Phosphorylation of the cardiac ryanodine receptor by Ca2+/calmodulin-dependent protein kinase II: the dominating twin of protein kinase A?
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Animals↗
Biomedical subjects
Burkert Pieske
Publications and source records attributed to Burkert Pieske.
Selective upregulation of beta1-adrenergic receptors and dephosphorylation of troponin I in end-stage heart failure patients supported by ventricular assist devices.
In terminal failing hearts, adrenergic receptors are downregulated and intracellular adrenergic signal transduction is inhibited. Mechanical circulatory support by ventricular assist devices (VAD) is used to bridge patients to heart transplantation. Mechanical unloading by VAD may induce reverse remodeling in heart transplantation (HTx) candidates. However, little is known on beta-adrenergic receptor subtype regulation and adrenergic signal transduction under VAD-support. We investigated paired myocardial samples from 16 VAD-supported patients and 9 non-failing donor hearts. We analyzed beta-adrenergic receptor subtype regulation by real-time PCR and radioligand binding and cardiac troponin I phosphorylation (by phospho-cTnI-specific antibodies). We found that the beta1-adrenergic receptor (beta1AR) is downregulated at VAD-implantation on mRNA and protein levels whereas the beta2-adrenergic receptor (beta2AR) was not. After VAD-support, beta1AR protein but not its mRNA was upregulated, whereas the degree of cTnI-phosphorylation was reduced. Upregulation of beta1AR was enhanced by beta blocking medication during VAD-support. However, in 9 out of 15 patients, beta1AR-density remained below the 0.25 percentile of donor hearts. VAD-support is associated with partial normalization of the betaAR-signal transduction pathways. This beneficial effect is related to a posttranscriptional increase in beta1AR-density.
Adult↗
Arginase: a modulator of myocardial function.
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Animals↗
Can brain natriuretic peptide be used to diagnose and manage patients with diastolic heart failure?
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Comment↗
Urocortin II enhances contractility in rabbit ventricular myocytes via CRF(2) receptor-mediated stimulation of protein kinase A.
OBJECTIVE: Urocortin II (UcnII), a peptide of the corticotropin-releasing factor (CRF) family, exerts profound actions on the cardiovascular system. Direct effects of UcnII on adult cardiomyocytes have not been evaluated before. Our aim was to characterize functional effects of UcnII on cardiomyocytes and to elucidate the underlying signaling pathway(s) and cellular mechanisms. METHODS: Rabbit ventricular cardiomyocytes were stimulated at 0.5 Hz (22-25 degrees C). Unloaded cell shortening (FS, edge detection), [Ca(2+)](i) transients (Fluo-4), and L-type Ca(2+) currents (I(Ca), whole-cell patch clamping) were measured. Sarcoplasmic reticulum (SR) Ca(2+) load was assessed by rapid application of caffeine (20 mmol/L). RESULTS: UcnII increased cell shortening and accelerated relaxation in a time- and concentration-dependent manner (EC(50): 10.7 nmol/L). The inotropic effect of UcnII was maximal at 100 nmol/L (35%+/-11% increase in FS, n=8, P<0.05). The inotropic and lusitropic actions of UcnII were largely eliminated by inhibition of CRF(2) receptors (10 nmol/L antisauvagine-30, n=5) or protein kinase A (PKA, 500 nmol/L H-89, n=5). UcnII increased [Ca(2+)](i) transient amplitude (by 63%+/-35%, n=7, P<0.05) and decreased the time constant for decay (from 800+/-63 to 218+/-27 ms, n=7, P<0.001). UcnII also increased SR Ca(2+) load (by 19%+/-7%, n=7, P<0.05) and fractional Ca(2+) release (from 57%+/-7% to 98%+/-2%, n=7, P<0.01). I(Ca) was augmented by 32.7%+/-10.0% (n=9, P<0.05) and the I(Ca)-V relationship was shifted by -15 mV during UcnII treatment. CONCLUSION: UcnII exerts positive inotropic and lusitropic effects in cardiomyocytes via CRF(2) receptor-mediated stimulation of PKA which augments I(Ca) and SR Ca(2+) load to increase SR Ca(2+) release and [Ca(2+)](i) transients.
Animals↗
N-terminal pro-brain natriuretic peptide or troponin testing followed by echocardiography for risk stratification of acute pulmonary embolism.
BACKGROUND: Brain natriuretic peptide (BNP) and N-terminal (NT)-proBNP have recently emerged as promising parameters for risk assessment in acute pulmonary embolism (PE). However, their positive predictive value is low, and the prognostic implications of NT-proBNP or troponin elevation alone are questionable. METHODS AND RESULTS: To determine whether the combination of NT-proBNP testing with echocardiography may identify both low-risk and high-risk patients with PE, we examined 124 consecutive patients with proved PE. All underwent echocardiography on admission to detect right ventricular dysfunction. NT-proBNP and troponin concentrations were measured in one core laboratory. The primary end point was death or major in-hospital complications. The cutoff level of 1000 pg/mL had a high negative predictive value (95% for a complicated course, 100% for death), but NT-proBNP > or =1000 pg/mL did not independently predict an adverse outcome. Combination of NT-proBNP testing with echocardiography identified 3 major risk groups. A positive echocardiogram was associated with a 12-fold elevation in complication risk compared with patients with low NT-proBNP (P=0.002), whereas NT-proBNP elevation without right ventricular dysfunction on echocardiography only slightly increased the risk of an adverse outcome (P=0.17). The combination of cardiac troponin testing with echocardiography yielded similar complication rates in the lowest-risk group and a similar magnitude of risk elevation for the highest-risk patients, but it also increased the number of intermediate-risk groups. CONCLUSIONS: Our results support a simple risk stratification algorithm for patients with PE, with the use of NT-proBNP or troponin testing as an initial step that should be followed by echocardiography if elevated levels of the biomarker are found.
Acute Disease↗
Increased SR Ca2+ cycling contributes to improved contractile performance in SERCA2a-overexpressing transgenic rats.
OBJECTIVE: Heart failure is associated with reduced function of sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2a) but increased function of sarcolemmal Na+/Ca2+ exchanger (NCX), leading to decreased SR Ca2+ content and loss of frequency-potentiation of contractile force. We reported that SERCA2a-overexpression in transgenic rat hearts (TG) results in improved contractility. However, it was not clear whether TG have improved contractility due to frequency-dependent improved SR Ca2+ handling. METHODS: Therefore, we characterized TG (n=35) vs. wild-type (WT) control rats (n=39) under physiological conditions (37 degrees C, stimulation rate <8 Hz). Twitch force, intracellular Ca2+ transients ([Ca2+]i), and SR Ca2+ content were measured in isolated muscles. The contribution of transsarcolemmal Ca2+ influx (I(Ca)) through L-type Ca2+ channels (LTCC) and reverse mode NCX (I(Na/Ca)) to Ca2+ cycling were studied in isolated myocytes. RESULTS: With increasing frequency, force increased in TG muscles by 168+/-35% (8 Hz; P<0.05) and SR Ca2+ content increased by maximally 118+/-31% (4 Hz; P<0.05). In WT, there was a flat force-frequency response without changes in SR Ca2+ content. Relaxation parameters of force and [Ca2+]i decay were accelerated at each frequency in TG vs. WT by approximately 10%. At prolonged rest intervals (<240 s), force and SR Ca2+ content increased significantly more in TG. Consequently, absolute SR Ca2+ content measured in myocytes was increased approximately 2-fold in TG. Transsarcolemmal Ca2+ fluxes estimated by I(Ca) (at 0 mV -10.2+/-1.1 vs. -16.9+/-1.3 pA/pF) and I(Na/Ca) (0.17+/-0.02 vs. 0.46+/-0.05 pA/pF) were decreased in TG vs. WT (P<0.05), whereas NCX and LTCC protein expression was only slightly reduced (P=n.s.). CONCLUSION: In summary, SERCA2a-overexpression improved contractility in a frequency-dependent way due to increased SR Ca2+ loading whereas transsarcolemmal Ca2+ fluxes were decreased.
Animals↗
Stretch-dependent modulation of [Na+]i, [Ca2+]i, and pHi in rabbit myocardium--a mechanism for the slow force response.
OBJECTIVE: Rabbit ventricular myocardium is characterized by a biphasic response to stretch with an initial, rapid increase in force followed by a delayed, slow increase in force (slow force response, SFR). The initial phase is attributed to increased myofilament Ca(2+) sensitivity, but the mechanisms of the delayed phase are only incompletely understood. We tested whether stretch-dependent stimulation of Na(+)/H(+) exchange (NHE1) and consecutive changes in pH(i) and/or [Na(+)](i) may underlie the SFR. METHODS: Isometric contractions of rabbit ventricular muscles were recorded in bicarbonate-containing Tyrode's (Tyrode) or bicarbonate-free HEPES-buffered solution (HEPES). Muscles were loaded with the Ca(2+) indicator aequorin, the pH indicator BCECF, or the Na(+) indicator SBFI and rapidly stretched from 88% (L(88)) to 98% (L(98)) of optimal length. The resulting immediate and slow increases in twitch force (1st phase and SFR) as well as changes in [Ca(2+)](i), [Na(+)](i), or pH(i) were quantified before and after inhibition of NHE1 by HOE 642 (3 microM) or reverse-mode Na(+)/Ca(2+) exchange (NCX) by KB-R 7943 (5 microM). RESULTS: In both Tyrode (n=21) and HEPES (n=22), developed force increased to approximately 160% during the 1st phase followed by a further increase to approximately 205% during the SFR. The SFR was accompanied by a 21% increase of the aequorin light transient (n=4; normalized to the 1st phase) and a approximately 3 mM increase in [Na(+)](i) (n=4-7). The SFR was also associated with an increase in pH(i). However, this increase was delayed and was significant only after the SFR had reached its maximum. The delayed pH(i) increase was larger in HEPES than in Tyrode. HOE 642 and/or KB-R 7943 reduced the SFR by approximately 30-40%. In addition, HOE 642 diminished the stretch-mediated elevation of [Na(+)](i) by 72% and the delayed alkalinization. CONCLUSIONS: The data are consistent with the hypothesis that SFR results from increases in [Ca(2+)](i) secondary to altered flux via NCX in part resulting from increases in [Na(+)](i) mediated by NHE1.
Animals↗
Insulin causes [Ca2+]i-dependent and [Ca2+]i-independent positive inotropic effects in failing human myocardium.
BACKGROUND: Insulin has been shown to exert positive inotropic effects in several in vitro and in vivo models, but signal transduction and substrate dependency remain unclear. We examined inotropic responses and signal transduction mechanisms of insulin in human myocardium. METHODS AND RESULTS: Experiments were performed in isolated trabeculae from end-stage failing hearts of 58 nondiabetic and 3 diabetic patients undergoing heart transplantation. The effect of insulin (0.3 and 3 IU/L) on isometric twitch force (37 degrees C, 1 Hz) was tested in the presence of glucose or pyruvate as energetic substrate. Furthermore, intracellular Ca2+ transients (aequorin method), sarcoplasmic reticulum (SR) Ca2+ content (rapid cooling contractures), and myofilament Ca2+ sensitivity (semiskinned fibers) were assessed. In addition, potential signaling pathways were tested by blocking glycolysis, PI-3-kinase, protein kinase C, diacylglycerol kinase, insulin-like growth factor-1 receptors, or transsarcolemmal Ca2+ entry via the Na+/Ca2+ exchanger. Insulin exerted concentration-dependent and partially substrate-dependent positive inotropic effects. The phosphatidylinositol-3-kinase inhibitor wortmannin and the Na2+/Ca2+ exchanger reverse-mode inhibitor KB-R7943 completely or partially prevented the functional effects of insulin. In contrast, insulin-like growth factor-1 receptor blockade, protein kinase C inhibition, and diacylglycerol kinase blockade were without effect. The inotropic response was associated with increases in intracellular Ca2+ transients, SR Ca2+ content, and increased myofilament Ca2+ sensitivity. CONCLUSIONS: Insulin exerts Ca2+-dependent and -independent positive inotropic effects through a phosphatidylinositol-3-kinase-dependent pathway in failing human myocardium. The increased [Ca2+]i originates at least in part from enhanced reverse-mode Na+/Ca2+ exchange and consequently increased SR-Ca2+ load. These nongenomic functional effects of insulin may be of clinical relevance, eg, during insulin-glucose-potassium infusions.
Actin Cytoskeleton↗
[Restrictive cardiomyopathy].
The group of restrictive cardiomyopathies (RKMP) includes a number of myocardial and endomyocardial diseases. These entities are characterized by stiffening of both ventricles resulting in a deterioration of ventricular filling and severe diastolic dysfunction. Systolic ventricular function is usually normal or only mildly impaired. Amyloidosis is the most prevalent underlying cause, but also sarcoidosis, hypereosinophilic syndrome with endocardial fibrosis and some inherited metabolic and storage diseases are common. Constrictive pericarditis is the main differential diagnosis, but noninvasive (magnetic resonance imaging, echocardiography, natriuretic peptides) and invasive diagnostic procedures allow separation of these two entities. Diagnosis of the underlying disease is mandatory for therapy of RKMP, as there are specific therapeutic options for symptom relief. Heart transplantation may be an option in end-stage heart failure.
Cardiomyopathy, Restrictive↗
Functional relevance of the stretch-dependent slow force response in failing human myocardium.
Stretch induces immediate and delayed inotropic effects in mammalian myocardium via distinct mechanosensitive pathways, but these effects are poorly characterized in human cardiac muscle. We tested the effects of stretch on immediate and delayed force response in failing human myocardium. Experiments were performed in muscle strips from 52 failing human hearts (37 degrees C, 1 Hz, bicarbonate buffer). Muscles were stretched from 88% of optimal length to 98% of optimal length. The resulting immediate and delayed (ie, slow force response [SFR]) increases in twitch force were assessed without and after blockade of the sarcoplasmic reticulum (SR; cyclopiazonic acid and ryanodine), stretch-activated ion channels (SACs; gadolinium, streptomycin), L-type Ca2+-channels (diltiazem), angiotensin II type-1 (AT1) receptors (candesartan), endothelin (ET) receptors (PD145065 or BQ123), Na+/H+ exchange (NHE1; HOE642), or reverse-mode Na+/Ca+ exchange (NCX; KB-R7493). We also tested the effects of stretch on SR Ca2+ load (rapid cooling contractures [RCCs]) and intracellular pH (in BCECF-loaded trabeculae). Stretch induced an immediate (<10 beats), followed by a slow (5 to 10 minutes), force response. Twitch force increased to 232+/-6% of prestretch value during the immediate phase, followed by a further increase to 279+/-8% during the SFR. RCC amplitude significantly increased, but pHi did not change during SFR. Inhibition of SACs, L-type Ca2+ channels, AT1 receptors, or ET receptors did not affect the stretch-dependent immediate or SFR. In contrast, the SFR was reduced by NHE1 inhibition and almost completely abolished by reverse-mode NCX inhibition or blockade of sarcoplasmic reticulum function. The data demonstrate the existence of a functionally relevant, SR-Ca2+-dependent SFR in failing human myocardium, which partly depends on NHE1 and reverse-mode NCX activation.
Biomechanical Phenomena↗
Improved systolic and diastolic myocardial function with intracoronary pyruvate in patients with congestive heart failure.
BACKGROUND: Pyruvate increases myocardial performance in isolated myocardium and improves hemodynamics in patients with congestive heart failure. AIMS: To investigate the influence of pyruvate on detailed parameters of systolic and diastolic left ventricular (LV) function. METHODS AND RESULTS: In patients with heart failure due to dilated cardiomyopathy (LVEF 30+/-4%, n=9) pyruvate was infused intracoronarily. LV function was analysed before, during and after application of different pyruvate concentrations using a LV-micromanometer catheter. LV volumes were determined using cine ventriculography. Pyruvate increased maximum rate of LV isovolumic pressure rise (Peak +dP/dt) from 802+/-106 to 1125+/-103 mmHg/s (P<0.05). Left ventricular end-diastolic pressure declined in parallel from 17+/-2 to 12+/-2 mmHg (P<0.05) and heart rate decreased from 79+/-4 to 72+/-5 min(-1) (P<0.05). Stroke volume index increased from 34+/-4 to 43+/-6 ml/m(2) (P<0.05), end-diastolic LV volume remained unchanged, thus left ventricular ejection fraction increased with pyruvate from 30+/-4 to 39+/-4% (P<0.05). Maximum rate of LV isovolumic pressure decline (Peak -dP/dt) was significantly increased with pyruvate (from 794+/-94 to 980+/-108 mmHg/s; P<0.05) and mean arterial pressure increased from 80+/-5 to 88+/-4 mmHg (P<0.05). Discontinuation of pyruvate resulted in immediate reversibility of its effects. CONCLUSION: Intracoronary pyruvate improves systolic and diastolic myocardial function and increases ejection fraction without increasing heart rate. Pyruvate thus exhibits the profile of a favourable inotropic agent, however, further investigation for the treatment of patients with acute heart failure is mandatory.
Adult↗
[Dyspnea and normal systolic function].
30-50% of patients presenting with symptoms of congestive heart failure exhibit a near normal left ventricular systolic function at rest, and an impaired diastolic function of the heart may be causative. Despite a better prognosis than in systolic heart failure, frequency of hospitalizations due to diastolic heart failure is comparable with systolic heart failure. According to the criteria of Vasan and Levy diagnosis of diastolic heart failure is probable, if symptoms and signs of heart failure are accompanied in proximity (within 72 h) by objective evidence of normal left ventricular systolic function. Newer echocardiographic techniques (e. g., tissue Doppler) aid to confirm the diagnosis and to determine the severity of dysfunction and may substitute invasive demonstration of impaired left ventricular relaxation, filling, compliance or stiffness for standardized diagnosis. Incorporation of biochemical test (BNP [brain natriuretic peptide]) allows differential diagnosis and may increase the accuracy of diagnosis. Due to inconsistent diagnostic criteria, data from prospective randomized controlled trials for the treatment of diastolic heart failure are rare. Basic principles include treatment of the underlying disease, i. e., control of hypertension, diabetes, or obstructive airway disease. Angiotensin 1 antagonists (ARB) have proven effective in regression of left ventricular hypertrophy (LIFE) and may reduce morbidity, but not mortality (CHARM). Maintenance of sinus rhythm, heart rate control (beta-blockers, calcium channel blockers) and anti-ischemic therapy may be indicated in view of pathophysiological aspects. Diuretics should be administered with caution in patients with symptoms of congestion, digitalis is not useful in the treatment of isolated diastolic heart failure. The results of ongoing trials (e. g., I-Preserve) may offer new therapeutic options, and evidence-based guidelines for the so far often unsatisfactory treatment of diastolic dysfunction/heart failure are awaited.
Diagnosis, Differential↗
Na(+)-Ca(2+) exchanger overexpression predisposes to reactive oxygen species-induced injury.
OBJECTIVE: In heart failure (HF), the generation of reactive oxygen species (ROS) is enhanced. It was shown that failing cardiac myocytes are more susceptible to ROS-induced damage, possibly due to increased expression of the sarcolemmal Na-Ca exchanger (NCX). METHODS: We investigated the consequences of increased expression levels of NCX in adult rabbit ventricular cardiomyocytes (via adenovirus-mediated gene transfer, Ad-NCX1-GFP) with respect to tolerance towards ROS. After 48-h incubation, cells were monitored for morphological changes on an inverted microscope. ROS were generated via hydrogen peroxide (H(2)O(2)) (100 micromol/l) and Fe(3+)/nitrilotriacetate (Fe(3+)/NTA, 100/200 micromol/l) for 4 min and cell morphology was followed over 30 min. [Na(+)](i) and [Ca(2+)](i) in native cells were measured using SBFI-AM and Indo1-AM, respectively. RESULTS: In native myocytes, exposure to ROS induced hypercontracture. This was accompanied by a 1.3-fold increase in diastolic Indo1 fluorescence ratio (P<0.05). Overexpression of NCX significantly enhanced development of hypercontracture. After 15 min, the percentage of cells that had undergone hypercontracture (F(hyper)) was 85+/-4% vs. only 44+/-10% in control cells (P<0.05). Inhibition of NCX-mediated Ca(2+) entry with KB-R7943 (5 micromol/l) reduced F(hyper) to 33+/-11% (P<0.05). [Na(+)](i) was increased 2.9-fold 1 min prior to hypercontracture (P<0.05). CONCLUSIONS: ROS-induced hypercontracture is due to Ca(2+) entry via NCX which could be triggered by a concomitant substantial increase in [Na(+)](i). Elevated NCX levels predispose to ROS-induced injury, a mechanism likely contributing to myocyte dysfunction and death in heart failure.
Adenoviridae↗
Determination of bleeding risk using genetic markers in patients taking phenprocoumon.
BACKGROUND: In patients on oral anticoagulation with warfarin, genetic variations of the cytochrome P 450-CYP2C9 have recently been associated with very low warfarin requirements. Patients needing low doses had an increased risk for bleeding complications. In Germany, phenprocoumon (having a similar metabolic pathway) is the most commonly employed vitamin K antagonist. Treatment is usually monitored by general practitioners (GPs). OBJECTIVES: To determine whether CYP2C9 variant alleles can serve as risk markers in general-practice patients anticoagulated with phenprocoumon. METHODS: All adult anticoagulated patients in 12 teaching general practices and one university outpatient clinic were to be recruited. Blood samples were taken from 185 patients during routine anticoagulation controls and tested for CYP2C9 mutations. Subjects answered a questionnaire concerning bleeding complications, drug intolerance, and personal and family medical history. Phenprocoumon dosages required for stable anticoagulation were recorded. Odds ratios (OR) with 95% confidence intervals (CI) were calculated based on 2-way cross-tabulations and multivariate logistic regression models, t-tests used where appropriate. RESULTS: Bleeding was reported by 19% of the patients, 2.2% of whom had suffered life-threatening bleeding. CYP2C9 variants were carried by 26.3% of 179 patients tested (17.9% *1/*2, 7.8% *1/*3, 0.6% *2/*3). While presence of a *2 allele was not associated with an increased risk (OR 0.35, CI 0.10-1.24), carriers of the rare *3 alleles had a higher risk of bleeding (OR 3.10, CI 1.02-9.40). With regard to bleeding, carrying CYP2C9*3 was highly specific (94%), though sensitivity was low at 17%; post-test probability of bleeding was 40%. CONCLUSIONS: CYP2C9*3 variants are associated with an increased bleeding risk in patients anticoagulated with phenprocoumon. Screening can identify patients with a high risk of bleeding. Appropriate clinical consequences (restricted indication for anticoagulation, careful induction, adjustment of target INR, closer monitoring or self-testing of INR) as well as the cost-effectiveness of screening for variant CYP2C9 with regard to patient outcomes should be subject of further research.
Aged↗
Stretch-dependent slow force response in isolated rabbit myocardium is Na+ dependent.
OBJECTIVE: Stretch induces functional and trophic effects in mammalian myocardium via various signal transduction pathways. We tested stretch signal transduction on immediate and slow force response (SFR) in rabbit myocardium. METHODS: Experiments were performed in isolated right ventricular muscles from adult rabbit hearts (37 degrees C, 1 Hz stimulation rate, bicarbonate-buffer). Muscles were rapidly stretched from 88% of optimal length (L88) to near optimal length (L98) for functional analysis. The resulting immediate and slow increases in twitch force (first phase and SFR, respectively) were assessed at reduced [Na+]o or without and with blockade of stretch activated ion channels (SACs), angiotensin-II (AT1) receptors, endothelin-A (ET(A)) receptors, Na+/H+-exchange (NHE1), reverse mode Na+/Ca2+-exchange (NCX), or Na+/K+-ATPase. The effects of stretch on sarcoplasmic reticulum Ca2+-load were characterized using rapid cooling contractures (RCCs). Intracellular pH was measured in BCECF-AM loaded muscles, and action potential duration (APD) was assessed using floating electrodes. RESULTS: On average, force increased to 216+/-8% of the pre-stretch value during the immediate phase, followed by a further increase to 273+/-10% during the SFR (n=81). RCCs significantly increased during SFR, whereas pH and APD did not change. Neither inhibition of SACs, AT1, or ET(A) receptors affected the stretch-dependent immediate phase nor SFR. In contrast, SFR was reduced by NHE inhibition and almost completely abolished by reduced [Na+]o or inhibition of reverse-mode NCX, whereas increased SFR was seen after raising [Na+]i by Na+/K+-ATPase inhibition. CONCLUSIONS: The data demonstrate the existence of a delayed, Na+- and Ca2+-dependent but pH and APD independent SFR to stretch in rabbit myocardium. This inotropic response appears to be independent of autocrine/paracrine AT1 or ET(A) receptor activation, but mediated through stretch-induced activation of NHE and reverse mode NCX.
Action Potentials↗
Serum and glucocorticoid inducible kinases in the regulation of the cardiac sodium channel SCN5A.
The serum and glucocorticoid inducible kinase SGK1 and its isoform SGK3 are both expressed in cardiac tissue. One of the functions of SGK1 is the phosphorylation and inactivation of the ubiquitin ligase Nedd4-2, which in turn could be shown to downregulate the voltage-gated Na+ channel SCN5A (hH1). The present study has been performed to test for a role of SGK1 and SGK3 in the regulation of SCN5A. To this end cRNA encoding the human Na+ channel SCN5A was injected into Xenopus laevis oocytes with or without cRNA encoding the wild-type kinases SGK1, the constitutively active kinase (S422D)SGK1, the inactive form K127NSGK1 or the wild-type SGK3. SCN5A currents were activated by coexpression of either wild-type SGK1 or SGK3 or the constitutively active S422DSGK1. In contrast, the inactive mutant K127NSGK1 significantly decreased the currents. Moreover, coexpression of SGK3 significantly altered SCN5A gating, i.e. it hyperpolarized the activation threshold and depolarized the prepotential required for 50% availability of the channel. Opposite shifts of gating properties were elicited by mutation of serine to alanine (S483ASCN5A and S663ASCN5A) in the SGK consensus sequences of SCN5A. The present observations disclose a role of the kinases SGK1 and SGK3 in the regulation of cardiac Na+ channels. As SGK1 is upregulated by glucocorticoids, mineralocorticoids and a variety of inflammatory mediators and both kinases are activated by insulin and IGF1, the kinases could mediate effects of those hormones and mediators on cardiac function.
Animals↗
[Na+]i handling in the failing human heart.
Proper contractile function of the heart depends on intact excitation-contraction processes and ion homeostasis of the myocytes. The Ca2+ ion activates contraction through its binding to troponin C. However, Ca2+ homeostasis is tightly linked to Na+ regulation because the primary mechanism for Ca2+ efflux in cardiac myocytes is via electrogenic Na+/Ca2+-exchange. While altered Ca2+-homeostasis has been demonstrated in animal models of heart failure and failing human cardiac tissue, the role of dysfunctional Na+ handling processes in altered excitation-contraction coupling remains obscure. Furthermore, altered Na+ handling has been implicated in a wide range of cellular processes, such as regulation of membrane potential, pH, and growth. This review will discuss (1) the evidence for altered [Na+]i homeostasis in the failing human heart, (2) how alterations in the Na+ electrochemical gradient can influence Ca2+ handling, contractile function, and a number of other cellular processes, and (3) the potential defects in Na+ channels and transporters that may underlie altered [Na+]i in the failing human heart.
Animals↗