Interactive Effects of Verapamil and Sympathetic Nerve Stimulation on the Sinus and AV Nodes in the Canine Hearts.

The interactive effects of verapamil and sympathetic nerve stimulation on the sinus and atrioventricular (AV) nodes were studied in 33 bilaterally stellectomized and vagotomized dogs. Sinus cycle length (SCL) and AV nodal conduction time (AH) at a constant drive cycle length (600 msec) were recorded. Sympathetic nerve stimulation (SNS) was performed for 40 sec by electrical stimulation of the right stellate ganglion. Several doses of verapamil (0.5-2.0μg/ml, 1.0ml) were injected directly into the sinus and AV node arteries at 20 sec of SNS. In addition, SNS was repeated at two different intensities after verapamil administration. Verapamil significantly increased the SCL and the AH in a dose dependent fashion, which was reduced by SNS. The reductions of SCL and AH by the SNS were attenuated in the presence of verapamil when the stimulation intensity was weak, but not when the stimulation intensity was strong. It is concluded that verapamil and sympathetic nerve activity interact antagonistically on the sinus node automaticity and AV nodal conduction.