Prolongation of the functional recovery of the sinoatrial (SA) nodal pacemaker activity after overdrive, "overdrive suppression," is determined by intrinsic pacemaker activity, pacemaker site, and SA conduction. We investigated the effects of E-4031, a blocker of a rapid type of the delayed rectifier K+ current (Ikr) and stimulation of the intracardiac parasympathetic nerves to the SA nodal region (SAP) on the prolongation of the first returning cycle length (1st RCL) after overdrive in autonomically decentralized hearts of open-chest anesthetized dogs. Second and third RCLs also were measured. We determined SA node recovery time (SNRT) and corrected SNRT (CSNRT) after atrial pacing at rates of 120, 150, and 200% of the control rate for 1 min and also determined SA conduction time (SACT) by the constant-atrial-pacing method. E-4031 (0.1-3 micromol/kg i.v.) increased the sinus cycle length (SCL) and SNRT dose dependently. However, E-4031 decreased CSNRT when the pacing rate was low or the number of pacing stimuli was small, although the agent did not induce a significant change in CSNRT when sufficient pacing stimuli were applied. E-4031 decreased SACT dose dependently. After E-4031 treatment. we observed changes in atrial electrical configurations, suggesting the possibility of pacemaker shift. When SAP stimulation increased SCL, SNRT, CSNRT, and SACT, E-4031 selectively inhibited the prolongation of SCL by SAP stimulation but did not affect the prolongation of CSNRT or SACT. These results suggest that functional recovery of the SA nodal pacemaker activity after overdrive is regulated by Ikr at least in part and that Ikr inhibition attenuates prolongation of the SCL but not the 1st RCL induced by parasympathetic nerve activation in the heart in situ.
