Aim: The occurrence of ventricular fibriUation (VF) is dependent on the deterioration of channelopathy in the myocardium. It is interesting to investigate molecular changes in relation to abrupt appearance of VF on repeffusion. We aimed to study whether changes in the expression of FKBP12.6 and SERCA2a and the endothelin (ET) system on reperfusion against ischemia were related to the rapid occurrence of VF and whether CPU86017, a class Ⅲ antiarrhythmic agent which blocksK IKr, IKs,and ICa.L, suppressed VF by correcting the molecular changes on repeffusion.Methods: Cardiomyopathy (CM) was produced by 0.4 mg/kg sc L-thyroxin for 10 d in rats, and subjected to 10 min coronary artery ligation/reperfusion on d 11. Expressions of the Ca2+ handling and ET system and calcium transients were conducted and CPU86017 was injected (4 mg/kg, sc) on d 6-10.Results: Ahigh incidence of VF was found on repeffusion of the rat CM hearts, but there was no VF before reperfusion. The elevation of diastolic calcium was significant in the CM myocytes and exhibited abnormality of the Ca2+ handling system. The rapid downregulation of mRNA and the protein expression of FKBP12.6 and SERCA2a were found on reperfusion in association with the upregulation of the expression of the endothelin-converting enzyme (ECE) and protein kinase A (PKA), in contrast, no change in the ryanodine type 2 receptor (RyR2), phospholamban (PLB),endothelin A receptor (ETAR), and iNOS was found. CPU86017 removed these changes and suppressed VEConclusion: Abrupt changes in the expression of FKBP12.6, SERCA2a, PKA, and ECE on reperfusion against ischemia, which are responsible for the rapid occurrence of VF, have been observed. These changes are effectively prevented by CPU86017.