Endocrinology Flashcards
Why are hormones occupying nuclear receptors slower acting?
Hormones that enter cells and occupy nuclear receptors activate gene transcription and translation with the production of new proteins. This takes minutes to hours.
Removal of the stimulus does not immediately reverse the cellular changes that require protein synthesis to cease and removal of the new protein. So these hormones have slow, sustained effects.
Describe the peptide hormones and where they’re secreted
These are short chains of amino acids (e.g. vasopressin), or larger as proteins (insulin). Secreted by the pituitary, parathyroid, heart, stomach, liver and kidneys.
Describe the amine hormones and where they’re secreted
these are derived from the amino acid tyrosine and are secreted from the thyroid and adrenal medulla. Includes the thyroid hormones and catecholamines.
Describe the steroid hormones and where they’re secreted
These are lipids derived from cholesterol. Testosterone for men and Estradiol (a form of oestrogen) for women. Secreted by the gonads, the adrenal cortex and the placenta
What are the catecholamines?
these are the adrenergic amine hormones epinephrine and norepinephrine.
Describe the action of the catecholamines
These amines are synthesised and stored in vesicles for exocytosis when required. They’re water soluble and act through surface receptors.
Describe the storage of thyroid hormone precursor
Precursor thyroglobulin is synthesised by the follicular cells of the thyroid and secreted into the colloid inside the thyroid follicles.
How are thyroid hormones produced from thyroglobulin?
Thyroglobulin has tyrosine residues where iodination occurs, producing mono and diiodotyrosine. These molecules are able to couple to produce T4 and T3
Describe the transport and action of the thyroid hormones
They are lipid soluble so are carried around the bloodstream bound to thyroxine-binding globulin. They act on nuclear receptors
What peptide hormone is bound to a transport molecule in the plasma?
Despite water solubility, growth hormone is bound to a plasma protein with the same amino acid sequence as the extracellular component of it cell-surface receptor.
Which hormones are hydrophilic?
Hydrophilic hormones includes the Protein and Peptide hormones including: Insulin (pancreas), thyroid stimulating hormone, growth hormone and prolactin (anterior pituitary), oxytocin and vasopressin (posterior pituitary), and calcitonin (thyroid). Plus hydrophilic amines including dopamine (CNS) and epinephrine (adrenal medulla).
Which hormones are hydrophobic?
Hydrophobic hormones includes the Steroids and some Amines. Steroids include oestrogen and testosterone (ovary and testis) and aldosterone and cortisol (adrenal cortex). Hydrophobic amines include thyroxine (thyroid).
Describe the hypothalamo-pituitary axis
Neurons from the hypothalamic nuclei release neurotransmitter (releasing hormones) into the capillary plexus in the median eminence just above the infundibulum - no blood brain barrier. The portal capillary plexus delivers the releasing hormones to the anterior pituitary, stimulating specific cells to secrete trophic hormones.
What is the role of the posterior pituitary?
contains nerve endings from neurons in the hypothalamic supraoptic and paraventricular nuclei. These secrete hormones into the posterior pituitary for storage and secretion later.
How does feedback control the hypothalamo-pituitary axis.
Increased trophic hormone feeds back to inhibit the hypothalamic secretion of releasing hormone. Increasing peripheral concentrations of gland hormone feedback to inhibit release of trophic hormone by the anterior pituitary cells and secretion of releasing hormone by the hypothalamus.
Give four reasons why endocrine axes fail
A defect in synthesis, secretion, action (e.g. receptors won’t bind), and the effector such that it doesn’t carry out the desired function.
Describe the hypothalamo-pituitary thyroid axis
ThyroidRH release from the paraventricular nucleus of the hypothalamus (in response to temperature) stimulates TSH from the anterior pituitary gland. This feeds back to inhibit TRH release and also stimulates the thyroid to secrete thyroxine, which feeds back to inhibit the secretion of both TSH & TRH
How is Thyroid stimulating hormone secreted?
TRH synthesized by neurons in the paraventricular nucleus is secreted from nerve endings in the median eminence when plasma thyroxine levels are low. It binds to G protein–coupled receptors on anterior pituitary TSH secreting basophilic cells, that acting through IP3 and diacylglycerol, elevate cell Ca2+ concentration which triggers the exocytotic release of TSH
Describe the hypothalamo-pituitary adrenal axis (HPA)
corticotropinRH synthesised by paraventricular nucleus neurons (in response to stress) stimulates ACTH release by the anterior pituitary, which feeds back to inhibit CRH release and stimulates the adrenal cortex to release cortisol, which in turn inhibits CRH and ACTH release.
How is adrenocorticotropic hormone secreted?
CRH synthesized by neurons in the paraventricular nucleus is secreted from nerve endings in the median eminence. It binds to G protein–coupled receptors on anterior pituitary basophilic, that acting through adenylyl cyclase, stimulate the cyclic AMP-dependent protein kinase A (PKA) signal transduction pathway, causing ACTH release.
What is the effect of vasopressin on ACTH secretion?
also synthesized by cells in the paraventricular nucleus. Some of these nerve endings also release vasopressin into the median eminence from which it moves to the anterior pituitary. Though not, itself, a significant stimulator of ACTH secretion, it augments the effects of CRH so that there is greater ACTH release.
Where is most vasopressin secreted?
The nerve endings of the posterior pituitary
What is ACTH derived from and what else can be derived from this?
ACTH is cleaved from pro-opiomelanocortin (POMC) – that also yields β-endorphin and α-MSH (regulates melanin production). Over expression of these POMC cleavage products can cause skin colour changes associated with pathology of the HPA.
How is testosterone secreted?
GnRH (gonadotropin–releasing hormone) synthesized by preoptic anterior hypothalamus neurons is secreted from their nerve endings in the median eminence when plasma testosterone is low (pulsatile). At anterior pituitary basophilic cells through portal circulation it binds to G protein–coupled receptors in the phosphoinositide system, elevating cell Ca2+concentration that triggers the exocytotic release of FSH and LH (inhibit further release). FSH stimulates spermatogenesis and LH testosterone production (which inhibits everything)
