Fig. 6. ( A ) Effect of 30 μm bupivacaine on action potential configuration at a constant pacing frequency of 1 Hz. ( B , D , and E ) Cumulative concentration-dependent effects of bupivacaine on maximum velocity of depolarization (Vmax), action potential duration (APD), and phase 1 amplitude, respectively. The experimental protocol was similar to that applied in the ropivacaine studies. Vmaxdata, fitted to the Hill equation, are shown in C . Symbols and bars represent mean ± SEM values obtained in eight myocytes. * Significant ( P < 0.05) change from control. Steady state frequency-dependent effects of 30 μm bupivacaine on Vmaxand action potential duration are presented in F and G , respectively (n = 5).