Endothelium-independent relaxation of the rat femoral artery caused by activation of histamine H2-receptors.

The effect of histamine on the endothelial and smooth muscle cells of isolated rings of the rat femoral artery and the receptor type involved in its development were examined. Relaxed rings did not respond to histamine (10(-8)-10(-4) mol/l). However, when contraction had been produced by phenylephrine, histamine (3 x 10(-7)-10(-4) mol/l) caused a concentration-dependent relaxation. The relaxant effect of histamine on the rat femoral artery was abolished by metiamide, but it was not affected by removal of the vascular endothelium, pyrilamine, atropine, sotalol, hemoglobin or methylene blue. In contrast, under the same experimental conditions, the relaxant effect of histamine on the rat mesenteric artery was strongly reduced by removal of the vascular endothelium, hemoglobin or methylene blue. These findings indicate that, in the rat femoral artery, unlike in several other rat large peripheral arteries, the histamine-induced relaxation is endothelium-independent and results from the activation of smooth muscle histamine H2-receptors. It is tentatively suggested that histamine H1-receptors are not present on the endothelial and smooth muscle cells of the rat femoral artery.