The effects of mesudipine (an analog of nifedipine), in comparison with those of verapamil and nifedipine, were studied on the normal fast action potentials (APs), the slow APs, and the contractions of guinea pig papillary muscles. In papillary muscles perfused with normal Tyrode solution, mesudipine depressed (10-7 and 10-6M) and abolished (10-5M) contractions within 10–12 min. The maximal upstroke velocity (+JOURNAL/jcph/04.02/00005344-198301000-00025/ENTITY_OV0312/v/2017-07-28T024844Z/r/image-pngmax) and overshoot of the fast APs were not affected, whereas the AP duration (at 50 and 90% repolarization, APD50, APD90) was shortened: APD90 by 30% and APD50 by 36% at 10-5M. In order to determine the effect of mesudipine on the slow APs, the fast Na+ channels were inactivated by partial depolarization (to about −40 mV) by perfusing with 25 m M K+-Tyrode solution. Slow-rising overshooting APs and contractions were elicited on stimulation when iso-proterenol (10-6M), theophylline (10-4M), or histamine (10-5M) were added. The slow-channel blockers verapamil (5 × 10-6M) and nifedipine (10-7M) completely blocked the slow APs. Mesudipine, at 4 × 10-8M, depressed +JOURNAL/jcph/04.02/00005344-198301000-00025/ENTITY_OV0312/v/2017-07-28T024844Z/r/image-pngmax, amplitude, and duration of the isoproterenol-induced slow APs, whereas at 10-7M, excitability was abolished within 12 min. The contractions accompanying the slow APs were depressed by mesudipine in a parallel manner. The effect of mesudipine on the slow APs was reversed by washout of the drug after 20–40 min. The dose/response curve for the mesudipine effect was shifted to the right in high Ca2+ concentration (5.4 m M). Lowering the stimulation frequency from 0.5 Hz (usual drive rate) to 0.1 Hz restored the slow APs blocked by 10-7M mesudipine (less than 20 min); after 20 min, the mesudipine block was independent of frequency. Mesudipine completely abolished the histamine-induced slow APs at 10-6M and the theophylline-induced slow APs at 3 × 10-7M. The results indicate that mesudipine has Ca2+ slow-channel blocking properties. This effect is not mediated by β-adrenergic receptor antagonism. Mesudipine has about the same potency as nifedipine and is about 10 times more potent than verapamil.