In addition to the known effects of PKA and cAMP, HERG channels are also modulated by PKC. The molecular mechanisms of this PKC-dependent process are not completely understood but do not depend on direct PKC-dependent phosphorylation of the channel.

Fluoxetine is a commonly prescribed antidepressant compound. Its action is primarily attributed to selective inhibition of the reuptake of serotonin (5-hydroxytryptamine) in the central nervous system. Although this group of antidepressant drugs is generally believed to cause fewer proarrhythmic side effects compared with tricyclic antidepressants, serious concerns have been raised by case reports of tachycardia and syncopes associated with fluoxetine treatment. To determine the electrophysiological basis for the arrhythmogenic potential of fluoxetine, we investigated the effects of this drug on cloned human ether-a-go-go-related gene (HERG) potassium channels heterologously expressed in Xenopus oocytes using the two-microelectrode voltage-clamp technique. We found that fluoxetine blocked HERG channels with an IC 50 value of 3.1 M. Inhibition occurred fast to open channels with very slow unbinding kinetics. Analysis of the voltage dependence of block revealed loss of inhibition at membrane potentials greater than 40 mV, indicating that channel inactivation prevented block by fluoxetine. No pronounced changes in electrophysiological parameters such as voltage dependence of activation or inactivation, or inactivation time constant could be observed, and block was not frequency-dependent. This is the first study demonstrating that HERG potassium channels are blocked by the selective serotonin reuptake inhibitor fluoxetine. We conclude that HERG current inhibition might be an explanation for the arrhythmogenic side effects of this drug.

Background-Protein kinases A (PKA) and C (PKC) are activated in ischemic preconditioning and heart failure, conditions in which patients develop arrhythmias. The native inward rectifier potassium current (IK 1 ) plays a central role in the stabilization of the resting membrane potential and the process of arrhythmogenesis. This study investigates the functional relationship between PKC and IK 1 . Methods and Results-In whole-cell patch-clamp experiments with isolated human atrial cardiomyocytes, the IK 1 was reduced by 41% when the nonspecific activator of PKC phorbol 12 myristate 13-acetate (PMA; 100 nmol/L) was applied. To investigate the effects of PKC on cloned channel underlying parts of the native IK 1 , we expressed Kir 2.1b heterologously in Xenopus oocytes and measured currents with the double-electrode voltage-clamp technique. PMA decreased the current by an average of 68%, with an IC 50 of 0.68 nmol/L. The inactive compound 4-␣-PMA was ineffective. Thymeleatoxin and 1-oleolyl-2-acetyl-sn-glycerol, 2 specific activators of PKC, produced effects similar to those of PMA. Inhibitors of PKC, ie, staurosporine and chelerytrine, could inhibit the PMA effect (1 nmol/L) significantly. After mutation of the PKC phosphorylation sites (especially S64A and T353A), PMA became ineffective. Conclusions-The human IK 1 in atrial cardiomyocytes and one of its underlying ion channels, the Kir 2.1b channel, is inhibited by PKC-dependent signal transduction pathways, possibly contributing to arrhythmogenesis in patients with structural heart disease in which PKC is activated. Key Words: ion channels Ⅲ signal transduction Ⅲ arrhythmia Ⅲ electrophysiology I n human cardiomyocytes, repolarization at the end of the action potential is caused by many different potassium currents, which can be classified as delayed outward and inward rectifiers. The inward rectifier potassium current (IK 1 ) reveals inwardly rectifying properties responsible for the terminal repolarization at the end of the cardiac action potential. The native IK 1 is composed of several different potassium channels with distinct single channel conductances of 20 pS, 35 pS and 10 pS. 1 Likewise, different gene families (Kir 2.1 , Kir 2.2 , and Kir 2.3 ) have been found in human heart encoding IK 1 . 2 The native IK 1 may be involved in arrhythmogenesis of coronary artery disease 3 and dilated cardiomyopathy. 4 Mutations of Kir 2.1 channels are associated with Anderson's syndrome, an inherited disease with an arrhythmia characterized by QT-prolongation, periodic paralysis, and dysmorphic features. 5 Within the Kir 2.1 family, several distinct channels have been cloned in chronological order. The first channel, named IRK 1 or MM-IRK 1 , was found in mouse tissue. 6 The rabbit channel RBHIK 1 revealed a 97% amino acid homology to MM-IRK 1 . 7 The human HH-IRK 1 , which has a 98% homology to MM-IRK 1 , was the first channel cloned from the human heart. 8 With the help of primers derived from IRK 1 , 2 channel sequences were identified in human atrial tissue; one was...