Adrenal Medullary Hormones
Cells in the adrenal medulla synthesize and secrete epinephrine and norepinephrine. The ratio of these two catecholamines differs considerably among species: in humans, cats and chickens, roughly 80, 60 and 30% of the catecholamine output is epinephrine. Following release into blood, these hormones bind adrenergic receptors on target cells, where they induce essentially the same effects as direct sympathetic nervous stimulation.
Synthesis and Secretion of Catecholamines
Synthesis of catecholamines begins with the amino acid tyrosine, which is taken up by chromaffin cells in the medulla and converted to norepinephrine and epinephrine through the following steps:
Norepinephine and epinephrine are stored in electron-dense granules which also contain ATP and several neuropeptides. Secretion of these hormones is stimulated by acetylcholine release from preganglionic sympathetic fibers innervating the medulla. Many types of "stresses" stimulate such secretion, including exercise, hypoglycemia and trauma. Following secretion into blood, the catecholamines bind loosely to and are carried in the circulation by albumin and perhaps other serum proteins.
Adrenergic Receptors and Mechanism of Action
The physiologic effects of epinephrine and norepinephrine are initiated by their binding to adrenergic receptors on the surface of target cells. These receptors are prototypical examples of seven-pass transmembrane proteins that are coupled to G proteins which stimulate or inhibit intracellular signalling pathways.
Complex physiologic responses result from adrenal medullary stimulation because there are multiple receptor types which are differentially expressed in different tissues and cells. The alpha and beta adrenergic receptors and their subtypes were originally defined by differential binding of various agonists and antagnonists and, more recently, by analysis of molecular clones.
|Receptor||Effectively Binds||Effect of Ligand Binding|
Physiologic Effects of Medullary Hormones
In general, circulating epinephrine and norepinephrine released from the adrenal medulla have the same effects on target organs as direct stimulation by sympathetic nerves, although their effect is longer lasting. Additionally, of course, circulating hormones can cause effects in cells and tissues that are not directly innervated. The physiologic consequences of medullary catecholamine release are justifiably framed as responses which aid in dealing with stress. These effects can be predicted to some degree by imagining what would be needed if, for example, you were trapped in Jurassic Park when the power went off. A listing of some major effects mediated by epinephrine and norepinephrine are:
- Increased rate and force of contraction of the heart muscle: this is predominantly an effect of epinephrine acting through beta receptors.
- Constriction of blood vessels: norepinephrine, in particular, causes widespread vasoconstriction, resulting in increased resistance and hence arterial blood pressure.
- Dilation of bronchioles: assists in pulmonary ventilation.
- Stimulation of lipolysis in fat cells: this provides fatty acids for energy production in many tissues and aids in conservation of dwindling reserves of blood glucose.
- Increased metabolic rate: oxygen consumption and heat production increase throughout the body in response to epinephrine. Medullary hormones also promote breakdown of glycogen in skeletal muscle to provide glucose for energy production.
- Dilation of the pupils: particularly important in situations where you are surrounded by velociraptors under conditions of low ambient light.
- Inhibition of certain "non-essential" processes: an example is inhibition of gastrointestinal secretion and motor activity.
Common stimuli for secretion of adrenomedullary hormones include exercise, hypoglycemia, hemorrhage and emotional distress.
< Previous: Functional Anatomy of the Adrenal Gland
Next: Adrenal Steroids >
Send comments to Richard.Bowen@colostate.edu