How does stochastic ryanodine receptor-mediated Ca leak fail to initiate a Ca spark?

Abstract

Spontaneous calcium (Ca) sparks are initiated by single ryanodine receptor (RyR) opening. Once one RyR channel opens, it elevates local [Ca] in the cleft space ([Ca](Cleft)), which opens other RyR channels in the same Ca release unit (CaRU) via Ca-induced Ca-release. Experiments by Zima et al. (J. Physiol. 588:4743-4757, 2010) demonstrate that spontaneous Ca sparks occur only when intrasarcoplasmic-reticulum (SR) [Ca] ([Ca](SR)) is above a threshold level, but that RyR-mediated SR Ca leak exists without Ca sparks well below this threshold [Ca](SR). We examine here how single RyR opening at lower [Ca](SR) can fail to recruit Ca sparks at a CaRU, while still contributing to SR Ca leak. We assess this using a physiologically detailed mathematical model of junctional SR Ca release in which RyR gating is regulated by [Ca](SR) and [Ca](Cleft). We find that several factors contribute to the failure of Ca sparks as [Ca](SR) declines: 1), lower [Ca](SR) reduces driving force and thus limits local [Ca](Cleft) achieved and the rate of rise during RyR opening; 2), low [Ca](SR) limits RyR open time (τ(O)), which further reduces local [Ca](Cleft) attained; 3), low τ(O) and fast [Ca](Cleft) dissipation after RyR closure shorten the opportunity for neighboring RyR activation; 4), at low [Ca](SR), the RyR exhibits reduced [Ca](Cleft) sensitivity. We conclude that all of these factors conspire to reduce the probability of Ca sparks as [Ca](SR) declines, despite continued RyR-mediated Ca leak. In addition, these same factors explain the much lower efficacy of L-type Ca channel opening to trigger local SR Ca release at low [Ca](SR) during excitation-contraction coupling. Conversely, all of these factors are fundamentally important for increasing the propensity for pro-arrhythmic Ca sparks and waves in cardiac myocytes at high [Ca](SR).

Figure 1

(A) Schematic illustration of CaRU RyR array (green spheres), Ca leak (upper), and a Ca spark (lower). (B) RyR gating scheme. (C) Open probability (P_o) of RyR as a function of [Ca]_Cleft for two levels of [Ca]_SR.

Figure 2

(A) Ca accumulation in the cleft space during single RyR opening with low ([Ca]_SR = 200 μM) and high ([Ca]_SR = 800 μM) SR Ca load. Circles indicate mean open time for each case. Slow increase of [Ca]_Cleft at 1–4 ms reflects slow accumulation of [Ca] at the submembrane space. (B) Normalized open-time distribution for simulated events.

Figure 3

(A) Probability of transition from the closed to the open state when RyR channel is exposed to [Ca]_Cleft for the indicated Δt ms. Circles indicate [Ca]_Cleft level if RyR opens for mean open time for each case (black, low SR Ca load;, red, high SR Ca load). (B) Probability that n RyR channels will open when the CaRU is exposed to [Ca]_Cleft for 2 ms with a high SR Ca load (800 μM). The orange line indicates the [Ca]_Cleft level if RyR opens for the mean open time. (C) Probability that n RyR channels open when the CaRU is exposed to [Ca]_Cleft for 1 ms with low SR Ca load (200 μM). The orange line indicates [Ca]_Cleft level if RyR opens for the mean open time.

Figure 4

[Ca]_SR dependence of RyR gating. (A) Mean open time versus [Ca]_SR. (B) [Ca]_Cleft versus [Ca]_SR when a single RyR opens for the mean open time. (C) Probability of transition from the closed to the open state when the RyR channel is exposed to [Ca]_Cleft for the mean open time. (D) Probabilities that n RyR channels open when the CaRU is exposed to [Ca]_Cleft for the mean open time.

Figure 5

Numerical experiment using the physiologically detailed model of Ca cycling. (A) Ca-spark frequency and SR Ca load. The SERCA pump (I_up) is decreased to simulate thapsigargin exposure. Ca spark frequency (bars) and SR Ca load (line) versus time. (B) Example of single CaRU activity from simulation in A. Number of open channels versus time. (Inset) A typical Ca spark.

Figure 6

SR Ca leak-load relationships. (A) Control SR Ca flux rates for total, Ca spark and nonspark Ca leak versus SR Ca load ([Ca]_SR). (B) Same analysis for model adjusted for HF. (C) Same analysis for model with ISO adjustments. (D) Same analysis when HF is exposed to ISO. Broken curves in B–D are data from A for comparison.

Figure 7

L-type I_Ca and induction of RyR channel opening. (A) Time course of [Ca]_cleft during opening of one or two Ca channels at different V_m. (B) Probabilities that n RyR channels will open when the CaRU is exposed to [Ca]_Cleft for 0.5 ms (open time) with high SR Ca load ([Ca]_SR = 800 μM). (C) Same as B but for [Ca]_SR = 200 μM.

Figure 8

(A) ECC gain versus [Ca]_SR at V_m = −20, 0, and +40 mV. Gain is defined by ∫ I_rel/∫ I_CaL. (B) Fractional release versus [Ca]_SR at V_m = −20, 0, and +40 mV. Fractional release is defined by ∫ I_rel/total SR Ca. (C) Normalized peak I_Ca and peak SR Ca release flux versus test V_m.