Calcium Modulates Herg Function by Acting at Two Distinct Binding Sites

Abstract

Reduction of the current carried by the cardiac potassium channel HERG can lead to Long QT syndrome, an arrhythmia characterized by a rapid heart rate and reduced cardiac output, which can, in certain situations, be fatal. The effect of extracellular electrolytes on the biophysical properties of the HERG channel have been studied in some detail. In particular, increases in extracellular calcium, magnesium and hydrogen have been shown to slow channel activation, increase channel deactivation, and shift the G-V curve to more positive voltages. A number of reports have also shown that hydrogen can reduce HERG current by a mechanism that does not involve an effect on channel deactivation and which likely involves pore block. We have shown previously that reducing extracellular potassium results in a significant increase in HERG current reduction by a number of extracellular divalent ions, including calcium, magnesium, cobalt, manganese, and hydrogen. We have also shown that current reduction of the HERG mutant G628CS631A is significantly reduced compared to WT at all voltages tested. We show here that the positive shift in the G-V curve of the WT HERG due to an increase in extracellular calcium is much less sensitive to potassium than the effect of extracellular potassium on current reduction. In addition, the G-V curve of the HERG double mutant G682CS631 (which shows reduced current reduction with an increase in extracellular calcium) is shifted to the right with an increase in extracellular calcium in a similar manner to the positive shift with increased extracellular calcium seen with the WT channel. Both of these findings are consistent two separate calcium binding sites: one site that interacts with the voltage sensor and one site that may interact with the outer pore of the HERG channel.