The Adrenal Glands

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The Adrenal Glands

_http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Adrenals.html_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Adrenals.html)

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

Both in anatomy and in function, they consist of two distinct regions:

* an outer layer, the adrenal cortex, which surrounds

* the adrenal medulla.

_Link to graphic showing the location and structure of the adrenal glands

(92K)._

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endocrines.gif)

The Adrenal Cortex

Using _cholesterol_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Cholesterol.html) as

the starting material, the cells of the adrenal cortex

secrete a variety of _steroid_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/S.html#steroid)

hormones. These fall into three classes:

* glucocorticoids (e.g., cortisol)

* _mineralocorticoids_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Adrenals.html#aldoster\

one) (e.g., aldosterone)

* _androgens_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Adrenals.html#androgen\

s) (e.g., testosterone)

Production of all three classes is triggered by the secretion of _ACTH_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pituitary.html#acth)

from the anterior lobe of the pituitary. These hormones achieve their effects

by:

 

* travelling through the body in the blood. Because they are so

hydrophobic, they must be carried bound to a _serum globulin_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/Blood.html#serum) .

* entering 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 dimer migrates into the nucleus (if it did not form there).

* The hormone-receptor dimer binds to specific hormone response

elements in DNA.

* These are specific DNA sequences in the _promoter_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Promoter.html) of

genes that will be

turned on (sometimes off) by the interaction.

* Other _transcription factors_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/T.html#transcription_f\

actor) are recruited to the

promoter and _gene transcription_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Transcription.html)

begins.

_Select this link for a stereo view of a steroid receptor (the

glucocorticoid receptor) dimer bound to the DNA of its response element._

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SteroidREs.html)

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

gluconeogenesis in the liver: the conversion of fat and protein into

_intermediate

metabolites_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/I/IntermediaryMetabolism\

..html) 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_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CMI.html) .

[_Discussion of mechanism_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellSignaling.html#TNF\

-a) ] For this reason

glucocorticoids are widely used in therapy:

* to reduce the _inflammatory_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/I/Inflammation.html)

destruction of rheumatoid arthritis and

other _autoimmune diseases_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/A.html#autoimmune_dise\

ase)

* to prevent the _rejection of transplanted organs_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CMI.html#graft)

* to control _asthma_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Asthma.html)

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 sodium ions

(Na+) into the blood. Water follows the salt and this helps maintain normal

blood pressure. _Link to discussion._

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/K/Kidney.html#nephron)

Aldosterone also

* acts on sweat glands to reduce the loss of sodium in perspiration;

* acts on taste cells to increase the sensitivity of the _taste buds_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Taste.html#salty)

to sources of sodium.

The secretion of aldosterone is stimulated by:

* a drop in the level of sodium ions in the blood;

* a rise in the level of potassium ions in the blood;

* _angiotensin II_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/K/KidneyHormones.html#re\

nin)

* _ACTH_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pituitary.html#acth)

(as is that of cortisol)

Androgens

The adrenal cortex secretes precursors to androgens such as testosterone.

In sexually-mature males, this source is so much lower than that of the

_testes_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Sexual_Reproduction.ht\

ml#Sex_Organs_of_the_Human_Male) 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 produce a masculine pattern of body hair and cessation of

_menstruation_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SexHormones.html#menst\

rual) .

Addison's Disease: Hyposecretion of the adrenal cortices

Addison's disease has many causes, such as

* destruction of the adrenal glands by infection;

* their destruction by an _autoimmune_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/A.html#autoimmune_dise\

ase) 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.It can be caused by:

* excessive production of _ACTH_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pituitary.html#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_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Allergies.html#Rheumat\

oidArthritis) or

* preventing the rejection of an _organ transplant_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Transplants.html#Corti\

costeroids) .

 

The Adrenal Medulla

The adrenal medulla consists of masses of neurons that are part of the

_sympathetic branch_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/PNS.html#sympathetic)

of the _autonomic nervous system_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/PNS.html#autonomic) .

Instead of releasing

their _neurotransmitters_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/N.html#neurotransmitte\

r) at a _synapse_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Synapses.html) , these

neurons release them into the

blood. Thus, although part of the nervous system, the adrenal medulla

functions as an endocrine gland.The adrenal medulla releases:

* adrenaline (also called epinephrine) and

* noradrenaline (also called norepinephrine)

Both are derived from the amino acid _tyrosine_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Tyr_phe.gif) .

Release of adrenaline and noradrenaline is triggered by nervous stimulation

in response to physical or mental stress. The hormones bind to _adrenergic

receptors_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellSignaling.html#GPC\

Rs) — _transmembrane proteins_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellMembranes.html#Tra\

nsmembraneProteins) in the

plasma membrane of many cell types.Some of the effects are:

* increase in the rate and strength of the heartbeat resulting in

increased _blood pressure_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Circulation2.html#Bloo\

d_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_

(http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Clotting.html) time

of the blood is reduced;

* increased ACTH secretion from the anterior lobe of the pituitary.

All of these effects prepare the body to take immediate and vigorous action.