Excitation-contraction Coupling Gain Research Articles

Progression of Heart Failure

The cardiac ryanodine receptor (RyR2)/Ca2+ release channel on the sarcoplasmic reticulum (SR) is regulated by evolutionarily highly conserved signaling pathways that control excitation-contraction (EC) coupling in the heart. Phosphorylation of RyR2 by cAMP-dependent protein kinase (PKA) plays a key role in regulating the channel in response to stress via activation of the sympathetic nervous system (the “fight-or-flight response”).1 Maladaptive PKA hyperphosphorylation of RyR2 in failing hearts alters channel function, which may cause depletion of SR Ca2+ and diastolic release of SR Ca2+. This can initiate delayed afterdepolarizations that trigger ventricular arrhythmias.1 Mutations in RyR2 recently have been identified in patients with catecholaminergic induced sudden cardiac death (SCD).2–4⇓⇓ There may be a direct link between the PKA hyperphosphorylation of RyR2 that occurs during the progression of heart failure and fatal cardiac arrhythmias. Regulation of cardiac EC coupling by the release of Ca2+ from the SR via RyR2 in cardiomyocytes, known as Ca2+-induced Ca2+ release (CICR), has been appreciated for more than a decade.5,6⇓ Furthermore, it is well known that the amplitude of the Ca2+ transient generated by SR Ca2+ release determines contractile force in cardiomyocytes. The systems that regulate SR Ca2+ release include: (1) the triggers (predominantly Ca2+ influx through the voltage-gated Ca2+ channel on the plasma membrane); (2) the SR Ca2+ release channel or type 2 RyR2; and (3) the SR Ca2+ reuptake pump (SERCA2a) and its regulator phospholamban. These systems (trigger, release, and reuptake) are modulated by signaling pathways, including the β-adrenergic receptor (β-AR) signaling pathway (ie, phosphorylation by PKA). Activation of the sympathetic nervous system in response to stress results in elevation of cAMP levels and activation of PKA. Phosphorylation of RyR2 may not correlate directly …

Ryanodine receptors, FKBP12, and heart failure

RyR2 function is regulated by highly conserved signaling pathways that modulate excitation-contraction (EC) coupling. cAMP dependent protein kinase (PKA) phosphorylation of RyR2 plays an important role in regulating channel function in response to stress signaled by the sympathetic nervous system (the classic "fight or flight response") (1). PKA phosphorylation of RyR2 induces dissociation of the regulatory protein FKBP12.6 resulting in channels with increased sensitivity to Ca2+-induced Ca2+ release. Under normal physiological conditions (no cardiac damage) PKA phosphorylation of RyR2 is part of an integrated physiological response that leads to increased EC coupling gain and increased cardiac output. PKA-hyperphosphorylation of RyR2 in failing hearts is a maladaptive response that results in depletion of FKBP12.6 from the RyR2 macromolecular complex and defective channel function (pathologically increased sensitivity to Ca2+-induced Ca2+ release) that may cause depletion of SR Ca2+ and diastolic release of SR Ca2+ that can initiate delayed after depolarizations (DADs) that trigger ventricular arrhythmias (1). RyR2 mutations in patients with catecholaminergic induced sudden cardiac death provide further evidence linking the sympathetic nervous system, RyR2 and ventricular arrhythmias (2-4). The chronic hyperadrenergic state of heart failure is associated with defective Ca2+ signaling in part due to PKA hyperphosphorylation of RyR2.

Ryanodine receptors, FKBP12, and heart failure.

RyR2 function is regulated by highly conserved signaling pathways that modulate excitation-contraction (EC) coupling. cAMP dependent protein kinase (PKA) phosphorylation of RyR2 plays an important role in regulating channel function in response to stress signaled by the sympathetic nervous system (the classic "fight or flight response") (1). PKA phosphorylation of RyR2 induces dissociation of the regulatory protein FKBP12.6 resulting in channels with increased sensitivity to Ca2+-induced Ca2+ release. Under normal physiological conditions (no cardiac damage) PKA phosphorylation of RyR2 is part of an integrated physiological response that leads to increased EC coupling gain and increased cardiac output. PKA-hyperphosphorylation of RyR2 in failing hearts is a maladaptive response that results in depletion of FKBP12.6 from the RyR2 macromolecular complex and defective channel function (pathologically increased sensitivity to Ca2+-induced Ca2+ release) that may cause depletion of SR Ca2+ and diastolic release of SR Ca2+ that can initiate delayed after depolarizations (DADs) that trigger ventricular arrhythmias (1). RyR2 mutations in patients with catecholaminergic induced sudden cardiac death provide further evidence linking the sympathetic nervous system, RyR2 and ventricular arrhythmias (2-4). The chronic hyperadrenergic state of heart failure is associated with defective Ca2+ signaling in part due to PKA hyperphosphorylation of RyR2.

Heart failure after myocardial infarction: altered excitation-contraction coupling.

Heart failure (HF) frequently follows the occurrence of myocardial infarction (MI). Questions about how HF develops and what cellular defects contribute to this dysfunction led to this study. Methods and Results-- MI was induced in rats by coronary artery ligation. Clinical examination of the post-MI (PMI) surviving animals indicated that they were in overt HF by all measures. Cellular examination of the cardiomyocytes by patch-clamp and confocal [Ca(2+)](i) imaging methods indicated that cellular function was significantly compromised. At the single-cell level, [Ca(2+)](i) transient amplitudes were reduced and contractions were decreased and slowed, although Ca(2+) current (I(Ca)) remained unchanged. The excitation-contraction coupling (ECC) gain function measured as Delta[Ca(2+)](i)/I(Ca) was significantly decreased. Ouabain, a cardiotonic steroid that blocks the Na(+),K(+)-ATPase and activates Ca(2+) entry via cardiac Na(+) channels, largely alleviated this defect. After MI, I(Ca) becomes less able to trigger release of Ca(2+) from the sarcoplasmic reticulum. This failure of ECC is a major factor contributing to the development of contractile dysfunction and HF in PMI animals. The improved ECC gain, enhanced Ca(2+) entry, and augmented Ca(2+) signaling due to cardiotonic steroids contribute to the beneficial effects of these agents.

Functional coupling between glycolysis and excitation-contraction coupling underlies alternans in cat heart cells.

Electromechanical alternans was characterized in single cat atrial and ventricular myocytes by simultaneous measurements of action potentials, membrane current, cell shortening and changes in intracellular Ca2+ concentration ([Ca2+]i). Using laser scanning confocal fluorescence microscopy, alternans of electrically evoked [Ca2+]i transients revealed marked differences between atrial and ventricular myocytes. In ventricular myocytes, electrically evoked [Ca2+]i transients during alternans were spatially homogeneous. In atrial cells Ca2+ release started at subsarcolemmal peripheral regions and subsequently spread toward the centre of the myocyte. In contrast to ventricular myocytes, in atrial cells propagation of Ca2+ release from the sarcoplasmic reticulum (SR) during the small-amplitude [Ca2+]i transient was incomplete, leading to failures of excitation-contraction (EC) coupling in central regions of the cell. The mechanism underlying alternans was explored by evaluating the trigger signal for SR Ca2+ release (voltage-gated L-type Ca2+ current, ICa,L) and SR Ca2+ load during alternans. Voltage-clamp experiments revealed that peak ICa,L was not affected during alternans when measured simultaneously with changes of cell shortening. The SR Ca2+ content, evaluated by application of caffeine pulses, was identical following the small-amplitude and the large-amplitude [Ca2+]i transient. These results suggest that the primary mechanism responsible for cardiac alternans does not reside in the trigger signal for Ca2+ release and SR Ca2+ load. beta-Adrenergic stimulation with isoproterenol (isoprenaline) reversed electromechanical alternans, suggesting that under conditions of positive cardiac inotropy and enhanced efficiency of EC coupling alternans is less likely to occur. The occurrence of electromechanical alternans could be elicited by impairment of glycolysis. Inhibition of glycolytic flux by application of pyruvate, iodoacetate or beta-hydroxybutyrate induced electromechanical and [Ca2+]i transient alternans in both atrial and ventricular myocytes. The data support the conclusion that in cardiac myocytes alternans is the result of periodic alterations in the gain of EC coupling, i. e. the efficacy of a given trigger signal to release Ca2+ from the SR. It is suggested that the efficiency of EC coupling is locally controlled in the microenvironment of the SR Ca2+ release sites by mechanisms utilizing ATP, produced by glycolytic enzymes closely associated with the release channel.

Dynamic modulation of excitation‐contraction coupling by protein phosphatases in rat ventricular myocytes.

1. The effects of the serine/threonine protein phosphatases (PP) type 1 and 2A on L-type Ca2+ current (ICa) and the intracellular [Ca2+]i transient were examined in rat ventricular myocytes. ICa was measured under voltage clamp using patch-type microelectrodes in the whole-cell mode with the cells in a steady state of sarcoplasmic reticulum (SR) Ca2+ loading. [Ca2+]i transients were measured simultaneously using the fluorescent Ca2+ indicator indo-1 (50 microM) which was added to the pipette filling solution along with PP-1 or PP-2A (4 units ml-1). 2. PP-1 had no effect on the ICa-V relationship but decreased the [Ca2+]i-voltage relationship (by 43% at 0 mV). PP-2A decreased both ICa-V (by 26% at 0 mV) and the [Ca2+]i transient-voltage (by 65% at 0 mV). Excitation-contraction coupling gain, defined as (delta [Ca2+]i/ICa), was decreased to 43% of control by PP-1 and to 29% of control by PP-2A at-28 mV. 3. Diastolic [Ca2+]i (i.e.[Ca2+]i measured immediately before each voltage clamp pulse) was not altered by PP-1 or PP-2A and neither phosphatase changed steady-state SR Ca2+ content, as measured with caffeine. 4. We conclude that the reduced [Ca2+]i transient following the application of PP-1 was due to reduced SR Ca2+ release channel activity. The effects of PP-2A, while more broadly based, were still consistent with a decrease in SR Ca2+ release channel activity. 5. Our experiments, combined with recent experiments by others, suggest that the basal state of contractility in heart is dynamically regulated by dephosphorylation and phosphorylation of the SR Ca2+ release channel.