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N Toda
In helical strips of human and monkey cerebral arteries, norepinephrine produced a greater contraction than that in dog cerebral arteries. In monkey cerebral arteries, phenylephrine and norepinephrine produced a similar magnitude of maximum contractions, although the ED50 value of phenylephrine was approximately 5.6 times greater than that of norepinephrine. Clonidine (up to 10(-5) M) did not produce contractions. Dog cerebral arteries responded to phenylephrine in high concentrations with a greater contraction than that induced by norepinephrine and to clonidine with significant contractions. Contractions induced by norepinephrine of human and monkey cerebral arteries were attenuated by low concentrations of prazosin but were not influenced by yohimbine in concentrations up to 10(-8) M. In contrast, norepinephrine-induced contractions of dog cerebral arteries were attenuated by yohimbine but were unaffected by prazosin. It appears that norepinephrine-induced contractions are mediated by alpha-2 adrenoceptors in dog cerebral arteries and by alpha-1 receptors in human and monkey cerebral arteries as well as monkey and dog mesenteric arteries. The relative unresponsiveness of monkey and dog cerebral arteries to adrenergic nerve stimulation may not be explained by a paucity of alpha adrenoceptors in neuroeffector junction.
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