The possible role of type II (cGMP-stimulated cAMP hydrolysis) phosphodiesterase (PDE) in the accentuated antagonism of muscarinic effects on heart rate during β-stimulation via endogenous nitric oxide (NO) was evaluated. The canine isolated sinoatrial node preparation was cross circulated with arterial blood of a support dog. The sinoatrial rate of the preparation was 96 ± 5 beats/min ( n = 16) at control. Methacholine (MCh; 0.01–1 μg) injected into the right coronary artery in a bolus fashion caused dose-dependent decreases in sinoatrial rate. Under an intra-arterial infusion of isoproterenol (1 μM), resulting in ∼50% increase in sinoatrial rate, MCh-induced decreases were markedly augmented from −18 ± 3% to −44 ± 4% at 0.3 mg of MCh. When NG-nitro-l-arginine methyl ester (100 μM) or NG-monomethyl-l-arginine (100 μM) were continuously infused, the augmented MCh-induced decreases in sinoatrial rate were significantly suppressed (−29 ± 3% or −25 ± 3%, respectively, P < 0.01). Pretreatment with either 3-isobutyl-1-methylxanthine (IBMX; 20 μM), a non-selective PDE inhibitor, or amrinone (20 μM), a selective type III (cGMP inhibited cAMP hydrolysis) PDE inhibitor, doubled the isoproterenol-induced increase in the sinoatrial rate. However, the augmented MCh-induced decreases in sinoatrial rate were significantly depressed by IBMX (from −23 ± 5% to −14 ± 1%, P < 0.01) but not by amrinone (to −20 ± 3%). These results suggest that MCh-induced accentuated antagonism in the sinoatrial node pacemaker activity can be modulated by endogenous NO via an activation of the type II cyclic GMP-stimulated cAMP PDE.
