MEDI7111 Lecture Notes - Lecture 5: Adrenocorticotropic Hormone, Aquaporin 2, Cytosol
Endocrine 5
The Hypothalamus & Pituitary Gland
The hypothalamus and pituitary gland act together to stimulate various hormonal axes
which influence many physiological and pathological processes. The pituitary gland is split
into a glandular anterior lobe and a neural posterior lobe which are structurally and
functionally different.
Anterior Pituitary
The anterior pituitary gland regulates the production of prolactin, thyroid hormones,
cortisol, insulin growth factor 1 (IGF1) and sex hormones through the secretion of
stimulating hormones. Neurons of the hypothalamus secrete releasing hormones into the
pituitary portal system stimulating the release of anterior pituitary hormones into the
systemic circulation. The exception to this rule is prolactin which is tonically inhibited (by
dopamine) rather than tonically stimulated. These hormones act on their specific target
gland/organ to elicit a secondary hormone secretion, which again uses the systemic
circulation to create an effect in a target organ (see below).
Posterior Pituitary
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The posterior pituitary is a neural extension of the brain which secretes oxytocin and anti-
diuretic hormone/vasopressin (ADH). ADH is a very important regulator of fluid balance; it
acts by inserting aquaporin 2 channels into the collecting duct and increasing sodium
resorption in the ascending loop of Henle, thus increasing the resorption of water from the
renal tubules. It also acts to increase vascular tone, however this effect is only evident as a
compensatory mechanism during hypovolaemic shock.
Oxytocin is virtually only relevant in pregnancy and labour, providing positive feedback to
increase the strength of uterine contractions. As oxytocin is routinely given to mothers in
labour, its physiological relevance is negligible.
HPA Axis
The hypothalamic-pituitary-adrenal axis is the term given to the hormonal chain that
stimulates the adrenal glands. This is the hormonal system covered in this set of notes, see
the following weeks for the other axes.
Hormones of the Adrenal Cortex
Aldosterone
Aldosterone is a mineralocorticoid hormone which predominantly acts on ENaC channels in
the late distal tubule/collecting duct to resorb sodium and excrete potassium, as well as
functioning in acid/base balance. Pathological levels of aldosterone cause hypokalaemia,
hyponatremia (increased intracellular volume) and metabolic alkalosis.
The regulation of aldosterone secretion is very complex with multiple systems interacting
to maintain electrolyte and fluid homeostasis. It is important to note that ACTH can also
increase aldosterone secretion, therefore excess pituitary/ectopic production of ACTH will
cause derangements in aldosterone function as well as cortisol.
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Document Summary
The hypothalamus and pituitary gland act together to stimulate various hormonal axes which influence many physiological and pathological processes. The pituitary gland is split into a glandular anterior lobe and a neural posterior lobe which are structurally and functionally different. The anterior pituitary gland regulates the production of prolactin, thyroid hormones, cortisol, insulin growth factor 1 (igf1) and sex hormones through the secretion of stimulating hormones. Neurons of the hypothalamus secrete releasing hormones into the pituitary portal system stimulating the release of anterior pituitary hormones into the systemic circulation. The exception to this rule is prolactin which is tonically inhibited (by dopamine) rather than tonically stimulated. These hormones act on their specific target gland/organ to elicit a secondary hormone secretion, which again uses the systemic circulation to create an effect in a target organ (see below). The posterior pituitary is a neural extension of the brain which secretes oxytocin and anti- diuretic hormone/vasopressin (adh).
