LECTURE 1 - intro to endocrine system Flashcards
Introduction to endocrine system
What is a hormone?
A substance secreted directly into the blood by specialised cells
What are the main endocrine glands?
- hypothalamus
- pituitary gland
- thyroid glands
- parathyroids
- adrenals
- pancreas
- ovary
- testes
What are the main types of signalling molecules?
- hormones
- neurotransmitters
- growth factors and cytokines
- fatty acid derivatives
How long are hormones stored for?
Peptides & proteins: day
Steroids & pseudo steroids: min-hour
Thyroid hormones: weeks
Catecholamines: day
How are different hormones secreted?
Peptides & proteins: exocytosis
Steroids & pseudo steroids: diffusion
Thyroid hormones: proteolysis
Catecholamines: exocytosis
Are different categories of hormones binding proteins?
Peptides & proteins: some
Steroids & pseudo steroids: all
Thyroid hormones: yes
Catecholamines: no
What are the half lives of various hormones?
Peptides & proteins: min-hour
Steroids & pseudo steroids: hours
Thyroid hormones: days
Catecholamines: sec-min
How long do various hormones take to work
Peptides & proteins: min-hour
Steroids & pseudo steroids: hour-day
Thyroid hormones: day
Catecholamines: seconds
What are the properties of peptide hormones?
- range from 10-20 amino acids to hundreds
- water soluble
- often produced as larger molecular weight precursors that get proteolytically cleaved to the active form of the hormone
How are protein hormones stored?
- can be in vesicles (or granules)
- exocytosis will occur in response to a signal
- storage is useful as can allow for a quick response
How are polypeptide hormones synthesised?
- mRNA on RER binds amino acids into peptide chain called preprohormone
- Chain is directed into ER lumen by signal sequence of amino acids
- Enzymes in the ER chop off signal sequence creating an inactive prohormone
- Prohormone passes from ER to Golgi apparatus
- Secretory vesicles (containing enzymes and prohormone) bud off the Golgi
- Secretory vesicle releases its contents by exocytosis
- Hormone moves into circulation for transport to its target
What is a prohormone?
an active hormone that requires an addition enzyme that can be localised around target tissue
What is the use of a prohormone?
Allows fine-tuning of hormone activation
How is insulin produced?
- produced in 3 chains (A,B,C)
- C chain serves as a link between A and B chains
- C chain facilitates efficient assembly, folding and processing of insulin
How are thyroid hormones synthesised?
- Thyroglobulin is synthesised and discharged into follicle lumen
- Iodide is trapped
- Iodide is oxidised to iodine
- Iodine is attached to tyrosine in colloid, forming DIT and MIT
- Iodinated tyrosines are linked together to form T3 and T4
- Thyroglobulin colloid is endocytosed and combined with a lysosome
- Lysosomal enzymes cleave T4 and T3 from thyroglobulin colloid and hormones diffuse into bloodstream
How are catecholamines synthesised?
- The medulla is a modified sympathetic ganglion derived from neuroectodermal cells
- The adrenal medulla is the source of the circulating catecholamine, adrenaline
- The medulla also secretes small amounts of noradrenaline, normally a neurotransmitter
- The dietary amino acid tyrosine is absorbed as tyrosine from the gut
- Tyrosine is also produced in the liver from dietary phenylalanine
How are steroid hormones synthesised?
Cholesterol is converted to Pregnenolone by P450Scc through 2 hydroxyls and one cleavage reaction
- cholesterol is transported into the mitochondria by StAR (steroidogenic acute regulatory protein) - this is the rate limiting step
Where are steroid hormones produced?
Adrenal cortex: glucocorticoids, mineralocorticoids
Woman:
Corpus lutetum: progestogens
Ovary: estrogens
Men
Testis: testosterone
