Biological Actions of Catecholamines

Catecholamines exert their effects through specific receptors on the target cell surface. However, the effects elicited depend on the type or subtype of receptor with which they interact. There are three types of catecholamine receptor dopamine, a-adrenergic, and )S-adrenergic. Each of these consists of at least two or more subtypes, which differ with respect to ligand affinity, tissue distribution, postreceptor events, and drug antagonists (Table 32-3). 

Circulating catecholamines have an estimated half-life of about one circulation time (20 seconds). They are rapidly taken up by various tissues (notably the liver), where they may exert their effects before they are inactivated. DA may be converted to NE after uptake by nerve endings, kidney, heart, and other tissues. Two enzymes responsible for inactivation of catecholamines are present in most tissues but are particularly abundant in the liver. Catechol-O-methyltransferase (COMT) is a cytosolic, Mg +-dependent enzyme that catalyzes methoxylation of catecholamines at the hydroxyl group at position 3. COMT 

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