Response of cerebral arteries to sympathetic stimulation during acute hypertension.

Our goal was to determine whether sympathetic stimulation during acute hypertension constricts large cerebral arteries and attenuates increases in cerebral microvascular pressure. We measured cerebral blood flow with microspheres and pressure in small pial arteries with a servonull device in anesthetized cats. During moderate hypertension, sympathetic stimulation had little effect on resistance of large or small cerebral vessels. During severe hypertension, sympathetic stimulation prevented passive decreases in resistance of large cerebral arteries and allowed pronounced constriction of small vessels. During hypertension, there was a large increase in pressure in small pial arteries. Although sympathetic stimulation prevented decreases in resistance of large arteries during severe hypertension, it did not attenuate increases in pressure in pial arteries approximately 200 micron in diameter and only modestly attenuated increases in pressure in pial arteries approximately 100 micron in diameter. These findings indicate that sympathetic stimulation has important effects on resistance of both large and small cerebral vessels during severe hypertension. Thus, although stimulation produces dilation of small cerebral vessels during normotension, sympathetic stimulation allowed constriction of small vessels during severe hypertension. These results also indicate that sympathetic stimulation does not prevent increases in pressure in small pial arteries. Thus, protection of the blood-brain barrier by stimulation of sympathetic nerves during hypertension is not the result of attenuation of increases in pial artery pressure.