Vasodilation refers to the widening of blood vessels^[1] resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When vessels dilate, the flow of blood is increased due to a decrease in vascular resistance. Therefore, dilation of arterial blood vessels (mainly arterioles) leads to a decrease in blood pressure. The response may be intrinsic (due to local processes in the surrounding tissue) or extrinsic (due to hormones or the nervous system). Additionally, the response may either be localized to a specific organ (depending on the metabolic needs of a particular tissue, as during strenuous exercise), or systemic (seen throughout the entire systemic circulation). Factors that result in vasodilation are termed vasodilators.

[edit] Examples and individual mechanisms

Vasodilation is the result of relaxation in smooth muscle surrounding the blood vessels. This relaxation, in turn, relies on removing the stimulus for contraction, which depends on intracellular calcium ion concentrations and, consequently, phosphorylation of the light chain of the contractile protein myosin. Thus, vasodilation mainly works either by lowering intracellular calcium concentration or the dephosphorylation of myosin. This includes stimulation of myosin light chain phosphatase and induction of calcium symporters and antiporters that pump calcium ions out of the intracellular compartment. This i ! Description ! Example |- | Hyperpolarization mediated (Calcium channel blocker) | Changes in the resting membrane potential of the cell affects the level of intracellular calcium through modulation of voltage sensitive calcium channels in the plasma membrane. | adenosine |- | cAMP mediated | Adrenergic stimulation results in elevated levels of cAMP and protein kinase A, which results in increasing calcium removal from the cytoplasm | prostacyclin |- | cGMP mediated (Nitrovasodilator) | Through stimulation of protein kinase G | nitric oxide |}

PDE5 inhibitors and potassium channel openers can also have similar results.

Compounds that mediate the above mechanisms may be grouped as endogenous and exogenous.

[edit] Endogenous

[edit] Exogenous vasodilators

• Absence of high levels of environmental noise
• Absence of high levels of illumination
• Adenocard - Adenosine agonist, primarily used as an anti-arrhythmic.
• Alpha blockers (block the vasoconstricting effect of adrenaline).
• Amyl nitrite and other nitrites are often used recreationally as a vasodilator, causing lightheadedness and a euphoric feeling.
• Atrial natriuretic peptide (ANP) - a weak vasodilator.
• Ethanol
• Histamine-inducers
  • Complement proteins C3a, C4a and C5a work by triggering histamine release from mast cells and basophil granulocytes.
• Nitric oxide inducers
  • Glyceryl trinitrate (commonly known as Nitroglycerin)
  • Isosorbide mononitrate & Isosorbide dinitrate
  • Pentaerythritol Tetranitrate (PETN)
  • Sodium nitroprusside
  • PDE5 inhibitors: these agents indirectly increase the effects of nitric oxide
    • Sildenafil (Viagra)
    • Tadalafil
    • Vardenafil
• Tetrahydrocannabinol (THC) - the major active chemical in marijuana. Its mild vasodilating effects redden the eyes of cannabis users.
• Theobromine.
• Papaverine an alkaloid found in the opium poppy papaver somniferum

[edit] Therapeutic uses

Vasodilators are used to treat conditions such as hypertension, where the patient has an abnormally high blood pressure, as well as angina and congestive heart failure, where maintaining a lower blood pressure reduces the patient's risk of developing other cardiac problems.^[5] Flushing may be a physiological response to vasodilators. Viagra, a phosphodiesterase inhibitor, works to increase blood flow in the penis through vasodilation. It may also be used to treat pulmonary arterial hypertension (PAH).

[edit] References