Quinidine blocks adenosine 5'-triphosphate-sensitive potassium channels in heart

The ATP-sensitive potassium channel current (I(K-ATP)) was studied in excised inside-out patches from rat ventricular cells at 20-23°C. The bath solution contained 140 mM KF, and the pipette solution contained 140 mM KCl and 1.2 mM MgCl₂. ATP (0.5 mM) in the bath inhibited I(K-ATP). In the absence of ATP, 10 μM quinidine decreased open probability 67 ± 1% (n = 6) at -50 mV and 28 ± 12% at -130 mV (n = 5) without affecting single channel conductance (48-52 pS). The block increased with 25 and 50 μM quinidine and could be reversed on washing quinidine for several minutes. Interburst (closed) intervals were increased by quinidine, whereas open and closed time distributions within bursts were not changed. We conclude that quinidine blocks I(K-ATP) in a ''slow'' and voltage-dependent manner in clinically relevant concentrations. Because of the postulated role for I(K-ATP) in cardiac ischemia, quinidine block of this channel may play a role in ischemic arrhythmias.