Figure 3.

The effects of elevated extracellular Ca²⁺ on HERG deactivation. A and B are families of current recordings measured in 1.8 and 10 mM Ca²⁺, respectively, using the voltage-clamp protocol shown at top right. The membrane potential was stepped from a holding potential of −80 to +50 mV for 2 s before stepping to the test potential between −130 and +20 mV. Traces in C are super-imposed tail currents measured at −60 mV taken from the records shown in A and B and scaled to the peak current level. The time constants shown in D were measured by fitting a monoexponential function (y = Ae ^−(t/τ) + c) to the initial segment of the tail currents at the indicated membrane potentials in 1.8 (•) or 10 (○) mM Ca²⁺. Time constants were not obtained at potentials less than −80 mV or greater than −105 mV due to the difficulty in fitting the small currents close to the K⁺ reversal potential. Each point represents the mean and SEM for three to four cells. The time constants were significantly different between 1.8 and 10 mM Ca²⁺ with P < 0.05 at all voltages tested.