The Ca2+ release units of ventricular myocytes are clusters of Ca2+ release channels (ryanodine receptors; RyRs) in the sarcoplasmic reticulum (SR) membrane, which arrange discretely and periodically inside the cell. Intra-cluster RyRs open and close together during a local Ca2+ release event, termed a Ca2+ spark, by a Ca2+-induced Ca2+ release (CICR) mechanism. A sparkles release has also been reported, presumably from single RyRs that release too little Ca2+ to open their neighbors.
Diastolic arrhythmias during heart failure are a leading cause of death in developed countries. A fraction of those arrhythmias originate at the SR, and are observed as spatially propagating, temporally regenerative, waves of Ca2+ release. It is noteworthy that, for regenerative CICR to occur, the Ca2+ leaked through one RyR cluster must reach the neighboring cluster in sufficient amounts, and this process repeat iteratively despite the intra-cluster distance and intra-cluster buffering. This indicates that, during heart failure, RyRs are dramatically sensitive to their environment. And indeed, fundamental publications have revealed that RyR are hyperphosphorylated during heart failure and that hyperphosphorylation leads to increased channel activity.
However, additional considerations greatly cloud the above picture. Diseased myocytes progressively decrease the SR Ca2+ load, leading to a mechanistic paradox. The behavior of diseased cells is counterintuitive because, as their total cellular Ca2+ decreases, the probability of spontaneous RyR openings and CICR should also decrease. Therefore, cells should be able to easily prevent the "extra" diastolic release. This stability is, in fact, observed in healthy cells regardless of the initial RyR tendency to open.
The current dissertation demonstrates, both for mouse and rabbit myocytes, that a substantial portion of the global RyR-mediated SR Ca2+ leak (Jleak) occurs in the form of non-sparks. Moreover: by using maximal β-adrenergic stimulation in rabbit healthy cells, it is demonstrated that the fraction of spark-mediated Jleak is increased relative to the basal levels of RyR phosphorylation, and that sparks are wider. Therefore, since more sparks occur for a given Jleak and sparks are wider, the chances of CICR and diastolic Ca2+ waves are maximized even in healthy cells. These results partially explain the success of β-blocker therapy in heart failure patients.