The adrenal medulla produces both the catecholamines, epinephrine (adrenaline) and norepinephrine (noradren-aline). Norepinephrine is also formed at adrenergic nerve endings. Norepinephrine is formed from the essential amino acid phenylalanine and epinephrine is formed by methyla-tion of norepinephrine. The enzyme for this only exists in the adrenal medulla. Catecholamines are broken down in the circulation to metanephrines which are then oxidised to form 4-hydroxy-3-methoxy mandelic acid (VMA). The catecholamines and their breakdown products are excreted through the kidney, half as metanephrines, one third as VMA and the remainder as free hormone.
Catecholamines are not essential for life but are essential for reaction to acute stress (preparation for 'flight or fight'). The sympathetic nervous system stimulates the adrenal medulla to release catecholamines directly into the circulation. In view of the anatomical position of the adrenal glands, this is directly into the upper inferior vena cava and directly to the heart for immediate effect. The main actions of catecholamines are:
• increase the heart rate;
• raise blood pressure;
• stimulate the metabolic rate;
• lower the threshold in the reticular formation of the brain re-inforcing the state of arousal;
• increase hepatic and skeletal muscle glycogenesis;
• mobilise free fatty acids.
Epinephrine and norepinephrine have different actions. Norepinephrine produces peripheral vasoconstriction in all peripheral vessels while epinephrine produces vasodilatation in liver and skeletal muscle resulting in a drop in total peripheral resistance. Norepinephrine therefore produces a rise in systolic and diastolic blood pressure while epinephrine causes an increase in pulse pressure.
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