Motor reactivity of isolated heart of the grass-snake (Natrix natrix L.).

Stimulation of the vagus nerve with a volley of electric impulses changed the action of grass-snake heart producing a negative chronotropic and inotropic effect. The effect of vagal stimulation was not different from the effect of acetylcholine administration and it was absent in the presence of atropine and hexamethonium. It was not possible to demonstrate sympathetic nervous fibres in the stimulated segment of the vagus nerve and trials of finding a separate nerve increasing the heart rate were unsuccessful. Parasympathicotonic agents caused bradycardia and a fall in the amplitude of cardiac contractions, and in sufficiently high doses they arrested the heart in diastole. The action of muscarine-like agents was stronger than that of nicotine, and the anticholinergic action of tubocurarine was weaker than that of atropine. Catecholamines exerted a positive inotropic and chronotropic effect which was completely blocked by propranolol in some tests only.