The Ca2+ leak paradox and “rogue ryanodine receptors”: SR Ca2+ efflux theory and practice

Abstract

Ca 2+ efflux from the sarcoplasmic reticulum (SR) is routed primarily through SR Ca 2+ release channels (ryanodine receptors, RyRs). When clusters of RyRs are activated by trigger Ca 2+ influx through L-type Ca 2+ channels (dihydropyridine receptors, DHPR), Ca 2+ sparks are observed. Close spatial coupling between DHPRs and RyR clusters and the relative insensitivity of RyRs to be triggered by Ca 2+ together ensure the stability of this positive-feedback system of Ca 2+ amplification. Despite evidence from single channel RyR gating experiments that phosphorylation of RyRs by protein kinase A (PKA) or calcium-calmodulin dependent protein kinase II (CAMK II) causes an increase in the sensitivity of the RyR to be triggered by [Ca 2+ ] i there is little clear evidence to date showing an increase in Ca 2+ spark rate. Indeed, there is some evidence that the SR Ca 2+ content may be decreased in hyperadrenergic disease states. The question is whether or not these observations are compatible with each other and with the development of arrhythmogenic extrasystoles that can occur under these conditions. Furthermore, the appearance of an increase in the SR Ca 2+ "leak" under these conditions is perplexing. These and related complexities are analyzed and discussed in this report. Using simple mathematical modeling discussed in the context of recent experimental findings, a possible resolution to this paradox is proposed. The resolution depends upon two features of SR function that have not been confirmed directly but are broadly consistent with several lines of indirect evidence: (1) the existence of unclustered or "rogue" RyRs that may respond differently to local [Ca 2+ ] i in diastole and during the [Ca 2+ ] i transient; and (2) a decrease in cooperative or coupled gating between clustered RyRs in response to physiologic phosphorylation or hyperphosphorylation of RyRs in disease states such as heart failure. Taken together, these two features may provide a framework that allows for an improved understanding of cardiac Ca 2+ signaling.

Optical single-channel resolution imaging of the ryanodine receptor distribution in rat cardiac myocytes

“…It has been estimated that a trigger level of Ϸ10 M is needed to explain the approximately 10 4 -fold increase in spark rates resulting from activated RyR clusters within the timescale of EC coupling (35). Computer simulations suggest that such calcium levels may exist within 100 nm of an activated cluster (32). With these figures in mind, we can envision a calcium spark incorporating more than one of the closely spaced RyR clusters.…”

Spontaneous Ca2+ sparks and Ca2+ homeostasis in a minimal model of permeabilized ventricular myocytes

“…Because the ventricular myocyte is assumed to be permeabilized, the precise value of the parameter kpm is unimportant so long as there is rapid equilibration of bulk myoplasmic Ca 2ϩ with the extracellular [Ca 2ϩ ] (cext). The assumed number of RyRs in each release site (N ϭ 10) was chosen to be consistent with estimates of the number of channels activated during a Ca 2ϩ spark (7,15,43). This is a smaller number of RyRs than previously reported in electron microscopic studies performed a decade ago (11,34) but is consistent with more recent estimates based on superresolution optical techniques and three-dimensional electron tomography (2,23 (21).…”

Mechanisms of Ca2+-Triggered Arrhythmias

“…The seemingly conflicting observation is that Ca 2+ -dependent arrhythmias are more prevalent in heart failure due to enhanced diastolic SR Ca 2+ release (Pogwizd et al, 2001), despite the decrease of SR Ca 2+ content (Kubalova et al, 2005). To explain this dichotomy, Sobie et al (2006) proposed an interesting hypothesis using a mathematical model based on the recent experimental findings. Their model predicts that (1) "rogue RyR2" 3 can operate almost invisibly to produce a fraction of the overall Ca 2+ leak and (2) coupled gating between clustered RyR2s is disrupted in response to physiologic phosphorylation or excessive phosphorylation of RyR2s in disease states such as heart failure.…”

“…This disequilibrium may evolve from (1) the reduced Ca 2+ efflux (primarily due to increased NCX current in forward mode), (2) increased SR Ca 2+ uptake (due to increased phosphorylation of phospholamban and/or SERCA2a expression), (3) increased Ca 2+ influx across the sarcolemma (primarily due to increased L-type Ca 2+ current), and (4) altered SR Ca 2+ buffering capacity (due to a calsequestrin mutation).SR Ca 2+ overload typically results in spontaneous SR Ca 2+ release via RyR2. As opposed to the "silent" Ca 2+ leak through rogue (or unclustered) RyR2 (Sobie et al, 2006), the diastolic Ca 2+ leak via clustered RyR2 can be experimentally visualized as increased Ca 2+ spark frequency, which, when high enough in a given volume of the cell, can initiate a Ca 2+ wave. Once the Ca 2+ wave has been initiated, the propagation of Ca 2+ wave will largely depend on the amount of SR Ca 2+ content.…”