Open channel block of hKv1.5 by psoralen from Heracleum moellendorffii Hance.

A furocoumarin derivative, psoralen (7H-furo[3,2-g][1]benzopyran-7-one), was isolated from the n-hexane fraction of Heracleum moellendorffii Hance. We examined the effects of psoralen on a human Kv1.5 potassium channel (hKv1.5) cloned from human heart and stably expressed in Ltk- cells. We found that psoralen inhibited the hKv1.5 current in a concentration-, use- and voltage-dependent manner with an IC50 value of 180 +/- 21 nM at +60 mV. Psoralen accelerated the inactivation kinetics of the hKv1.5 channel, and it slowed the deactivation kinetics of the hKv1.5 current resulting in a tail crossover phenomenon. These results indicate that psoralen acts on the hKv1.5 channel as an open channel blocker. Furthermore, psoralen prolonged the action potential duration of rat atrial muscles in a dose-dependent manner. Taken together, the present results strongly suggest that psoralen may be an ideal antiarrhythmic drug for atrial fibrillation.

Direct block of cloned hKv1.5 channel by cytochalasins, actin-disrupting agents.

The action of cytochalasins, actin-disrupting agents on human Kv1.5 channel (hKv1.5) stably expressed in Ltk(-) cells was investigated using the whole cell patch-clamp technique. Cytochalasin B inhibited hKv1.5 currents rapidly and reversibly at +60 mV in a concentration-dependent manner with an IC(50) of 4.2 microM. Cytochalasin A, which has a structure very similar to cytochalasin B, inhibited hKv1.5 (IC(50) of 1.4 microM at +60 mV). Pretreatment with other actin filament disruptors cytochalasin D and cytochalasin J, and an actin filament stabilizing agent phalloidin had no effect on the cytochalasin B-induced inhibition of hKv1.5 currents. Cytochalasin B accelerated the decay rate of inactivation for the hKv1.5 currents. Cytochalasin B-induced inhibition of the hKv1.5 channels was voltage dependent with a steep increase over the voltage range of the channel's opening. However, the inhibition exhibited voltage independence over the voltage range in which channels are fully activated. Cytochalasin B produced no significant effect on the steady-state activation or inactivation curves. The rate constants for association and dissociation of cytochalasin B were 3.7 microM/s and 7.5 s(-1), respectively. Cytochalasin B produced a use-dependent inhibition of hKv1.5 current that was consistent with the slow recovery from inactivation in the presence of the drug. Cytochalasin B (10 microM) also inhibited an ultrarapid delayed rectifier K(+) current (I(K,ur)) in human atrial myocytes. These results indicate that cytochalasin B primarily blocks activated hKv1.5 channels and endogenous I(K,ur) in a cytoskeleton-independent manner as an open-channel blocker.