Book of Abstracts - New Frontiers 2022

Abstracts of oral presentations

EFFECT OF RyR GATING ON ELEMENTARY CALCIUM RELEASE OF CARDIAC MYOCYTES

A. Zahradníková, B. Iaparov, I. Baglaeva, I. Zahradník

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

Calcium release in cardiac myocytes is provided by ryanodine receptors (RyRs) clustered in calcium release sites (CRSs) of dyads. The physiological trigger for RyR activation is an increase in the local Ca 2+ concentration. In the absence of elevated [Ca 2+ ], RyR opening frequency is extremely low due to tonic inhibition of the RyR by Mg 2+ ions that compete with Ca 2+ ions at the RyR activation site and also directly inhibit RyR at the inhibition site [1]. Of importance are indications that RyR dysfunction in some pathophysiological states may be due to changes in the regulation of RyR activity by Mg 2+ [2]. To elucidate the effect of Ca 2+ and Mg 2+ binding on RyR activity, and the consequences it has on the formation of calcium release events (CREs), we developed a mathematical model of RyR gating that incorporated the allosteric interaction between the binding of Ca 2+ or Mg 2+ to the RyR activation site and channel opening, as well as the inhibition by Mg 2+ at the RyR inhibition site. The parameters of the model were extracted from published experimental data and tested by simulations [3]. Changes in Mg 2+ binding had a prominent effect on RyR open probability, mean open time, and mean closed time, as well as on the rate of activation. Mg 2+ acted as a strong inhibitor of RyR opening, and a change in Mg 2+ binding and unbinding rates had a profound effect on RyR gating kinetics, open probability, and rate of activation. The RyR gating model was inserted into an in silico model of the calcium release site, based on a quantitative description of RyR placement and calcium diffusion. Calcium release events (CREs) were simulated at different strengths and kinetics of Mg 2+ binding to RyRs and analysed [3]. The characteristics of CREs reacted sensitively and specifically to changes in the effective coupling strength, a parameter that jointly characterized RyR placement, Ca 2+ flux, and RyR gating. Increased Mg 2+ unbinding rate from the RyR activation site and decreased Mg 2+ binding rate to the RyR inhibition site contributed the most to the increased effective coupling strength and the increased frequency of spontaneous sparks. The results of simulations revealed the role of Mg 2+ ions as a protector of the CRS from spontaneous activation in the absence of an external stimulus. This finding is of principal physiological importance since excessive calcium release may cause not only arrhythmias but also glucose intolerance and neuronal disorders [4]. References [1] Zahradníková A. et al., Am J Physiol Cell Physiol. 285: C1059 -70, 2003.

[2] Guo W.,et al., J Biol Chem 295: 15622 – 15635, 2020. [3] Iaparov B. et al., Front Physiol. 12: 805956, 2022. [4] Sleiman Y et al., Cell Death Dis. 12 (2021), 1041.

Keywords: calcium release, cardiac myocyte, ryanodine receptor, calcium release site, mathematical model

Funding: Supported by grants SAV-TUBITAK JRP/2019/836/RyRinHeart, VEGA 2/0182/21, and IMTS: 313011V344.

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