Showing papers by "David A. Eisner published in 2007"

The sarcoplasmic reticulum and arrhythmogenic calcium release

Abstract: There is much evidence showing that some lethal ventricular arrhythmias arise from waves of Ca(2+) release from the sarcoplasmic reticulum (SR) that propagate along cardiac cells. The purpose of this review is to discuss the mechanism of production of these waves and how they depend on the properties of the SR Ca(2+) release channel or ryanodine receptor (RyR). The best-known method of producing Ca(2+) waves is by increasing the Ca(2+) content of the cell by either increasing Ca(2+) influx or decreasing efflux. Once SR Ca(2+) content reaches a threshold level a Ca(2+) wave is produced. Altering the properties of the RyR affects the threshold level of Ca(2+) required to produce a wave. Patients with a mutation in the RyR suffer from catecholaminergic polymorphic ventricular tachycardia, and this may be due to a decrease in the SR Ca(2+) threshold for wave production. Heart failure has also been suggested to result in Ca(2+) waves due to a leak of Ca(2+) through the RyR. We review the finding that these changes in RyR function will only result in Ca(2+) waves in the steady state if some other mechanism maintains the SR Ca(2+) content. The review concludes with a description of potential mechanisms for treating arrhythmias produced by Ca(2+) waves.

Increasing Ryanodine Receptor Open Probability Alone Does Not Produce Arrhythmogenic Calcium Waves: Threshold Sarcoplasmic Reticulum Calcium Content Is Required

Abstract: Diastolic waves of Ca(2+) release have been shown to activate delayed afterdepolarizations as well as some cardiac arrhythmias. The aim of this study was to investigate whether increasing ryanodine receptor open probability alone or in the presence of beta-adrenergic stimulation produces diastolic Ca release from the sarcoplasmic reticulum (SR). When voltage-clamped rat ventricular myocytes were exposed to caffeine (0.5 to 1.0 mmol), diastolic Ca(2+) release was seen to accompany the first few stimuli but was never observed in the steady state. We attribute the initial phase of diastolic Ca(2+) release to a decrease in the threshold SR Ca(2+) content required to activate Ca(2+) waves and its subsequent disappearance to a decrease of SR content below this threshold. Application of isoproterenol (1 micromol/L) increased the amplitude of the systolic Ca(2+) transient and also the SR Ca(2+) content but did not usually produce diastolic Ca(2+) release. Subsequent addition of caffeine, however, resulted in diastolic Ca(2+) release. We estimated the time course of recovery of SR Ca(2+) content following recovery from emptying with a high (10 mmol/L) concentration of caffeine. Diastolic Ca(2+) release recommenced only when SR content had increased back to its final level. We conclude that increasing ryanodine receptor open probability alone does not produce arrhythmogenic diastolic Ca(2+) release because of the accompanying decrease of SR Ca(2+) content. beta-Adrenergic stimulation increases SR content and thereby allows the increased ryanodine receptor open probability to produce diastolic Ca(2+) release. The implications of these results for arrhythmias associated with abnormal ryanodine receptors are discussed.

Does nitric oxide modulate cardiac ryanodine receptor function? Implications for excitation–contraction coupling

Abstract: Nitric oxide (NO) is a highly reactive, free radical signalling molecule that is constitutively released in cardiomyocytes by both the endothelial and neuronal isoforms of nitric oxide synthase (eNOS and nNOS, respectively). There are increasing data indicating that NO modulates various proteins involved in excitation-contraction coupling (ECC), and here we discuss the evidence that NO may modulate the function of the ryanodine receptor Ca(2+) release channel (RyR2) on the cardiac sarcoplasmic reticulum (SR). Both constitutive isoforms of NOS have been shown to co-immunoprecipitate with RyR2, suggesting that the channel may be a target protein for NO. eNOS gene deletion has been shown to abolish the increase in spontaneous Ca(2+) spark frequency in cardiomyocytes exposed to sustained stretch, whereas the effect of nNOS-derived NO on RyR2 function remains to be investigated. Single channel studies have been performed with RyR2 reconstituted in planar lipid bilayers and exposed to various NO donors and, under these conditions, NO appears to have a dose-dependent, stimulatory effect on channel open probability (P(open)). We discuss whether NO has a direct effect on RyR2 via covalent S-nitrosylation of reactive thiol residues within the protein, or whether there are downstream effects via cyclic nucleotides, phosphodiesterases, and protein kinases. Finally, we consider whether the proposed migration of nNOS from the SR to the sarcolemma in the failing heart may have consequences for the nitrosative vs. oxidative balance at the level of the RyR2, and whether this may contribute to an increased diastolic Ca(2+) leak, depleted SR Ca(2+) store, and reduced contractility in heart failure.

Regulation of systolic [Ca2+]i and cellular Ca2+ flux balance in rat ventricular myocytes by SR Ca2+, L-type Ca2+ current and diastolic [Ca2+]i

Abstract: The force-frequency response is an important physiological mechanism regulating cardiac output changes and is accompanied in vivo by beta-adrenergic stimulation. We sought to determine the role of sarcoplasmic reticulum (SR) Ca2+ content and L-type current (ICa-L) in the frequency response of the systolic Ca2+ transient alone and during beta-adrenergic stimulation. Experiments (on single rat ventricular myocytes) were designed to be as physiological as possible. Under current clamp stimulation SR Ca2+ content increased in line with stimulation frequency (1-8 Hz) but the systolic Ca2+ transient was maximal at 6 Hz. Under voltage clamp, increasing frequency decreased both systolic Ca2+ transient and ICa-L. Normalizing peak ICa-L by altering the test potential decreased the Ca2+ transient amplitude less than an equivalent reduction achieved through changes in frequency. This suggests that, in addition to SR Ca2+ content and ICa-L, another factor, possibly refractoriness of Ca2+ release from the SR contributes. Under current clamp, beta-adrenergic stimulation (isoprenaline, 30 nm) increased both the Ca2+ transient and the SR Ca2+ content and removed the dependence of both on frequency. In voltage clamp experiments, beta-adrenergic stimulation still increased SR Ca2+ content yet there was an inverse relation between frequency and Ca2+ transient amplitude and ICa-L. Diastolic [Ca2+]i increased with stimulation frequency and this contributed substantially (69.3 +/- 6% at 8 Hz) to the total Ca2+ efflux from the cell. We conclude that Ca2+ flux balance is maintained by the combination of increased efflux due to elevated diastolic [Ca2+]i and a decrease of influx on IC-L) on each pulse.

Na/Ca exchange: regulator of intracellular calcium and source of arrhythmias in the heart.

Abstract: The major effect of Na/Ca exchange (NCX) on the systolic Ca transient is secondary to its effect on the Ca content of the sarcoplasmic reticulum (SR). SR Ca content is controlled by a mechanism in which an increase of SR Ca produces an increase in the amplitude of the systolic Ca transient. This, in turn, increases Ca efflux on NCX as well as decreasing entry on the L-type current resulting in a decrease of both cell and SR Ca content. This control mechanism also changes the response to other maneuvers that affect excitation-contraction coupling. For example, potentiating the opening of the SR Ca release channel (ryanodine receptor, RyR) with caffeine produces an immediate increase in the amplitude of the systolic Ca transient. However, this increases efflux of Ca from the cell on NCX and then decreases SR Ca content until a new steady state is reached. Changing the activity of NCX (by decreasing external Na) changes the level of SR Ca reached by this mechanism. If the cell and SR are overloaded with Ca then Ca waves appear during diastole. These waves activate the electrogenic NCX and thereby produce arrhythmogenic-delayed afterdepolarizations. A major challenge is how to remove this arrhythmogenic Ca release without compromising the normal systolic release. We have found that application of tetracaine to decrease RyR opening can abolish diastolic release while simultaneously potentiating the systolic release.

Sarcoplasmic reticulum and mitochondria in cardiac pathophysiology

Abstract: Muscle cells, and in particular cardiomyocytes, consume a large amount of chemical energy to produce mechanical work. Myofilament contraction is regulated and synchronized within the cell by rapid, transient changes in the cytosolic Ca2+ concentration. Most of this calcium is released from the sarcoplasmic reticulum (SR), which has a large surface for Ca2+ transport, in close contact with the myofilaments. In between the myofilaments, cardiomyocytes are packed with mitochondria. These generate the energy needed for myofilament contraction, but serve also as an additional store for Ca2+. Thus, SR and mitochondria interact physiologically to control cellular Ca2+ homeostasis. It is partly through the Ca2+ loading of the mitochondria that their energy production is matched with the contractile energy demand. Mitochondria are now recognized as an essential effector structure determining cell death and survival. This is not only because of their central role in cellular energy metabolism, but also as key players in survival and death signalling. Alterations in the function of the SR are now known to be of primary importance for the development of hypertrophy and heart failure and in the genesis of life-threatening arrhythmias. Therefore, interaction between SR and mitochondria not only determines cardiomyocyte function under physiological conditions, but also is relevant for the development of cardiomyocyte dysfunction or death under pathophysiological conditions. The present Spotlight Issue of Cardiovascular Research aims to update our understanding of these important and rapidly evolving aspects of cardiac physiology and pathophysiology through a series of invited reviews and original contributions. In this issue, Orchard and Brette review the structure and function of t-tubules, their contribution to Ca2+ transients in cooperation with the SR, and the pathophysiological importance of their remodelling in … *Corresponding author. Tel: +34 93 489 4038; fax: +34 93 489 4032. E-mail address : dgdorado{at}ir.vhebron.net

Does the adenosine A2A receptor stimulate the ryanodine receptor

Abstract: A recent paper in Cardiovascular Research from Hove-Madsen and colleagues [1] has investigated the role of the adenosine A2A receptor in human atrial myocytes. Elegant confocal images show that this receptor is localized at the level of the Z line in the myocyte. Their paper then proceeds to address an important physiological question; the role of this receptor. No effect was found on the amplitude or voltage-dependence of the L-type Ca current. However agonists of the adenosine A2A receptor increased the frequency of both Ca sparks and Ca waves, phenomena that result from Ca induced Ca release (CICR) from the sarcoplasmic reticulum (SR). There was no change in either the amount of Ca extruded from the cell during each Ca wave or the SR Ca content (as assessed by the integral of the caffeine evoked Na–Ca exchange, NCX, current). Based on these …