Pituitary Anatomy and Physiology Flashcards
Describe the anatomical position of the pituitary gland
In the pituitary fossa (sella turcica) in the sphenoid bone.
What connects the pituitary to the hypothalamus
The pituitary stalk
How does venous blood drain from the pituitary gland
By a number of veins into the cavernous sinus
What is the anterior pituitary gland derived from
An upward growth of the ectoderm of the roof of the oropharynx (Rathke’s pouch) which becomes pinched off
What is the posterior pituitary gland derived from
Down growth of neuroectoderm of the floor of the third ventricle
Name the hormones secreted by the anterior pituitary
Growth hormone (GH) Prolactin Adrenocorticotrophin hormone (ACTH) Thyroid stimulating hormone (TSH) Luteinizing hormone (LH) and follicle stimulating hormone (FSH)
What is the most abundant hormone secreted by the anterior pituitary gland
Growth hormone
What does GH stimulate
hepatic synthesis and secretion of insulin-like growth factor 1
Epiphyseal prechondrocyte differentiation and linear bone growth in children
lipolysis
increases protein synthesis
antagonises insulin action
phosphate, water and sodium retention
How does GH work
It binds to its plasma membrane receptor (usually in the liver) which leads to receptor dimerisation
What happens after receptor dimerisation ?
It is followed by a phosphorylation cascade mediated by Janus kinds (JAK) and components of the signal transduction and activators of transcription (STAT) family, which translocate to the nucleus and regulate target gene expression
How is the secretion of GH stimulated
BY hypothalamic GH releasing hormone
How does hypothalamic GH releasing hormone work
It acts via a G-protein coupled receptor and increases cyclic AMP leaves
What factors may stimulate GH release
stress
exercise
sleep
prolonged fasting
What inhibits GH secretion
IGF-1 and hypothalamic somatostatin
Describe the release of GH
pulsatile
What does prolactin stimulate
the proliferation of the breast lobule-alveolar epithelium and lactation
What does prolactin suppress
Gonadotrophin-releasing hormone (GnRH)
Pituitary gonadotrophin secretion
How does prolactin work
In the same way as GH - It binds to its plasma membrane receptor which leads to receptor dimerisation which is followed by a phosphorylation cascade
What inhibits prolactin secretion
Hypothalamic dopamine binding to D2 receptors
What stimulates prolactin secretion
hypothalamic thyrotrophin releasing hormone (TRH) and others including oestrogen, opiates, serotonin and ACh
When might prolactin levels be high
in stress during sleep following a suckling stimulus exercise meals sex epileptic fit
When are the prolactin secretory peaks
during rapid eye movement sleep (between 4 and 6am)
What does ACTH do
It stimulates the conversion of cholesterol to pregnenolone in the zone fasciculate and reticular
therefore stimulates the production of cortisol and adrenal androgens
How does ACTH work
It binds to its plasma membrane receptor in the zone fasciculata of the adrenal gland which results in the activation of adenylate cyclase
What does the activation of adenylate cyclase result in
an increased cyclic AMP production which then results in stimulation of steroidogenic acute regulatory protein
what does steroidogenic acute regulatory protein do
It mediates the transport of cholesterol through the cytosol to the inner mitochondrial membrane where it is converted to pregnenolone
What is the rate limiting step in cortisol synthesis
the transport of cholesterol through the cytosol to the inner mitochondrial membrane, where it is converted to pregnenolone
What else does the activation of adenylate cycles result in
the unregulated gene expression of other enzymes involved in steroid synthesis
What stimulates ACTH
hypothalamic corticotrophin-releasing hormone (CRH) as well as other factors, including stress
What inhibits ACTH
cortisol
When is there a peak of cortisol
8am
What does TSH stimulate
Every step in thyroid hormone synthesis
also stimulates the expression of many genes in thyroid tissue and causes thyroid hyperplasia and hypertrophy
What inhibits TSH synthesis
T4 and T3
What are some other factors that may reduce TSH secretion
dopamine
somatostatin
acute non-thyroidal illness
increased human chorionic gonadotrophin (early pregnancy
What do LH and FSH do in males
LH stimulates the production of testosterone
FSH stimulates spermatogenesis (alongside testosterone)
How do the gonadotrophin work in males
Binding of LH and FSH to their plasma membrane receptors results in a stimulation of adenylate cyclese and increased cAMP production
LH stimulates testosterone synthesis by acting on the StAR protein which delivers cholesterol to the inner mitochondrial membrane where it is converted to pregnenolone
What stimulates the release of LH and FSH
pulsatile release of hypothalamic GnRH
What does testosterone inhibit
hypothalamic GNRH and pituitary LH production
How does LH and FSH work in females
LH stimulates the early steps in steroidogenesis and the production of androgens in ovarian theca cells
What does the LH surge induce
ovulation and thereafter maintains the secretory function of the corpus luteum
What does FSH stimulate
the recruitment and growth of ovarian follicles and their secretion of estradiol
What does FSH stimulate
The aromatase enzyme that converts androgens to oestrogen
What is the secretion of FSH and LH fundamentally under the control of
negative feedback control by ovarian steroids (oestradiol) and by inhibin
What does the oestrogen peak cause
the LH surge
What are the hormones secreted by the posterior pituitary
Oxytocin
vasopressin (ADH)
How are vasopressin and oxytocin synthesised and packaged
In granules in the cell bodies of specific magnocellular neurones in the supraoptic and paraventricular nuclei of the hypothalamus
What stimulates vasopressin secretion
an increase in serum osmolality
decreased extracullar volume
decreased BP
stress
What inhibits vasopressin secretion
alcohol and cold
How does Vasopressin cause generation of cyclic AMP and activation of intracellular protein kinases
It binds to the VR2 membrane receptors on the distal renal tubular cells (in the collecting duct) causing activation of adenylate cyclase
How does vasopressin cause urine to become concentrated
Insertion of water channel proteins into the tubular membrane, allowing a flow of solute free water from the hypotonic luminal fluid into the hypertonic renal interstitium
What happens at high concentrations
Vasopressin binds to VR1 receptors on vascular smooth muscle and causes vasoconstriction
Where else is vasopressin secreted into
the portal circulation
What does vasopressin stimulate
ACTH release from the pituitary
What stimulates oxytocin release
vaginal stimulation caused by the fetus during parturition, sex or nipple stimulation during lactation
What is oxytocin released inhibited
Stress
What does oxytocin stimulate
contractions of the uterine muscle which helps deliver the foetus and the placenta
What is another function of oxytocin
Stimulate the contraction of the myoepithelial cells that surround the alveoli in the breast to aid milk ejection
