Book of Abstracts - New Frontiers 2022

Abstracts of oral presentations

DETERMINANTS OF RyR-RyR COUPLING STRENGTH IN CARDIAC CALCIUM RELEASE SITES

B. Iaparov , I. Zahradník, A. Zahradníková

Dept. of Cellular Cardiology, Institute of Experimental Endocrinology, Biomedical Research Center, Slovak Academy of Sciences, Bratislava, Slovakia

Recent electron tomography and super-resolution microscopy of cardiac muscle cells has shown that the distribution of RyR in cardiac dyads is non-uniform in contrast to previous views that considered a checkerboard pattern of RyR placement. To test how the placement of RyRs contributes to the formation of calcium sparks, which cannot be addressed experimentally, we performed in silico simulations of calcium release events (CREs) on a large parameter set of models of calcium release sites. The models covered the observed range of RyR number, density, and spatial placement in cardiac dyads. RyR activity in the models was approximated by a two state gating model conforming to the published single RyR channel properties such as sensitivity to allosteric activation by Ca2+ ions and sensitivity to competitive/non-competitive inhibition by Mg2+ ions. Calcium dynamics in the calcium release site (CRS) model were approximated using the linearized buffered diffusion approximation and a constant single RyR channel calcium current. The spatial placement of RyRs in the calcium release site models was quantitatively characterized by RyR vicinity defined as an average reciprocal distance between RyRs normalized on the number of RyRs [1]. The simulated responses of CRS models to a random RyR channel opening, i.e., the calcium release events (CRE), were classified according to their amplitude histograms as a quark when no other RyR was activated, as a blip when few more RyRs were activated, and as a spark when many RyRs were activated. Each calcium release event was characterized by the amplitude and time to peak. The relative occurrence of individual CRE types was evaluated. The characteristics of CREs did not correlate well with individual determinants of CRS models; however, they correlated in a dose-response manner with the coupling strength between RyRs of respective models defined as a weighted product of the RyR vicinity and single-channel calcium current amplitude [1]. This in silico observation explains how the distribution of RyRs in dyads affects their spontaneous calcium releasing activity in synergy with the single RyR calcium current. From the physiological viewpoint, the simulations revealed how the sparse placement of RyRs or a small calcium current may lead to an increased spontaneous calcium leak from the sarcoplasmic reticulum and a decreased spontaneous spark occurrence during diastole. The introduction of the concept of RyRs coupling strength made it possible to quantitatively estimate the spark probability for a given dyad. It revealed that under the modelling conditions, the resulting RyR coupling strength is dominated by the RyR vicinity over the single-channel calcium current. [1] B. Iaparov, I. Zahradnik, A. Moskvin and A. Zahradnikova, J. Gen. Physiol. 153 (2021), e202012685.

Keywords: cardiac dyad, calcium spark, ryanodine receptor, calcium release site, mathematical modelling

Funding:The research was supported by grants VEGA 2/0143/17 and APVV-15-0302.

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