to those observed in hearts of wild-type littermates maximally stimulated with the beta-agonist isoproterenol. These findings indicate that phospholamban acts as a critical repressor of basal myocardial contractility and may be the key phosphoprotein in mediating the heart's contractile responses to beta-adrenergic agonists.

Protein kinase C activity is altered in diabetic rat hearts

An elevation in diacylglycerol content in the myocardium from diabetic rats has been reported. Since diacylglycerol is known to be an important second messenger in activating protein kinase C, we carried out a study to investigate the status of protein kinase C activity in the hearts of Wistar diabetic rats. Our results showed that protein kinase C activity was significantly increased in the membrane fraction of diabetic hearts compared with controls, and the increased activity was accompanied by a decrease in cytosolic protein kinase C activity in these diabetic hearts. The increase in the membrane-bound protein kinase C activity thus appears to be due to translocation of the enzyme from the cytosolic to the membrane fraction. These results indicate that the development of diabetic cardiomyopathy is accompanied with a high membrane-bound protein kinase C level.

Protein kinase C phosphorylates a 15 kDa protein but not phospholamban in intact rat cardiac myocytes

In the present study the effects of the protein kinase C activator 12-O-tetradecanoylphorbol 13-acetate (TPA) as well as the alpha- and beta-adrenoceptor agonists methoxamine and isoproterenol on protein phosphorylation of intact rat cardiac myocytes were investigated. TPA, isoproterenol and methoxamine were shown to stimulate phosphorylation of a 15 kDa protein. EC50 for TPA and isoproterenol were 4 x 10(-8) M and 5 x 10(-9) M respectively. The time course of phosphorylation by TPA and isoproterenol greatly differed, revealing a maximal phosphorylation (2.9-fold) after 10 min and 1 min respectively. Cell fractionation showed a significant enrichment of the 15 kDa protein in a crude membrane preparation. While the 15 kDa protein was the only phosphoprotein stimulated by TPA and methoxamine, isoproterenol additionally enhanced the 32Pi incorporation into four proteins corresponding to 6 kDa (phospholamban), 28 kDa, 97 kDa and 140 kDa. Furthermore, dephosphorylation of a 21 kDa substrate upon beta-adrenoceptor stimulation was observed. Phospholamban phosphorylation was effectively (max. 9.1-fold) stimulated by isoproterenol (EC50 of 5 x 10(-9) M), reaching a maximal phosphorylation state within 1 min. The present study clearly demonstrates: (1) TPA stimulates the phosphorylation of a membrane-localized 15 kDa protein and this effect can be mimicked by both isoproterenol and methoxamine; (2) TPA, in contrast to isoproterenol, does not change the phosphorylation state of phospholamban. Whilst phospholamban under in vitro conditions is known to be a substrate for protein kinase C, it does not appear to be accessible for the enzyme in intact cardiac myocytes.

Effect of alpha-adrenergic stimulation on activation of protein kinase C and phosphorylation of proteins in intact rabbit hearts

The intracellular events and specifically the role of protein kinase C-mediated protein phosphorylation, after alpha-adrenergic receptor stimulation of the heart, are not well understood. We examined the phosphorylation of sarcolemmal, sarcoplasmic reticular, myofibrillar, and cytosolic proteins in perfused beating rabbit hearts on activation of protein kinase C by phenylephrine. Perfusion of rabbit hearts with phenylephrine was associated with a positive inotropic response, which was dose and time dependent. Maximal stimulation (1.54-fold increase in +dP/dt) was obtained with 10 microM phenylephrine at 4 minutes. Examination of the activity levels of protein kinase C in these hearts revealed a redistribution of this activity from the cytosolic to the membranous fraction, suggesting the activation of this enzyme in vivo. Prazosin, an alpha 1-adrenergic antagonist, prevented the increase in the inotropy and the redistribution of protein kinase C activity mediated by phenylephrine. Examination of the degree of phosphorylation of membranous, myofibrillar, and cytosolic proteins revealed that activation of protein kinase C in vivo was associated with increased phosphorylation of a 15-kd sarcolemmal protein and a 28-kd cytosolic protein. There were no increases in the degree of phosphorylation of phospholamban in the sarcoplasmic reticulum and of troponin I, troponin T, and C protein in the myofibrils, although these proteins were found to be substrates for protein kinase C in vitro. These findings provide evidence that protein kinase C is activated in response to alpha-adrenergic stimulation and that activation is associated with increased phosphorylation of a 15-kd sarcolemmal protein and a 28-kd cytosolic protein in the myocardium.

Aging: changes in cardiac alpha 1-adrenoceptor responsiveness and expression

Several investigators have reported a diminished responsiveness of senescent cardiac muscle to norepinephrine and beta-adrenoceptor agonists. In contrast, relatively little is known regarding the effects of aging on myocardial actions mediated specifically by alpha-adrenoceptor stimulation. Thus, the current study examined aging-dependent changes in: (a) the inotropic response to methoxamine, an alpha-adrenoceptor agonist; (b) characteristics of myocardial alpha 1-adrenoceptors as monitored by specific [3H]prazosin binding; and (c) steady state levels of alpha 1-adrenoceptor mRNA as determined by Northern blot analysis. Cardiac preparations were isolated from 4-, 14-, and 25-month-old F344 rats. An aging-associated decline was observed in the maximum positive inotropic effect elicited by methoxamine in right ventricular strips (160 +/- 23, 134 +/- 13 and 79 +/- 26% increase above control developed tension in 4, 14 and 25 months, respectively) with no change in ED50 values. [3H]Prazosin binding to ventricular sarcolemmal membranes revealed a reduction in receptor number (82 +/- 7, 69 +/- 6 and 59 +/- 5 fmol/mg protein in 4, 14 and 25 months, respectively); the apparent dissociation constant was not affected. Steady state levels of alpha 1-adrenoceptor mRNA decreased progressively between 4 and 25 months of age (14- and 25-month levels were approximately 71 and 38% of 4 months, respectively), while steady state levels of beta-actin mRNA did not change with age. These results suggest that the aging-related decline in alpha 1-adrenergic responsiveness in rat ventricular muscle is mediated, at least in part, by a decrease in cardiac alpha 1-adrenoceptor density.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin and phorbol ester stimulate phosphorylation of a 15,000 dalton membrane protein in rat diaphragm in a similar manner

The effects of insulin on the phosphorylation of a 15 kilodalton (kDa) membrane protein in rat diaphragm in situ have been investigated. Incubation of the diaphragm with insulin or tumor-promoting phorbol ester increased the 32P-labelling of the 15 kDa protein at serine residues by 50 +/- 8% and 64 +/- 11%, (mean +/- S.E.), respectively. Thermolytic peptide mapping of the 15 kDa protein after insulin treatment of the diaphragm yielded two major phosphopeptides, one of which was absent from digests from control diaphragms. The same two phosphopeptides were identified after incubation of the diaphragm with phorbol ester and after phosphorylation of sarcolemma in vitro with [gamma-32P]ATP and protein kinase C. Additional experiments indicated that pretreatment of diaphragms with insulin or phorbol ester both increased the state of phosphorylation of the 15 kDa sarcolemma protein on phosphorylation sites regulated by protein kinase C. The stimulatory effect of insulin was decreased by staurosporine or by preincubation of the diaphragms with phorbol esters. These results indicate that the insulin-induced increases in protein kinase C activity previously found in rat diaphragm (Walaas et al. (1987) FEBS Lett. 220, 311-318) may be involved in insulin-mediated regulation of phosphorylation of the 15 kDa protein in situ.

Inhibition of phosphorylation of troponin I in rat heart by adenosine and 5'-chloro-5'-deoxyadenosine

We have investigated the effects of adenosine on protein phosphorylation in extracts of rat heart. Incubation of a myofibrillar fraction with [gamma-32P]ATP resulted in the phosphorylation of several proteins by endogenous protein kinases. The adenosine analog 5'-chloro-5'-deoxyadenosine inhibited the phosphorylation of a 29 kD protein in this preparation. The protein was identified as cardiac troponin I (cTnI) by two-dimensional gel electrophoresis, using purified cTnI as standard. Addition of the catalytic subunit of cAMP-dependent protein kinase to the myofibrillar fraction increased phosphorylation of cTnI; this increase was inhibited by 5'-chloro-5'-deoxyadenosine and adenosine. Phosphorylation of purified cTnI by the catalytic subunit was also inhibited by 5'-chloro-5'-deoxyadenosine. Under these conditions used, 50% inhibition of phosphorylation by either endogenous or exogenous kinase was observed at approximately 50 microM 5'-chloro-5'-deoxyadenosine or adenosine. The inhibition described here occurred independently of catecholamines. The effects of ADP, AMP, and adenine on cTnI phosphorylation are also described.

Alpha-adrenergic regulation of phosphoinositide metabolism and protein kinase C in isolated cardiac myocytes

alpha 1-Adrenergic regulation of phosphoinositide metabolism and protein kinase C translocation was studied in isolated rat cardiac myocytes. Exposure of [3H]inositol-labeled myocytes to norepinephrine in the presence of propranolol caused a dose-dependent increase in [3H]inositol phosphates. Norepinephrine also increased the level of membrane-associated protein kinase C from approximately 10% of total activity to 18%, with a dose response similar to that for generation of inositol phosphates. Depolarization of myocytes with 30 mM KCl had no effect on inositol phosphates or membrane-associated protein kinase C but potentiated the effect of submaximal norepinephrine on both parameters. The potentiation of protein kinase C translocation was amplified when extracellular Ca2+ was increased to 4 mM, resulting in membrane association of one-third of the total cellular activity. These data show that activation of protein kinase C occurs during alpha 1-adrenergic stimulation of cardiac myocytes and that elevation of intracellular Ca2+ amplifies this effect at least in part through increased phosphoinositide metabolism.

Modulation of alpha-adrenergic receptors and their intracellular coupling in the ischemic heart

The alpha 1-adrenergic receptor exists as at least two distinct subtypes, alpha 1a and alpha 1b. Based on hydrophobic exclusion studies and limited proteolysis of the cloned receptor, it appears to possess characteristics analogous to other membrane-bound receptors including seven membrane spanning domains, three extracellular, and three intracellular loops, with extensive glycosylation near the extracellular amino terminus. Although the receptor is coupled to phospholipase C in cardiac myocytes, with activation resulting in the production of inositol trisphosphate (IP3) and diacylglycerol, recent findings suggest that the receptor may also be linked to phospholipase A2, phospholipase D, and cyclic nucleotide phosphodiesterase. The alpha 1-adrenergic receptor has been shown to increase in response to myocardial ischemia in a number of different species and to mediate not only positive inotropic effects, but also to contribute substantially to arrhythmogenesis. The increase in alpha 1-adrenergic receptors can also occur in isolated adult ventricular myocytes in response to hypoxia, a mechanism which appears to be secondary to the sarcolemmal accumulation of long-chain acylcarnitines. This increase in alpha 1-adrenergic receptors in hypoxic myocytes is also linked to an enhanced increase in IP3 in response to receptor stimulation. These and other findings obtained in vivo during ischemia suggest that alpha 1-adrenergic mechanisms can become prominent in myocardium under pathophysiologic conditions in which a depressed contractile state exists and may therefore serve as a secondary inotropic system. However, the arrhythmogenic effects of stimulation of the alpha 1-adrenergic receptor in the ischemic heart in man may contribute substantially to arrhythmogenesis and, thereby, to the incidence of sudden cardiac death.