Histamine Receptors in the Smooth Muscle of Human Internal Mammary Artery and Saphenous Vein

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Abstract
The effects of histamine were characterized and compared in the vascular smooth muscle of two human isolated blood vessels, the human internal mammary artery (HIMA) and the human saphenous vein (HSV). Segments of these vessels were obtained during aortocoronary bypass surgery and their intimal surface was rubbed in order to eliminate any possible influence of the endothelium. Histamine contracted both types of vessels in a concentration‐dependent manner and this effect was antagonized by the H1receptor antagonists mepyramine and cicletanine. In the case of HIMA only this antagonism was found to be competitive (pA2values of 9.3 and 7.7 for mepyramine and cicletanine, respectively). Histamine‐induced contractions were not significantly affected by phentolamine (0.3 μM). In HSV, but not HIMA, indomethacin (5 μM) significantly depressed histamine‐induced contractions (by about 30%). In the presence of the H2receptor antagonist cimetidine (10 μM), concentration‐response curves of histamine‐induced contractions were significantly shifted to the left in both HIMA and HSV, suggesting the presence of H2receptors mediating relaxation. HIMA and HSV precontracted by noradrenaline could be partially and concentrationdependently relaxed by histamine, only in the presence of a H1receptor antagonist. This relaxation was inhibited by cimetidine. The results show that in de‐endothelialized HIMA and HSV histamine induced mainly contraction which is sensitive to the H1receptor antagonists. Only in HIMA, nevertheless, was competitive antagonism established. In addition, histamine‐induced relaxation, antagonized by cimetidine, could be demonstrated in both precontracted vessels, indicating the presence of H2receptors.