Sudden onset of fast discharge in the presence of isoproterenol. In panel a, the non-spontaneous SAN was driven at 6 min-1 in 11.2 mM [K+]o. Isoproterenol was administered between the downward arrows. In panel b, the first 3 APs were driven, but once the fast sudden rhythm started, the drive was discontinued. The sudden cessation of the fast rhythm is shown in panel d. In panel e, the same SAN was driven at 6 min-1, but the drive was discontinued prior to isoproterenol administration. In panel f, the arrow points to the ThVos that initiated the sudden fast rhythm. The fast rhythm ceased during recovery and a driven AP is shown in panel h. The means and SEM are shown in inset 1. Other explanations are as in the legend of Fig. 1.