Abstract
Isolated segments of cat extracranial and intracranial arteries were tested simultaneously for circular motor activity in an aerated Krebs-Ringer organ bath at constant temperature and pH. Histamine produced a strong contraction (about 850 dyn) in the extracranial arteries but a considerably weaker contraction, with a high half maximum response, in the intracranial arteries. The mode of inhibition of the latter response by antihistaminic compounds (chlorpheniramine and mepyramine) showed the response to be nonspecific; the contraction in the extracranial arteries was inhibited in a competitive manner, demonstrating the presence of histamine H1 receptors. The dilatory response was studied after the vessels had been given a tonic contraction with serotonin. Histamine produced similar dilatory effects (about 200 dyn) in both types of arteries and competitive inhibitions were obtained with burimamide, showing that the dilation was mediated through histamine H2 receptors. Dissociation constants were calculated for the receptor-antagonist complex.
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