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

View ORCID ProfileAnna V. Maltsev, View ORCID ProfileVictor A. Maltsev
doi: https://doi.org/10.1101/2021.11.19.469309
Anna V. Maltsev
1School of Mathematics, Queen Mary University of London, London, United Kingdom
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  • For correspondence: a.maltsev@qmul.ac.uk
Victor A. Maltsev
2Laboratory of Cardiovascular Science, Intramural Research Program, National Institute on Aging, NIH, 251 Bayview Blvd, Baltimore, Maryland 21224
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Abstract

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 β-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.

Summary The 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.

Competing Interest Statement

The authors have declared no competing interest.

Copyright 
The copyright holder for this preprint is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. All rights reserved. No reuse allowed without permission.
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Posted November 20, 2021.
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Disorder in Ca2+ Release Unit Locations Confers Robustness but Cuts Flexibility of Heart Pacemaking
Anna V. Maltsev, Victor A. Maltsev
bioRxiv 2021.11.19.469309; doi: https://doi.org/10.1101/2021.11.19.469309
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Disorder in Ca2+ Release Unit Locations Confers Robustness but Cuts Flexibility of Heart Pacemaking
Anna V. Maltsev, Victor A. Maltsev
bioRxiv 2021.11.19.469309; doi: https://doi.org/10.1101/2021.11.19.469309

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