Ciguatoxins alter RyR2 gating, intracellular Ca²⁺ handling and cardiac function in human iPSC-derived cardiomyocytes and rats

Ciguatoxins (CTXs) cause ciguatera, a seafood-borne intoxication frequently associated with cardiovascular manifestations, but the underlying cardiac mechanisms remain poorly understood. We investigated the effects of purified CTX-1B and CTX-3 C in human induced pluripotent stem-cell-derived cardiomyocytes (hiPSC-CMs), porcine cardiac ryanodine receptor type 2 (RyR2) channels, and rats. In hiPSC-CMs, CTX-1B and CTX-3 C (0.3–5 nM) rapidly increased diastolic intracellular Ca²⁺ and altered spontaneous Ca²⁺ transients. At 5 nM, CTXs increased transient frequency, reduced transient amplitude, impaired contractile performance, and disrupted sarcomeric organisation. In planar lipid bilayers, CTX-1B (1 nM) prolonged RyR2 openings under resting cytosolic Ca²⁺ conditions and remodelled channel gating after activation with ATP (25 µM). Molecular docking predicted a CTX-1B interaction region within the RyR2 transmembrane domain, close to ligand-sensitive regulatory elements, supporting state-dependent modulation of channel gating. Cardiac sheets exposed to CTX showed reduced spontaneous field-potential activity and contraction. In rats, intravenous CTX-1B administration (0.13 ng/g bodyweight) caused bradycardia, premature atrial and ventricular contractions, atrioventricular conduction abnormalities, and atrial flutter. Echocardiography revealed impaired myocardial deformation and relaxation despite preserved ejection fraction. These findings identify altered RyR2 gating and intracellular Ca²⁺ dysregulation as plausible complementary contributors to CTX cardiotoxicity and extend the established mechanism based on persistent voltage-gated Na⁺-channel activation. Combined disruption of sarcolemmal excitability and intracellular Ca²⁺ release could explain the acute electrical and mechanical cardiac abnormalities associated with ciguatera.