Changes

===Parathyroid Hormone (PTH)===

===Parathyroid Hormone (PTH)===

Synthesis of PTH is from a preprohormone of 115 amino acids into a  [[Prohormones - Anatomy & Physiology|prohormone]] of 90 amino acids. This prohormone is then packaged into vesicles as the 84 amino acid PTH molecule. It is secreted by the chief cells of the parathyroid gland continuously with a basal secretory rate of around 25% of the maximum possible rate. Secretion rate increases with a '''decrease in serum ionised calcium''' (hypocalcemia). This is a direct [[Negative Feedback - Anatomy & Physiology|negative feedback]] mechanism.

Synthesis of PTH is from a preprohormone of 115 amino acids into a  [[Prohormones - Anatomy & Physiology|prohormone]] of 90 amino acids. This prohormone is then packaged into vesicles as the 84 amino acid PTH molecule. It is secreted by the chief cells of the parathyroid gland continuously with a basal secretory rate of around 25% of the maximum possible rate. Secretion rate increases with a '''decrease in serum ionised calcium''' (hypocalcemia). Regulation of PTH is highly sensitive due to membrane receptors on Chief Cells coupled to G-Proteins. Receptor stimulation decreases secretion; this is therefore a direct [[Negative Feedback - Anatomy & Physiology|negative feedback]] mechanism. The half-life of PTH in circulation is short - less than 10 minutes which also allows tight regulation of calcium levels.

** This is a highly sensitive system due to membrane receptors on Chief Cells coupled to G-Proteins. Receptor stimulated decreases secretion.

 

* The half-life of PTH in circulation is short - less than 10 minutes which allows tight regulation of calcium levels.

PTH is metabolised in the liver and kidneys.

*PTH is metabolised in the liver and kidneys.

PTH leads to increased calcium levels in the blood by actions on:

PTH leads to increased calcium levels in the blood by actions on:

* '''Bone'''

* '''Bone'''

**'''Fast Phase''' - This phase begins in minutes and progressively increases for hours. PTH acts on existing osteoblasts and osteocytes to increase calcium uptake from the ''bone fluid''. Nearby calcium phosphate crystals replace the calcium which has been removed from the bone fluid.

**'''Fast Phase''' - This phase begins in minutes and progressively increases for hours. PTH acts on existing osteoblasts and osteocytes to increase calcium uptake from the ''bone fluid''. Nearby calcium phosphate crystals replace the calcium which has been removed from the bone fluid.