Fig. 5. The increases in both frequency and amplitude of heartbeat induced by Hoa-SK I (A,C) and Hoa-SK II (B,D) were rapid in onset, and relatively rapid in time to return to baseline. Shown are examples from a single, representative preparation, in which it can be seen that the peak effect was reached within less than 5 min. Wash-out took somewhat longer, but activity had returned to baseline within 10–12 min.