HISTAMINE ACTIONS IN DOG RETINAL CENTRAL ARTERIES AS COMPARED TO THOSE IN MIDDLE CEREBRAL AND TEMPORAL ARTERIES

Purpose. Mechanisms underlying the relaxant response to histamine were compared in isolated dog retinal arteries (branch of internal and ext ernal carotid arteries), middle cerebral arteries (branch of internal carotid artery) and superficial temporal arteries (branch of external carotid artery). Methods. Changes in the isometric tension of helical strips of the arteries with and without the endothelium were recorded. Results. Histamine produced concentration-related biphasic (phasic an d sustained) relaxations in retinal arterial strips contracted partial ly with prostaglandin (PG)F-2 alpha. Relaxations induced by histamine were not dependent on the endothelium. Treatment with cimetidine atten uated the sustained relaxation, whereas chlorpheniramine or indomethac in depressed the phasic relaxation. In addition, the phasic relaxant r esponse to histamine was attenuated by tranylcypromine, a PGI(2) synth esis inhibitor. In contrast, the amine-induced relaxant responses in d og middle cerebral arterial branch and temporal arteries were markedly suppressed by cimetidine alone. Conclusions. In dog retinal arteries, the phasic relaxation caused by histamine is mediated by PGI(2) in as sociation with activation of the H-1 receptor subtype in subendothelia l tissues, possibly smooth muscle, and the sustained relaxation is evo ked by direct stimulation of the H-2 receptor subtype in smooth muscle . The histamine-induced relaxation in temporal and distal middle cereb ral arteries is associated solely with a stimulation of H-2 receptors in smooth muscle.