How to Detect Ca Quarks

Nearly 2 decades ago, local Ca 2 release events during diastole were first observed and named Ca sparks.1 These were originally attributed to large releases from single ryanodine receptor (RyR) channels.1 Subsequently, Lipp and Niggli proposed that Ca sparks could be the summation of smaller releases recruited from within the same cluster and coined the term calcium quarks.2 Arduous experiments from this group and others have indeed recorded very small release events proposed to be from smaller numbers of RyRs; however, such reports have remained limited.3,4 In this issue of Circulation Research, evidence is presented to show how small release events from within a RyR cluster are a key component of sarcoplasmic reticulum (SR) Ca release.5 Physiologically, during depolarization, Ca sparks are triggered throughout the cells by a small influx of Ca from the sarcolemmal Ca channels. The resultant synchronous rise in Ca is observed as the systolic Ca transient. This global Ca release has been used to derive a measure of RyR function termed the “gain” of coupling between trigger Ca influx and Ca release. This yields a whole-cell measure of RyR function; changes in this gain infer changes in the RyR behavior, which may be attributable to several factors, including the SR Ca load, or the sensitivity of the channels themselves (see Heinzel et al6 for review). As a more direct measure for RyR behavior, diastolic Ca sparks have been characterized, in particular in studies of pathological conditions. Measurements of diastolic RyR behavior have been used to calculate loss of Ca from the SR and how this would affect the SR Ca load (eg, in Neef et al7). Alternatively, diastolic SR Ca loss or “leak” can be measured directly and used as a measure of the function of RyR behavior where it would normally act to balance uptake by the SR Ca pump (eg, in Shannon et al8). The “quark” like behavior and its involvement within this framework, meanwhile, has remained unresolved. Recently, measurements of both cytosolic and SR Ca changes have been combined to uncover global and local depletions of SR Ca .9 Brochet et al have used this method to characterize the Ca depletion that occurs during a Ca spark and termed it a Ca blink.10 This approach further contributed to the study of the kinetics of intra-SR depletion and refilling at subcellular level.11 In this issue of Circulation Research, Brochet et al5 revisit the Ca blink measurement to further investigate spontaneous RyR Ca release from the SR at rest. This study focuses on the elusive Ca quark, proposed to originate from single or very small clusters of RyRs associated at the Z-line. The theory of Ca release from sparks has thus come full circle, reinstating the much earlier hypothesis from the group that single RyRs or small groups within a cluster of RyRs can elicit spontaneous Ca release. This sheds new light on how 100 tightly coupled RyRs can coordinate an all-or-nothing response to L-type Ca channel influx, in a graded manner. The present study opens up yet more questions, such as how these events can influence Ca sparks, as well as how dominant is their role in regulating the Ca load of the SR. Spontaneous “quarky” Ca release (QCR) events, as opposed to those triggered by Ca -induced Ca release3,4 have previously gone undetected because of the low signalto-noise ratio inherent to confocal Ca measurement. The common mode of spontaneous spark detection is based on thresholding above this noise floor such that “false” event detection is minimized. In doing so, small events are lost as they are of similar amplitude to the noise. The present study shows the merit of pairing cytosolic (sparks) and SR (blinks) Ca measurements leading to an order of magnitude improvement in rejection of false sparks. This improved, if rather demanding, method, therefore, allows the study of these QCR events, possibly answering some existing questions surrounding non–spark-mediated Ca release. In 2 recent studies, the relevance of non–spark-mediated Ca release during diastole has been highlighted. Two separate methods have recently proposed that Ca spark– mediated release was the “tip of the iceberg” with regard to diastolic regulation of SR Ca . Santiago et al12 have recently shown how sparks may only account for 50% of SR Ca leak. Zima et al13 came to a similar conclusion by monitoring both cytosolic and SR Ca , as in the present study. While they found a similar relationship at physiological Ca loads, they noted that non–spark-mediated leak dominated at lower SR Ca levels. This was extended to show that this relationship was altered in an animal model of heart failure. This model showed a Ca spark frequency that was 21% higher, whereas the nonspark SR Ca leak increased by 40%. The data of Brochet et al5 help unify the results of these 2 studies, suggesting QCR could be the common pathway responsible for these measurements. The QCR events detected by Brochet et al5 are compatible with the theory of “rogue” RyRs proposed by Sobie et al.14 The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Laboratory of Experimental Cardiology, Department of Cardiovascular Medicine, University of Leuven, Belgium. Correspondence to Niall Macquaide, PhD, or to Karin R. Sipido, MD, PhD, Laboratory of Experimental Cardiology, KUL, Campus Gasthuisberg O/N 7th Floor, Herestraat 49, B-3000 Leuven, Belgium. E-mail [email protected] or [email protected] (Circ Res. 2011;108:154-156.) © 2011 American Heart Association, Inc.