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

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Physiology of Microglia

Spontaneous local increases in the concentration of intracellular calcium, called "calcium sparks," were detected in quiescent rat heart cells with a laser scanning confocal microscope and the fluorescent calcium indicator fluo-3 Estimates of calcium flux associated with the sparks suggest that calcium sparks result from spontaneous openings of single sarcoplasmic reticulum (SR) calcium-release channels, a finding supported by ryanodine-dependent changes of spark kinetics At resting intracellular calcium concentrations, these SR calcium-release channels had a low rate of opening (approximately 00001 per second) An increase in the calcium content of the SR, however, was associated with a fourfold increase in opening rate and resulted in some sparks triggering propagating waves of increased intracellular calcium concentration The calcium spark is the consequence of elementary events underlying excitation-contraction coupling and provides an explanation for both spontaneous and triggered changes in the intracellular calcium concentration in the mammalian heart

https://science.sciencemag.org/content/sci/262/5134/740.full.pdf

Calcium sparks: elementary events underlying excitation-contraction coupling in heart muscle

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

https://www.physiology.org/doi/pdf/10.1152/physrev.1999.79.3.763

Sodium/Calcium Exchange: Its Physiological Implications

https://www.cell.com/cell/pdf/0092-8674(95)90359-3.pdf

SCN5A mutations associated with an inherited cardiac arrhythmia, long QT syndrome

Background: Transverse (t)-tubules enable close coupling between L-type calcium (Ca) channels and ryanodine receptors (RyR) to facilitate triggered Ca release throughout the cell. In heart failure (HF) there is disruption of the t-tubule network that contributes to dyssynchronous Ca release. Despite the importance of t-tubules in triggering Ca release in the atria of large mammals, little is known about Ca release sites and how they are altered in HF. Objective: To investigate Ca release in the healthy and failing sheep atria and examine how this may be contributed to by ttubule and RyR remodelling. Methods: HF was induced in sheep by right ventricular tachypacing and left atrial myocytes isolated from control and HF animals. Cells were loaded with fluo-3 and stimulated under current clamp control. RyR structure was assessed using stochastic optical reconstruction microscopy (STORM). Results: In control atrial myocytes, triggered Ca release occurred at discrete sites on the surface and in the centre of the cell associated with t-tubules. In HF, t-tubule loss was accompanied by a reduction in central Ca release sites. As such, triggered Ca release was restricted to the cell surface with central Ca release decreased and reliant on propagation. Ca transient amplitude was decreased in both triggered and propagated sites in HF, and the difference between triggered and propagated release was exacerbated. The coefficient of variation for Ca release was greater in HF indicating variable amplitude of release between beats. As RyR cluster properties can influence Ca release, their structure was assessed. Atrial RyRs predominantly localise to the z-line but also to the cell surface. Compared to control, RyR clusters were smaller, more fragmented and further apart in HF which could perturb both triggered and propagated Ca release. Conclusion: Our data suggests loss of t-tubules, decreased Ca release at triggered and propagated sites and RyR cluster remodelling all contribute to the decrease in the atrial systolic Ca transient and dyssynchrony observed in HF.

Po-614-04 stabilizing cardiac ryanodine receptor with dantrolene prevents binge alcohol and caffeine induced ventricular tachyarrhythmias

Positive aspects of negative data

Transverse (t)-tubules drive the rapid and synchronous Ca2+ rise in cardiac myocytes. The virtual complete loss of atrial t-tubules in heart failure (HF) decreases Ca2+ release. It is unknown if or how atrial t-tubules can be restored and if restored t-tubules are functional. 

Sheep were tachypaced to induce HF and recovered when pacing was stopped. Serial block face Scanning Electron Microscopy and confocal imaging were used to understand t-tubule ultrastructure and function. Candidate proteins involved in atrial t-tubule recovery were identified by western blot and causality determined using expression studies.

Sheep atrial t-tubules reappeared following recovery from HF. Despite being disordered (branched, longer and longitudinally arranged) recovered t-tubules triggered Ca2+ release and were associated with restoration of systolic Ca2+. Telethonin and myotubularin abundance correlated with t-tubule density and altered the density and structure of BIN1-driven tubules in neonatal myocytes. Myotubularin had a greater effect, increasing tubule length and branching, replicating that seen in the recovery atria.

Recovery from HF restores atrial t-tubules and systolic Ca2+ and myotubularin facilitates this process. Atrial t-tubule restoration could present a new and

https://www.biorxiv.org/content/biorxiv/early/2021/10/28/2021.10.28.466162.full.pdf

Disordered yet functional atrial t-tubules on recovery from heart failure

https://rupress.org/jgp/article-pdf/154/3/e202213123/1428802/jgp_202213123.pdf

2022

https://rupress.org/jgp/article-pdf/155/4/e202313385/1450445/jgp_202313385.pdf

David A. Eisner

Papers

Po-614-04 stabilizing cardiac ryanodine receptor with dantrolene prevents binge alcohol and caffeine induced ventricular tachyarrhythmias

Positive aspects of negative data

Disordered yet functional atrial t-tubules on recovery from heart failure

2022

The Cranefield Awards 2022.