6. Adrenal cortex - hormones and physiology Flashcards
Anatomy of the adrenal gland
Adrenal glands lie on top of the kidneys
Divided into inner adrenal medulla (10%) and the outer adrenal cortex (90%)
What does the medulla do?
Medulla is concerned with stress response (fight or flight) but not essential for life
What is the cortex associated with?
The cortex is concerned with stress, sodium and glucose homeostasis. A functioning adrenal cortex is essential for life.
What does the adrenal cortex synthesise?
The adrenal cortex synthesises many different hormones of a similar chemical structure (steroid hormones) which are derived from cholesterol from the diet or synthesised within the gland itself.
Testosterone, cortisol, eostrogen and progesterone are all steroids
What are the three layers of the adrenal cortex?
Outer: zona glomerulosa
Middle: zona fasciculata
Inner: zona reticularis
Cells within the different areas possess different enzymes and therefore synthesise different adrenocortical hormones
What are the effects of steroid hormones
Classified according to their predominant action:
glucocorticoids e.g. cortisol control sugars
mineralocorticoids e.g. aldosterone control minerals Na+ and K+
What hormones are made in which layers of the adrenal cortex?
Aldosterone: in the zona glomerulosa because it has 18-hydroxylase enzyme
17a-hydroxypregnalone and 17a-hydroxyprogesterone and hormones derived from them: zona fasciculata and zona reticularis because have 17a-hydroxylase
cortisol - zona fasciculata
androgens - zona reticularis
Does the adrenal cortex secrete sex hormones?
Under normal circumstances, the adrenal cortex secretes small quantities of male sex hormones (androgens) such as dehydroepiandrosterone, androstenedione and testosterone, and the femal eostregens e.g. oestradiol. Only significant in adrenal disorders
Are mineralocorticoids and glucocorticoids synthesis, secretion and actions controlled together?
They are controlled independently
How is glucocorticoid secretion controlled?
The hypothalamus releases CRH which stimulates the anterior pituitary to release ACTH
ACTH stimulates the adrenal cortex to releases cortisol
Cortisol has a negative feedback effect on CRH release from the hypothalamus
What are the patterns of ACTH and cortisol secretion?
The secretion of ACTH is pulsatile, it peaks in the early morning and at the time of waking, and nadirs in the middle of the night.
There is also increased secretion at times of prolonged stress.
Cortisol secretion shows the same pattern but the peak and nadir occurs approx 2 hours later than those of ACTH
This pattern is related to sleep- wake patterns: disrupted by shift work and long-haul travel
Transport of glucocorticoids
- 10% of cortisol is transported in a free, active form
- 75% bound to corticosteroid binding globulin (CBG or transcortin)
- 15% albumin-bound
the same proteins also transport the other glucocorticoids and progesterone
How does pregnancy affect glucocorticoid transport
Pregnancy associated with an increase in CBG which results in a compensatory increase in circulating plasma cortisol concentrations
The amount of free cortisol remains stable
Describe the metabolism of the adrenal steroids
Steroid hormones are glucoronidated in the liver to make them water soluble then excreted in the urine
How do steroid hormones including glucocorticoids produce their effects?
By acting on intracellular receptors and alterations in gene expression: inevitably results in a delay of hours to days, but in some cases cortisol has rapid effects e.g. feedback inhibition of ACTH secretion
Negative feedback isn’t based on gene expression but on membrane effects so doesn’t take days and can be instantaneous