Biology subjects

Maltsev, V. A.

Publications and source records attributed to Maltsev, V. A..

14 recordsLinked to original sources

bioRxiv2026

Proton tunneling at the ryanodine receptor Ca2+ activation site provides temperature-invariant noise for robust Ca2+-induced Ca2+ release

Ryanodine receptor (RyR)-mediated Ca2+-induced Ca2+ release (CICR) is a regenerative trigger-to-release mechanism used in diverse cell types. RyRs amplify small Ca2+ signals into large, localized surges that drive autonomous oscillation, excitation-contraction coupling, pulsatile secretion, neurotransmitter release, or memory formation. Robust RyR function depends on the probability of RyR recruitment remaining within an operating range as physiological conditions change. This raises a basic question: what stabilizes the microscopic Ca2+-sensitive opening step of RyR across temperature and cellular state? Here we tested that proton tunneling at the conserved RyR2 Ca2+-activation site contributes a temperature-stable stochastic component to channel opening. To this end we developed a multiscale quantum-structural pipeline that estimates the per-channel modulation amplitude from cryo-EM geometry under stated assumptions. We applied this pipeline to a rabbit sinoatrial node cell model with explicit Ca2+ release unit architecture. Holding this RyR quantum noise constant across 25-37 {circ}C preserves pacemaker rhythmicity at low temperature, whereas a Q10-scaled classical noise results in irregular pacemaking activity as amplitude falls below the coherence-resonance range. Experimentally in isolated rabbit sinoatrial node cells, Ca2+ transient rate increased with temperature, but inter-transient interval variability did not, consistent with our numerical model predictions. These results support a model in which proton tunneling stabilizes the regenerative RyR-mediated CICR step. In autonomous oscillators, such as sinoatrial node cells, this manifests as robust rhythmicity. In triggered systems, it is expected to stabilize CICR gain, release synchrony, and release fidelity. The importance of this shared microscopic mechanism is evidenced by the conservation of the Ca2+-activation site across RyR isoforms for >600 million years of evolution. Significance StatementThis work suggests that RyR-based CICR networks can harness quantum-conditioned stochasticity. Proton tunneling at a conserved RyR activation site contributes a temperature-stable source of microscopic variability, and CICR amplifies that variability into robust physiological output. In pacemaker cells, this appears as stable rhythmic timing. More broadly, it provides a concrete route by which atomic-scale quantum events can influence macroscopic cellular function.

biophysics↗

bioRxiv2025

A New Fight-or-Flight Pacemaker Mechanism via Ryanodine Receptor Abundance and Superclustering

The sinoatrial node is the primary cardiac pacemaker. Individual sinoatrial node cells (SANCs) generate spontaneous rhythmic action potentials (APs) that initiate each heartbeat. The mechanism of SANC automaticity and its modulation by autonomic nervous system are based on the coupled function of molecules of both the cell membrane (ion channels, exchangers, and pumps) and the sarcoplasmic reticulum (SR), which generates rhythmic local Ca releases (LCRs). While LCRs are generated by ryanodine receptors (RyRs), the molecular-scale RyR network structure remains unknown. Here we performed single-molecule localization of RyRs via direct Stochastic Optical Reconstruction Microscopy (dSTORM) in rabbit SANCs in basal conditions and 5 minutes after {beta}-adrenergic receptor ({beta}AR) stimulation by isoproterenol. RyRs form clusters of various sizes, with a mean density of 67.7{+/-}13.2 RyR/m2. (Mean{+/-}SEM, 6 cells). While the majority of cluster sizes ranged from 3 to 32 RyRs, each cell had a few substantially larger clusters (>76 RyRs), dubbed superclusters. {beta}AR stimulation significantly increased the RyR density to 119.1{+/-}22.6 RyR/m2 (8 cells, p<0.05) and created more superclusters. Our new numerical SANC model showed that superclustering substantially decreased the AP cycle length (APCL) by creating Ca release hotspots that initiated larger LCRs under any condition. Increasing RyR density prolonged APCL in the basal state but shortened APCL during {beta}AR stimulation. With no change in RyR network, {beta}AR stimulation of only SR Ca pump and ion currents shortened APCL on average from 414.9 to 284 ms. When realistic higher RyR density and superclustering were added to the model, APCL further shortened to 231.9 ms. Thus, dynamic nanoscale changes in RyR network provide a new powerful pacemaker mechanism. This mechanism may help explain athletic bradycardia at rest and high exercise heart rates, with its deterioration contributing to the age-related loss of heart rate reserve. Author summaryThe heartbeat begins in the sinoatrial node, the hearts natural pacemaker. During stress or exercise, pacemaker cells accelerate their firing rate via {beta}-adrenergic ("fight-or-flight") stimulation. Although Ca signals are known to drive this acceleration via electrogenic Na/Ca exchanger, how the underlying ryanodine receptor (RyR) Ca release channels are organized at the nanoscale and how this organization changes during stimulation remain unclear. Here, we used super-resolution microscopy (dSTORM) to visualize individual RyRs in rabbit pacemaker cells. We found that RyRs form heterogeneous clusters that include rare, very large "superclusters." {beta}-adrenergic stimulation increased overall RyR density, enlarged clusters, and markedly increased the number of superclusters. To understand how these structural changes affect pacemaker function, we developed a computational sinoatrial node cell model in which each RyR was represented explicitly and arranged according to our imaging data. Numerical model simulations revealed that superclusters act as Ca-release hotspots that fire early during diastolic depolarization and recruit neighboring RyR clusters to fire via Ca-induce-Ca-release mechanism. When combined with {beta}-adrenergic enhancement of Ca cycling and membrane currents, the superclustering and RyR abundance synergistically accelerated action potential firing. Together, our results identified dynamic nanoscale RyR network reorganization as a key structural mechanism in fight-or-flight response.

biophysics↗

bioRxiv2024

Unique spatiotemporal synchronization solutions of heterogeneous local Ca2+ dynamics underlie the formation of each impulse that emerges from the cardiac sinoatrial node

We used both linear and nonlinear analyses to determine how information processing within and among incessant heterogeneous local Ca2+ oscillation (LCO) results in formation of rhythmic impulses in mouse SAN ex vivo. Phase analysis delineated a network of functional pacemaker cell clusters, distinguished by their LCO amplitudes, kinetics, and phases. Cross-talk of LCO dynamics within the network culminated in rhythmic SAN global Ca2+ transients (CaTs), having a mean rate and rhythm identical to that recorded by the reference sharp electrode in the right atria, indicating that CaTs are induced by global SAN electrical impulses. Initial conditions of each impulse and subsequent LCO ensemble evolution during an impulse differed from each other, associated with an apparent stochastic process (carrying a degree of uncertainty) within network. A small pacemaker cluster located near the crista terminalis (CT) exhibited the highest degrees of intrinsic power, earliest rotor-like energy transfer, most frequent point-to-point instability, earliest acrophase, greatest impulse-to-impulse variability within the SAN functional cluster network. Unique, variable small-world network Ca2+ information sharing within and among all clusters during initial and terminal impulse phases, created a unique solution for each impulse, while preserving the identity of each cluster (a highly efficient form of information processing at low wiring and energy costs). Cross-recurrence analysis verified that LCO dynamics within the small cluster near CT were more stochastic and less deterministic than those of the other clusters, indicating that this small cluster took the lead in the initiation of the SAN impulse and that the others followed.

cell biology↗

bioRxiv2024

Cardiac Pacemaker Cells Harness Stochastic Resonance to Ensure Fail-Safe Operation at Low Rates Bordering on Sinus Arrest

BACKGROUNDThe sinoatrial node (SAN) is the primary pacemaker of the heart. Recent high-resolution imaging showed that synchronized action potentials (APs) exiting the SAN emerge from heterogeneous signals, including subthreshold signals in non-firing (dormant) cells. This raises a new question in cardiac biology: how do these signals contribute to heartbeat generation? Here, we tested the hypothesis that pacemaker cells harness stochastic resonance to ensure fail-safe operation, especially at low rates bordering on sinus arrest. METHODSMembrane potential and Ca signals were measured using perforated-patch recordings in rabbit SAN cells exposed to sine-wave or white-noise currents. Additionally, we imaged Ca signals in intact mouse SAN tissue and performed multiscale model simulations at the subcellular, cellular, and tissue levels. RESULTSIn addition to classical synchronized Ca transients, SAN tissue exhibited heterogeneous local Ca signals of different kinetics. Noise currents, mimicking the heterogeneous natural cell environment, restored AP firing in dormant cells and substantially improved the rate and rhythm of those firing infrequently and irregularly. The benefit followed a bell-shaped curve: the performance improved but then declined, demonstrating a hallmark of stochastic resonance. Rhythmic AP generation in response to sine-wave currents of different frequencies defined a resonance spectrum in SAN cells, reflecting their ability to respond via stochastic resonance to specific frequency components embedded in noise. Cholinergic stimulation shifted the resonance spectrum and responses to noise toward lower frequencies across all amplitudes tested, rendering cells unresponsive to higher-frequency signals while enabling more effective processing of slower signals. Both the numerical models and simultaneous recordings of membrane potential and Ca dynamics demonstrated that stochastic resonance is amplified by coupled electrical and Ca signaling, enhancing AP generation at low noise levels. Adding noise currents to the cell and tissue models allowed firing under conditions where they otherwise would have stopped. CONCLUSIONSSAN cells harness stochastic resonance amplified by coupled membrane-Ca signaling to ensure rhythmic heartbeat initiation, especially at low rates. This new signaling mechanism could help avoid sinus arrest when heart slows but noise increases, such as during parasympathetic stimulation, bradyarrhythmia, or aging.

biophysics↗

bioRxiv2024

Structure-Function Relationship of the Ryanodine Receptor Cluster Network in Sinoatrial Node Cells

The rate of spontaneous action potentials (APs) generated by sinoatrial node cells (SANC) is regulated by local Ca2+ release (LCR) from the sarcoplasmic reticulum via Ca2+ release channels (ryanodine receptors, RyRs). LCR events propagate and self-organize within the network of RyR clusters (Ca release units, CRUs) via Ca-induced-Ca-release (CICR) that depends on CRU sizes and locations: while larger CRUs generate stronger release signals, the networks topology governs signal diffusion and propagation. This study used super-resolution structured illumination microscopy to image the 3D network of CRUs in rabbit SANC. The peripheral CRUs formed a spatial mesh, reflecting the cell surface geometry. Two distinct subpopulations of CRUs were identified within each cell, with size distributions conforming to a two component Gamma mixture model. Furthermore, neighboring CRUs exhibited repulsive behavior. Functional properties of the CRU network were further examined in a novel numerical SANC model developed using our experimental data. Model simulations revealed that heterogeneities in both CRU sizes and locations facilitate CICR and increase AP firing rate in a cooperative manner. However, these heterogeneities reduce the effect of {beta}-adrenergic stimulation in terms of its relative change in AP firing rate. The presence of heterogeneities in both sizes and locations allows SANC to reach higher absolute AP firing rates during {beta}-adrenergic stimulation. Thus, the CICR facilitation by heterogeneities in CRU sizes and locations regulates and optimizes cardiac pacemaker cell operation under various physiological conditions. Dysfunction of this optimization could be a key factor in heart rate reserve decline in aging and disease.

biophysics↗

bioRxiv2023

A novel conceptual model of heart rate autonomic modulation based on a small-world modular structure and heterogeneous innervation of the sinoatrial node

The current theory of cardiac pacemaker rate modulation by the autonomic nervous system is based on the concept that a primary pacemaker cell or a group of cells in the center of the sinoatrial node (SAN) can change its AP firing rate within a broad range, driving respective myocardium contractions to commensurate body demands. Experimental data show, however, that pacemaker cells are extremely heterogeneous, with different areas of the SAN or cell clusters specializing to drive APs at specific rates. Thus, higher heart rates under stress are mainly driven by cell clusters in superior SAN, whereas low rates by cell clusters in inferior SAN, with basal state rates generated somewhere in the middle of the node. Cells within different clusters feature different intrinsic electrophysiological and Ca cycling properties, sympathetic and parasympathetic innervation, and vasculature, thereby supporting effective shift of the system to an optimal rate (under given conditions) accompanied by respective shifts in the leading pacemaker site. Thus, the popular single-cell-based pacemaker theory does not capture this complex emerging paradigm of pacemaker function of SAN tissue revealed by recent experimental studies. Here we propose a more realistic, conceptual model of heart rate autonomic modulation based on these studies. Our new model (the gear model) simulates the SAN as a brain-like structure featuring a small world of loosely connected clusters (functional modules) of tightly coupled cells, modeled as Maltsev-Lakatta coupled-clock system. One module (the higher chronotropic gear) generates higher AP rates in basal state and under {beta}-adrenergic stimulation, but its activity is strongly suppressed by parasympathetic stimulation. The other module (the lower gear) generates lower rates and has low sensitivity to parasympathetic stimulation. Such modular, gear-like system reproduces the respective shifts of the leading pacemaker site observed experimentally and features a wide range of rate modulation and robust function whilst conserving energy. In perspective, future refinement and application of this new pacemaker tissue mechanism will provide better understanding of cardiac pacemaker function, its deterioration in aging and disease, and ultimately the creation of new therapies to treat sick sinus syndrome and other SAN function-related cardiac arrythmias.

biophysics↗

bioRxiv2022

The paradigm shift: heartbeat initiation without the pacemaker cell

The current dogma about the heartbeat origin is based on "the pacemaker cell", a specialized cell residing in the sinoatrial node (SAN) that exhibits spontaneous diastolic depolarization triggering rhythmic action potentials (APs). Recent high-resolution imaging, however, demonstrated that Ca signals and APs in the SAN are heterogeneous, with many cells generating APs of different rates and rhythms or even remaining non-firing (dormant cells), i.e. generating only subthreshold signals. Here we numerically tested a hypothesis that a community of dormant cells can generate normal automaticity, i.e. "the pacemaker cell" is not required to initiate rhythmic cardiac impulses. Our model includes (i) non-excitable cells generating oscillatory local Ca releases and (ii) an excitable cell lacking automaticity. While each cell in isolation was not "the pacemaker cell", the cell system generated rhythmic APs: the subthreshold signals of non-excitable cells were transformed into respective membrane potential oscillations via electrogenic Na/Ca exchange and further transferred and integrated (computed) by the excitable cells to reach its AP threshold, generating rhythmic pacemaking. Conclusions: Cardiac impulse is an emergent property of the SAN cellular network and can be initiated by cells lacking intrinsic automaticity. Cell heterogeneity, weak coupling, subthreshold signals, and their summation are critical properties of the new pacemaker mechanism, i.e cardiac pacemaker can operate via a signaling process basically similar to that of "temporal summation" happening in a neuron with input from multiple presynaptic cells. The new mechanism, however, does not refute the classical pacemaker cell-based mechanism: both mechanisms can co-exist and interact within SAN tissue.

biophysics↗

bioRxiv2022

Adenosine Reduces Sinoatrial Node Cell AP Firing Rate by uncoupling its Membrane and Calcium Clocks

AO_SCPLOWBSTRACTC_SCPLOWThe spontaneous action potential (AP) firing rate of sinoatrial nodal cells (SANC) is regulated by a system of intracellular Ca2+ and membrane ion current clocks driven by Ca2+-calmodulin-activated adenylyl cyclase-protein kinase A (PKA) signaling. The mean AP cycle length (APCL) and APCL variability inform on the effectiveness of clock coupling. Endogenous ATP metabolite adenosine (ado) binds to adenosine receptors that couple to Gi protein-coupled receptors, reducing spontaneous AP firing rate via G{beta}{gamma} signaling that activates an membrane-clock outward current, IKACh. Ado also inhibits adenylyl cyclase activity via Gi signaling, impacting cAMP-mediated PKA-dependent protein phosphorylation and intracellular Ca2+ cycling. We hypothesize that in addition to IKAdo activation, ado signaling impacts Ca2+ via Gi signaling and that both effects reduce AP firing rate by reducing the effectiveness of the Ca2+ and membrane clock coupling. To this end, we measured Ca2+ and membrane potential characteristics in enzymatically isolated single rabbit SANC. 10 {micro}M ado substantially increased both the mean APCL (on average by 43%, n=10) and AP beat-to-beat variability from 5.1{+/-}1.7% to 7.2{+/-}2.0% (n=10) measured via membrane potential and 5.0{+/-}2.2 to 10.6{+/-}5.9 (n=40) measured via Ca2+ (assessed as the coefficient of variability, CV=SD/mean). These effects were mediated by hyperpolarization of the maximum diastolic membrane potential (membrane clock effect) and suppression of diastolic spontaneous, local Ca2+ releases (LCRs) (Ca2+ clock effect): as LCR size distributions shifted from larger to smaller values, the time of LCR occurrence during diastolic depolarization (LCR period) became prolonged, and the ensemble LCR Ca2+ signal became reduced. The tight linear relationship of coupling between LCR period to the APCL in the presence of ado "drifted" upward and leftward, i.e. for a given LCR period, APCL was prolonged, becoming non-linear indicating clock uncoupling. An extreme case of uncoupling occurred at higher ado concentrations (>100 {micro}M): small stochastic LCRs of the Ca2+ clock failed to self-organize and synchronize to the membrane clock, thus creating a failed attempt to generate an AP resulting in arrhythmia and cessation of AP firing. Thus, the effects of ado to activate G{beta}{gamma} and IKACh, Ado and to activate Gi, suppressing adenylyl cyclase activity, both contribute to the ado-induced increase in the mean APCL and APCL variability by reducing the fidelity of clock coupling and AP firing rate.

physiology↗

bioRxiv2021

Functional heterogeneity of cell populations increases robustness of pacemaker function in a numerical model of the sinoatrial node tissue

Each heartbeat is initiated by specialized pacemaker cells operating within the sinoatrial node (SAN). While individual cells within SAN tissue exhibit substantial heterogeneity of their electrophysiological parameters and Ca cycling, the role of this heterogeneity for cardiac pacemaker function remains mainly unknown. Here we investigated the problem numerically in a 25x25 square grid of coupled-clock Maltsev-Lakatta cell models and tested the hypothesis that functional heterogeneity of cell populations increases robustness of SAN function. The tissue models were populated by cells with different degree of heterogeneity of the two key model parameters of the coupled-clock system, maximum L-type Ca current conductance (gCaL) and sarcoplasmic reticulum Ca pumping rate (Pup). Our simulations showed that in the areas of Pup-gCaL parametric space at the edge of the system stability where action potential (AP) firing was absent or dysrhythmic in tissues populated by identical cells, rhythmic AP generation was rescued in tissues populated by cells with uniformly random distributions of gCaL or Pup (but keeping the same average values). This effect to increase robust AP generation was synergistic with respect to heterogeneity in both gCaL and Pup and was further strengthened by clustering of cells with higher gCaL or Pup. The effect of functional heterogeneity was not due to a simple summation of activity of intrinsically firing cells naturally present in SAN; rather AP firing cells locally and critically interacted with non-firing/dormant cells. When firing cells prevailed, they recruited many dormant cells to fire, strongly enhancing overall SAN function. And vice versa, prevailing dormant cells suppressed AP firing in cells with intrinsic automaticity and halted SAN automaticity.

systems biology↗

bioRxiv2021

Disorder in Ca2+ Release Unit Locations Confers Robustness but Cuts Flexibility of Heart Pacemaking

Excitation-contraction coupling kinetics are dictated by the rate and rhythm of the excitations generated by sinoatrial-nodal cells. These cells generate local Ca releases (LCRs) that activate Na/Ca exchanger current, which accelerates diastolic depolarization and determines the rate and rhythm of the excitations. The LCRs are generated by clusters of ryanodine receptors, Ca release units (CRUs), residing in the sarcoplasmic reticulum. While the spatial CRU distribution in pacemaker cells exhibits substantial heterogeneity, it remains unknown if it has any functional importance. Using numerical modeling, here we showed that with a square lattice distribution of CRUs, Ca-induced-Ca-release propagation during diastolic depolarization is insufficient for pacemaking within a broad lower range of realistic ICaL densities. Allowing each CRU to deviate from its original lattice position fundamentally changes the model behavior: during diastolic depolarization sparks propagate, forming LCRs observed experimentally. As disorder in the CRU positions increases, the CRU distribution exhibits larger empty spaces but simultaneously CRU clusters, as in Poisson clumping. Propagating within the clusters, Ca release becomes synchronized, increasing AP firing rate and reviving pacemaker function within lower ICaL densities at which cells with lattice CRU distribution were dormant/non-firing. However, cells with fully disordered CRU positions cannot reach low firing rates and their {beta}-adrenergic receptor stimulation effect was substantially decreased. Thus, order/disorder in CRU locations regulates Ca release propagation and could be harnessed by pacemaker cells to regulate their function. Excessive disorder is expected to limit heart rate range that may contribute to heart rate range decline with age and in disease. SummaryThe present numerical modeling study shows that disorder in locations of Ca release units in cardiac pacemaker cells has substantial functional impact by creating release clusters, similar to Poisson clumping, and opportunity of Ca release to propagate within the clusters.

biophysics↗

bioRxiv2021

Universal inverse square relationship between heart rate variability and heart rate

In our previous study, we analyzed heart rate variability and heart rate from a large variety of cardiac preparations (including humans, living animals, Langendorff-perfused isolated hearts, and single sinoatrial nodal cells) in diverse species, combining our data with those of previously published articles. The analysis revealed that regardless of conditions, heart rate variability (for the purposes of the study assessed as standard deviation of beat-to-beat intervals) vs. heart rate follows a universal exponential decay-like relationship. Numerical simulations of diastolic interval variability by adding a randomly fluctuating term (Iper) to net current revealed a similar relationship. In the present study, using a Taylor series, we found that this relationship is, in fact, inverse square, and we derive an explicit formula for the standard deviation (sd) of the cycle length (CL) as a function of heart rate (HR) with biophysically meaningful parameters: sd(CL)=sd(Iper)*(60,000/mean(HR) -APD)^2/({Delta}V*C), where CL is in ms, HR in beats per minute, Iper in pA, APD in ms is an average AP duration of pacemaker cells, C in pF is cell membrane capacitance, and {Delta}V is the magnitude of diastolic depolarization in mV. This relationship gives direct insight into heart rate variability mechanisms at the basic level of individual pacemaker cells, i.e. their intrinsic CL variability linked to stochastic operation of ion channels (both Ca release and cell membrane channels) generating Iper. Our explicit formula may be also used for a more precise biomedical interpretation of heart rate variability after respective corrections for heart rate.

biophysics↗

bioRxiv2021

β-adrenergic stimulation synchronizes a broad spectrum of action potential firing rates of cardiac pacemaker cells towards a higher population average

The heartbeat is initiated by pacemaker cells residing in the sinoatrial node (SAN). SAN cells generate spontaneous action potentials (APs), i.e. normal automaticity. The sympathetic nervous system increases heart rate commensurate with cardiac output demand via stimulation of SAN {beta}-adrenergic receptors ({beta}AR). While SAN cells reportedly represent a highly heterogeneous cell population, the current dogma is that in response to {beta}AR stimulation all cells increase their spontaneous AP firing rate in a similar fashion. The aim of the present study was to investigate cell-to-cell variability in the responses of a large population of SAN cells. We measured {beta}AR responses among 166 single SAN cells isolated from 33 guinea pig hearts. In contrast to the current dogma, the SAN cell responses to {beta}AR stimulation substantially varied. In each cell, changes in AP cycle length highly correlated (R2=0.97) with the AP cycle length before {beta}AR stimulation. While, as expected, on average the cells increased their pacemaker rate, greater responses were observed in cells with slower basal rates, and vice versa, cells with higher basal rates showed smaller responses, no responses, or even decreased their rate. Thus, {beta}AR stimulation synchronizes the operation of the SAN cell population towards a higher average rate, rather than uniformly shifting the rate in each cell, creating a new paradigm of {beta}AR-driven fight-or-flight response among individual pacemaker cells.

biophysics↗

bioRxiv2020

Self-similar action potential cycle-to-cycle variability of Ca2+ and current oscillators in cardiac pacemaker cells

Ca2+ and Vm transitions occurring throughout AP cycles in sinoatrial nodal (SAN) cells are cues that: (1) not only regulate activation states of molecules operating within criticality (Ca2+ domain) and limit-cycle (Vm domain) mechanisms of a coupled-clock system that underlies SAN cell automaticity; (2) but are also regulated by the activation states of the clock molecules they regulate. In other terms, these cues are both causes and effects of clock molecular activation (recursion). Recently, we demonstrated that Ca2+ and Vm transitions during AP cycles in single SAN cells isolated from mice, guinea pigs, rabbits and humans are self-similar (obey a power law) and are also self-similar to trans-species AP firing intervals of these cells in vitro, to heart rate in vivo, and to body mass. Neurotransmitter stimulation of {beta} adrenergic receptor or cholinergic receptor initiated signaling in SAN cells modulates their AP firing rate and rhythm by impacting on the degree to which SAN clocks couple to each other, creating the broad physiologic range of SAN cell mean AP firing intervals and firing interval variabilities. Here we show that Ca2+ and Vm domain kinetic transitions (time to AP ignition in diastole and 90% AP recovery) occurring within given AP, the mean AP firing intervals, and AP firing interval variabilities within time-series of APs in 230 individual SAN cells are self-similar (obey power laws). In other terms, these long-range correlations inform on self-similar distributions of order among SAN cells across the entire broad physiologic range of SAN AP firing intervals, regardless of whether autonomic receptors of these cells are stimulated or not, and regardless of the type (adrenergic or cholinergic) of autonomic receptor stimulation. These long-range correlations among distributions of Ca2+ and Vm kinetic functions that regulate SAN cell clock coupling during each AP cycle in different individual, isolated SAN cells not in contact with each other. Our numerical model simulations further extended our perspectives to the molecular scale and demonstrated that many ion currents also behave self-similar across autonomic states. Thus, to ensure rapid flexibility of AP firing rates in response to different types and degrees of autonomic input, nature "did not reinvent molecular wheels within the coupled-clock system of pacemaker cells", but differentially engaged or scaled the kinetics of gears that regulate the rate and rhythm at which the "wheels spin" in a given autonomic input context.

biophysics↗

bioRxiv2020

Synchronized cardiac impulses emerge from multi-scale, heterogeneous local calcium signals within and among cells of heart pacemaker tissue

ABSTRACTBackground The current paradigm of Sinoatrial Node (SAN) impulse generation: (i) is that full-scale action potentials (APs) of a common frequency are initiated at one site and are conducted within the SAN along smooth isochrones; and (ii) does not feature fine details of Ca2+ signalling present in isolated SAN cells, in which small subcellular, subthreshold local Ca2+ releases (LCRs) self-organize to generate cell-wide APs.Objectives To study subcellular Ca2+ signals within and among cells comprising the SAN tissue.Methods We combined immunolabeling with a novel technique to detect the occurrence of LCRs and AP-induced Ca2+ transients (APCTs) in individual pixels (chonopix) across the entire mouse SAN images.Results At high magnification, Ca2+ signals appeared markedly heterogeneous in space, amplitude, frequency, and phase among cells comprising an HCN4+/CX43- cell meshwork. The signalling exhibited several distinguishable patterns of LCR/APCT interactions within and among cells. Apparently conducting rhythmic APCTs of the meshwork were transferred to a truly conducting HCN4-/CX43+ network of straited cells via narrow functional interfaces where different cell types intertwine, i.e. the SAN anatomical/functional unit. At low magnification, the earliest APCT of each cycle occurred within a small area of the HCN4 meshwork and subsequent APCT appearance throughout SAN pixels was discontinuous.Conclusions We have discovered a novel, microscopic Ca2+ signalling paradigm of SAN operation that has escaped detection using low-resolution, macroscopic tissue isochrones employed in prior studies: APs emerge from heterogeneous subcellular subthreshold Ca2+ signals, resembling multiscale complex processes of impulse generation within clusters of neurons in neuronal networks.Condensed abstract By combining immunolabeling with a novel optical technique we detected markedly heterogenous Ca2+signals within and among cell clusters of an HCN4+/CX43- meshwork in mouse sinoatrial node. These Ca2+ signals self-organized and transferred, throughout the node, to projections from an HCN4-/CX43+ network connected to a highly organized, rapidly conducting part of the CX43+ network. Thus, APs emerge from heterogeneous, subthreshold Ca2+ signaling not detected in low-resolution macroscopic isochrones. Our discovery requires a fundamental paradigm shift from concentric impulse propagation initiated within a leading site, to a multiscale/complex process, resembling the emergence of organized signals from heterogeneous local signals within neuronal networks.Competing Interest StatementThe authors have declared no competing interest.Abbreviations listAPaction potentialAPCTAP-induced Ca2+ transientChronopixchrono-pixelCX43Connexin 43 (Gap junction alpha-1 protein)FFTFast Fourier TransformHCN4Hyperpolarization-Activated Cyclic Nucleotide-Gated Channel 4ICaLL-type Ca2+ currentICaTT-type Ca2+ currentIVCInferior Vena CavaLCRLocal Ca2+ ReleaseNCXNa+/Ca2+ exchangerPBSPhosphate-Buffered SalinepreBötCpreBötzinger ComplexROIRegion of InterestSANSinoatrial NodeSVCSuperior Vena CavaView Full Text

physiology↗