Characterization of inward-rectifier K+ channel inhibition by antiarrhythmic piperazine

Piperazine protects the rat heart against sudden cardiac death from barium chloride-induced ventricular fibrillation

Fifteen of 20 Wistar albino rats were treated with various doses of the anthelmintic agent piperazine citrate (15, 30, and 60 mg/kg body weight). All 20 rats were subsequently given barium chloride, 20 mg/kg. The 5 rats (25%) that did not receive piperazine citrate developed ventricular fibrillations after barium chloride was administered to them, via one of the external jugular veins, and died shortly thereafter. The remaining 75% of the rats were fully protected by all the doses of piperazine citrate employed for the study. Barium chloride did not produce any dysrhythmic phenomenon in the piperazine-protected rats. Conversely, sinus rhythm was maintained in the electrocardiogram of all the rats, with every P wave followed by a normal QRS-T complex. This may portend a novel application for an old drug.

The molecular basis of chloroquine block of the inward rectifier Kir2.1 channel

Although chloroquine remains an important therapeutic agent for treatment of malaria in many parts of the world, its safety margin is very narrow. Chloroquine inhibits the cardiac inward rectifier K(+) current I(K1) and can induce lethal ventricular arrhythmias. In this study, we characterized the biophysical and molecular basis of chloroquine block of Kir2.1 channels that underlie cardiac I(K1). The voltage- and K(+)-dependence of chloroquine block implied that the binding site was located within the ion-conduction pathway. Site-directed mutagenesis revealed the location of the chloroquine-binding site within the cytoplasmic pore domain rather than within the transmembrane pore. Molecular modeling suggested that chloroquine blocks Kir2.1 channels by plugging the cytoplasmic conduction pathway, stabilized by negatively charged and aromatic amino acids within a central pocket. Unlike most ion-channel blockers, chloroquine does not bind within the transmembrane pore and thus can reach its binding site, even while polyamines remain deeper within the channel vestibule. These findings explain how a relatively low-affinity blocker like chloroquine can effectively block I(K1) even in the presence of high-affinity endogenous blockers. Moreover, our findings provide the structural framework for the design of safer, alternative compounds that are devoid of Kir2.1-blocking properties.