The Adrenal Glands

The adrenal glands are two small structures situated one atop each kidney.

• an outer layer, the adrenal cortex, which surrounds
• the adrenal medulla.

The Adrenal Cortex

• glucocorticoids (e.g., cortisol)
• mineralocorticoids (e.g., aldosterone)
• androgens (e.g., testosterone)

Production of all three classes is triggered by the secretion of ACTH from the anterior lobe of the pituitary.

• diffusing from the blood into all cells
• binding to their receptor - a protein present in the cytoplasm and/or nucleus of "target" cells
• The hormone-receptor complex binds to a second to form a dimer.
• The hormone-receptor complex migrates into the nucleus (if it did not form there).
• The hormone-receptor complex binds to specific hormone response elements in DNA
• These are specific DNA sequences in the promoter of genes that will be turned on (sometimes off) by the interaction.
• Other transcription factors are recruited to the promoter and gene transcription begins.

Glucocorticoids

The glucocorticoids get their name from their effect of raising the level of blood sugar (glucose). One way they do this is by stimulating the liver to convert fat and protein into intermediate metabolites that are ultimately converted into glucose.

The most abundant glucocorticoid is cortisol (also called hydrocortisone).

Cortisol and the other glucocorticoids also have a potent anti-inflammatory effect on the body. They depress the immune response, especially cell-mediated immune responses.

• to reduce the inflammatory destruction of rheumatoid arthritis and other autoimmune diseases
• to prevent the rejection of transplanted organs
• to control asthma

Mineralocorticoids

The mineralocorticoids get their name from their effect on mineral metabolism. The most important of them is the steroid aldosterone. Aldosterone acts on the kidney promoting the reabsorption of salt into the blood and the maintenance of normal blood pressure.

• angiotensin II
• ACTH (as is that of cortisol)
• a high level of potassium ions in the blood

Androgens

In sexually-mature males, this source is so much lower than that of the testes that it is probably of little physiological significance. However, excessive production of adrenal androgens can cause premature puberty in young boys.

In females, the adrenal cortex is a major source of androgens. Their hypersecretion may cause some masculinization in adult females, producing a masculine pattern of body hair and cessation of menstruation.

Addison's Disease: Hyposecretion of the adrenal cortices

• destruction of the adrenal glands by infection
• their destruction by an autoimmune attack
• an inherited mutation in the ACTH receptor on adrenal cells

The essential role of the adrenal hormones means that a deficiency can be life-threatening. Fortunately, replacement therapy with glucocorticoids and mineralocorticoids can permit a normal life.

Cushing's Syndrome: Excessive levels of glucocorticoids

In Cushing´s syndrome, the level of adrenal hormones, especially of the glucocorticoids, is too high.

• excessive production of ACTH by the anterior lobe of the pituitary.
• excessive production of adrenal hormones themselves (e.g., because of a tumor), or (quite commonly)
• as a result of glucocorticoid therapy for some other disorder such as
  • rheumatoid arthritis or
  • preventing the rejection of an organ transplant

The Adrenal Medulla

The adrenal medulla consists of masses of neurons that are part of the sympathetic branch of the autonomic nervous system. Instead of releasing their neurotransmitters at a synapse, these neurons release them into the blood. Thus, although part of the nervous system, the adrenal medulla functions as an endocrine gland.

• adrenaline (also called epinephrine) and
• noradrenaline (also called norepinephrine)

Both are derived from the amino acid tyrosine.

Release of adrenaline and noradrenaline is triggered by nervous stimulation in response to physical or mental stress. The hormones bind to adrenergic receptors - transmembrane proteins in the plasma membrane of many cell types.

• increase in the rate and strength of the heartbeat resulting in increased blood pressure
• blood shunted from the skin and viscera to the skeletal muscles, coronary arteries, liver, and brain
• rise in blood sugar
• increased metabolic rate
• bronchi dilate
• pupils dilate
• hair stands on end ("gooseflesh" in humans)
• clotting time of the blood is reduced
• increased ACTH secretion from the anterior lobe of the pituitary.