Ryanodine Receptor Adaptation

signal activates specialized Ca 2 1 release channels, the ryanodine receptors (RyRs), in the sarcoplasmic reticulum (SR). This process is called Ca 2 1 -induced Ca 2 1 release (CICR). Intuitively, the CICR process should be self-regenerating because the Ca 2 1 released from the SR should feedback and activate further SR Ca 2 1 release. However, the CICR process is precisely controlled in the heart and, consequently, some sort of negative control mechanism(s) must exist to counter the inherent positive feedback of the CICR process. Defining the nature of this negative control has been a focus of investigation for decades. Several mechanisms have been suggested including all of the following: Ca 2 1 -dependent inactivation, adaptation, stochastic attrition, “fateful” inactivation, SR Ca 2 1 depletion, and coupled RyR gating. These mechanisms are generally regarded as being mutually exclusive (i.e., alternative). An emerging and more sophisticated view is that the required negative control is probably provided by a synergy of mechanisms, not a single mechanism. In this perspective, we focus on the origin of Ca 2 1 dependent inactivation and adaptation of single cardiac RyR channels. Specific concerns about the adaptation phenomenon are addressed and a comprehensive unifying view of RyR Ca 2 1 regulation is forwarded. We conclude that the steady-state Ca 2 1 dependence, high Ca 2 1 inactivation and low Ca 2 1 adaptation are three distinct manifestations of the same underlying mechanism, Ca 2 1 -dependent modal RyR channel gating.