The dilating effect of histamine on pial arteries of cats and its mediation by H2 receptors.

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Abstract
We studied the effect of histamine and H1 or H2 blockers on the diameter of pial arteries (39-227 micron) using microapplication into the perivascular space. Concentration-response curves for histamine 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(-7) to 10(-3) M). The H1 blocker, mepyramine, was not vasoactive in the concentration range from 10(-7) to 5 X 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.