Electrophysiological effects of droperidol on sinoatrial nodal fibers

The influence of 1 and 5 mg/l of droperidol on rabbit sinus node intracellular recordings was evaluated. At 1 mg/l the drug merely prolonged the spontaneous basic cycle by reducing phase 4 slope of the action potential. At the higher concentration, droperidol induced a more pronounced effect of these variables and after 20 min perfusion, it also decreased the upstroke velocity and the threshold potential. Further exposure continued to reduce spontaneous rate, until irregular failure of impulse initiation and subthreshold oscillations appeared. After 45 min exposure to droperidol, only subthreshold oscillations could be recorded. Isoproterenol (0.2 mg/l) rapidly restored normal action potentials, while acetylcholine (0.5 mg/l) caused an immediate arrest leaving a stable level of membrane potential. The activity of droperidol in 1 mg/l concentration could be dependent on a partial blockade of the sodium leakage current, while at higher concentrations of 5 mg/l the drug seems to provoke a total blockade of Na--Ca slow channels. Subthreshold oscillations could be explained by changes in K conductance together with a remanent of the sodium leakage current.