Nitroglycerin: A volatile vasodilator which relieves ANGINA PECTORIS by stimulating GUANYLATE CYCLASE and lowering cytosolic calcium. It is also sometimes used for TOCOLYSIS and explosives.Vasodilator Agents: Drugs used to cause dilation of the blood vessels.Drug Tolerance: Progressive diminution of the susceptibility of a human or animal to the effects of a drug, resulting from its continued administration. It should be differentiated from DRUG RESISTANCE wherein an organism, disease, or tissue fails to respond to the intended effectiveness of a chemical or drug. It should also be differentiated from MAXIMUM TOLERATED DOSE and NO-OBSERVED-ADVERSE-EFFECT LEVEL.Administration, Cutaneous: The application of suitable drug dosage forms to the skin for either local or systemic effects.Vasodilation: The physiological widening of BLOOD VESSELS by relaxing the underlying VASCULAR SMOOTH MUSCLE.Administration, Sublingual: Administration of a soluble dosage form by placement under the tongue.Nicorandil: A derivative of the NIACINAMIDE that is structurally combined with an organic nitrate. It is a potassium-channel opener that causes vasodilatation of arterioles and large coronary arteries. Its nitrate-like properties produce venous vasodilation through stimulation of guanylate cyclase.Coronary Vessels: The veins and arteries of the HEART.Pentaerythritol Tetranitrate: A vasodilator with general properties similar to NITROGLYCERIN but with a more prolonged duration of action. (From Martindale, The Extra Pharmacopoeia, 30th ed, p1025)Dogs: The domestic dog, Canis familiaris, comprising about 400 breeds, of the carnivore family CANIDAE. They are worldwide in distribution and live in association with people. (Walker's Mammals of the World, 5th ed, p1065)Aldehyde Dehydrogenase: An enzyme that oxidizes an aldehyde in the presence of NAD+ and water to an acid and NADH. This enzyme was formerly classified as EC 184.108.40.206.Tilt-Table Test: A standard and widely accepted diagnostic test used to identify patients who have a vasodepressive and/or cardioinhibitory response as a cause of syncope. (From Braunwald, Heart Disease, 7th ed)Spasm: An involuntary contraction of a muscle or group of muscles. Spasms may involve SKELETAL MUSCLE or SMOOTH MUSCLE.Blood Pressure: PRESSURE of the BLOOD on the ARTERIES and other BLOOD VESSELS.Angina Pectoris: The symptom of paroxysmal pain consequent to MYOCARDIAL ISCHEMIA usually of distinctive character, location and radiation. It is thought to be provoked by a transient stressful situation during which the oxygen requirements of the MYOCARDIUM exceed that supplied by the CORONARY CIRCULATION.Peripheral Nervous System Agents: Drugs that act principally at one or more sites within the peripheral neuroeffector systems, the autonomic system, and motor nerve-skeletal system. (From Smith and Reynard, Textbook of Pharmacology, 1991, p75)Coronary Circulation: The circulation of blood through the CORONARY VESSELS of the HEART.Ointments: Semisolid preparations used topically for protective emollient effects or as a vehicle for local administration of medications. Ointment bases are various mixtures of fats, waxes, animal and plant oils and solid and liquid hydrocarbons.Ferricyanides: Inorganic salts of the hypothetical acid, H3Fe(CN)6.Vascular Resistance: The force that opposes the flow of BLOOD through a vascular bed. It is equal to the difference in BLOOD PRESSURE across the vascular bed divided by the CARDIAC OUTPUT.Nitric Oxide: A free radical gas produced endogenously by a variety of mammalian cells, synthesized from ARGININE by NITRIC OXIDE SYNTHASE. Nitric oxide is one of the ENDOTHELIUM-DEPENDENT RELAXING FACTORS released by the vascular endothelium and mediates VASODILATION. It also inhibits platelet aggregation, induces disaggregation of aggregated platelets, and inhibits platelet adhesion to the vascular endothelium. Nitric oxide activates cytosolic GUANYLATE CYCLASE and thus elevates intracellular levels of CYCLIC GMP.Hemodynamics: The movement and the forces involved in the movement of the blood through the CARDIOVASCULAR SYSTEM.Heart Rate: The number of times the HEART VENTRICLES contract per unit of time, usually per minute.Brachial Artery: The continuation of the axillary artery; it branches into the radial and ulnar arteries.Coronary Vasospasm: Spasm of the large- or medium-sized coronary arteries.Acetylcholine: A neurotransmitter found at neuromuscular junctions, autonomic ganglia, parasympathetic effector junctions, a subset of sympathetic effector junctions, and at many sites in the central nervous system.Endothelium, Vascular: Single pavement layer of cells which line the luminal surface of the entire vascular system and regulate the transport of macromolecules and blood components.Nitric Oxide Donors: A diverse group of agents, with unique chemical structures and biochemical requirements, which generate NITRIC OXIDE. These compounds have been used in the treatment of cardiovascular diseases and the management of acute myocardial infarction, acute and chronic congestive heart failure, and surgical control of blood pressure. (Adv Pharmacol 1995;34:361-81)Isosorbide Dinitrate: A vasodilator used in the treatment of ANGINA PECTORIS. Its actions are similar to NITROGLYCERIN but with a slower onset of action.Diltiazem: A benzothiazepine derivative with vasodilating action due to its antagonism of the actions of CALCIUM ion on membrane functions.Nitrates: Inorganic or organic salts and esters of nitric acid. These compounds contain the NO3- radical.Cyclic GMP: Guanosine cyclic 3',5'-(hydrogen phosphate). A guanine nucleotide containing one phosphate group which is esterified to the sugar moiety in both the 3'- and 5'-positions. It is a cellular regulatory agent and has been described as a second messenger. Its levels increase in response to a variety of hormones, including acetylcholine, insulin, and oxytocin and it has been found to activate specific protein kinases. (From Merck Index, 11th ed)Syncope, Vasovagal: Loss of consciousness due to a reduction in blood pressure that is associated with an increase in vagal tone and peripheral vasodilation.Nitroprusside: A powerful vasodilator used in emergencies to lower blood pressure or to improve cardiac function. It is also an indicator for free sulfhydryl groups in proteins.Papaverine: An alkaloid found in opium but not closely related to the other opium alkaloids in its structure or pharmacological actions. It is a direct-acting smooth muscle relaxant used in the treatment of impotence and as a vasodilator, especially for cerebral vasodilation. The mechanism of its pharmacological actions is not clear, but it apparently can inhibit phosphodiesterases and it may have direct actions on calcium channels.Muscle Relaxation: That phase of a muscle twitch during which a muscle returns to a resting position.Guanylate Cyclase: An enzyme that catalyzes the conversion of GTP to 3',5'-cyclic GMP and pyrophosphate. It also acts on ITP and dGTP. (From Enzyme Nomenclature, 1992) EC 220.127.116.11.Methylene Blue: A compound consisting of dark green crystals or crystalline powder, having a bronze-like luster. Solutions in water or alcohol have a deep blue color. Methylene blue is used as a bacteriologic stain and as an indicator. It inhibits GUANYLATE CYCLASE, and has been used to treat cyanide poisoning and to lower levels of METHEMOGLOBIN.Injections, Intra-Arterial: Delivery of drugs into an artery.Regional Blood Flow: The flow of BLOOD through or around an organ or region of the body.Infusions, Intravenous: The long-term (minutes to hours) administration of a fluid into the vein through venipuncture, either by letting the fluid flow by gravity or by pumping it.Double-Blind Method: A method of studying a drug or procedure in which both the subjects and investigators are kept unaware of who is actually getting which specific treatment.Scorpion Stings: The effects, both local and systemic, caused by the bite of SCORPIONS.Coronary Disease: An imbalance between myocardial functional requirements and the capacity of the CORONARY VESSELS to supply sufficient blood flow. It is a form of MYOCARDIAL ISCHEMIA (insufficient blood supply to the heart muscle) caused by a decreased capacity of the coronary vessels.Fissure in Ano: A painful linear ulcer at the margin of the anus. It appears as a crack or slit in the mucous membrane of the anus and is very painful and difficult to heal. (Dorland, 27th ed & Stedman, 25th ed)Cyanamide: A cyanide compound which has been used as a fertilizer, defoliant and in many manufacturing processes. It often occurs as the calcium salt, sometimes also referred to as cyanamide. The citrated calcium salt is used in the treatment of alcoholism.Radial Artery: The direct continuation of the brachial trunk, originating at the bifurcation of the brachial artery opposite the neck of the radius. Its branches may be divided into three groups corresponding to the three regions in which the vessel is situated, the forearm, wrist, and hand.Hydralazine: A direct-acting vasodilator that is used as an antihypertensive agent.Blood Flow Velocity: A value equal to the total volume flow divided by the cross-sectional area of the vascular bed.Coronary Angiography: Radiography of the vascular system of the heart muscle after injection of a contrast medium.Vasoconstriction: The physiological narrowing of BLOOD VESSELS by contraction of the VASCULAR SMOOTH MUSCLE.S-Nitrosothiols: A group of organic sulfur-containing nitrites, alkyl thionitrites. S-Nitrosothiols include compounds such as S-NITROSO-N-ACETYLPENICILLAMINE and S-NITROSOGLUTATHIONE.Aorta: The main trunk of the systemic arteries.Muscle, Smooth, Vascular: The nonstriated involuntary muscle tissue of blood vessels.