Post-Overdrive Suppression of the Rabbit Sinus Node Pacemaker Cells : Symposium on Sinus Node Function and Its Abnormality

Effects of rapid stimulation (overdrive) on the isolated rabbit sinus node pacemaker cells were examined in order to elucidate the mechanism of post-overdrive suppression. Small specimens of sinus node tissue having a size of approximately half or less of the length constant of sinus nodes were driven at a fast rate through single sucrose gap methods for 10 to 120 seconds. Under the control perfusion (35°C oxygenated Tyrode's solution), overdrive was followed by a slight suppression of spontaneous activity of the preparations. The post-overdrive suppression was accompanied by the decrease in maximum diastolic potential (MDP), overshoot (OS) and maximum rate of depolarization ((dV/dt)max) as well as a slowing of the rate of diastolic depolarization. The values of MDP, OS, (dV/dt)max and the spontaneous activity returned to the pre-overdrive level within a few seconds. Atropine (2 × 10-6g/ml) did not influence the effects of overdrive. After ouabain administration (2 × 10-7g/ml) or in the low temperature perfusate (25°C), the effects of overdrive were potentiated and a marked post-overdrive suppression characterized by a long pause of over several seconds was obtained. The decrease in MDP, OS and (dV/dt)max following overdrive was also accentuated in both experimental conditions. These results would suggest that the post-overdrive suppression of sinus node is, at least partly, due to some ionic shifts following overdrive and may be potentiated by the metabolic dysfunction of pacemaker cells.