Sympathetic Cerebral Vasoconstriction Blocked by Adrenergic Alpha Receptor Antagonists

Alpha receptor adrenergic antagonists were used in chloralose-anesthetized dogs to block cerebral vasoconstriction elicited by stimulation of the sympathetic innervation of the cerebral vessels. A venous outflow technique was used to measure cerebral blood flow with an electromagnetic flow transducer. The brain's arterial supply was left undisturbed. The experiments were performed in both open and closed chest animals and in animals with open and closed craniums. The left sympathetic stellate ganglion was stimulated for 60 seconds at 15 Hz, 3 msec pulse duration, and 3 to 9 v intensity. Control stimulation produced a 65.8% decrease in cerebral blood flow. Dibozane, I or 2 mg/kg, or phentolamine, 2 mg/kg, was then administered, and stimulations were repeated with the same stimulus parameters. After alpha receptor blockade, cerebral blood flow decreased only 3.5% with sympathetic stimulation. The effective blockade by two different alpha receptor antagonists, of a different structure, indicated that an alpha receptor was responsible for the sympathetically mediated cerebral vasoconstriction. In conclusion, stimulation of the sympathetic innervation of the cerebral vessels results in a marked decrease in cerebral blood flow which is blocked by alpha receptor antagonists.