n. a catecholamine neurotransmitter and adrenal hormone that is the end product of the metabolism of the dietary amino acid tyrosine. It is synthesized primarily in the adrenal medulla by methylation of norepinephrine, which itself is formed from dopamine by the action of the enzyme dopamine β-hydroxylase. As a hormone, it is secreted in large amounts when an individual is stimulated by fear, anxiety, or a similar stress-related reaction. As a neurotransmitter, it is the primary stimulant of both alpha-adrenergic receptors and beta-adrenergic receptors. Thus, it increases the heart rate and force of heart contractions, relaxes bronchial and intestinal smooth muscle, and produces varying effects on blood pressure as it acts both as a vasodilator and vasoconstrictor. Also called adrenaline.