EFFECTS OF CALCIUM-CHANNEL BLOCKERS ON SPONTANEOUS ELECTRICAL-ACTIVITY OF FRESHLY ISOLATED 3-DAY-OLD EMBRYONIC CHICK VENTRICLE

The effects of four major types of organic Ca2+ channel blockers, vera pamil, nifedipine, diltiazem and fendiline and of tetrodotoxin (TXX), a fast Na+ channel blocker, on the action potential (AP) of fleshly is olated 3-day-old embryonic chick ventricle (3d ECV) were investigated to resolve the controversy about the ionic basis of upstroke. The APs were characterized by a maximum diastolic potential (MDP) of -60 mV, a n overshoot (E(ov)) of 16 mV and a maximum upstroke velocity (+V-max) of 42 V s(-1) All four Ca2+ channel blockers (0.1-40 mu M) and TTX (0. 1-80 nM) produced a dose-dependent reduction in +V-max and E(ov). MDP was also reduced by Ca2+ channel blockers in a dose-dependent manner b ut was unaffected by TTX. A significant linear correlation between MDP and +V-max was observed for verapamil (r = 0.99), nifedipine (r = 0.9 9), diltiazem (r = 0.96) and fendiline (r = 0.98). Surprisingly, all C a2+ channel blockers produced a dose-dependent positive chronotropic e ffect leading to cessation of firing at high doses (20-40 mu M). In pr eparations becoming quiescent with high doses of verapamil (20-40 mu M ), elevated extracellular concentrations of Ca2+ (up to 9.6 mM) and is oproterenol (0.5-40 mu M) failed to restore spontaneous APs. Electrica l stimulation also failed to elicit APs in preparations inhibited by v erapamil, diltiazem and fendiline. The inhibition of +V-max by TTX dem onstrates that fast Na+ channels were involved in the upstroke of AP i n 3d ECV. Voltage-dependent inactivation of fast Na+ channels during d epolarization (reduction in MDP) by the Ca2+ channel blockers explains their inhibitory effect on +V-max and indicates that L-type Ca2+ chan nels had no significant role in the upstroke. A positive chronotropic effect of the Ca2+ channel blockers further suggests that slow Ca2+ ch annels are not involved in automaticity in freshly isolated 3d ECV.