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H 2 -receptors predominantly mediate pial arteriolar dilatation in response to histamine, but the reaction of pial veins to histamine has not been clearly identified. In anesthetized cats, we examined responses of pial veins and arterioles to perivascular microapplication of histamine and specific histamine H 1  and H 2  receptor agonists. Arterioles were very sensitive to the H 2 -receptor agonist impromidine, with significant dilatation (+16%) occurring at concentrations as low as 10 −10  m. Arteriolar responses to H 1  receptor stimulation by 2,2-pyridylethylamine were small, even at high concentrations. The order of potency and maximum dilatations found for the receptor agonists were: H 2  (43%) &gt; histamine (28%) &gt; H 1  (17%). By contrast, pial veins did not respond to histamine or the receptor agonists. The results indicate that pial venomotor activity to histamine is negligible, and suggest a sparse distribution of histamine receptors on the outer surfaces of pial veins.

Cerebral Circulation and Histamine: 2. Responses of Pial Veins and Arterioles to Receptor Agonists