The Dilating Effect of Histamine on Pial Arteries of Cats and Its Mediation by H2 Receptors

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We studied the effect of histamine and H1 or H2blockers on the diameter of pial arteries (39-227 μm) using microapplication into the perivascular space. Concentration-response curves for hiatamine showed dilations which started at 10-7 M and were maximal at 10-5 and 10-4 M. The H2 blocker, cimetidine, induced no vascular reaction over the whole concentration range tested (10-7to 10-3M). The H1 blocker, mepyramine, was not vasoactive in the concentration range from 10-7 to 5 × 10-5 M and evoked dilations at higher concentrations. The concentration-response curve for histamine was only slightly displaced by 10-5 M mepyramine but was significantly shifted to the right by 10-5 M cimetidine. The dilating effect of histamine could be reduced in a stepwise manner by increasing concentrations of cimetidine. These findings are in accordance with a selective antagonism between histamine and cimetidine at the H2 receptors of smooth muscle cells of pial arteries. The insignificant role of H1 receptors in histamine-induced dilations is supported by the finding that a combination of H1 and H2 blockers resulted in the same reduction of histamine-induced dilation as did the application of the H2 blocker. Ore Res 44:161-165, 1979

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