Clemastine inhibits both the vasoconstrictor and vasodilator effects of histamine. Adrenalone is an adrenergic agonist used as a topical vasoconstrictor and hemostatic. With respect to vascular smooth muscle, there exist both vasoconstrictor and vasodilator autacoids. Angiotensin II is a potent direct vasoconstrictor and a stimulator of aldosterone release. It converts the hormone angiotensin I to the active vasoconstrictor angiotensin II. More specifically, it can refer to vasodilator action and vasoconstrictor action. The vasoconstrictor thromboxane is also synthesized from arachidonic acid. Angiotensin II is potent vasoconstrictor in a substrate concentration-dependent manner. In this way he established the existence of vasomotor nerves, both vasodilator and vasoconstrictor. Phenylephrine hydrochloride at 0.25% is used as a vasoconstrictor in some suppository formulations. Ornipressin is a vasoconstrictor, haemostatic and renal agent. Thromboxane is a vasoconstrictor and a potent hypertensive agent, and it facilitates platelet aggregation. It is an inhibitor of nitric oxide synthase and hence a vasoconstrictor. It possesses structural similarity to several neurotransmitters, and has biological activity as a vasoconstrictor. When injected, serotonin acts as a vasoconstrictor, which could potentially be lethal to small animals.