Role of endothelium and nitric oxide in histamine‐induced responses in human cranial arteries and detection of mRNA encoding H1‐ and H2‐receptors by RT‐PCR

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Abstract
Histamine induces relaxation of human cranial arteries. Studies have revealed that the relaxant histamine H1‐receptor predominates in human cerebral and the H2‐receptor in temporal arteries, while H1‐ and H2‐receptors are of equal importance in the middle meningeal artery. The purpose of the present study was to examine the role of the endothelium and nitric oxide in histamine‐induced responses and to show the presence of mRNA encoding H1‐ and H2‐receptors in human cranial arteries. Electrophoresis of polymerase chain reaction (PCR) products from human cerebral, middle meningeal and temporal arteries, demonstrated products corresponding to mRNA encoding both H1‐ and H2‐receptors in arteries with and without endothelium. The amplified PCR products were sequenced and showed 100% homology with the published sequences of these histamine receptors. A sensitive in vitro system was used to study vasomotor responses to histamine. In precontracted cerebral, middle meningeal and temporal arteries with and without endothelium, histamine caused a concentration‐dependent relaxation with Imax values between 87% and 81% and pIC50 values between 8.14 and 7.15. In arteries without endothelium the histamine‐induced relaxation was significantly less potent (Imax values between 87% and 66% and pIC50 values between 7.01 and 6.67) than in cranial arteries with an intact endothelium. The addition of histamine to arteries without endothelium and pretreated with the histamine H2‐antagonist, cimetidine (10−5 M), caused a concentration‐dependent contraction of the cranial arteries with Emax values between 86% and 29% and pEC50 values between 7.53 and 6.77. This contraction was blocked by the histamine H1‐receptor antagonist, mepyramine (10−7 M), and even turned into a relaxation with Imax values between 84% and 14% and pIC50 values between 7.42 and 5.86. The nitric oxide synthase inhibitor NG‐nitro‐L‐arginine methyl ester (L‐NAME, 3×10−5 M) significantly inhibited the relaxant response to histamine in cerebral and temporal arteries (pIC50 values between 7.43 and 7.13). The combined treatment with L‐NAME (3×10−5 M) and cimetidine (10−5 M) caused a further displacement of the concentration‐response curve (pIC50 values between 7.14 and 6.57) and decreased the maximum relaxant responses in all three cranial arteries (Imax values between 62% and 39%). In conclusion, this is the first study which show mRNA encoding histamine H1‐ and H2‐receptors in human cranial arteries. The results indicate that histamine‐induced relaxation of human cranial arteries is partially mediated via an endothelial H1‐receptor coupled to the production of nitric oxide and partially via a H2‐receptor associated with the smooth muscle cells. In addition, there is evidence for a contractile H1‐receptor in the smooth muscle cells in these arteries. British Journal of Pharmacology (1997) 121, 41–48; doi:10.1038/sj.bjp.0701097