Fig. 3. Both Hoa-SK I and Hoa-SK II (10^–6 mol l^-1) enhanced activity of the isolated heart. Shown are recordings from a force-displacement transducer in two preparations. (A,B) Recordings from a preparation in which the heartbeat was very regular. Each of the peptides evoked an increase in both frequency and amplitude of the heartbeat, although the effects were more pronounced with Hoa-SK II than with Hoa-SK I. (C,D) Recordings from a second preparation in which the heartbeat showed periodically irregularities. In addition to increasing the frequency and amplitude of heart contractions, Hoa-SK I and Hoa-SK II regularized heart beat frequency and amplitude when the heartbeat was initially irregular. Bar, 10 s for all recordings.