Ca2+ Heterogeneity Within a Ca2+ Spark

Abstract

The Ca2+ spark is the elemental Ca2+ release event in excitation-contraction coupling. The synchronized summation of thousands (10,000 to 20,000 per cell) of these local Ca2+ release events will give rise to the normal Ca2+ transient that underlies contraction. Ca2+ sparks represent a widely found Ca2+ release process among different types of tissue (muscles, neurons and even non-excitable cells) and species (rat, mouse, guinea-pig, rabbit, dog, cat, human) although the details are tissue and species specific. Ca2+ sparks are the local events showing the increase of Ca2+ in the cytosol following activation of ryanodine receptors (RyR) at the junctional sarcoplasmic reticulum (jSR). This increase of [Ca2+] in the cytosol is matched by a decrease of Ca2+ within the jSR (Ca2+ blink). Ca2+ blinks were detected by using the low affinity Ca2+ indicator fluo-5N loaded into the SR. Ca2+ blinks are even more localized events (FWHM=1 μM versus 2.2 μM for sparks) than Ca2+ sparks, reflecting the inner structure of the SR. Simultaneous visualization of Ca2+ sparks and Ca2+ blinks has allowed the detection of a new sub-population of Ca2+ release events that are smaller than Ca2+ sparks and these have been called “quarky Ca2+ release events (QCR)” (Brochet et al., 2011) because they are intermediate in size between Ca2+ release by a single RyR channel and the entire cluster of RyR in the jSR. QCR events have also been hypothesized to be commingled with the dynamics of Ca2+ release during a spark. Here, we visualize heterogeneity of Ca2+ release within a spark. The importance of this observation and its relationship with the organization of RyR at the jSR will be discussed. These results provide important new understanding of cardiac Ca2+ signaling.