Contribution of vagal blockade to the tachycardia induced by the antimuscarinic agents atropine and pirenzepine.

1. The cardiac cholinergic blockade and the chronotropic effect of the widely differing antimuscarinic drugs atropine and pirenzepine were investigated in the dog. 2. In conscious dogs, suppression of the parasympathetic system with atropine (0.2 mg kg-1 h-1) causes marked brief cardioacceleration (234 +/- 13 beats min-1) while pirenzepine (3 mg kg-1 h-1) causes moderate but persistent cardioacceleration (179 +/- 13 beats min-1). After suppression of the influence of the cardiac sympathetic system these cardioaccelerator effects are attenuated, particularly those of pirenzepine. 3. The effects of vagal stimulation are blocked completely and persistently by both agents. 4. When the cardioinhibitory action of the vagus nerve is blocked by pirenzepine the induced tachycardia can be increased by atropine, which causes an additional cardioacceleration (25 +/- 9 beats min-1). 5. These results show that the tachycardia induced by antimuscarinic agents is not only due to vagal blockade. The different mechanisms which may be involved are discussed. The results suggest that pirenzepine can suppress cholinergic influence on the heart more electively than atropine, which induced an 'excess tachycardia'. Also, intrinsic heart rate can be approached more closely when pirenzepine is used to suppress the parasympathetic system, than with atropine.