Class IA antiarrhythmic drug quinidine was one of the first clinically used compounds to terminate atrial fibrillation and acts as multichannel inhibitor with well-documented inhibitory effects on several cardiac potassium channels. In the mammalian heart, heteromeric assembly of Kir2.1-2.3 channels underlies IK1 current. While a low-affinity block of quinidine on Kir2.1 has already been described, a comparative analysis of effects on other Kir2.x channels has not been performed to date. Therefore, we analyzed the effects of quinidine on wild type and mutant Kir2.x channels in the Xenopus oocyte expression system. Quinidine exerted differential inhibitory effects on Kir2.x channels with the highest affinity towards Kir2.3 subunits. Onset of block was slow and solely reversible in Kir2.2 subunits. Quinidine inhibited Kir2.x currents in a voltage-independent manner. By means of comparative Ala-scanning mutagenesis we further found that residue E224, F254, D259 and E299 are essential for quinidine block in Kir2.1 subunits. Analogously, quinidine mediated Kir2.3 inhibition by binding corresponding residues E216, D247, D251 and E291. In contrast, Kir2.2 current block merely involved corresponding residue D260. Using channel mutants with altered PIP2 affinities, we were able to demonstrate that high PIP2 affinities (i.e. Kir2.3 I214L) correlate with low quinidine sensitivity. Inversely, mutant channels interacting only weakly with PIP2 (i.e. Kir2.1 K182Q and L221I) are prone to a higher inhibitory effect. Thus, we conclude that inhibition of Kir2.x channels by quinidine is mediated by joint modes of action involving direct cytoplasmic pore block and an impaired channel stabilization via interference with PIP2.
- antiarrhythmic drugs
- cardiac electrophysiology
- cardiovascular disease
- cardiovascular drugs
- inwardly rectifying potassium channels
- The American Society for Pharmacology and Experimental Therapeutics