Heterogeneous responses to vasodilators of dog proximal and distal middle cerebral arteries

Responses to various vasodilators were compared in helical strips of proximal and distal middle cerebral arteries isolated from the same dogs, which were partially precontracted with prostaglandin (PG) F2 alpha or K+. Nicotine-induced relaxations, selectively blocked by hexamethonium, were greater in the proximal than in the distal portion, whereas K+ (5 mM)-induced relaxations, reversed to contractions by ouabain, were greater in the distal portion. Angiotensin II and PGI2 relaxed distal middle cerebral arteries to a greater extent; the difference in relaxations induced by the octapeptide in large and small arteries was more evident. Relaxations induced by isoproterenol and adenosine were greater in distal arteries; in contrast, those induced by nitroglycerin were greater in proximal arteries. Acetylcholine and verapamil relaxed the arteries of both portions to a similar extent. It may be concluded that proximal middle cerebral artery functions are regulated by dilator nerves more intensely than the distal artery functions, and the electrogenic Na+ pump in cell membrane of distal middle cerebral arteries leaves more room for being activated under the experimental conditions used. Angiotensin II appears to liberate more PGI2 from the distal cerebral arteries.