Ii

Normetanephrine

Metanephrine

COMT

Vanillylmandelic acid

FIGURE 65-4. Degradation of norepinephrine and epinephrine. MAO = monoamine oxidase; COMT = catechol O-methyl-transferase.

(normal, 0.6 to 2 nmol/L) originates from the adrenal medulla, because the major part of the norepinephrine is secreted from the sympathetic nerves. In contrast, plasma epinephrine (normal, 0.1 to 0.3 nmol/L) is secreted only by the adrenal medulla. The half-life of plasma epinephrine and norepinephrine is very short (1 to 2 minutes), and the variations in plasma norepinephrine are a reflection of variations in sympathetic tone. Because of the effective mechanism for reuptake into sympathetic nerves, venous plasma catecholamines show variations related to the sampling site. More recently, measurement of plasma normetanephrine and metanephrine has been shown to be more reliable for evaluating hypersecretion of catecholamines from the adrenal medulla because their half-life in plasma is much longer than norepinephrine and epinephrine.17

Physiologic Effects

Catecholamines exert their effect on specific adrenergic receptors. These receptors are transmembrane proteins known to be encoded by separate genes. Initially a- and p-adrenergic receptors were identified and their subtypes have been characterized. The a, receptors mediate vascular smooth muscle contraction. (3 preceptor stimulation increases heart rate and myocardial contractility, whereas (J2 receptors are involved in smooth muscle relaxation. The (J3 receptors regulate lipolysis and energy expenditure.

Catecholamines influence almost all tissues and organs in the body. Catecholamines have profound cardiovascular and metabolic effects and also influence the secretion of many hormones.5 The major effects are cardiovascular, with contraction of blood vessels and increasing heart rate and force. Catecholamines also exert effects on extravascular smooth muscle, causing both contraction and relaxation. In addition, catecholamines affect metabolism by increasing oxygen consumption and heat production, and they also regulate the mobilization of glucose and fat stores.

Basal secretion of catecholamines from the adrenal medulla is low. Substantial stimulatory conditions such as trauma and surgical stress are required to increase catecholamine secretion from the adrenal medulla. The release of epinephrine is of interest in special conditions.18 Epinephrine is released from the adrenal medulla during an operation at the time of intubation and skin incision. High levels of epinephrine are also found during traumatic events, such as myocardial infarction and pain or fear. During severe hypoglycemia, epinephrine levels may be up to 50 times the basal level, owing to stimulation of glucose-sensitive neurons in the central nervous system.

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