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

The Ca content of the sarcoplasmic reticulum (s.r.) was measured in voltage-clamped rat ventricular myocytes from the integral of the Na-Ca exchange current evoked by applying caffeine to release the s. r. Ca content. Following spontaneous release of Ca from the s.r., the s.r. Ca content was decreased. The magnitude of this decrease was equal to that of the amount of calcium directly measured to have been pumped out of the cell during the spontaneous release. Following a spontaneous release, the s.r. Ca content recovered linearly. These results are shown to be consistent with the hypothesis that the frequency of spontaneous release is determined by the time taken for the cell and s.r. to reaccumulate the Ca2+ ions pumped out of the cell during spontaneous release.

A measurable reduction of s.r. ca content follows spontaneous ca release in rat ventricular myocytes

1. The intracellular Ca2+ concentration ([Ca2+]i) was measured in mesenteric artery smooth muscle cells using the fluorescent indicator indo-1. 2. Noradrenaline (1-10 microM) produced a transient increase in [Ca2+]i. This response was unaffected by the removal of external calcium suggesting that the bulk of the increase in [Ca2+]i produced by noradrenaline is due to release from an intracellular store. 3. The maintained application of caffeine (10 mM) produced a transient rise in [Ca2+]i. The rate of relaxation was slower than that of the noradrenaline response. If caffeine was removed at the peak of the rise in [Ca2+]i then [Ca2+]i recovered more quickly than was the case in both the maintained response to noradrenaline and that to caffeine. 4. In the presence of noradrenaline, caffeine or thapsigargin elevated [Ca2+]i. However, if thapsigargin or caffeine was added first, the subsequent application of noradrenaline did not increase [Ca2+]i, suggesting that only part of the caffeine-sensitive store is sensitive to noradrenaline. 5. The recovery of [Ca2+]i during the application of caffeine was unaffected by the removal of external sodium suggesting that Na+-Ca2+ exchange is not important in the reduction in [Ca2+]i. The addition of lanthanum (1 mM) did, however, greatly slow [Ca2+]i recovery. 6. We conclude that the three major factors responsible for removing Ca2+ ions from the cytoplasm are: (i) a caffeine- and noradrenaline-sensitive store (43%), (ii) a caffeine-sensitive but noradrenaline-insensitive store (36%), and (iii) a sarcolemmal Ca(2+)-ATPase (16%). Finally, a 5% contribution remains to be accounted for.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1995.sp020514

Factors controlling changes in intracellular Ca2+ concentration produced by noradrenaline in rat mesenteric artery smooth muscle cells.

We have studied the kinetics of decay of cardiac Ca transients elicited by either caffeine or electrical stimulation. The decay of the caffeine-evoked increase of intracellular Ca concentration ([Ca2+]i) could not be fit by a single exponential. A two exponential fit showed an initial rapid component. The rate of decay of total Ca was calculated from measured free Ca and the buffering properties of the cell. There was no initial rapid phase of decay of total Ca. We conclude that the rapid phase of decay of free Ca is due to a decrease of Ca buffering power at elevated [Ca2+]i. In contrast, the decay of the Ca transient, produced by voltage-clamp depolarization or field stimulation was fit by a single exponential. We suggest that these apparently simple kinetics arise because a tendency to saturation at elevated [Ca2+]i of the sarcoplasmic reticulum Ca-ATPase offsets the decrease of Ca buffering power. These data show the importance of Ca buffers as well as transporters in determining the kinetics of changes of [Ca2+]i.

The role of intracellular Ca buffers in determining the shape of the systolic Ca transient in cardiac ventricular myocytes.

Spontaneous contraction of uterine smooth muscle is enhanced by alkalinization and depressed by acidification. We have investigated the ionic currents responsible for this in single myometrial cells. Intracellular acidification (20 mM butyrate) at constant external pH depressed the magnitude of the calcium current to 58±6% of control, but had little effect on outward currents. Similar but slower effects were also observed when the extracellular pH was lowered to 6.9 (56±9% of control). Correspondingly, when the intracellular or extracelluar pH was elevated (20 mM NH4Cl or pH 7.9 respectively) the calcium current magnitude increased (165±15 % in NH4Cl; 136±2 % at pH 7.9) and there was, again, no effect on the outward currents. These observations are consistent with the effects of pH on spontaneous contractile activity being due to an effect on the membrane calcium current.

Changes of pH affect calcium currents but not outward potassium currents in rat myometrial cells

Cardiac contraction is initiated by Ca2+ leaving the sarcoplasmic reticulum through the ryanodine receptor (RyR). Although opening of the RyR can be modified by various ligands, these have no maintained effect on contraction. We conclude that modulation of the RyR controls sarcoplasmic reticulum Ca2+ content rather than cytoplasmic Ca2+ concentration.

David A. Eisner

Papers

A measurable reduction of s.r. ca content follows spontaneous ca release in rat ventricular myocytes

Factors controlling changes in intracellular Ca2+ concentration produced by noradrenaline in rat mesenteric artery smooth muscle cells.

The role of intracellular Ca buffers in determining the shape of the systolic Ca transient in cardiac ventricular myocytes.

Changes of pH affect calcium currents but not outward potassium currents in rat myometrial cells

No Role for the Ryanodine Receptor in Regulating Cardiac Contraction