NO is involved in MCh-induced accentuated antagonism via type IIPDE in thecanine blood-perfused SA node

The possible role of type II (cGMP-stimulated cAMP hydrolysis) phosphodiest erase (PDE) in the accentuated antagonism of muscarinic effects on heart ra te during beta -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 5 16) at control. Methacholine (MCh; 0.01-1 mug) inj ected into the right coronary artery in a bolus fashion caused dose-depende nt decreases in sinoatrial rate. Under an intra-arterial infusion of isopro terenol (1 muM), resulting in similar to 50% increase in sinoatrial rate, M Ch-induced decreases were markedly augmented from -18 +/- 3% to -44 +/- 4% at 0.3 mg of MCh. When N-G-nitro-L-arginine methyl ester (100 muM) or N-G-m onomethyl-L-arginine (100 muM) 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-iso butyl-1-methylxanthine (IBMX; 20 <mu>M), a non-selective PDE inhibitor, or amrinone (20 muM), a selective type III (cGMP inhibited cAMP hydrolysis) PD E inhibitor, doubled the isoproterenol-induced increase in the sinoatrial r ate. However, the augmented MCh-induced decreases in sinoatrial rate were s ignificantly depressed by IBMX (from -23 +/- 5% to -14 +/- 1%, P < 0.01) bu t not by amrinone (to -20 <plus/minus> 3%). These results suggest that MCh- induced accentuated antagonism in the sinoatrial node pacemaker activity ca n be modulated by endogenous NO via an activation of the type II cyclic GMP - stimulated cAMP PDE.