Reactivity in human cerebral artery: species variation.

It is becoming obvious that the reactivity of vascular smooth muscle to vasoactive agents is not homogeneous in different arteries (cerebral vs. other arteries) from the same animal species or in cerebral arteries from different species. In this communication, the reactivity of human cerebral arteries to norepinephrine (NE), dopamine (DA), small amounts of K+ and ouabain and their mechanisms of action are compared with those in monkey and dog cerebral arteries. NE produces moderate contractions in the primate arteries, mediated by alpha 1 adrenoceptors, and slight contractions in the dog arteries, possibly mediated by alpha 2 receptors. DA relaxes human and monkey cerebral arteries but contracts the dog arteries. The primate artery relaxation mediated by dopaminergic receptors is large enough to predominate over the alpha 1 receptor-mediated contraction. Minute amounts of K+ preferentially relax cerebral arteries from humans, monkeys, and dogs, possibly activating the electrogenic Na+ pump. Ouabain, a Na+ pump inhibitor, contracts these cerebral arteries with low concentrations. Human and monkey cerebral arteries respond similarly to vasoactive agents presented so far, and appear to share the same mechanisms of action; however, the responsiveness of dog cerebral arteries differs.