Hemolysate inhibits cerebral artery relaxation

In helical strips of dog middle cerebral arteries partially contracted with prostaglandin (PG) F2 alpha, relaxations induced by angiotensin-II, possibly mediated by PGI2, and those induced by PGH2 were reversed to a contraction or markedly reduced by treatment with hemolysate, which, however, attenuated the PGI2-induced relaxation only slightly. The relaxant response of human middle cerebral arterial strips to PGH2 was also suppressed by hemolysate. Dog and monkey middle cerebral arteries responded to transmural electrical stimulation and nicotine with transient relaxations, which were quite susceptible to tetrodotoxin and hexamethonium, respectively; the relaxations were abolished almost completely by hemolysate and methylene blue. On the other hand, the relaxant response of dog cerebral arteries to a low concentration of K+ was not influenced by hemolysate or by methylene blue, but was reversed to a contraction by treatment with ouabain. Relaxations induced by substance-P and nitroglycerin were markedly inhibited by hemolysate; removal of endothelium abolished the relaxation by substance-P, but did not influence the nitroglycerin-induced relaxation. Hemolysate may interfere with the biosynthesis of PGI2 in the vascular wall, thereby reversing the relaxation induced by angiotensin-II and PGH2 to a contraction. Relaxations induced by electrical and chemical stimulation of vasodilator nerves innervating cerebral arteries appear to be elicited by a mechanism dependent on cellular cyclic guanosine monophosphate (GMP), like that underlying the substance-P-induced and nitroglycerin-induced relaxation. These actions of hemolysate may be involved in the genesis of cerebral vasospasm after subarachnoid hemorrhage.