• Hormonal control of Ca2 homeostasis W4 Flashcards
Parathyroid Hormone
• Key player in Ca2+ homeostasis
• Peptide hormone (84 amino acids) - classified as a protein
Parathyroid Hormone secretion
From the parathyroid glands and secretion increases when plasma [ca2+] falls
This hormone acts to increase plasma [ca2+]
Decreased Ca equals increased PTH
What is ca2+ monitored by
Directly monitored by the parathyroid cels via Ca-sensing receptors in a neg feedback loop
What does Parathyroid Hormone acts on
Bone, kidneys and GIT(indirectly)
Main action on bones
Negative feedback loop of calcium monitoring
Secret PTH= increased plasma Ca= decrease PTH secretion
Where is the thyroid/parathyroid glands and how many?
At the back of the neck and there are four parathyroid glands within the thyroid tissue
Effect of PTH on bone
PTH leads to a release of Ca from the bone to increase plasma Ca levels
Two phases of the effect of PTH on the bone
Fast and slow exchange
What occurs during fast exchange
PTH induces a rapid effluent of Ca from a small labile pool in the fluid within the canaliculi of the bone
That of PTH receptors on osteoblast and osteocytes and when they are stimulated this increases CAMP and leads to a movement of Ca into the cells from the bone fluid
Gap junctions allow this movement via osteocytes and out through osteoblasts into the plasma
PTH receptors are…
GPCR coupled to Gs
What happens during slow exchange?
This pathway is activated under conditions of more prolonged hypokalaemia
When there is a decrease of Ca for a long time we need to be more drastic
PTH activate blast to increase RANKL expression
RANKL will increase the number of osteoclasts and this increases different differentiation in bone marrow and decreases apoptosis of osteoclasts
The osteoclasts increase bone resorption which increases plasma Ca
Osteoblast building activity is inhibited
Plasma PO4 Levels are also increased
Balance is restored when Ca Absorption in the GIT is increased in the long-term term
Effect of PTH on the kidneys
PTH has a direct effect on kidney epithelial cells and promotes calcium retention as well as phosphate excretion
Causes an activation of vitamin D
Calcitonin
Is the antagonistic hormone to PTH
Is secreted by C cells of the thyroid gland
Secretion is increased when there is an increase in plasma Ca
It is a polypeptide of 32 amino acids
Receptors are also GPCR that increase CAMP
Calcitonin aims to decrease levels of extracellular/plasma calcium by:
Decreasing calcium of movement from the canaliculi fluid into the plasma
Inhibiting osteoclast activity
Inhibiting absorption of calcium and phosphate in the kidney
Vitamin D
Is a pre-hormone that following metabolism to active hormones increases calcium absorption in the GIT
It is a steroid like hormone (hydrophobic)
Is synthesised in the skin in response to sunlight and also absorbed from diet
Vit D in the liver and kidney
Initially inactive and activated in the liver and kidney
One OH group added in the liver and stored until required
Second OH group added in kidney - caused by PTH
