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

1. The carotid body chemoreceptors are stimulated in situ by cyanide (CN-), which mimics the effect of hypoxia. We have shown that CN- increases a calcium-dependent potassium conductance (gK(Ca)) in single type I cells dissociated from the carotid body of the rabbit. We have now used the Ca2(+)-sensitive fluorophore, Fura-2, to measure intracellular Ca2+ directly in single type I cells. 2. CN- reversibly increased [Ca2+]i from approximately 90 nM to a mean of approximately 200 nM. Some of this Ca2+ originated from an intracellular store, which was depleted by exposure to Ca2(+)-free solutions. Prolonged application of CN- caused a sustained increase in [Ca2+]i, suggesting that CN- impairs the removal or sequestration of Ca2+. 3. pHi measured with the dye BCECF (2,7-bis(2-carboxyethyl)-5(and-6)-carboxyfluorescein) did not change consistently in response to CN-, although pHi changed predictably in response to both ammonium chloride and to acidification of the superfusate with CO2. 4. Potassium-induced depolarization (35 mM-K+) caused a large, cadmium-sensitive rise in [Ca2+]i. The K(+)-induced Ca2+ load was used to study the regulation of [Ca2+]i. 5. The clearance of a Ca2+ load was slowed either by removal of [Na+]o or by application of CN-. This shows that both a Na+-Ca2+ exchange and an energy-dependent process or processes contribute to the regulation of [Ca2+]i. 6. Carbachol (CCh, 10-100 microM), which also hyperpolarizes type I cells, caused a small transient rise in [Ca2+]i, indicating release from an exhaustible intracellular pool. The response to CN- was unaffected by prior or continued exposure to CCh, suggesting that the two stimuli operate by distinct mechanisms. 7. The increased gK(Ca) seen in type I cells in response to CN- thus reflects a change in cellular Ca2+ homeostasis. The rise in [Ca2+]i presumably underlies the documented increase in transmitter release from the carotid body in response to CN-. If chemotransduction is a consequence of the release of transmitters from the type I cell, the response of the carotid body to CN-, and possibly also to hypoxia, is thus a direct consequence of the energy dependence of Ca2+ homeostasis in the type I cell.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1989.sp017769

Measurements of intracellular Ca2+ in dissociated type I cells of the rabbit carotid body

Voltage clamp and tracer flux data: effects of a restricted extracellular space

Rationale: Spontaneous Ca 2+ release (SCR) from the sarcoplasmic reticulum can cause delayed afterdepolarizations and triggered activity, contributing to arrhythmogenesis during β-adrenergic stimulation. Excessive beat-to-beat variability of repolarization duration (BVR) is a proarrhythmic marker. Previous research has shown that BVR is increased during intense β-adrenergic stimulation, leading to SCR. Objective: We aimed to determine ionic mechanisms controlling BVR under these conditions. Methods and Results: Membrane potentials and cell shortening or Ca 2+ transients were recorded from isolated canine left ventricular myocytes in the presence of isoproterenol. Action-potential (AP) durations after delayed afterdepolarizations were significantly prolonged. Addition of slowly activating delayed rectifier K + current (I Ks ) blockade led to further AP prolongation after SCR, and this strongly correlated with exaggerated BVR. Suppressing SCR via inhibition of ryanodine receptors, Ca2+/calmodulin-dependent protein kinase II inhibition, or by using Mg 2+ or flecainide eliminated delayed afterdepolarizations and decreased BVR independent of effects on AP duration. Computational analyses and voltage-clamp experiments measuring L-type Ca 2+ current (I CaL ) with and without previous SCR indicated that I CaL was increased during Ca 2+ -induced Ca 2+ release after SCR, and this contributes to AP prolongation. Prolongation of QT, T peak -T end intervals, and left ventricular monophasic AP duration of beats after aftercontractions occurred before torsades de pointes in an in vivo dog model of drug-induced long-QT1 syndrome. Conclusions: SCR contributes to increased BVR by interspersed prolongation of AP duration, which is exacerbated during I Ks blockade. Attenuation of Ca 2+ -induced Ca 2+ release by SCR underlies AP prolongation via increased I CaL. These data provide novel insights into arrhythmogenic mechanisms during β-adrenergic stimulation besides triggered activity and illustrate the importance of I Ks function in preventing excessive BVR.

Diastolic Spontaneous Calcium Release from the Sarcoplasmic Reticulum Increases Beat-to-Beat Variability of Repolarization in Canine Ventricular Myocytes after β-Adrenergic Stimulation

Analysis of cellular calcium fluxes in cardiac muscle to understand calcium homeostasis in the heart.

We have investigated the mechanisms responsible for the changes of systolic Ca2+ that occur in voltage-clamped rat ventricular myocytes during acidosis produced by application of the weak acid butyrate (30 mM). Intracellular pH regulation was inhibited with dimethylamiloride (bicarbonate-free solution). The application of butyrate produced an intracellular acidification of 0.33 pH units. This was accompanied by a decrease in systolic Ca2+ to about 50% of control. However, within 2 min, systolic Ca2+ returned to control levels. The decrease in systolic Ca2+ was accompanied by a decrease in the Na+-Ca2+ exchange current observed on repolarisation so that the calculated Ca2+ efflux on Na+-Ca2+ exchange was less than the entry on the L-type Ca2+ current. The magnitude of the Na+-Ca2+ exchange current recovered along with systolic Ca2+ until it equalled the Ca2+ entry on the L-type Ca2+ current. From the measurement of Ca2+ fluxes, it was calculated that, during acidosis, the cell gains 121.6+/-16.2 micromol l(-1) of Ca2+. This is equal to the measured increase of sarcoplasmic reticulum (SR) calcium content obtained by applying caffeine (20 mM) and integrating the resulting Na+-Ca2+ exchange current. We conclude that the recovery of the amplitude of the systolic Ca2+ transient is due to decreased SR calcium release, resulting in reduced Ca2+ efflux from the cell leading to increased SR calcium content.

David A. Eisner

Papers

Measurements of intracellular Ca2+ in dissociated type I cells of the rabbit carotid body

Voltage clamp and tracer flux data: effects of a restricted extracellular space

Diastolic Spontaneous Calcium Release from the Sarcoplasmic Reticulum Increases Beat-to-Beat Variability of Repolarization in Canine Ventricular Myocytes after β-Adrenergic Stimulation

Analysis of cellular calcium fluxes in cardiac muscle to understand calcium homeostasis in the heart.

The effect of acidosis on systolic Ca2+ and sarcoplasmic reticulum calcium content in isolated rat ventricular myocytes