Endocrine physiology Flashcards
(14 cards)
Where is calcitonin produced? What is its function?
C cells in thyroid gland (parafollicular)
Function = decrease calcium (counters PTH). Acts on bone to decrease resorption (minimal effects on gut/kidneys)
May also act on appetite centre to reduce intake
Give 3 actions of thyroperoxidase.
- Catalyses oxidisation of iodide (transported into thyroid follicular cell) into oxidised iodide
- Catalyses incorporation of oxidised iodide into thyroglobulin (organification)
- Catalyses coupling of tyrosine residues to form T3 and T4
Give 4 actions of TSH. What kind of receptor does it have on the thyroid cell.
- Stimulates sodium:iodide pump to increase iodide uptake into thyroid follicular cell
- Increases size and number of thyroid follicular cells
- Stimulates change of thyroid epithelial cells from cuboidal to columnar and increases their infolding to increase SA
- Increases proteolysis of thyroglobulin
Acts via receptor which stimulates adenyl cyclase - cAMP formed -> activates protein kinase SECOND MESSENGER
Which thyroid hormone (T3/T4) is more potent and enters cells more rapidly? How is one converted into the other?
T3 is more potent, enters cells faster, has less affinity for proteins
T4 converted to T3 by outer ring deiodination
How does most thyroid hormone circulate - free or protein bound?
Protein bound mostly to thyroxine binding protein (not cats) = reservoir (as not bio active)
Describe the process of thyroid hormone secretion from thyroid follicular cells.
- Hormones need to be cleaved from thyroglobulin -> pseudopods from follicular cell membrane extend into colloid and endocytose it
- Lysosomes fuse -> become phagolysosomes and proteases hydrolyse T3/T4 from TG
- T3/T4 leave cell by diffusion
- ~75% of DIT/MIT not used and liberated, deiodinated by deiodinase enzyme - recycled to make T3/T4 again
- Some thyroglobulin binds to megalin and released in blood
Where is PTH secreted from?
Chief cells in parathyroid gland
How do chief cells sense calcium concentrations?
Calcium sensing receptors - G protein coupled receptor. If high calcium levels then calcium binds to CSR - induces phospholipases which SUPPRESS PTH secretion
Where does PTH act and what are its effects?
Overall action = increase calcium, decrease phosphate
- Kidneys - predominantly ascending limb of loop of Henle and distal tubule. Acts via second messenger system (cAMP formed) which then induces Ca channels in luminal membrane to INCREASE calcium reabsorption and increase phosphate excretion
- Gut (indirectly): stimulates 1 alpha hydroxylase enzyme which increases formation of calcitriol (1,25 OH-D) in kidneys, which then increases Ca and Po4 absorption in GIT
- Bone to increase osteoclastic activity which mobilises Ca and Po4 from bone
In insulin synthesis in beta cells of I.O.L, describe the 3 stages of insulin formed.
- Preproinsulin - made by ribosomes on ER
- Proinsulin - preproinsulin cleaved in ER to make proinsulin
- Insulin - proinsulin cleaved in Golgi to make insulin then stored in secretory granules until needed
Describe the basic structure of insulin.
A chain
B chain
These chains are joined by disulphide links
C-peptide - role unclear, has no insulin activity
What type of receptor is the insulin receptor? When it binds describe what happens.
Enzyme linked receptor
Insulin binds to alpha units - connected to beta units by disulphide bridges
When insulin binds causes beta units to be phosphorylated
This activates tyrosine kinase
TK then phosphorylates other enzymes including insulin receptor substrates (IRSs)
These IRS allow movement of glucose transporters to cell membrane
How does insulin promote/cause glycogen storage in the liver?
Insulin inactivates the phosphorylase enzyme which means that glycogen can’t be broken down
Insulin also increases the activity of glycogen synthase
Also increases activity of glucokinase (catalyses conversion of glucose to glucose-6-phosphate so that glucose can’t escape cell)
