Determinants of Ca2+ release restitution: Insights from genetically altered animals and mathematical modeling

Abstract

Each heartbeat is followed by a refractory period. Recovery from refractoriness is known as Ca2+ release restitution (CRR), and its alterations are potential triggers of Ca2+ arrhythmias. Although the control of CRR has been associated with SR Ca2+ load and RYR2 Ca2+ sensitivity, the relative role of some of the determinants of CRR remains largely undefined. An intriguing point, difficult to dissect and previously neglected, is the possible independent effect of SR Ca2+ content versus the velocity of SR Ca2+ refilling on CRR. To assess these interrogations, we used isolated myocytes with phospholamban (PLN) ablation (PLNKO), knock-in mice with pseudoconstitutive CaMKII phosphorylation of RYR2 S2814 (S2814D), S2814D crossed with PLNKO mice (SDKO), and a previously validated human cardiac myocyte model. Restitution of cytosolic Ca2+ (Fura-2 AM) and L-type calcium current (ICaL; patch-clamp) was evaluated with a two-pulse (S1/S2) protocol. CRR and ICaL restitution increased as a function of the (S2-S1) coupling interval, following an exponential curve. When SR Ca2+ load was increased by increasing extracellular [Ca2+] from 2.0 to 4.0 mM, CRR and ICaL restitution were enhanced, suggesting that ICaL restitution may contribute to the faster CRR observed at 4.0 mM [Ca2+]. In contrast, ICaL restitution did not differ among the different mouse models. For a given SR Ca2+ load, CRR was accelerated in S2814D myocytes versus WT, but not in PLNKO and SDKO myocytes versus WT and S2814D, respectively. The model mimics all experimental data. Moreover, when the PLN ablation-induced decrease in RYR2 expression was corrected, the model revealed that CRR was accelerated in PLNKO and SDKO versus WT and S2814D myocytes, consistent with the enhanced velocity of refilling, SR [Ca2+] recovery, and CRR. We speculate that refilling rate might enhance CRR independently of SR Ca2+ load.

Role of Reduced Sarco-Endoplasmic Reticulum Ca2+-ATPase Function on Sarcoplasmic Reticulum Ca2+ Alternans in the Intact Rabbit Heart

“…The magnitude and recovery of RyR Ca 2+ release are known to be highly dependent on SR Ca 2+ load, and although we assume that SERCA inhibition decreases SR Ca 2+ content, we cannot measure absolute SR Ca 2+ concentration with our method. However, recent data indicate that the velocity of SR Ca 2+ refilling may affect SR Ca 2+ release restitution, independent of SR Ca 2+ load ( Cely-Ortiz et al, 2020 ), and this mechanism may also be involved here.…”

Cardiac Alternans: From Bedside to Bench and Back

“…70,71 It is this recovery period that underlies the CaT restitution curve shown in Figure 3F. 72 Since SR refilling is faster than RyR recovery, 70,73 the CaT restitution curve is mainly determined by RyR recovery kinetics, mediated by regions of the RyR protein that sense the intraluminal C, Simulation demonstrating the Ca signaling hierarchy of quarks (q), sparks (s), and spark clusters (c) in a computer model of a ventricular myocyte. 66 In the time sequence illustrated, one of the clusters at the left eventually propagates as a mini-wave (w) by recruiting adjacent clusters.…”

“…The second key feature is that once a CRU fires and releases its local SR Ca store, there is a time delay before it becomes available to fire again, called Ca spark restitution. 70 , 71 It is this recovery period that underlies the CaT restitution curve shown in Figure 3 F. 72 Since SR refilling is faster than RyR recovery, 70 , 73 the CaT restitution curve is mainly determined by RyR recovery kinetics, mediated by regions of the RyR protein that sense the intraluminal SR Ca levels, either directly 74 or through interactions with intraluminal accessory SR proteins such as triadin, junction, and calsequestrin. 75 With this background, it is now possible to understand the 2 main mechanisms that cause CaT alternans 56 , 63 , 68 , 76 – 79 and how they synergize with each other at fast heart rates.…”

“…67 E, The fractional SR Ca release curve measured in a rabbit ventricular myocyte. 69 F, Ca transient (CaT) restitution curves at 2 different extracellular [Ca] o recorded from mouse ventricular myocytes using the S1S2 extrastimulus method 72. CaM indicates calmodulin; CaMKII, Ca/calmodulin-dependent protein kinase II; and NCX, Na-Ca exchange.…”

Apoptosis en el corazón prediabético: Rol de la mitocondria y la comunicación retículo sarcoplasmático-mitocondria

“…Por ejemplo, los ratones transgénicos S2814D(Van Oort et al, 2010) poseen el sitio Ser 2814 del RyR2 mutado a aspartato y, por lo tanto, el RyR2 se encuentra pseudofosforilado de forma constitutiva; mientras los ratones S2814A(Chelu et al, 2009) poseen el sitio Ser 2814 del RyR2 mutado a alanina y, por lo tanto, el RyR2 no puede ser fosforilado en ese sitio. La pseudofosforilación del RyR2 produce un aumento en la actividad máxima de apertura del RyR2(Cely-Ortiz et al, 2020), mientras la inhibición de la fosforilación disminuye la probabilidad de apertura (P0) del canal. Cabe destacar que la fosforilación del canal modula el efecto de Ca 2+ en el RyR2 sin tener la capacidad inherente para abrir o cerrar el canal per se(Camors & Valdivia, 2014).…”