A comparison of changes in atropine-induced tachycardia and atropine concentration in conscious dogs.

In the conscious dog, after intravenous injection of 0.2 mg/kg 3H-labelled atropine the progressive decrease of the heart rate was well correlated with the disappearance of atropine from the blood. But, with atropine infusion (0.2 mg/kg/h) the tachycardia decreased progressively in spite of an increase in the blood concentration of atropine. Differences appeared between the normal animals or those pretreated with propranolol or reserpine, and the animals which had undergone bilateral thoracic sympathectomy. In the stellectomized animals atropine induced a smaller but much more lasting cardiac acceleration than under the other experimental conditions. It seems that only peripheral factors are involved in the complex interactions between the adrenergic and cholinergic nervous systems at the sinus node level.