In vitro and in vivo activities of anticholinergics on the cholinoceptors of the cardiac pacemaker cells.

The tachycardia-inducing effects of atropine, N-methylscopolammonium and N-butylscopolammonium were studied in the conscious dog, both in the presence and absence of sympathetic innervation. These effects were compared with the anticholinergic effects of the drugs on the muscarinic cholinoceptors of guinea-pig isolated atria. The order of potency for inhibition of the effects of acetylcholine on the isolated atria was N-methylscopolammonium greater than atropine greater than N-butylscopolammonium. This order was the same for the tachycardia-inducing potencies. Maximal tachycardia with these three drugs was less marked in the absence of sympathetic innervation than in its presence. However, the doses of atropine and of N-butylscopolammonium required to block the effects of vagal influence were greater. Sympathectomy had no effect on the vagolytic potency of N-methylscopolammonium. In the case of atropine and N-methyl-scopolammonium there was good correlation between anticholinergic and tachycardia-inducing potencies. The tachycardiac effects of these compounds can be explained by their anticholinergic effect. The tachycardia-inducing potency of N-butylscopolammonium was greater than its anticholinergic potency.