Modification by manganese ions and verapamil of the responses of the atrioventricular node to norepinephrine.

In isolated and cross-circulated canine A-V node preparations single injections of Mn2+ (1-30 mumol) and verapamil (0.3-10 nmol) into the posterior septal artery (PSA) caused a dose-dependent increase in the A-V conduction time and, in large doses, produced A-V conduction block. The A-V conduction time increased by Mn2+ and verapamil is resotred by 1-norepinephrine (1-NE) injected into the PSA. The A-V nodal tachycardia induced by 1-NE is suppressed by Mn2+ but not by verapamil. These indicate the following: (1) the slow Ca2+ channel plays an important role in A-V conduction; (2) the positive dromotropic response of the A-V node to 1-NE may have some relations to the Ca2+ channel; (3) the A-V nodal tachycardia induced by 1-NE may be mediated by the Ca2+ channel. Possible causes which produced differences between Mn2+ and verapamil in the effects in antagonizing the 1-NE-induced A-V nodal tachycardia are discussed.