Introduction to endo Flashcards
What is an endocrine gland?
Group of cells secreting messenger molecules directly into the bloodstream
What is a hormone?
Bioactive messengers secreted into blood
What are the three modes of hormone action?
Endocrine: action on target cells at a distance from source
Paracrine: action on nearby target cells
Autocrine: action on self (immediate source)
What are the main differences between the endocrine system and the nervous system?
Endo: Effect spread across many target cells throughout body
Neuro: Effect restricted to target cells that are localised and innovated
Endo: Has a long time-span (seconds-days)
Neuro: Effect generated within milliseconds
What are the classical endocrine glands?
Pituitary Parathyroids Thyroid Adrenals Pancreas Gonads (GI tract)
What are the three types of hormone?
Proteins (polypeptide) Hormones
Steroid hormones (derrived from cholesterol)
Miscellaneous
What differentiates polypeptide and protein hormones?
Polypeptide: <50aa and single chain
Protein: >50aa w/more complex structure
Describe the process of Protein hormone synthesis:
Using ACTH as an example
- Signal needed to activate transcription of DNA to produce mRNA of pro-hormone e.g. ACTH needs POMC mRNA to be transcribed in corticotroph cells
- mRNA translated e.g. POMC produced
- Pro-hormone (POMC) transported from ER to Golgi apparatus for packaging of hormone with cleavage enzymes that can liberate active hormone
- Enzymes in vesicles cleave pro-hormone, producing active hormone e.g. ACTH from POMC, and the vesicles remain near the cell membrane
- Vesicles fuse with membrane in secretion
Describe the process of Protein hormone signalling:
Require extracellular target
Target cell has complementary receptor for hormone e.g. ACTH Receptors on adrenal gland for ACTH
G-protein coupled receptor activated by hormone with GaS subunit dissociating, modifying intracellular signalling e.g. Adenylate cyclase to increase cAMP levels to transduce signal e.g. Linking to steroid hormone synthesis via protein kinase A which activates cholesterol esterase and the StAR protein
Describe the production of Steroid hormones:
Cholesterol enters cells from LDLs (lipoprotein lipase) which are transported into cell, and the cholesterol is removed and stored as fatty acid esters in cytoplasmic vesicles (clear on histology)
Esterase used to liberate cholesterol from fatty acid esters, and StAR (steroidogenic acute regulatory) protein allows the cholesterol to enter the mitochondria - rate limiting step
Steroidogenic enzyme pathway in mitochondria converts cholesterol to steroids via pregnenolone - balance of enzymes depends on type of tissue (e.g. Gonads vs Adrenals)
Produced steroid hormone very lipid soluble so can freely diffuse across cell membrane to circulation - not stored within cells
What is the difference in transport of protein/steroid hormones across membranes?
Proteins packaged in vesicles which exocytose
Steroids very lipid soluble so can freely diffuse across membrane
How are steroid hormones stored in the blood?
Bound to plasma proteins e.g. albumin (weakly)
Bound to specific binding proteins e.g. Corticosteroid binding globulin for cortisol (strongly)
Describe the amounts of bound steroid hormones in blood and what this allows:
Most of a hormone is bound, but some is free
Only free hormone can enter tissued.
An equilibrium exists between free and bound hormone
What type of hormone does the anterior pituitary exclusively produce?
Protein/Polypeptide Hormones
Where are steroid hormones made?
Mitochondria
