Type 1 ryanodine receptor (RyR1) is a Ca ²⁺ release channel in the sarcoplasmic reticulum in skeletal muscle. In excitation–contraction (E-C) coupling, RyR1 opens by depolarization of transverse tubule membrane via physical interaction with dihydropyridine receptor, which is referred to as depolarization-induced Ca ²⁺ release (DICR). RyR1 can also be gated via Ca ²⁺ -induced Ca ²⁺ release (CICR), in which binding of Ca ²⁺ directly opens the channel. Thus, RyR1 has two Ca ²⁺ release modes; DICR and CICR, but the physiological role of CICR has been a matter of debate: whether CICR can amplify Ca ²⁺ signals in E-C coupling. To address this issue, we created a mouse model carrying a mutation in the Ca ²⁺ -binding site in RyR1 (RyR1-E3896A), which selectively inhibits CICR. Surprisingly, the homozygous RyR1-E3896A mice show no appreciable changes in E-C coupling, ex vivo muscle contraction, in vivo muscle performance, or muscle fiber type. Gain-of-function mutations in RyR1 cause malignant hyperthermia (MH), which is a lethal disease triggered by inhalational anesthetics. The E3896A mutation conferred resistance to isoflurane-induced MH episodes and severe heat stroke triggered by environmental heat stress. Our data suggest that RyR1-mediated CICR plays a negligible role in E-C coupling of normal skeletal muscle but may increase the risk for muscle diseases when excessively activated.

Background and Purpose: Aberrant activation of type 2 ryanodine receptors (RyR2) causes lethal arrhythmias, such as catecholaminergic polymorphic ventricular tachycardia (CPVT). Developing drugs that suppress RyR2 hyperactivation may be key to novel arrhythmia treatments. This study evaluated the antiarrhythmic potential of Ryanozole, a recently developed novel RyR2 modulator with high affinity and selectivity, using CPVT mouse models harbouring mutant RyR2s. Experimental Approach: In vitro effects of Ryanozole were evaluated by endoplasmic reticulum (ER) Ca ²⁺ ‐based assay and [ ³ H]ryanodine‐binding assay using RyR2‐expressing HEK293 cells. Two lines of mice with different arrhythmia severities, RyR2‐R420W and RyR2‐K4750Q, were employed for in vivo and ex vivo assessments. Intracellular Ca ²⁺ signals were analysed in isolated cardiomyocytes using Cal‐520. Antiarrhythmic effects were evaluated by electrocardiography (ECG) under catecholaminergic challenge in anaesthetized mice and during spontaneous arrhythmias in conscious mice. ECG and echocardiographic parameters were evaluated before and after drug administration. Key Results: Ryanozole inhibited both wild‐type and mutant RyR2s with an IC ₅₀ of 15–40 nM. Inhibition was more potent at lower cytosolic Ca ²⁺ concentrations. Ryanozole suppressed Ca ²⁺ waves and Ca ²⁺ sparks without affecting action potential‐evoked Ca ²⁺ transients. Ryanozole effectively prevented adrenaline‐induced arrhythmias and rapidly terminated ongoing spontaneous arrhythmias during daily activity. Importantly, Ryanozole did not impair cardiac conduction or contractility, unlike conventional antiarrhythmic drugs. Conclusion and Implications: Ryanozole preferentially suppresses RyR2 under diastolic Ca ²⁺ conditions, thereby preventing the arrhythmogenic trigger of aberrant Ca ²⁺ release. This mechanism may provide potent antiarrhythmic effects while preserving cardiac function. Ryanozole is a potential therapeutic candidate for RyR2‐mediated arrhythmias, such as CPVT.

In adults, expressed in renal cancer (ERC)/mesothelin is exclusively expressed in the mesothelial cells lining the pleural, pericardial, and peritoneal cavities, yet its function under physiological conditions is unknown. To explore this, we studied ERC expression in wild-type (WT) mice at different developmental stages by immunohistochemistry and analyzed the ultrastructure of the mesothelium in WT and Erc-knockout (KO) mice via electron microscopy. Additionally, cardiopulmonary function in adult WT and Erc-KO mice was assessed using echocardiography and the forced oscillation technique (FOT). During embryonic development in WT mice, ERC expression was detected in the epicardium as early as embryonic day (E)12.5 but was absent in the pleura until E18.5. The timing of expression appeared to coincide with the active maturation of these organs, which implied a potential role in cardiopulmonary development. Electron microscopy revealed that microvilli on the mesothelium of Erc-KO mice were immature compared to those of WT mice. Based on these findings, we hypothesized that ERC might contribute to cardiopulmonary function; however, echocardiography and FOT did not reveal any functional differences between WT and Erc-KO mice. This suggests that ERC has limited functional relevance under physiological conditions.