Effects of PI-103 and Iso on canine SN automaticity. (A and B) Representative transmembrane potentials recorded from spontaneously beating SN preparations. After baseline recording in control Tyrode’s solution, tissue was superfused for 180 min with (A) control Tyrode’s solution or (B) Tyrode’s solution containing 1 µM PI-103. Then 0.1 µM Iso was added for 5 min (+Iso). The last records were made 15 min after Iso washout (-Iso). The numbers at the top of each record show the rate in beats per minute (b/min). (C and D) Summary data showing effects of PI-103 and Iso on rate and MDP, respectively. (E) Action potential traces from (B) recorded at baseline, following superfusion with PI-103, and after addition of Iso are superimposed, and shown with an expanded time scale. To better compare the slopes of diastolic depolarization, MDP of all traces is set at the same level. (F) Summary data for EDD. *, P < 0.05 versus PI-103 baseline; †, P < 0.05 versus 180 min control; ‡, P < 0.05 versus respective baseline, 180 min, and -Iso (n = 6 per group). Summarized data show mean ± SEM.