The effect of atropine on acetylcholine-induced depressor responses in adult domestic fowl.

In pentobarbital-anaesthetized, spontaneously breathing adult chickens, intravenous administration of acetylcholine, histamine and 5-HT produced a dose-related fall in carotid arterial pressure, rise in central venous pressure and highly variable effects on heart rate. Atropine at low doses (0.1 to 0.2 mg/kg) selectively antagnoized depressor-responses to acetylchline. The blockade persisted for 20 to 30 min. At medium doses (0.5 to 2 mg/kg) atropine strongly antagonized acetylcholine for 50 to 110 min, and also partially antagonized responses to 5-HT and histamine. At higher doses (6 mg/kg), atropine-induced cholinergic blockade persisted for 280 min. It also nonselectively and significantly blocked responses to histamine and 5-HT. Therefore, it is concluded that the effectiveness and persistence of cholinergic blockade in chickens depends upon the dose of atropine used. It seems to rule out the opinion that high doses of atropine are required for cholinergic blockade in chickens.