The chromaffin cells of the adrenal medulla secrete small, non-steroid hormones called catecholamines. These are adrenaline and noradrenaline, secreted in a 4:1 ratio respectively. Other hormones are produced (e.g. leu-enkephalin and met-enkephalin) but their function is not clear.
Adrenaline and noradrenaline, when released into the bloodstream, act on a wide range of tissues such as the heart, lungs, gut and eyes to produce the following effects:
The release of noradrenaline and adrenaline from the chromaffin cells is under the control of the sympathetic nervous system. This part of the nervous system is not under voluntary control and is activated in situations of sudden stress, such as conflict or fear (the so-called 'fight or flight' response).
Adrenaline and noradrenaline are rapidly broken down to inactive molecules.
In 1993, scientists found a peptide that was derived from the adrenal medulla and named it 'adrenomedullin'. Research has shown that this peptide is expressed in a multitude of tissues in the body and has many actions. The main physiological effects seem to be on the vascular system, growth and development, the endocrine system, the kidney and the nervous system. It is thought that adrenomedullin is a regulatory peptide that affects the secretory activity of the adrenal cortex. Adrenomedullin is also thought to inhibit ACTH production from the pituitary.