Beta-adrenergic stimulation of pig myocytes with decreased cytosolic free magnesium prolongs the action potential and enhances triggered activity

Magnesium, Oxidative Stress, Inflammation, and Cardiovascular Disease

Hypomagnesemia is commonly observed in heart failure, diabetes mellitus, hypertension, and cardiovascular diseases. Low serum magnesium (Mg) is a predictor for cardiovascular and all-cause mortality and treating Mg deficiency may help prevent cardiovascular disease. In this review, we discuss the possible mechanisms by which Mg deficiency plays detrimental roles in cardiovascular diseases and review the results of clinical trials of Mg supplementation for heart failure, arrhythmias and other cardiovascular diseases.

Effect of Carvacrol, TRP Channels Modulator, on Cardiac Electrical Activity

Despite the wide application of carvacrol (CAR) in medicines, dietary supplements, and foods, there is still insufficient electrophysiological data on the mechanisms of action of CAR, particularly with regard to heart function. Therefore, in this study, we attempted to elucidate whether CAR, whose inhibitory effect on both cardiac and vascular TRPM7 and L-type Ca²⁺ currents has been demonstrated previously, could modify cardiac electrical activity. We used a combination of optical mapping and microelectrode techniques to track the action potentials (APs) and the spread of electrical activity in a Langendorff-perfused rabbit heart model during atrial/endo/epicardial pacing. Simultaneously, ECG recordings were acquired. Because human trials on CAR are still lacking, we tested the action of CAR on human ventricular preparations obtained from explanted hearts. Activation time (AT), AP duration (APD), and conduction velocity maps were constructed. We demonstrated that at a low concentration (10 μM) of CAR, only marginal changes in the AP parameters were observed. At higher concentrations (≥100 μM), a decrease in AP upstroke velocity (dV/dt_max), suggesting inhibition of Na⁺ current, and APD (at 50 and 90% repolarization) was detected; also slowing in the spread of electrical signals via the atrioventricular node was observed, suggesting impaired functioning of Ca²⁺ channels. In addition, a decrease in the T-wave amplitude was seen on the ECG, suggesting an impaired repolarization process. Nevertheless, those changes occurred without a significant impact on the resting membrane potential and were reversible. We suggest that CAR might play a role in modulating cardiac electrical activity at high concentrations.

Estrogen and calcium handling proteins: new discoveries and mechanisms in cardiovascular diseases

Estrogen deficiency is considered to be an important factor leading to cardiovascular diseases (CVDs). Indeed, the prevalence of CVDs in postmenopausal women exceeds that of premenopausal women and men of the same age. Recent research findings provide evidence that estrogen plays a pivotal role in the regulation of calcium homeostasis and therefore fine-tunes normal cardiomyocyte contraction and relaxation processes. Disruption of calcium homeostasis is closely associated with the pathological mechanism of CVDs. Thus, this paper maps out and summarizes the effects and mechanisms of estrogen on calcium handling proteins in cardiac myocytes, including L-type Ca²⁺ channel, the sarcoplasmic reticulum Ca²⁺ release channel named ryanodine receptor, sarco(endo)plasmic reticulum Ca²⁺-ATPase, and sodium-calcium exchanger. In so doing, we provide theoretical and experimental evidence for the successful design of estrogen-based prevention and treatment therapies for CVDs.

Arrhythmias, elicited by catecholamines and serotonin, vanish in human chronic atrial fibrillation

Atrial fibrillation (AF) is the most common heart rhythm disorder. Transient postoperative AF can be elicited by high sympathetic nervous system activity. Catecholamines and serotonin cause arrhythmias in atrial trabeculae from patients with sinus rhythm (SR), but whether these arrhythmias occur in patients with chronic AF is unknown. We compared the incidence of arrhythmic contractions caused by norepinephrine, epinephrine, serotonin, and forskolin in atrial trabeculae from patients with SR and patients with AF. In the patients with AF, arrhythmias were markedly reduced for the agonists and abolished for forskolin, whereas maximum inotropic responses were markedly blunted only for serotonin. Serotonin and forskolin produced spontaneous diastolic Ca(2+) releases in atrial myocytes from the patients with SR that were abolished or reduced in myocytes from the patients with AF. For matching L-type Ca(2+)-current (ICa,L) responses, serotonin required and produced ∼ 100-fold less cAMP/PKA at the Ca(2+) channel domain compared with the catecholamines and forskolin. Norepinephrine-evoked ICa,L responses were decreased by inhibition of Ca(2+)/calmodulin-dependent kinase II (CaMKII) in myocytes from patients with SR, but not in those from patients with AF. Agonist-evoked phosphorylation by CaMKII at phospholamban (Thr-17), but not of ryanodine2 (Ser-2814), was reduced in trabeculae from patients with AF. The decreased CaMKII activity may contribute to the blunting of agonist-evoked arrhythmias in the atrial myocardium of patients with AF.

Ca efflux via the sarcolemmal Ca ATPase occurs only in the t-tubules of rat ventricular myocytes

The transverse (t-) tubule network is an important site for Ca influx and release during excitation-contraction coupling in cardiac ventricular myocytes; however, its role in Ca extrusion is less clear. The present study was designed to investigate the relative contributions of Ca extrusion pathways across the t-tubule and surface membranes. Ventricular myocytes were isolated from the hearts of adult male Wistar rats and detubulated using formamide. Intracellular Ca was monitored using fluo-3 and confocal microscopy. Caffeine (20 mmol/L) was used to induce SR Ca release; carboxyeosin (20 μmol/L) and nickel (10 mmol/L) were used to inhibit the sarcolemmal Ca ATPase and Na/Ca exchanger (NCX) respectively. Carboxyeosin decreased the rate constant of decay of the caffeine-induced Ca transient in control cells, but had no effect in detubulated cells, suggesting that Ca extrusion via the Ca ATPase occurs only across the t-tubule membrane. However nickel decreased the rate constant of the caffeine-induced Ca transient in control and detubulated cells, although its effect was greater in control cells, suggesting that Ca extrusion via NCX occurs across the surface and t-tubule membranes. The PKA inhibitor H-89 (10 μmol/L) was used to investigate the role of basal PKA activity in Ca extrusion; H-89 appeared to have no effect on Ca extrusion via the Ca ATPase, but reduced Ca extrusion via NCX at the t-tubules but not the surface membrane. Thus it appears that Ca extrusion via the sarcolemmal Ca ATPase occurs only at the t-tubules, and is not regulated by basal PKA activity, while Ca extrusion via NCX occurs across both the surface and t-tubule membranes, but predominantly across the t-tubule membrane due, in part, to localised stimulation of NCX by PKA at the t-tubules. This may be important in heart disease, in which changes in t-tubule structure and protein phosphorylation occur.

Alterations of atrial electrophysiology induced by electrolyte variations: combined computational and P-wave analysis

AIMS

Haemodialysis (HD) therapy represents a unique model to test in vivo, in humans, the effects of changes in plasma ionic concentrations. Episodes of paroxysmal atrial fibrillation (AF) often occur during the treatment. We investigated the effects of HD-induced electrolyte variations on atrial electrophysiology by analysing ECG P-wave duration (PWd), which reflects atrial conduction velocity (CV), and simulated atrial action potential (AP).

METHODS AND RESULTS

In 20 end-stage renal disease patients PWd (signal-averaged ECG), heart rate (HR), blood pressure, Na(+), K(+), Ca(2+), and Mg(2+) plasma concentrations were measured before and after HD session. The Courtemanche computational model of human atrial myocyte was used to simulate the atrial AP. AP upstroke duration (AP(ud)), AP duration and atrial cell effective refractory period (ERP) were computed. Extracellular electrolyte concentrations and HR were imposed to the average values measured in vivo. HD decreased K(+) (from 4.9 +/- 0.5 to 3.9 +/- 0.4 mmol/L, P < 0.001) and Mg(2+) (0.92 +/- 0.08 to 0.86 +/- 0.05 mmol/L, P < 0.05), and increased Na(+) (139.8 +/- 3.4 to 141.6 +/- 3.1 mmol/L, P < 0.05) and Ca(2+) (1.18 +/- 0.09 to 1.30 +/- 0.07 mmol/L, P < 0.001) plasma concentrations. PWd systematically increased in all the patients after HD (131 +/- 11 to 140 +/- 12 ms, P < 0.001), indicating an intra-atrial conduction slowing. PWd increments were inversely correlated with K(+) variations (R = 0.73, P < 0.01). Model-based analysis indicated an AP(ud) increase (from 2.58 to 2.94 ms) after HD, coherent with experimental observations on PWd, and a reduction of ERP by 12 ms.

CONCLUSION

Changes of plasma ionic concentrations may lead to modifications of atrial electrophysiology that can favour AF onset, namely a decrease of atrial CV and a decrease of atrial ERP.

High extracellular [Mg2+]-induced increase in intracellular [Mg2+] and decrease in intracellular [Na+] are associated with activation of p38 MAP kinase and ERK2 in guinea-pig heart

High extracellular Mg(2+) concentrations ([Mg(2+)](o)) caused a remarkable concentration-dependent and reversible increase in intracellular Mg(2+) concentrations ([Mg(2+)](i)) in beating and quiescent guinea-pig papillary muscles, accompanied by a definite decrease in intracellular Na(+) concentrations ([Na(+)](i)). A change in 1 mm [Mg(2+)](o) evoked a direct change in 0.0161 mm [Mg(2+)](i) and an inverse change in 0.0263 mm [Na(+)](i). Imipramine completely abolished the high [Mg(2+)](o)-induced decrease in [Na(+)](i) and remarkably diminished the high [Mg(2+)](o)-induced increase in [Mg(2+)](i) in papillary muscles. High [Mg(2+)](o) also produced a significant activation of p38 mitogen-activated protein (MAP) kinase and extracellular signal-related kinase 2 (ERK2) that was inhibited by pretreatment with imipramine. These results suggest that the high [Mg(2+)](o)-induced increase in [Mg(2+)](i) could be coupled with the decrease in [Na(+)](i), which might involve activation of the reverse mode of Na(+)-Mg(2+) exchange, accompanied by activation of p38 MAP kinase and ERK2 in the guinea-pig heart.

Muscarinic modulation of the sodium-calcium exchanger in heart failure

BACKGROUND

The Na-Ca exchanger (NCX) is a critical calcium efflux pathway in excitable cells, but little is known regarding its autonomic regulation.

METHODS AND RESULTS

We investigated beta-adrenergic receptor and muscarinic receptor regulation of the cardiac NCX in control and heart failure (HF) conditions in atrially paced pigs. NCX current in myocytes from control swine hearts was significantly increased by isoproterenol, and this response was reversed by concurrent muscarinic receptor stimulation with the addition of carbachol, demonstrating "accentuated antagonism." Okadaic acid eliminated the inhibitory effect of carbachol on isoproterenol-stimulated NCX current, indicating that muscarinic receptor regulation operates via protein phosphatase-induced dephosphorylation. However, in myocytes from atrially paced tachycardia-induced HF pigs, the NCX current was significantly larger at baseline but less responsive to isoproterenol compared with controls, whereas carbachol failed to inhibit isoproterenol-stimulated NCX current, and 8-Br-cGMP did not restore muscarinic responsiveness. Protein phosphatase type 1 dialysis significantly reduced NCX current in failing but not control cells, consistent with NCX hyperphosphorylation in HF. Protein phosphatase type 1 levels associated with NCX were significantly depressed in HF pigs compared with control, and total phosphatase activity associated with NCX was significantly decreased.

CONCLUSIONS

We conclude that the NCX is autonomically modulated, but HF reduces the level and activity of associated phosphatases; defective dephosphorylation then "locks" the exchanger in a highly active state.

Regulation of cation channels in cardiac and smooth muscle cells by intracellular magnesium

Magnesium regulates various ion channels in many tissues, including those of the cardiovascular system. General mechanisms by which intracellular Mg(2+) (Mg(i)(2+)) regulates channels are presented. These involve either a direct interaction with the channel, or an indirect modification of channel function via other proteins, such as enzymes or G proteins, or via membrane surface charges and phospholipids. To provide an insight into the role of Mg(i)(2+) in the cardiovascular system, effects of Mg(i)(2+) on major channels in cardiac and smooth muscle cells and the underlying mechanisms are then reviewed. Although Mg(i)(2+) concentrations are known to be stable, conditions under which they may change exist, such as following stimulation of beta-adrenergic receptors and of insulin receptors, or during pathophysiological conditions such as ischemia, heart failure or hypertension. Modifications of cardiovascular electrical or mechanical function, possibly resulting in arrhythmias or hypertension, may result from such changes of Mg(i)(2+) and their effects on cation channels.

Intravenous Cocaine and QT Variability

BACKGROUND

Dynamic instability in cardiac repolarization may contribute to drug-induced arrhythmogenesis. We hypothesized that intravenous cocaine would significantly destabilize repolarization as measured by QT variability.

METHODS AND RESULTS

Twenty-nine cocaine-experienced volunteers not seeking treatment for cocaine addiction received randomized, sequential intravenous infusions of placebo or cocaine (20 and 40 mg). Five-minute epochs of digitized ECG were recorded 10 minutes before, during, and at intervals following the infusions. QT variability was measured using a semiautomated method and expressed as the log ratio of normalized QT variance to normalized heart rate variance (QTVI). Seventeen subjects received a repeat course of cocaine infusions 1 week later. Placebo infusion resulted in a small but significant increase in QTVI, while cocaine caused a highly significant, dose-dependent increase in QTVI that peaked at 10 minutes and dissipated by 45 minutes following infusion (P < 0.0001). The increase in QTVI was reproducible at 1 week (P = 0.8).

CONCLUSIONS

Cocaine injection results in a significant dose-dependent increase in QT variability as indexed by QTVI. This destabilizing effect on repolarization may increase vulnerability to reentrant arrhythmias and may partially explain an increased risk of sudden cardiac death associated with cocaine use.

An evaluation of the impact of oral magnesium lactate on the corrected QT interval of patients receiving sotalol or dofetilide to prevent atrial or ventricular tachyarrhythmia recurrence

BACKGROUND

Intravenous magnesium reduces the QTc interval of patients receiving ibutilide. Whether oral magnesium can reduce the QTc interval associated with oral sotalol and dofetilide is not known. This study was undertaken to evaluate the impact of oral magnesium on the QTc interval and whether an inherent intracellular magnesium deficiency exists among patients with arrhythmias.

METHODS

Participants receiving sotalol or dofetilide for atrial or ventricular arrhythmias were randomized to receive magnesium l-lactate (504 mg elemental magnesium daily, Niche Pharmaceuticals, Roanoke, TX) or placebo for 48 hours. A 12-lead electrocardiogram (ECG) was obtained at baseline, 3 hours, and 51 hours after dosing to correspond to the Tmax after oral ingestion. The QTc interval was measured from the ECGs and compared between groups. Intracellular magnesium concentrations were determined by energy-dispersive x-ray analysis at baseline and 51 hours after dosing (Intracellular Diagnostics, Inc., Foster City, CA).

RESULTS

The QTc interval reductions from baseline were greater in the magnesium group than placebo at 3 and 51 hours (P = 0.015 and P < 0.001, respectively). Sixty-three percent of patients (regardless of experimental group) had baseline intracellular magnesium concentrations below the normal reference range of 33.9-41.9 mEq/IU, with an average level of 32.6 +/- 2.2 mEq/IU.

CONCLUSIONS

Oral magnesium l-lactate raises intracellular magnesium concentrations and lowers the QTc interval of patients receiving sotalol or dofetilide.

Effects of Intravenous Magnesium in a Prolonged QT Interval Model of Polymorphic Ventricular Tachycardia Focus on Transmural Ventricular Repolarization

BACKGROUND

This study was performed to clarify the antiarrhythmic effects of magnesium sulfate (Mg(++)) in a prolonged QT interval canine model of polymorphic ventricular tachyarrhythmia (VTA).

METHODS

In six experiments in a canine model of prolonged QT by anthopleurin-A, Mg(++) was administered in boluses of 0.2 mL/kg during repetitive episodes of self-terminating polymorphic VTA or frequent premature ventricular complexes (PVCs). The distribution of ventricular repolarization across the left ventricular(LV) wall and dispersion of transmural repolarization were analyzed before, and 30 and 120 seconds after Mg(++) administration, during ventricular pacing at 100 bpm. Transmural unipolar electrograms were recorded from multipolar needle electrodes, and local activation-recovery intervals (ARI) were measured.

RESULTS

Mg(++) rapidly eliminated self-terminating polymorphic VTA and all isolated PVCs. During ventricular pacing at 100 bpm, Mg(++) caused modest shortening of ARI at all recording sites. Since the magnitude of ARI shortening was greater at mid-myocardial sites than at other ventricular sites, mean transmural ARI dispersion decreased from 80 +/- 22 to 45 +/- 18 ms within 30 seconds after Mg(++) injection. However, this effect was transient, and, at 120 seconds after Mg(++) administration, ARI had increased all sites and transmural ARI dispersion lengthened to 65 +/- 18 ms. Besides suppression of triggered premature activity, homogenization of transmural ventricular repolarization was associated with the antiarrhythmic effects of intravenous Mg(++) in this model.

CONCLUSION

Since these effects were transient, a continuous intravenous infusion of Mg(++) is preferred to prevent recurrences of VTA.

Sudden death in familial polymorphic ventricular tachycardia associated with calcium release channel (ryanodine receptor) leak

BACKGROUND

Familial polymorphic ventricular tachycardia (FPVT) is characterized by exercise-induced arrhythmias and sudden cardiac death due to missense mutations in the cardiac ryanodine receptor (RyR2), an intracellular Ca2+ release channel required for excitation-contraction coupling in the heart.

METHODS AND RESULTS

Three RyR2 missense mutations, P2328S, Q4201R, and V4653F, which occur in Finnish families, result in similar mortality rates of approximately 33% by age 35 years and a threshold heart rate of 130 bpm, above which exercise induces ventricular arrhythmias. Exercise activates the sympathetic nervous system, increasing cardiac performance as part of the fight-or-flight stress response. We simulated the effects of exercise on mutant RyR2 channels using protein kinase A (PKA) phosphorylation. All 3 RyR2 mutations exhibited decreased binding of calstabin2 (FKBP12.6), a subunit that stabilizes the closed state of the channel. After PKA phosphorylation, FPVT-mutant RyR2 channels showed a significant gain-of-function defect consistent with leaky Ca2+ release channels and a significant rightward shift in the half-maximal inhibitory Mg2+ concentration (IC50). Treatment with the experimental drug JTV519 enhanced binding of calstabin2 to RyR2 and normalized channel function.

CONCLUSIONS

Sympathetic activation during exercise induces ventricular arrhythmias above a threshold heart rate in RyR2 mutation carriers. Simulating the downstream effects of the sympathetic activation by PKA phosphorylation of RyR2 channels containing these FPVT missense mutations produced a consistent gain-of-function defect. RyR2 function and calstabin2 depletion were rescued by JTV519, suggesting stabilization of the RyR2 channel complex may represent a molecular target for the treatment and prevention of exercise-induced arrhythmias and sudden death in these patients.

Protein kinase A hyperphosphorylation increases basal current but decreases beta-adrenergic responsiveness of the sarcolemmal Na+-Ca2+ exchanger in failing pig myocytes

The sodium-calcium exchanger (NCX) protein is the major cardiac calcium extrusion mechanism and is upregulated in heart failure (HF). NCX expression level and functional activity as regulated by beta-adrenergic receptor (beta-AR) stimulation in swine with and without tachycardia-induced heart failure were studied. The Ni2+-sensitive NCX current was measured in myocytes from HF and control animals in the basal state or in the presence of isoproterenol, forskolin, 8-Br-cAMP, okadaic acid, or protein phosphatase type 1. Western blot analysis revealed a significant increase in both the 120-kDa (29%) and 80-kDa (69%) fragments in HF (P<0.05 versus control). Despite this modest increase in protein, the basal peak outward NCX current was increased almost 5-fold in HF (P<0.05 versus control). Stimulation with isoproterenol, however, increased the control currents to a significantly greater extent than HF (500% increase in control versus 100% increase in HF, P<0.01); peak stimulated current was not different in HF and control. This reduction in responsiveness to beta-AR stimulation was refractory to forskolin, 8-Br-cAMP, or okadaic acid stimulation. In vitro protein kinase A back-phosphorylation revealed higher phosphorylation capacity of NCX protein in control versus HF, consistent with increased phosphorylation in vivo (hyperphosphorylation) in HF. Protein phosphatase type 1 exposure resulted in a significant reduction (73%) in peak basal current in HF (compared with no significant difference in controls), confirming that the increased basal NCX current in HF is predominantly attributable to hyperphosphorylation. NCX expression and activity are thus increased in HF, although beta-AR responsiveness is decreased because of NCX hyperphosphorylation.