Anticholinergic action of quinidine sulfate in the rabbit atrioventricular node

Anticholinergic action of quinidine sulfate was electrophysiologically studied by recording spontaneous action potentials and membrane current of the rabbit atrioventricular node. In the presence of 0.1 mumol/l carbachol, the spontaneous activity of the atrioventricular nodal preparations was markedly inhibited, whereas subsequent addition of 1, 5 and 20 mumol/l quinidine restored automaticity in a concentration-dependent manner. In some preparations, quinidine at concentrations of 5 mumol/l and higher slowed the spontaneous activity by its direct membrane action even in the presence of carbachol. The dose-response curve for acetylcholine action on the spontaneous firing frequency showed that one molecule of acetylcholine bound to one muscarinic receptor of the atrioventricular node cell (Hill coefficient = 1.2). A parallel shift of this curve towards higher acetylcholine concentrations was observed at 0.03, 0.1 and 0.3 mumol/l but not at 1 and 3 mumol/l quinidine, suggesting a noncompetitive antagonism of quinidine against acetylcholine. Voltage clamp experiments revealed that 5 mumol/l quinidine reduced the slow inward current, hyperpolarization-activated inward current, and delayed rectifying K+ current, through its membrane actions. Quinidine at this concentration almost completely suppressed the acetylcholine-activated K+ current, which showed a relaxation phenomenon. Hence, the direct blockage of the acetylcholine-activated K+ current by quinidine was considered responsible for the anticholinergic action of this drug. We conclude that quinidine is a non-specific ionic channel blocker that inhibits all the membrane currents in the atrioventricular node including the acetylcholine-activated K+ current.

Membrane actions of quinidine sulfate in the rabbit atrioventricular node studied by voltage clamp method

The effects of quinidine (0.01-20 micrograms/ml) on spontaneous action potentials and membrane current systems of the rabbit atrioventricular node were studied. At therapeutic concentrations, this drug decreased the action potential amplitude, the maximal diastolic potential, the threshold potential as well as the maximal rate of depolarization and showed a negative chronotropic action. Quinidine, at 5 micrograms/ml, decreased the peak slow inward current by 30.2%, increased its time constant of inactivation by 27.3%, shifted the steady-state inactivation curve toward more negative membrane potentials by 2.8 mV and decreased its fully activated current. Quinidine exerted not only resting but also use-dependent blocking actions on the slow inward current. The depression of the action potential upstroke can thus be explained by the reduction in this current. The outward K+ tail current was decreased by 65.4% and its deactivation time constant was increased by 19.0%. These effects may have contributed to the prolongation of the action potential duration, the reduction in the maximal diastolic potential and the slowing of diastolic depolarization. Quinidine shifted the steady-state activation curve for this K+ current toward hyperpolarization by as much as 7.8 mV. It decreased the hyperpolarization-activated inward current by 16.3% and increased its activation time constant by 10.1%, but this current appeared to play a small role in reducing the rate of diastolic depolarization. These observations indicate that, depending upon dose, quinidine has the potential to decrease all of the time- and voltage-dependent ionic currents in atrioventricular nodal cells.