Excitation-contraction coupling in skeletal and cardiac muscles shares some basic features, and has some tissue-specific characteristics as well. The most important common feature underlying both tissues is that the ryanodine receptor (RyR) plays a central role in the activation process. In the skeletal muscle RyR (RyR1), the voltage sensing by the T-system voltage sensor activates the SR Ca²⁺ release channel by mediation of a physical interaction between the DHP receptor and the RyR1. In the cardiac muscle RyR (RyR2), the voltage-sensing process opens the T-system Ca²⁺ channel, causes Ca²⁺ flux into the cytoplasm, and this Ca²⁺ activates the RyR2. However, only a limited amount of information is available about how the excitation signal received by the RyR leads to the channel opening and contraction in normal and disease conditions. An essential step for the understanding of such a mechanism is to identify the critical regulatory domains involved. In searching for these key domains, particular attention was paid to the fact that the reported sites of point mutations in the skeletal (MH, CCD) and cardiac (CPVT and ARVD2) diseases are localized in three rather restricted regions (‘hot’ Regions 1-3)[1], suggesting that at least these three ‘hot’ domains are critical domains worth investigating. A large amount of data accumulated by our recent work support a ‘domain-switch’ hypothesis [2] that involves inter-domain interactions between Region 1 and Region 2 of RyRs serving as a key mechanism for Ca²⁺ channel regulation (designated as ‘domain switch’). In short, in the resting or non-activated state, Regions 1 and 2 make close contact via several sub-domains. The conformational constraints imparted by the ‘zipped’ configuration of these two domains stabilize the closed state of Ca²⁺ channel. Under usual stimulating conditions, the inter-domain contacts are weakened leading to an ‘unzipped’ or channel activating configuration. If mutation occurs in one of these domains, the inter-domain interaction will weaken even under resting conditions, causing a partial ‘unzipping’; this results in a lowering of the energy barrier necessary for channel opening. Such a partially ‘unzipped’ domain pair is readily turned to its fully opened configuration by weaker-than-normal stimuli, causing the phenotype usually seen in the diseased states (increased sensitivity to the activation signal, incomplete closure of the channel at the resting state, etc.). Synthetic peptides and antibodies corresponding to the domain switch cause domain unzipping and activate Ca²⁺ channels. The use of the peptides, as a structural and functional probe, permitted us to identify the sites of their interaction, monitor local conformational changes (the extent of domain unzipping) using a fluorescent probe attached in a site-directed manner, and follow the outcome of domain unzipping to the Ca²⁺ channel function. We found a close parallelism between the extent of domain unzipping and the extent of channel activation. Furthermore, dantrolene, the therapeutic agent used to treat MH, was found to reverse domain unzipping and prevents channel dysfunctions produced by the weakened inter-domain interaction. All of these findings well support the concept that the inter-domain interaction between Region 1 and Region 2 is the governing mechanism of Ca²⁺ channel regulation; aberration of this mechanism is the primary cause of RyR-linked muscle diseases, and it represents a new therapeutic target. The recent studies on the cardiac RyR2 [3] suggest that the same domain unzipping hypothesis accounts for the mechanism of the development of cardiac hypertrophy. The remaining important question concerns Region 3, where an increasing number of RyR1 and RyR2 mutations have been reported in recent years, but whose role has not yet been thoroughly investigated. According to the recent report [4], the 3722-4610 region of RyR2, designated as I-domain, may be involved in specific inter-domain interactions. Our preliminary studies have shown that the peptide probe matching the C-terminal area of Resion 3 binds to the N-terminal portion of Region-3 and modulates the channel gating in a Ca²⁺-dependent manner, consistent with the view that the interaction between the N-terminal portion of Region 3 and the channel pore region is involved in channel gating. Abbreviations: ARVD2, arrhythmogenic right ventricular dysplasia type-2; CCD, central core disease; CPVT, catecholaminergic polymorphic ventricular tachycardia; DHP, dihydropyridine; MH, malignant hyperthermia.