EFFECT OF CERVICAL SYMPATHETIC-NERVE STIMULATION ON CANINE CAROTID-SINUS REFLEX

In the chloralose-anesthetized dog the carotid sinus on 1 side of the neck was isolated vascularly. Pressure in the isolated sinus [carotid sinus pressure (CSP)], ECG and systemic arterial pressure were recorded. Both vagosympathetic trunks were cut and the contralateral common carotid artery was occluded or the contralateral sinus nerve was cut to reduce reflex buffering of arterial pressure changes. By varying CSP from 50-250 mm Hg the full range of the reflex response was examined. Electrical stimulation of the peripheral end of the cut ipsilateral cervical sympathetic nerve brought about a rapid decrease in mean arterial pressure (MAP) and heart rate (HR) at lower CSPs, no change in these variables at midrange CSPs and a gradual increase at higher CSPs, such that the gain of the reflex was reduced (1.89 .+-. 0.19-1.33 .+-. 0.15 mm Hg/mm Hg). The decrease in MAP and HR at lower CSPs implies an increase in baroreceptor activity whereas the converse would appear to occur at higher CSPs. These responses attained a maximum value at low stimulus frequencies (< 10 Hz).