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Cardiovascular Pharmacology Concepts |
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Alpha-Adrenoceptor Agonists (a-agonists)
General Pharmacology Alpha-adrenoceptor agonists (a-agonists) bind to a-receptors on vascular smooth muscle and induce smooth contraction and vasoconstriction, thus mimicking the effects of sympathetic adrenergic nerve activation to the blood vessels.
Alpha-agonists constrict both arteries and veins; however, the vasoconstrictor effect is more pronounced in the arterial resistance vessels. Constriction of the resistance vessels (small arteries and arterioles) increases systemic vascular resistance, whereas constriction of the venous capacitance vessels increases venous pressure. Specific Drugs and Therapeutic Uses Most a-agonists are relatively selective for either a1 or a2-adrenoceptors. In addition to the drugs listed below, there are other sympathomimetic drugs that have a-agonist properties in addition to their b-adrenoceptor agonist properties. These drugs include the naturally occurring catecholamines, dopamine, epinephrine and norepinephrine, as well as catecholamine analogs such as dobutamine. (Go to www.rxlist.com for specific drug information).
Side Effects and Contraindications Alpha1-agonists can cause headache, reflex bradycardia, excitability, and restlessness. Because alpha1-agonists produce systemic vasoconstriction, the work of the heart increases. If the coronary circulation is impaired, as in patients with coronary artery disease, the decrease in myocardial oxygen supply/demand ratio can precipitate angina. Preparations used as nasal decongestants can cause a rebound effect (increased congestion) after a couple days of use. Side effects of centrally acting α2-adrenoceptor agonists include sedation, dry mouth and nasal mucosa (because of increased vagal activity), bradycardia, orthostatic hypotension, and impotence. Constipation, nausea and gastric upset are also associated with the sympatholytic effects of these drugs. Fluid retention and edema is also a problem with chronic therapy; therefore, concurrent therapy with a diuretic is necessary. Sudden discontinuation of clonidine can lead to rebound hypertension, which results from excessive sympathetic activity. Revised 03/15/07 |
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DISCLAIMER: These materials are for educational purposes only, and are not a source of medical decision-making advice. © 2005-2008Ed Richard E. Klabunde, all rights reserved. |