noradrenaline (syn. norepinephrine) is the neurotransmitter of the peripheral sympathetic nervous system, responsible for so-called adrenergic neurotransmission, at nerve endings in the heart, in smooth (involuntary) muscle, and in many glands. It is one of the catecholamines. Tyrosine, an amino acid present in body fluids, is taken up into the adrenergic nerve terminal where it is acted upon by an enzyme, tyrosine hydroxylase, to form DOPA; this is converted to dopamine and in turn to noradrenaline. The neurotransmitter is stored in small vesicles, awaiting release. Arrival of the impulse at the nerve terminal releases vesicles, freeing the noradrenaline to exert its effects. To terminate the action of the transmitter, some noradrenaline is oxidized to inactive material, but most is taken back up into the nerve terminal and stored for later use. There are two types of cell membrane receptors with which noradrenaline interacts, termed a and b, each group having several subtypes. Activation of b-receptors in smooth muscle causes relaxation, while activation of a-receptors causes contraction of smooth muscle, except for that in the intestine. Activation of b-receptors in the heart causes an increase in both the force and rate of contraction. Noradrenaline is also an important transmitter in many parts of the central nervous system, where it is involved in arousal, blood pressure regulation, and mood.

Alan W. Cuthbert

noradrenaline (norepinephrine) A hormone produced by the adrenal glands and also secreted from nerve endings in the sympathetic nervous system as a chemical transmitter of nerve impulses (see neurotransmitter). Many of its general actions are similar to those of adrenaline, but it is more concerned with maintaining normal body activity than with preparing the body for emergencies.

noradrenaline Hormone secreted by nerves in the autonomic nervous system and by the adrenal glands. It slows the heart rate and constricts small arteries, thus raising the blood pressure. It is used therapeutically to combat the fall in blood pressure that accompanies shock.

noradrenaline (norepinephrine) (nor-ă-dren-ă-lin) n. a hormone, closely related to adrenaline, secreted by the medulla of the adrenal gland and also released as a neurotransmitter by sympathetic nerve endings. Among its many actions are constriction of small blood vessels leading to an increase in blood pressure, increase in the rate and depth of breathing, and relaxation of the smooth muscle in intestinal walls.

noradrenaline Hormone secreted by the adrenal medulla together with adrenaline; also a neurotransmitter. Physiological effects similar to those of adrenaline. Also known as norepinephrine.