Electrophysiologic effects of ambasilide in canine isolated cardiac muscle driven at 1 Hz and in rabbit sinoatrial (SA) node preparations were determined by standard microelectrode technique. Ambasilide (10(-7)-10(-5) M) produced concentration-dependent increases in action potential duration measured at -80 mV (APD-80) repolarization time in canine ventricular muscle and in Purkinje fibers. APD measured at -20 mV (APD-20) was also increased in ventricular muscle, but it shortened with 10(-5) M in Purkinje fibers; at this concentration, there was a negligible change in the amplitude and the maximum upstroke velocity (Vmax) of action potentials and in the resting membrane potential. With the stimulation frequency between 30/min and 120/min, ambasilide (10(-5) M) produced a parallel increase in APD-80 as well as in APD-20 of ventricular muscle. In Purkinje fibers, the prolonging effect of ambasilide on APD-80 was more pronounced at lower stimulation frequency, whereas APD-20 shortened at all stimulation frequencies. Ambasilide at 10(-5) M also produced a prolongation of the effective refractory period (ERP) in Purkinje fibers. The drug produced no significant change in the frequency-dependent relationship between ERP and APD-80. A small but significant frequency-dependent inhibition of Vmax was noted in both ventricular muscle and Purkinje fibers. When the stimulus cycle length was reduced from 1,000 to 300 ms, Vmax in ventricular muscle decreased by 8.4 +/- 3.4% in control solution but by 22.6 +/- 5.6% after 10(-5) M ambasilide (n = 8, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)