Vasomotor effects of histamine on bovine and equine basilar arteriesin vitro

The vasomotor effects of histamine on isolated bovine and equine basilar arteries were examined. Histamine induced contractions in both these preparations. The maximal response to and pEC₅₀ value for histamine of the equine artery were larger than those of bovine tissue. Similar results were obtained with endothelium-denuded basilar arteries. Diphenhydramine (H₁-receptor antagonist) inhibited histamine-induced contractions of the basilar arteries from both species in a concentration-dependent manner and its pA ₂ values (with 95% confidence limits) were 7.61 (7.39–7.83) and 8.15 (8.01–8.29) for the bovine and equine preparations, respectively. Cimetidine (H₂-receptor antagonist) slightly potentiated histamine-induced contractions of bovine, but not equine, basilar arteries. 2-Thiazolylethylamine (H₁-receptor agonist) induced contractions in both preparations, whereas impromidine (H₂-receptor agonist) induced weak relaxation of the bovine, but not the equine, tissue. These findings indicate that bovine basilar arterial smooth muscle cells possess H₁- and H₂-receptors. Stimulation of the former results in contraction, whereas stimulation of the latter results in weak relaxation. Equine basilar arterial smooth muscle cells possess H₁-receptors, stimulation of which results in contraction.Abbreviations CR concentration ratio - EC₅₀ concentration producing 50% maximal response - pA ₂ negative logarithm of the molar concentration of antagonist that produces a twofold rightward shift of a concentration-response curve - pEC₅₀ negative logarithm of EC₅₀ - PGF₂ prostaglandin F₂ - PGI₂ prostaglandin I₂