HISTAMINE INDUCES ENDOTHELIUM-DEPENDENT RELAXATION OF BOVINE RETINAL ARTERIES

The effect of histamine on bovine isolated retinal small arteries (internal diameter, approximately 240-mu-m) was studied. Histamine induced a concentration-dependent relaxation in 43 of 53 vessels. The histamine-induced relaxation involves primarily activation of H-1-receptors, with H-2-receptors also affected as evidenced by the effect of selective histamine-receptor agonists and antagonists. The histamine-induced relaxation was dependent on the endothelium and seem to involve release of both endothelium-derived relaxing factor (EDRF) and a product which was inhibited by indomethacin, probably prostaglandin I2 (prostacyclin). The development of tachyphylaxis to the action of histamine seemed to rely on desensitization of the vascular smooth muscle cells to the relaxing effect of EDRF.