Clemastine inhibits both the vasoconstrictor and vasodilator effects of histamine. Endothelial cells are responsible for releasing vasoconstriction and vasodilator factors, thus modifying tone. Nitric oxide is a potent pulmonary vasodilator, and thus reduces PVR and improves venous return. Nitrite is also important in biochemistry as a source of the potent vasodilator nitric oxide. In this way he established the existence of vasomotor nerves, both vasodilator and vasoconstrictor. Nitric oxide is a vasodilator causing arterioles to expand and draw in more blood. The most prominent vasodilator is nitric oxide (termed endothelium-derived relaxing factor for this reason). Prostaglandin E1 is in the vasodilator family of medications. It also enhances angiogenesis and is a potent renal vasodilator. Nifedipine is more a potent vasodilator and more effective in angina. It inhibits platelet activation and is also an effective vasodilator. Epidural anesthesia and hyperbaric oxygen therapy also have vasodilator effect. Mixing it with amyl nitrite, another vasodilator, is particularly dangerous and potentially fatal. It is a potent peptide vasodilator and can function in the transmission of nociception. Sodium nitroprusside is primarily used as a vasodilator.