Identification of IKr and its trafficking disruption induced by probucol in cultured neonatal rat cardiomyocytes

The human ether-a-go-go-related gene (hERG) encodes a channel that conducts the rapidly activating delayed rectifier K+ current (I-Kr), which is important for cardiac repolarization. Mutations in hERG reduce I-Kr and cause congenital long QT syndrome (LQTS). More frequently, common medications can reduce I-Kr and cause LQTS as a side effect. Protein trafficking abnormalities are responsible for most hERG mutation-related LQTS and are recently recognized as a mechanism for drug-induced LQTS. Whereas hERG trafficking has been studied in recombinant expression systems, there has been no reported study on cardiac I-Kr trafficking at the protein level. In the present study, we identified that I-Kr is present in cultured neonatal rat ventricular myocytes and can be robustly recorded using Cs+ as the charge carrier. We further discovered that 4,4'-(isopropylidenedithio)-bis-(2,6-di-t-butylphenol) (probucol), a cholesterol-lowering drug that induces LQTS, disrupted I-Kr trafficking and prolonged the cardiac action potential duration. Probucol did not directly block I-Kr. Probucol also disrupted hERG trafficking and did not block hERG channels expressed in human embryonic kidney 293 cells. We conclude that probucol induces LQTS by disrupting ether-a-go-go-related gene trafficking, and that primary culture of neonatal rat cardiomyocytes represents a useful system for studying native I-Kr trafficking.