Of the ions involved in the intricate workings of the heart, calcium is considered perhaps the most important. It is crucial to the very process that enables the chambers of the heart to contract and relax, a process called excitation-contraction coupling. It is important to understand in quantitative detail exactly how calcium is moved around the various organelles of the myocyte in order to bring about excitation-contraction coupling if we are to understand the basic physiology of heart function. Furthermore, spatial microdomains within the cell are important in localizing the molecular players that orchestrate cardiac function.

Cardiac excitation–contraction coupling

Objective To attempt to determine the relative value of preclinical cardiac electrophysiology data (in vitro and in vivo) for predicting risk of torsade de pointes (TdP) in clinical use. Methods Published data on hERG (or I(Kr)) activity, cardiac action potential duration (at 90% repolarisation; APD(90)), and QT prolongation in dogs were compared against QT effects and reports of TdP in humans for 100 drugs. These data were set against the free plasma concentrations attained during clinical use (effective therapeutic plasma concentrations; ETPC(unbound)). The drugs were divided into five categories: (1) Class Ia and III antiarrhythmics; (2) Withdrawn from market due to TdP; (3) Measurable incidence/numerous reports of TdP in humans; (4) Isolated reports of TdP in humans; (5) No reports of TdP in humans. Results Data from hERG (or I(Kr)) assays in addition to ETPC(unbound) data were available for 52 drugs. For Category 1 drugs, data for hERG/I(Kr) IC(50), APD(90), QTc in animals and QTc in humans were generally close to or superimposed on the ETPC(unbound) values. This relationship was uncoupled in the other categories, with more complex relationships between the data. In Category 1 (except amiodarone), the ratios between hERG/I(Kr) IC(50) and ETPC(unbound) (max) ranged from 0.1- to 31-fold. Similar ranges were obtained for drugs in Category 2 (0.31- to 13-fold) and Category 3 (0.03- to 35-fold). A large spread was found for Category 4 drugs (0.13- to 35700-fold); this category embraced an assortment of mechanisms ranging from drugs which may well be affecting I(Kr) currents in clinical use (e.g. sparfloxacin) to others such as nifedipine (35700-fold) where channel block is not involved. Finally, for the majority of Category 5 drugs there was a >30-fold separation between hERG/I(Kr) activity and ETPC(unbound) values, with the notable exception of verapamil (1.7-fold), which is free from QT prolongation in man; this is probably explained by its multiple interactions with cardiac ion channels. Conclusions The dataset confirms the widely-held belief that most drugs associated with TdP in humans are also associated with hERG K(+) channel block at concentrations close to or superimposed upon the free plasma concentrations found in clinical use. A 30-fold margin between C(max) and hERG IC(50) may suffice for drugs currently undergoing clinical evaluation, but for future drug discovery programmes, pharmaceutical companies should consider increasing this margin, particularly for drugs aimed at non-debilitating diseases. However, interactions with multiple cardiac ion channels can either mitigate or exacerbate the prolongation of APD and QT that would ensue from block of I(Kr) currents alone, and delay of repolarisation per se is not necessarily torsadogenic. Clearly, an integrated assessment of in vitro and in vivo data is required in order to predict the torsadogenic risk of a new candidate drug in humans.

Relationships between preclinical cardiac electrophysiology, clinical QT interval prolongation and torsade de pointes for a broad range of drugs : Evidence for a provisional safety margin in drug development

Mitochondria play an important role in cell death and cardioprotection. During ischemia, when ATP is progressively depleted, ion pumps cannot function resulting in a rise in calcium (Ca2+), which f...

/pdf/mechanisms-underlying-acute-protection-from-cardiac-ischemia-12u9l7d6d9.pdf

Mechanisms Underlying Acute Protection From Cardiac Ischemia-Reperfusion Injury

Calcium (Ca) is a universal intracellular second messenger. In muscle, Ca is best known for its role in contractile activation. However, in recent years the critical role of Ca in other myocyte processes has become increasingly clear. This review focuses on Ca signaling in cardiac myocytes as pertaining to electrophysiology (including action potentials and arrhythmias), excitation-contraction coupling, modulation of contractile function, energy supply-demand balance (including mitochondrial function), cell death, and transcription regulation. Importantly, although such diverse Ca-dependent regulations occur simultaneously in a cell, the cell can distinguish distinct signals by local Ca or protein complexes and differential Ca signal integration.

Calcium cycling and signaling in cardiac myocytes.

Ventricular arrhythmias and contractile dysfunction are the main causes of death in human heart failure (HF). In a rabbit HF model reproducing these same aspects of human HF, we demonstrate that a 2-fold functional upregulation of Na(+)-Ca(2+) exchange (NaCaX) unloads sarcoplasmic reticulum (SR) Ca(2+) stores, reducing Ca(2+) transients and contractile function. Whereas beta-adrenergic receptors (beta-ARs) are progressively downregulated in HF, residual beta-AR responsiveness at this critical HF stage allows SR Ca(2+) load to increase, causing spontaneous SR Ca(2+) release and transient inward current carried by NaCaX. A given Ca(2+) release produces greater arrhythmogenic inward current in HF (as a result of NaCaX upregulation), and approximately 50% less Ca(2+) release is required to trigger an action potential in HF. The inward rectifier potassium current (I(K1)) is reduced by 49% in HF, and this allows greater depolarization for a given NaCaX current. Partially blocking I(K1) in control cells with barium mimics the greater depolarization for a given current injection seen in HF. Thus, we present data to support a novel paradigm in which changes in NaCaX and I(K1), and residual beta-AR responsiveness, conspire to greatly increase the propensity for triggered arrhythmias in HF. In addition, NaCaX upregulation appears to be a critical link between contractile dysfunction and arrhythmogenesis.

Arrhythmogenesis and contractile dysfunction in heart failure: Roles of sodium-calcium exchange, inward rectifier potassium current, and residual beta-adrenergic responsiveness.

The control of intracellular calcium is central to regulation of contractile force in cardiac muscle. This review illustrates how analysis of the control of calcium requires an integrated approach in which several systems are considered. Thus, the calcium content of the sarcoplasmic reticulum (SR) is a major determinant of the amount of Ca(2+) released from the SR and the amplitude of the Ca(2+) transient. The amplitude of the transient, in turn, controls Ca(2+) fluxes across the sarcolemma and thence SR content. This control of SR content influences the response to maneuvers that modify, for example, the properties of the SR Ca(2+) release channel or ryanodine receptor. Specifically, modulation of the open probability of the ryanodine receptor produces only transient effects on the Ca(2+) transient as a result of changes of SR content. These interactions between various Ca(2+) fluxes are modified by the Ca(2+) buffering properties of the cell. Finally, we predict that, under some conditions, the above interactions can result in instability (such as alternans) rather than ordered control of contractility.

Integrative Analysis of Calcium Cycling in Cardiac Muscle

The aim of this work was to investigate whether beat-to-beat alternation in the amplitude of the systolic Ca 2+ transient (Ca 2+ alternans) is due to changes of sarcoplasmic reticulum (SR) Ca 2+ content, and if so, whether the alternans arises due to a change in the gain of the feedback controlling SR Ca 2+ content. We found that, in rat ventricular myocytes, stimulating with small (20 mV) depolarizing pulses produced alternans of the amplitude of the Ca 2+ transient. Confocal measurements showed that the larger transients resulted from propagation of Ca 2+ waves. SR Ca 2+ content (measured from caffeine-evoked membrane currents) alternated in phase with the alternans of Ca 2+ transient amplitude. After a large transient, if SR Ca 2+ content was elevated by brief exposure of the cell to a Na + -free solution, then the alternans was interrupted and the next transient was also large. This shows that changes of SR Ca 2+ content are sufficient to produce alternans. The dependence of Ca 2+ transient amplitude on SR content was steeper under alternating than under control conditions. During alternation, the Ca 2+ efflux from the cell was also a steeper function of SR Ca 2+ content than under control. We attribute these steeper relationships to the fact that the larger responses in alternans depend on wave propagation and that wave propagation is a steep function of SR Ca 2+ content. In conclusion, alternans of systolic Ca 2+ appears to depend on alternation of SR Ca 2+ content. This, in turn results from the steep dependence on SR Ca 2+ content of Ca 2+ release and therefore Ca 2+ efflux from the cell as a consequence of wave propagation.

Sarcoplasmic Reticulum Calcium Content Fluctuation Is the Key to Cardiac Alternans

1. Intracellular calcium concentration ([Ca2+]i) and Na(+)-Ca2+ exchange currents were measured in calcium-overloaded voltage-clamped rat ventricular myocytes loaded with the Ca(2+)-sensitive fluorescent indicator indo-1. Sarcoplasmic reticulum (SR) Ca2+ content was measured from the integral of the caffeine-evoked current. In cells that had spontaneous SR Ca2+ release in 1 mM external Ca2+ concentration ([Ca2+]o)i raising [Ca2+]o increased the frequency of release with no effect on SR Ca2+ content. In quiescent cells, increased [Ca2+]o produced spontaneous Ca2+ release associated with increased SR Ca2+ content. Further increase of [Ca2+]o had no effect on SR Ca2+ content. The amount of Ca2+ leaving the cell during each release was constant over a wide range of frequencies and [Ca2+]o values. It appears there is a maximum level of SR Ca2+ content, perhaps because spontaneous Ca2+ release results when the content reaches a threshold. 2. From the relationship between [Ca2+]i and Na(+)-Ca2+ exchange current during a caffeine response, it is possible to estimate the changes in Na(+)-Ca2+ exchange current expected from a change of [Ca2+]i. The data show that the calcium oscillations contribute a significant fraction of the total extra Ca2+ efflux induced by increasing [Ca2+]o. Raising [Ca2+]o decreased the rate of calcium removal from the cell as measured from the rate of decay of the caffeine response, suggesting that both inhibition of Ca2+ efflux and increased Ca2+ entry account for the Ca2+ overload at elevated [Ca2+]o. 3. Inhibiting spontaneous SR Ca2+ release increases resting [Ca2+]i. The Ca2+ efflux is identical to that in the presence of release. It is concluded that spontaneous release of calcium, although potentially arrhythmogenic, is an effective way to activate Ca2+ efflux in overloaded conditions and minimizes any increase of diastolic tension.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-7793.1997.003bo.x

Measurement of sarcoplasmic reticulum Ca2+ content and sarcolemmal Ca2+ fluxes in isolated rat ventricular myocytes during spontaneous Ca2+ release.

This review discusses the mechanism and regulation of Ca release from the cardiac sarcoplasmic reticulum Ca is released through the Ca release channel or ryanodine receptor (RyR) by the process of calcium-induced Ca release (CICR) The trigger for this release is the L-type Ca current with a small contribution from Ca entry on the Na-Ca exchange Recent work has shown that CICR is controlled at the level of small, local domains consisting of one or a small number of L-type Ca channels and associated RyRs Ca efflux from the sr in one such unit is seen as a 'spark' and the properties of these sparks produce controlled Ca release from the sr A major factor controlling the amount of Ca released from the sr and therefore the magnitude of the systolic Ca transient is its Ca content The Ca content depends on both the properties of the sr and the cytoplasmic Ca concentration Changes of sr Ca content and the Ca released affect the sarcolemmal Ca and Na-Ca exchange currents and this acts to control cell Ca loading and the sr Ca content The opening probability of the RyR can be regulated by various physiological mediators as well as pharmacological compounds However, it is shown that, due to compensatory changes of sr Ca, modifiers of the RyR only produce transient effects on systolic Ca We conclude that, although the RyR can be regulated, of much greater importance to the control of Ca efflux from the sr are effects due to changes of sr Ca content

The control of Ca release from the cardiac sarcoplasmic reticulum: regulation versus autoregulation

1. The effects of modulating Ca2+-induced Ca2+ release (CICR) in single cardiac myocytes were investigated using low concentrations of caffeine (< 500 microM) in reduced external Ca2+ (0.5 mM). Caffeine produced a transient potentiation of systolic [Ca2+]i (to 800 % of control) which decayed back to control levels. 2. Caffeine decreased the steady-state sarcoplasmic reticulum (SR) Ca2+ content. As the concentration of caffeine was increased, both the potentiation of the systolic Ca2+ transient and the decrease in SR Ca2+ content were increased. At higher concentrations, the potentiating effect decayed more rapidly but the rate of recovery on removal of caffeine was unaffected. 3. A simple model in which caffeine produces a fixed increase in the fraction of SR Ca2+ which is released could account qualitatively but not quantitatively for the above results. 4. The changes in total [Ca2+] during systole were obtained using measurements of the intracellular Ca2+ buffering power. Caffeine initially increased the fractional release of SR Ca2+. This was followed by a decrease to a level greater than that under control conditions. The fraction of systolic Ca2+ which was pumped out of the cell increased abruptly upon caffeine application but then recovered back to control levels. The increase in fractional loss is due to the fact that, as the cytoplasmic buffers become saturated, a given increase in systolic total [Ca2+] produces a larger increase in free [Ca2+] and thence of Ca2+ efflux. 5. These results confirm that modulation of the ryanodine receptor has no maintained effect on systolic Ca2+ and show the interdependence of SR Ca2+ content, cytoplasmic Ca2+ buffering and sarcolemmal Ca2+ fluxes. Such analysis is important for understanding the cellular basis of inotropic interventions in cardiac muscle.

Modulation of CICR has no maintained effect on systolic Ca2+: simultaneous measurements of sarcoplasmic reticulum and sarcolemmal Ca2+ fluxes in rat ventricular myocytes.

Mary E. Díaz

Papers

Integrative Analysis of Calcium Cycling in Cardiac Muscle

Sarcoplasmic Reticulum Calcium Content Fluctuation Is the Key to Cardiac Alternans

Measurement of sarcoplasmic reticulum Ca2+ content and sarcolemmal Ca2+ fluxes in isolated rat ventricular myocytes during spontaneous Ca2+ release.

The control of Ca release from the cardiac sarcoplasmic reticulum: regulation versus autoregulation

Modulation of CICR has no maintained effect on systolic Ca2+: simultaneous measurements of sarcoplasmic reticulum and sarcolemmal Ca2+ fluxes in rat ventricular myocytes.