Abstract: Experiments were performed to compare the effects of exogenous adenosine (1 μmol/ kg ,i v), vagal stimulation (30 Hz, 4 V, 10 ms) and hypoxia (20%O₂, 10 min) on guinea pig hearts (n=52). It was observed that all of these intervents caused prolongation of sinus cycle length, A-V conduction time and even high degree of A-V block. The site of block was all localized in the proximal portion of the AVN region. Aminophylline (5 mg/kg,iv) antago-nised the effects of adenosine. Vagal stimulation did not result in the increases in contents of myocardial adenosine occurred during hypoxia. Neither arrhythmias nor myocardial adeon-sine increases caused by hypoxia were blocked by atropinization(0. 2 mg/kg, iv). We con-clude that both high myocardial adenosine level and high vagal tone may result in sinus bradycardia and A-V block.

Key words: adenosine, vagal nerve, aminophylline, atropine, myocardial hypoxia