Principles of endocrinology Flashcards
Endocrine systems; Hormone classification; Hormone synthesis; Hormone signalling; Hormone homeostasis
Define endocrine gland
Group of cells that secrete hormones to the bloodstream.
Define endocrinology
The study of endocrine glands and secretions.
Define hormone
Bioactive messengers secreted by endocrine glands into blood.
(Not simple metabolite or energy substrate)
Define neurotransmitter
Endogenous chemicals that enable one neurone to transfer a signal from itself to a target cell.
Define neurosecretion
Secretion of chemical messengers from neurones
What distinguishes endocrine from paracrine and autocrine systems?
Action of the hormones secreted:
- Endocrine - action on target cells at a distance from a source
- Paracrine - action on nearby target cells
- Autocrine - action on self
What are the 3 main differences between the endocrine and nervous systems?
- Release of chemical hormones vs chemical neurotransmitter
- Effect spread across many target cells throughout body vs restricted to localised target cells that are innervated
- Effect can take place over a long time span vs generated in milliseconds
Name the 6 classic endocrine glands
1) Pituitary
2) Parathyroids
3) Thyroid
4) Adrenals
5) Pancreas
6) Gonads (and GI tract)
How are hormones classified?
1) protein/polypeptide hormones
- protein derived
2) Steroid hormones
- derived from cholesterol
- Intact or broken steroid nucleus
3) Misc.
- amines
How are protein hormones synthesised?
1) Transcription
2) mRNA to RER
3) Translation of pre-prohormone
4) Cleavage of pre-prohormone by proteolytic enzymes in RER to prohormone
5) Prohormone packed into golgi vesicle
6) Prohormone broken down into hormone
7) Secretion via exocytosis
How are steroid hormones synthesised?
1) Derived from cholesterol
2) LDL carries cholesterol to cell for storage as fatty acid esters
3) Cholesterol esterase liberates cholesterol
4) Cholesterol taken to mitochondria by Steroidogenic Acute Regulatory protein (StAR)
5) Hormone produced in mitochondria and SER
6) Lipid soluble steroid diffuses out of cell immediately
7) Steroid binds to protein in blood
How are hormones transported?
Most hormones secreted into systemic circulation
hypothalamus releases into→hypophyseal portal system
Water soluble → bloodstream
Insoluble steroids and thyroid hormones →
bound to plasma proteins or transport proteins
Name the 6 more recently identified endocrine glands
Kidneys Heart/blood Liver Brain Adipose tissue Placenta
What is the purpose of hormones bound to transport proteins?
Act as a reservoir so that bound hormone in dynamic equilibrium with small amount of free hormone
Protects against rapid changes on hormone concentration
What is the main receptor for peptide hormones and where is it located?
G-Protein Coupled Receptors
Plasma membrane of target cells
What is the mechanism of action of a G protein receptor in the case of ACTH?
1) Hormone binds to receptor
2) G-protein dissociates from receptor
3) G protein activates enzyme adenylate cyclase
4) AC converts ATP to cAMP
5) cAMP activates protein kinase A (PKA)
6) PKA acts as 2nd messenger producing an effect
How does PKA upregulate cortisol synthesis?
Protein Kinase A phosphorylates StAR protein, upregulating cortisol synthesis
What is the main receptor for steroid and thyroid hormones?
Intracellular receptors inside the target cells
Steroids are lipid soluble and can penetrate plasma membranes via diffusion
What is the mechanism of action of steroid hormones?
Act in nucleus as transcription factors to increase or decrease the rate of gene transcription
Define positive feedback
When end products of an action cause more of that action to occur in a feedback loop
Define negative feedback
When a deviation from the normal levels induces a response to correct the change and restore the normal state
How are hormone systems controlled?
Most often closed loop feedback systems - predominantly negative feedback
Via interaction between hypothalamus and pituitary and target organs
Peripheral hormones from target organs feed back to HP system ∴ regulating target hormone production
