A vagolytic action of neuromuscular blocking agents at the pacemaker of the isolated guinea pig atrium.

To examine the basis of tachycardia seen clinically with some neuromuscular blocking agents, the potencies of d-tubocurarine, dimethyltubocurarine, gallamine, and pancuronium in antagonizing the effects of vagal stimulation on the guinea pig atrial pacemaker were determined and expressed as an ED50 for vagal blockade. These ED50 values were compared with the respective potency values of these agents at the motor endplate. This comparison showed that in clinical doses, gallamine and pancuronium may reach levels that produce vagal blockade. Comparison with atropine indicated that the vagolytic action of the neuromuscular blocking agents was not attributable to receptor occlusion, but reflected instead an action on the vagus nerve itself.