🔴 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.

🔴 Mechanism of Action 🔴

🔺 Vitamin D :-

🔸1. Calcitriol enhances absorption of calcium and phosphate from intestine.

This is brought about by synthesis of calcium channels and carrier protein for Calcium called Calcium Binding Protein (CaBP) or Calbindin. Vitamin D shows it's action through Cytoplasmic receptor (VDR) .

🔸2. Calcitriol enhances resorption of calcium and phosphate from bone by promoting recruitment and differentiation of osteoclasts precursors in the bone remodeling units, but mature osteoclasts lack the VDR i.e. Vitamin D receptor.

🔸3.Caltriol enhances tubular reabsorption of Calcium and phosphate in the kidney, but action is less marked than PTH.

🔸4. Other actions :- Lymphokine production , proliferation and differentiation of epidermal cells, neuronal and skeletal muscle function have also been demonstrated.