Characterization of histamine receptors in isolated pig basilar artery by functional and radioligand binding studies

Histamine receptors in pig basilar arteries were investigated in vitro by radioligand binding assays and by measuring the contractile and relaxant responses to histamine. Histamine and 2-pyridylethylamine (H1-agonist) induced concentration-dependent contractions, whereas impromidine (H2-agonist) induced concentration-dependent relaxations. These responses were independent of the presence of endothelial cells. Diphenhydramine (H1-antagonist) partially reversed the histamine-induced contractions to relaxations. Cimetidine (H2-antagonist) potentiated the contraction in a concentration-dependent manner. In the presence of cimetidine, the pEC50 value of histamine for the contraction was 6.30, and diphenhydramine competitively antagonized the histamine-induced contractions (pA2, 7.77). In the presence of diphenhydramine, the pEC50 value of histamine for the relaxation was 5.93, and cimetidine competitively antagonized the histamine-induced relaxations (pA2, 6.62). In the binding studies, the Kd value of [3H]mepyramine was 2.1 nM and the Bmax value was 95.6 fmol/mg protein. A competition experiment with diphenhydramine showed that the pKi value (7.51) was similar to the pA2 value. The Kd value for [3H]cimetidine was 126.0 nM and the Bmax value was 459.8 fmol/mg protein. The pKd (6.90) for [3H]cimetidine was similar to the pA2 for cimetidine. The Hill coefficients for these experiments were not significantly different from unity. The present findings indicate that the number of H1-receptors, in terms of the Bmax value for [3H]mepyramine, is smaller than that of H2-receptors, in terms of the Bmax value for [3H]cimetidine. However, the contractile response to histamine is predominantly mediated through stimulation of H1-receptors on vascular smooth muscle cells in pig basilar artery.