Increased Risk for Atrial Alternans in Rabbit Heart Failure: The Role of Ca/Calmodulin-Dependent Kinase II and Inositol-1,4,5-trisphosphate Signaling.

Heart failure (HF) increases the probability of cardiac arrhythmias, including atrial fibrillation (AF), but the mechanisms linking HF to AF are poorly understood. We investigated disturbances in Ca signaling and electrophysiology in rabbit atrial myocytes from normal and failing hearts and identified mechanisms that contribute to the higher risk of atrial arrhythmias in HF. Ca transient (CaT) alternans-beat-to-beat alternations in CaT amplitude-served as indicator of increased arrhythmogenicity.

Activation of small conductance Ca -activated K channels suppresses Ca transient and action potential alternans in ventricular myocytes.

At the cellular level, cardiac alternans is observed as beat-to-beat alternations in contraction strength, action potential (AP) morphology and Ca transient (CaT) amplitude, and is a risk factor for cardiac arrhythmia. The (patho)physiological roles of small conductance Ca -activated K (SK) channels in ventricles are poorly understood. We tested the hypothesis that in single rabbit ventricular myocytes pharmacological modulation of SK channels plays a causative role for the development of pacing-induced CaT and AP duration (APD) alternans.

Mechanism of carvedilol induced action potential and calcium alternans.

Carvedilol is a nonspecific β-blocker clinically used for the treatment of cardiovascular diseases but has also been shown to have profound effects on excitation-contraction coupling and Ca signaling at the cellular level. We investigate the mechanism by which carvedilol facilitates Ca transient (CaT) and action potential duration (APD) alternans in rabbit atrial myocytes. Carvedilol lowered the frequency threshold for pacing-induced CaT alternans and facilitated alternans in a concentration-dependent manner.

L-type Ca channel recovery from inactivation in rabbit atrial myocytes.

Adaptation of the myocardium to varying workloads critically depends on the recovery from inactivation (RFI) of L-type Ca channels (LCCs) which provide the trigger for cardiac contraction. The goal of the present study was a comprehensive investigation of LCC RFI in atrial myocytes. The study was performed on voltage-clamped rabbit atrial myocytes using a double pulse protocol with variable diastolic intervals in cells held at physiological holding potentials, with intact intracellular Ca release, and preserved Na current and Na /Ca exchanger (NCX) activity.

Triggered Ca Waves Induce Depolarization of Maximum Diastolic Potential and Action Potential Prolongation in Dog Atrial Myocytes.

Background: We have identified a novel form of abnormal Ca wave activity in normal and failing dog atrial myocytes which occurs during the action potential (AP) and is absent during diastole. The goal of this study was to determine if triggered Ca waves affect cellular electrophysiological properties.

Methods: Simultaneous recordings of intracellular Ca and APs allowed measurements of maximum diastolic potential and AP duration during triggered calcium waves (TCWs) in isolated dog atrial myocytes.

Kindlin-2 modulates MafA and β-catenin expression to regulate β-cell function and mass in mice.

β-Cell dysfunction and reduction in β-cell mass are hallmark events of diabetes mellitus. Here we show that β-cells express abundant Kindlin-2 and deleting its expression causes severe diabetes-like phenotypes without markedly causing peripheral insulin resistance. Kindlin-2, through its C-terminal region, binds to and stabilizes MafA, which activates insulin expression.

Metabolic Inhibition Induces Transient Increase of L-type Ca Current in Human and Rat Cardiac Myocytes.

Metabolic inhibition is a common condition observed during ischemic heart disease and heart failure. It is usually accompanied by a reduction in L-type Ca channel (LTCC) activity. In this study, however, we show that metabolic inhibition results in a biphasic effect on LTCC current (I) in human and rat cardiac myocytes: an initial increase of I is observed in the early phase of metabolic inhibition which is followed by the more classical and strong inhibition.

Action potential shortening rescues atrial calcium alternans.

Key Points: Cardiac alternans refers to a beat-to-beat alternation in contraction, action potential (AP) morphology and Ca transient (CaT) amplitude, and represents a risk factor for cardiac arrhythmia, including atrial fibrillation. We developed strategies to pharmacologically manipulate the AP waveform with the goal to reduce or eliminate the occurrence of CaT and contraction alternans in atrial tissue. With combined patch-clamp and intracellular Ca measurements we investigated the effect of specific ion channel inhibitors and activators on alternans.

Metabolic inhibition reduces cardiac L-type Ca2+ channel current due to acidification caused by ATP hydrolysis.

Metabolic stress evoked by myocardial ischemia leads to impairment of cardiac excitation and contractility. We studied the mechanisms by which metabolic inhibition affects the activity of L-type Ca2+ channels (LTCCs) in frog ventricular myocytes. Metabolic inhibition induced by the protonophore FCCP (as well as by 2,4- dinitrophenol, sodium azide or antimycin A) resulted in a dose-dependent reduction of LTCC current (ICa,L) which was more pronounced during β-adrenergic stimulation with isoprenaline.

Alternans in atria: Mechanisms and clinical relevance.

Atrial fibrillation is the most common sustained arrhythmia and its prevalence is rapidly rising with the aging of the population. Cardiac alternans, defined as cyclic beat-to-beat alternations in contraction force, action potential (AP) duration and intracellular Ca release at constant stimulation rate, has been associated with the development of ventricular arrhythmias. Recent clinical data also provide strong evidence that alternans plays a central role in arrhythmogenesis in atria.

Membrane potential determines calcium alternans through modulation of SR Ca load and L-type Ca current.

Alternans is a risk factor for cardiac arrhythmia, including atrial fibrillation. At the cellular level alternans is observed as beat-to-beat alternations in contraction, action potential (AP) morphology and magnitude of the Ca transient (CaT). It is widely accepted that the bi-directional interplay between membrane voltage and Ca is crucial for the development of alternans, however recently the attention has shifted to instabilities in cellular Ca handling, while the role of AP alternation remains poorly understood.

Ca(2+)-activated chloride channel activity during Ca(2+) alternans in ventricular myocytes.

Cardiac alternans, defined beat-to-beat alternations in contraction, action potential (AP) morphology or cytosolic Ca transient (CaT) amplitude, is a high risk indicator for cardiac arrhythmias. We investigated mechanisms of cardiac alternans in single rabbit ventricular myocytes. CaTs were monitored simultaneously with membrane currents or APs recorded with the patch clamp technique.

Calcium-activated chloride current determines action potential morphology during calcium alternans in atrial myocytes.

Cardiac alternans--periodic beat-to-beat alternations in contraction, action potential (AP) morphology or cytosolic calcium transient (CaT) amplitude--is a high risk indicator for cardiac arrhythmias and sudden cardiac death. However, it remains an unresolved issue whether beat-to-beat alternations in intracellular Ca(2+) ([Ca(2+)]i ) or AP morphology are the primary cause of pro-arrhythmic alternans. Here we show that in atria AP alternans occurs secondary to CaT alternans.

The mechanisms of calcium cycling and action potential dynamics in cardiac alternans.

Rationale: Alternans is a risk factor for cardiac arrhythmia, including atrial fibrillation. At the cellular level alternans manifests as beat-to-beat alternations in contraction, action potential duration (APD), and magnitude of the Ca(2+) transient (CaT). Electromechanical and CaT alternans are highly correlated, however, it has remained controversial whether the primary cause of alternans is a disturbance of cellular Ca(2+) signaling or electrical membrane properties.

Optical mapping at increased illumination intensities.

Voltage-sensitive fluorescent dyes have become a major tool in cardiac and neuro-electrophysiology. Achieving high signal-to-noise ratios requires increased illumination intensities, which may cause photobleaching and phototoxicity. The optimal range of illumination intensities varies for different dyes and must be evaluated individually.

Photo-control of excitation waves in cardiomyocyte tissue culture.

Azobenzene photoswitches were recently reported to control the activity of neural cells and heart beat in leeches. Here, we report photocontrol of excitation of cultured cardiomyocytes that have been made light sensitive by using the addition of azobenzene trimethylammonium bromide (AzoTAB). The trans-isomer of AzoTAB reversibly suppresses spontaneous activity and propagation of excitation waves, whereas the cis-isomer has no detectable effect on the electrical properties of cardiomyocytes.

Gap junction channels exhibit connexin-specific permeability to cyclic nucleotides.

Gap junction channels exhibit connexin dependent biophysical properties, including selective intercellular passage of larger solutes, such as second messengers and siRNA. Here, we report the determination of cyclic nucleotide (cAMP) permeability through gap junction channels composed of Cx43, Cx40, or Cx26 using simultaneous measurements of junctional conductance and intercellular transfer of cAMP. For cAMP detection the recipient cells were transfected with a reporter gene, the cyclic nucleotide-modulated channel from sea urchin sperm (SpIH).

Cataracts are caused by alterations of a critical N-terminal positive charge in connexin50.

Purpose: To elucidate the basis of the autosomal dominant congenital nuclear cataracts caused by the connexin50 mutant, CX50R23T, by determining its cellular distribution and functional behavior and the consequences of substituting other amino acids for arginine-23.

Methods: Connexin50 (CX50) mutants were generated by PCR and transfected into HeLa or N2a cells. Expressed CX50 protein was detected by immunoblot analysis and localized by immunofluorescence.

Human connexin26 and connexin30 form functional heteromeric and heterotypic channels.

Mutations in GJB2 and GJB6, the genes that encode the human gap junction proteins connexin26 (Cx26) and connexin30 (Cx30), respectively, cause hearing loss. Cx26 and Cx30 are both expressed in the cochlea, leading to the potential formation of heteromeric hemichannels and heterotypic gap junction channels. To investigate their interactions, we expressed human Cx26 and Cx30 individually or together in HeLa cells.

Effect of hyperosmolarity on beta2-adrenergic stimulation in human atrium.

The purpose of the present study was to determine whether extracellular osmotic pressure modulates beta2-adrenergic stimulation of the contraction force and L-type Ca2+ current in human atrial myocytes. Experiments were performed on human atrial trabeculae and myocytes isolated from the right atrium. We have studied the effect of salbutamol (SAL), a beta2-adrenoceptor agonist, on peak tension (P), time to half peak tension (tc), time to half relaxation (tr), resting tension (contracture) (C) and L-type calcium current (ICaL) under isosmotic (345 mOsm) and hyperosmotic (525 mOsm in experiments for P, and 405 mOsm for ICaL) conditions.