Parasympathetic inhibition of sympathetic effects on sinus rate in anesthetized dogs

The intracardiac parasympathetic neural elements that control sinus rate are found in the fatty tissue overlying the atrial junctions of the right pulmonary veins of mammalian hearts. We refer to these nerves as the sinus rate-related parasympathetic nerves (SRRPN). Thus, to elucidate the role of SRRPN, we studied the effects of cervical vagus stimulation on the positive chronotropic responses to cardiac sympathetic nerve stimulation and isoproterenol infusion before and after the SRRPN were removed in the open-chest anesthetized dog heart. Before SRRPN denervation, cervical vagus stimulation suppressed the sinus rate and the positive chronotropic response to sympathetic nerve stimulation or isoproterenol infusion. After SRRPN denervation, cervical vagus stimulation hardly decreased the sinus rate. On the other hand, even after SRRPN denervation, cervical vagus stimulation suppressed the rate increased by sympathetic stimulation. Cervical vagus stimulation also attenuated the sinus rate increased by isoproterenol. The inhibition by vagus stimulation of the chronotropic response to sympathetic stimulation was greater than that of the response to isoproterenol. The attenuation by cervical vagus stimulation was abolished by atropine. These results suggest that 1) a small number of vagus nerves to the sinoatrial nodal area different from the SRRPN decrease the sinus rate increased by adrenergic interventions and 2) the same activation that causes relatively small effects on sinus rate is capable of causing much larger changes in sinus rate during increased sympathetic tone or in the case of beta-adrenoceptor agonist treatment in the heart in situ.