Electrophysiologic effects of the IK1 inhibitor PA‐6 are modulated by extracellular potassium in isolated guinea pig hearts

Abstract The pentamidine analog PA‐6 was developed as a specific inward rectifier potassium current ( I K 1 ) antagonist, because established inhibitors either lack specificity or have side effects that prohibit their use in vivo. We previously demonstrated that BaCl 2 , an established I K 1 inhibitor, could prolong action potential duration (APD) and increase cardiac conduction velocity (CV). However, few studies have addressed whether targeted I K 1 inhibition similarly affects ventricular electrophysiology. The aim of this study was to determine the effects of PA‐6 on cardiac repolarization and conduction in Langendorff‐perfused guinea pig hearts. PA‐6 (200 nm) or vehicle was perfused into ex‐vivo guinea pig hearts for 60 min. Hearts were optically mapped with di‐4‐ANEPPS to quantify CV and APD at 90% repolarization (APD 90 ). Ventricular APD 90 was significantly prolonged in hearts treated with PA‐6 (115 ± 2% of baseline; P 90 during hypokalemia (2 mmol/L [K+] o ), although to a lesser degree than observed at 4.56 mmol/L [K+] o . In contrast, the effect of PA‐6 on CV was more pronounced during hypokalemia, where transverse CV with PA‐6 (24 ± 2 cm/sec) was significantly faster than with vehicle (13 ± 3 cm/sec, P 90 , whereas its effect on CV was modest. During hypokalemia, PA‐6 prolonged APD 90 to a lesser degree, but profoundly increased CV. Thus, in intact guinea pig hearts, the electrophysiologic effects of the I K 1 inhibitor, PA‐6, are [K+] o ‐dependent.