CEREBRAL VASCULAR-RESPONSES TO PHYSIOLOGICAL STIMULATION OF SYMPATHETIC PATHWAYS IN CATS

This study was designed to determine whether physiological activation of sympathetic pathways affects cerebral blood flow (CBF) and the integrity of the blood-brain barrier during hypertension. Increased sympathetic activity was induced in anesthetized cats by sinoaortic deafferentation (SAD), a procedure that results in acute, severe neurogenic hypertension. Sympathetic innervation of one cerebral hemisphere was interrupted by sectioning the superior cervical ganglion or the cervical sympathetic trunk. CBF was measured with microspheres, and disruption of the blood-brain barrier was evaluated using Evans blue dye. Following SAD, mean arterial pressure increased abruptly from 142 ± 8 (mean ±SE) to 226 ± 9mm Hg, and total CBF increased from 31 ± 2 to 99 ± 15ml/min per 100g. Blood flow and the extent of disruption of the blood-brain barrier were less in the innervated hemibrain than in the denervated hemibrain. The influence of sympathetic nerves was ost pronounced in cortical gray matter: blood flow was 24 ± 4% less (P<0.05) in the cortical gray matter of the innervated hemibrain 1 minute following SAD. Additional studies were performed to determine whether physiological stimulation of sympathetic nerves constricts cerebral vessels during hemorrhagic hypotension in cats. During hypotension, blood flow was significantly lower in several regions of the innervated hemibrain compared to the denervated hemibrain, but the effect was very small. These results provide evidence for neural regulation of CBF when sympathetic tone is augmented by physiological stimuli during acute hypertension and hypotension. The data represent the first demonstration of a reduction in CBF when sympathetic nerves are activated physiologically during acute severe hypertension.