🔴 Mechanism of Action 🔴

🔺 Parathyroid Hormone :-

The PTH receptor is a G protein coupled receptor when activation increases cAMP formation and intracellular
Calcium in target cells. In bone, the target cell is the osteoblast because PTH receptors are
not expressed on the surface of osteoclasts.

Acting on the osteoblast, PTH induces a factor ' Receptor for activation of nuclear factor-KB-ligand' (RANKL)
which diffuses and
combines with RANK on osteoclast precursors
and transforms them into osteoclasts as well
as activates the osteoclasts.

In addition, birth rate of bone remodeling units into which osteoclasts are recruited is enhanced. Formation of the remodeling pit is followed
by osteoblastic deposition of new bone into it.

PTH enhances proliferation and differentiation of preosteoblasts and deposition of osteoid as well.

Bone resorption predominates when high concentrations o f PTH are present continuously, but intermittent exposure to low concentrations has the opposite effect.

🔴 Mechanism of Action 🔴

🔺Calcitonin :-

🔸Actions of calcitonin are generally apposite to PTH.
It inhibits bone resorption by direct action on osteoclasts -----decreasing their ruffled surface which forms contact with the resorptive pit.

🔸Calcitonin inhibits oroximal tubular reabsorption of calcium and phosphate by direct action on kidney.

🔸The actions of calcitonin are mediated through a G-protein coupled receptor (CTR) and increase in cAMP formation, but it's target cells are different than PTH.