The effect of supranodose vagotomy on the hexamethonium-resistant bradycardia in the anaesthetized rabbit

Previous studies from this laboratory have established that electrical stimulation of non-myelinated axons in the rabbit vagus nerve produces a bradycardia which is unaffected by the nicotinic ganglion blocker hexamethonium. The present study was undertaken to determine whether this effect is mediated by afferent or efferent axons. A unilateral supranodose vagotomy was performed on four New Zealand White rabbits, one further animal served as a sham-operated control. Fourteen days later the effects of vagal nerve stimulation (10 Hz, 20 s) were assessed. On the operated side, where the supranodose vagotomy would have led to the degeneration of efferent axons, vagal stimulation had no effect on heart rate. The integrity of afferent axons was demonstrated by recording both electrically evoked volleys and characteristically normal patterns of afferent activity from the nerve. On the unoperated side the bradycardia produced by vagal stimulation was consistent with previous studies. It is concluded that the hexamethonium-resistant bradycardia evoked by stimulation of the rabbit vagus is mediated by non-myelinated preganglionic efferent axons.