Histamine receptors in brain vessels of guinea‐pig:in‐vitropharmacology and ligand binding

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Abstract
The subtype of histamine receptors in brain vessels of guinea‐pig has been characterized by ligand binding andin‐vitropharmacology using selective antagonists. In the basilar artery histamine caused a concentration‐related contraction with an EC50of 1.6 ± 0.3, μ m. H1‐receptor blockade with mepyramine and chlorpheniramine caused a displacement to the right of the histamine concentration‐response curve with an apparentKDof 0.4 and 4.6 nM respectively, whereas H2‐receptor blockade with cimetidine was without effect. Histamine did not induce any dilatatory responses of vessels procontracted by 60 mM potassium‐containing buffer in the presence or absence of histamine antagonists. Ligand‐binding studies with [3H]mepyramine yielded aKDvalue of 5.5 nM in pial vessel membranes and 1.7 nM in the choroid plexus, confirming the presence of H1‐receptors. Nimodipine caused a concentration‐related blockade of histamine‐induced contractions. Omission of Ca2+from the extracellular medium for 30 min reduced the contractile responses to histamine in the basilar artery by 96%. Subsequent addition of Ca2+caused concentration‐related contractions which were inhibited by nimodipine. Thus, the histamine H1‐receptor activation in guinea‐pig basilar artery is coupled to dihydropyridine‐sensitive Ca2+channels.

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