Effect of lidocaine and procaine on the action potential of canine sinus nodal cells.

Effects of local anesthetics (lidocaine and procaine) on the canine sinus node were studied using isolated right atrial preparations which were obtained from puppies adn perfused with oxygenated Tyrode solution through the dorsal right atrial artery. Action potential of the sinus nodal area which was characterized by the slow rate of phase O and diastolic depolarization, was recorded using the conventional microelectrode technique, simultaneously with atrial contraction. Lidocaine (10-300 micrograms) prolonged the cycle length and duration of action potential, decreased the rate of phase 4 depolarization and amplitude of action potential, and shifted the threshold potential and maximal diastolic potential to the depolarizing direction. The atrial contraction decreased simultaneously. In higher doses, sinoatrial block was occasionally observed in 8 of 55 experiments. Procaine (10-300 micrograms) had similar negative chronotropic and inotropic effects and induced sinoatrial block in 7 of 57 experiments, but unlike lidocaine, procaine produced positive chronotropic and inotropic effects following the initial negative responses in 16 of 57 experiments. The sinus node acceleration was accompanied by an increase in the rate of phase 4 depolarization. Chronotropic changes of the sinus nodal cells by these drugs were accompanied by the changes in both the rate of phase 4 depolarization and duration of the action potential.