Effects of bimakalim on human cardiac action potentials: Comparison with guinea pig and nicorandil and use-dependent study

Electrophysiologic effects of K(ATP) channel openers (KCOs) are rarely studied for tissue and species specificity, and use-dependent investigations in human tissues are lacking. We therefore investigated in vitro the concentration-dependent effects of the KCO bimakalim [from 10 nM to 10 μM, at 1,000 ms of cycle length (CL) and 37°C] on human (atrium, n = 4, and ventricle, n = 6) and guinea pig (atrium, n = 7, and ventricle, n = 6) transmembrane action potential (AP). The frequency relation (from CL 1,600 to 300 ms, 31°C) of human atrial AP duration 90% (APD90) shortening (10 μM vs. baseline, n = 7) also was determined. A parallel study was performed with the KCO nicorandil (from 10 nM to 1 mM, n = 3) in human atrial APs, at 31 °C. Resting membrane potential and maximal upstroke velocity of AP were not modified by bimakalim at maximal concentration, whereas AP amplitude was decreased in both guinea pig preparations (p < 0.05); APD90 was shortened in all tissues (p < 0.01). Median effective concentration (EC50) for APD90 shortening at 37°C was 0.54 and 2.74 μM in atrial and ventricular human tissue, respectively, and 8.55 and 0.89 μM in atrial and ventricular guinea pig tissue, respectively. In human atrial tissue at 31°C, EC50 with bimakalim was 0.39 μM; a much higher value was seen with nicorandil (210 μM). Bimakalim (10 μM)-induced APD90 shortening as a function of stimulation rate was greatest at longest CL. Evidence is provided for (a) species (human vs. guinea pig) and tissue (atrium vs. ventricle) differential AP sensitivity to bimakalim; (b) an ≃500-fold higher efficacy of bimakalim versus nicorandil to shorten human atrial APD90; and (c) normal use- dependence of human atrial APD90 shortening with bimakalim at 10 μM.