Comparison of vasodilator effects on human middle cerebral artery constriction induced by vasoconstrictors or activated platelets.

Experiments were performed on de-endothelized perfused human middle cerebral artery constricted with noradrenaline, serotonin, prostaglandin F2 alpha and collagen-aggregated platelets, all of which provoked similar vasoconstriction; however, the ability of dipyridamole, prostacyclin and nifedipine to reduce the constriction was significantly less marked when induced by activated platelets. The decrease in drug antispasmatic effects was less marked for nifedipine than for other vasodilators. It was suggested that the resistance of platelet-induced vasoconstriction to vasodilator action depends on platelets ability to release several vasoactive substances simultaneously.