Human Biochemistry Volume 2 - Murray R. 1993

Biochemistry of Intra- and Intercellular Communication
Adrenal Medullary Hormones
Storage and Secretion of Catecholamines

Storage

The adrenal medulla contains chromaffin granules—Organelles capable of the Biosynthesis, uptake, storage, and secretion of catecholamines. In addition to catecholamines, these granules contain several other substances, including ATP-Mg²⁺, DBH, Ca²⁺, and the protein chromogranin A. Catecholamines enter the granules via an ATP-dependent transport mechanism and bind to the nucleotide in a 4:1 ratio (hormone to ATP). Norepinephrine is stored in these granules, but it can exit them and undergo methylation; the resulting epinephrine is then incorporated into a new population of granules.

Secretion

Neural stimulation of the adrenal medulla triggers the fusion of chromaffin granules with Cell/30.html">The Plasma Membrane, thereby driving the release of norepinephrine and epinephrine via exocytosis. This process is calcium-dependent and, much like other exocytotic pathways, is stimulated by cholinergic and ß-adrenergic agents and inhibited by a-adrenergic agents (Fig. 49.2). Catecholamines and ATP are released in the exact same proportion as they occur within the granules. This also holds true for other components, including DBH, calcium, and chromogranin A.

Neuronal reuptake of catecholamines serves as a vital mechanism that ensures, on the one hand, hormone conservation, and on the other, the rapid termination of hormonal or neurotransmitter activity. Unlike sympathetic nerves, the adrenal medulla lacks a mechanism for the reuptake and storage of released catecholamines. Epinephrine secreted by the adrenals reaches The Liver and skeletal Muscles, but is rapidly metabolized thereafter. Only a very small fraction of norepinephrine reaches distant Tissues. Catecholamines circulate in plasma weakly associated with albumin. They have a very short lifespan, with a biological half-life of 10–30 s.