Physiologic Effects of Medullary Hormones 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.

These receptors are prototypical examples of seven-pass transmembrane proteins that are coupled to G proteins which stimulate or inhibit intracellular signalling pathways.Ĭomplex physiologic responses result from adrenal medullary stimulation because there are multiple receptor types which are differentially expressed in different tissues and cells. The physiologic effects of epinephrine and norepinephrine are initiated by their binding to adrenergic receptors on the surface of target cells. Following secretion into blood, the catecholamines bind loosely to and are carried in the circulation by albumin and perhaps other serum proteins.Īdrenergic Receptors and Mechanism of Action Many types of 'stresses' stimulate such secretion, including exercise, hypoglycemia and trauma. Secretion of these hormones is stimulated by acetylcholine release from preganglionic sympathetic fibers innervating the medulla. Norepinephine and epinephrine are stored in electron-dense granules which also contain ATP and several neuropeptides. 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: Synthesis and Secretion of Catecholamines Following release into blood, these hormones bind adrenergic receptors on target cells, where they induce essentially the same effects as direct sympathetic nervous stimulation.

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. Endocrine System > Adrenal Glands Adrenal Medullary HormonesĬells in the adrenal medulla synthesize and secrete epinephrine and norepinephrine.

Adrenal medulla function