Antiarrhythmic effects of magnesium on rat papillary muscle and guinea pig ventricular myocytes

miRNAs Regulate hERG

BACKGROUND

The human ether-a-go-go-related gene (hERG) is the major molecular component of the rapidly activating delayed rectifier K⁺ current (I_kr ). Impairment of hERG function is believed to be a mechanism causing long-QT syndromes (LQTS). Growing evidences have shown that microRNAs (miRNAs) are involved in functional modulation of the hERG pathway. The purpose of this study was to screen and validate miRNAs that regulate the hERG pathway. The miRNAs identified in this study will provide new tools to assess the mechanism of LQTS.

METHODS

Six miRNAs were selected by algorithm predictions based on potential interaction with hERG. The effects of each miRNA on hERG were assessed by use of the Dual-Luciferase Reporter assay system, qRT-PCR, Western blotting, and confocal fluorescence microscopy. Furthermore, whole-cell patch clamp technique was used to validate the effect of miR-103a-1 on the electrophysiological characteristic of the I_kr of the hERG protein channel.

RESULTS

miR-134, miR-103a-1, miR-143, and miR-3619 significantly downregulated luciferase activity (P < 0.05) in a reporter test system. These 4 miRNAs significantly suppressed expression of hERG mRNA and protein in U2OS cells (P < 0.05).Corresponding AMOs rescued expression of hERG mRNA and protein. Confocal microscopy showed that all 4 miRNAs reduced the expression of both immature and mature hERG protein. miR-103a-1 decreased the maximum current and tail current amplitudes of hERG channel.

CONCLUSIONS

Expression and functions of hERG are regulated by specific miRNAs.

Diastolic spontaneous calcium release from the sarcoplasmic reticulum increases beat-to-beat variability of repolarization in canine ventricular myocytes after β-adrenergic stimulation

RATIONALE

Spontaneous Ca(2+) release (SCR) from the sarcoplasmic reticulum can cause delayed afterdepolarizations and triggered activity, contributing to arrhythmogenesis during β-adrenergic stimulation. Excessive beat-to-beat variability of repolarization duration (BVR) is a proarrhythmic marker. Previous research has shown that BVR is increased during intense β-adrenergic stimulation, leading to SCR.

OBJECTIVE

We aimed to determine ionic mechanisms controlling BVR under these conditions.

METHODS AND RESULTS

Membrane potentials and cell shortening or Ca(2+) transients were recorded from isolated canine left ventricular myocytes in the presence of isoproterenol. Action-potential (AP) durations after delayed afterdepolarizations were significantly prolonged. Addition of slowly activating delayed rectifier K(+) current (I(Ks)) blockade led to further AP prolongation after SCR, and this strongly correlated with exaggerated BVR. Suppressing SCR via inhibition of ryanodine receptors, Ca(2+)/calmodulin-dependent protein kinase II inhibition, or by using Mg(2+) or flecainide eliminated delayed afterdepolarizations and decreased BVR independent of effects on AP duration. Computational analyses and voltage-clamp experiments measuring L-type Ca(2+) current (I(CaL)) with and without previous SCR indicated that I(CaL) was increased during Ca(2+)-induced Ca(2+) release after SCR, and this contributes to AP prolongation. Prolongation of QT, T(peak)-T(end) intervals, and left ventricular monophasic AP duration of beats after aftercontractions occurred before torsades de pointes in an in vivo dog model of drug-induced long-QT1 syndrome.

CONCLUSIONS

SCR contributes to increased BVR by interspersed prolongation of AP duration, which is exacerbated during I(Ks) blockade. Attenuation of Ca(2+)-induced Ca(2+) release by SCR underlies AP prolongation via increased I(CaL.) These data provide novel insights into arrhythmogenic mechanisms during β-adrenergic stimulation besides triggered activity and illustrate the importance of I(Ks) function in preventing excessive BVR.

Protective effects of magnesium supplementation on metabolic energy derangements in lipopolysaccharide-induced cardiotoxicity in mice

Metabolic derangements and bioenergetic failure are major contributors to sepsis-induced multiple organ dysfunctions. Due to the well known role of magnesium (Mg) as a cofactor in many enzymatic reactions that involve energy creation and utilization, the present investigation was directed to estimate the cardioprotective effect of Mg supplementation in lipopolysaccharide (LPS)-induced metabolic energy changes in mice. Oral doses of Mg aspartate (20 or 40 mg/kg) were administered once daily for 7 day. Mice were then subjected to a single intraperitoneal injection of LPS (2 mg/kg). Plasma was separated 3 h after LPS injection for determination of creatine kinase-MB activity. Animals were then sacrificed and the hearts were separated for estimation of tissue thiobarbituric acid reactive substances, reduced glutathione, lactate, pyruvate, adenine nucleotides, creatine phosphate and cardiac Na(+),K(+)-ATPase activity. Finally, electron microscopic examination was performed to visualize the protective effects of Mg pretreatment on mitochondrial ultrastructure. In general, the higher dose of Mg was more effective than the lower dose in ameliorating creatine kinase-MB elevation and the state of oxidative stress, lactate accumulation, pyruvate reduction as well as preserving creatine phosphate, adenine nucleotides and Na(+),K(+)-ATPase activity. Moreover, the higher dose of Mg provided a significant cardioprotection against the mitochondrial ultrastructural changes. Mg therapy can afford a significant protection against metabolic energy derangements and mitochondrial ultrastructural changes induced by LPS cardiotoxicity in mice.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Ca²⁺-induced delayed afterdepolarizations are triggered by dyadic subspace Ca2²⁺ affirming that increasing SERCA reduces aftercontractions

Ca(2+)-induced delayed afterdepolarizations (DADs) are depolarizations that occur after full repolarization. They have been observed across multiple species and cell types. Experimental results have indicated that the main cause of DADs is Ca(2+) overload. The main hypothesis as to their initiation has been Ca(2+) overflow from the overloaded sarcoplasmic reticulum (SR). Our results using 37 previously published mathematical models provide evidence that Ca(2+)-induced DADs are initiated by the same mechanism as Ca(2+)-induced Ca(2+) release, i.e., the modulation of the opening of ryanodine receptors (RyR) by Ca(2+) in the dyadic subspace; an SR overflow mechanism was not necessary for the induction of DADs in any of the models. The SR Ca(2+) level is better viewed as a modulator of the appearance of DADs and the magnitude of Ca(2+) release. The threshold for the total Ca(2+) level within the cell (not only the SR) at which Ca(2+) oscillations arise in the models is close to their baseline level (∼1- to 3-fold). It is most sensitive to changes in the maximum sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pump rate (directly proportional), the opening probability of RyRs, and the Ca(2+) diffusion rate from the dyadic subspace into the cytosol (both indirectly proportional), indicating that the appearance of DADs is multifactorial. This shift in emphasis away from SR overload as the trigger for DADs toward a multifactorial analysis could explain why SERCA overexpression has been shown to suppress DADs (while increasing contractility) and why DADs appear during heart failure (at low SR Ca(2+) levels).

pH-dependent modulation of intracellular free magnesium ions with ion-selective electrodes in papillary muscle of guinea pig

A change in pH can alter the intracellular concentration of electrolytes such as intracellular Ca²⁺ and Na⁺ ([Na⁺]_i) that are important for the cardiac function. For the determination of the role of pH in the cardiac magnesium homeostasis, the intracellular Mg²⁺ concentration ([Mg²⁺]_i), membrane potential and contraction in the papillary muscle of guinea pigs using ion-selective electrodes changing extracellular pH ([pH]_o) or intracellular pH ([pH]_i) were measured in this study. A high CO₂-induced low [pH]_o causes a significant increase in the [Mg²⁺]_i and [Na⁺]_i, which was accompanied by a decrease in the membrane potential and twitch force. The high [pH]_o had the opposite effect. These effects were reversible in both the beating and quiescent muscles. The low [pH]_o-induced increase in [Mg²⁺]_i occurred in the absence of [Mg²⁺]_o. The [Mg²⁺]_i was increased by the low [pH]_i induced by propionate. The [Mg²⁺]_i was increased by the low [pH]_i induced by NH₄Cl-prepulse and decreased by the recovery of [pH]_i induced by the removal of NH₄Cl. These results suggest that the pH can modulate [Mg²⁺]_i with a reverse relationship in heart, probably by affecting the intracellular Mg²⁺ homeostasis, but not by Mg²⁺ transport across the sarcolemma.