Direct block of human ether-a-go-go-related gene potassium channels by caffeine

The human ether-a-go-go-related gene (hERG) potassium channel is expressed in a variety of cell types, including neurons, tumor cells, and cardiac myocytes. In the heart, it is important for repolarization of the cardiac action potential. Attenuation of hERG current can cause long QT syndrome and cardiac arrhythmias such as torsades de pointes. Caffeine is frequently used as a pharmacological tool to study calcium-dependent transduction pathways in cellular preparations. It raises cytosolic calcium by opening ryanodine receptors and may also inhibit phosphodiesterases to increase cytosolic cAMP. In this study, we show 5 mM caffeine rapidly and reversibly attenuates hERG currents expressed in human embryonic kidney 293 cells to 61.1 +/- 2.2% of control. Caffeine-dependent inhibition of hERG current is not altered by raising cAMP with forskolin, buffering cytosolic calcium with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, or inhibition of protein kinase C. Thus, the effects of caffeine are unlikely to be mediated by cAMP or intracellular calcium-dependent mechanisms. Further experiments showed caffeine directly blocks hERG in an open state-dependent manner. Furthermore, caffeine inhibition is greatly reduced by the pore mutants Y562A and F656A hERG, which disrupt block of most previously tested hERG antagonists. Thus, caffeine attenuates hERG currents by binding to a drug receptor located within the inner cavity of the channel. Dietary intake of caffeine is unlikely to cause long QT syndrome because plasma concentrations do not reach sufficiently high levels to significantly inhibit hERG currents. However, the effects of caffeine have implications for its use in examining calcium-dependent pathways in cellular preparations expressing hERG.