HISTAMINE H-2 RECEPTOR-MEDIATED ENDOTHELIUM-DEPENDENT RELAXATION IN CANINE SPINAL ARTERY

The mechanisms of histamine-induced relaxation were investigated in is olated canine spinal branches of intercostal artery (SBICA). Histamine (10(-7) to 10(-)4 M) and dimaprit (3 x 10(-6) to 3 x 10(-4) M) produc ed concentration-dependent relaxation in the SBICA which had been cont racted by 5 x 10(-6) M norepinephrine, whereas 2-pyridylethylamine cau sed only a very small relaxation. The histamine- and dimaprit-induced relaxation were inhibited by famotidine but not by diphenhydramine. In the SBICA without endothelium, histamine elicited very little relaxat ion. Aspirin (5 x 10(-5) M) and AA 861 (10(-5) M) did not affect the h istamine-induced relaxation. Treatment with L-NMMA (3 x 10(-5) M) Or m ethylene blue (10(-5) M) significantly suppressed the acetylcholine-in duced relaxation of the SBICA but not the histamine-induced one. These results suggest that histamine produces relaxation in the isolated ca nine SBICA through stimulation of H-2-receptors on the endothelium. Th e relaxing mediator(s) released by histamine seems to be neither endog enous prostaglandins, lipoxygenase products, nor nitric oxide.