Cerebral Circulation and Histamine: 1. Participation of Vascular H1- and H2-Receptors in Vasodilatatory Responses to Carotid Arterial Infusion

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Abstract
We examined the cerebral circulatory effects of intra-carotid infusion of histamine or its receptor agonists in anesthetized rats. Cerebral blood flow was measured by two methods: an intracarotid 133Xe clearance technique and tissue sampling after systemic administration of the diffusible tracer, [14C]iodoantipyrine. Brain metabolic responses were estimated by the 2-deoxyglucose method and tissue sampling. Intracarotid infusion of histamine when the blood-brain barrier was intact did not increase cerebral blood flow. Following disruption of the blood-brain barrier by carotid injection of hypertonic urea, histamine evoked dose-dependent increases (133Xe clearance method) in cerebral blood flow to a maximum of 50% (20 μg min−1 kg−1); histamine produced increases in blood flow to areas supplied by the internal carotid artery, e.g., thalamus and parietal cortex ([14C]iodoantipyrine method). Both classes of histamine receptors (H1 and H2) participated in mediating increases in cerebral blood flow after blood-brain barrier opening. Mepyramine (H1-antagonist) and metiamide (H2-antagonist) attenuated blood flow responses to histamine infusion (133Xe clearance method); metiamide was the more effective blocking agent. Pyridylethylamine (H1-agonist) and dimaprit (H2-agonist) both caused increases in regional cerebral blood flow ([14C]iodoantipyrine method). Histamine infusion after blood-brain barrier opening did not increase cerebral glucose consumption. We suggest that increases in cerebral blood flow produced by histamine are the result of stimulation of vascular H1- and H2-receptors, rather than a secondary response to metabolic activation.