🔴 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.

🔴 Mechanism of Action 🔴

🔺 Bisphophonates (BPNs) :-

Bisphophonates are the anlogues of pyrophosphate.

They inhinit bone resorption and have ability to prevent osteoporosis.

🔸a)BPNs have strong affinity for calcium phosphate and have selective action in calcified tissue.
Bisphosphonates localised in the acidic zone under the osteoclasts due to their high affinity for calcium ions are released from bone surface due to high acidity the bisphosphonates are also released and are internalized by endocytosis.
This results in 👇👇
---- Accelerated apoptosis of osteoclasts reducing their number.
----Distribution of cytoskeleton and ruffled border of osteoclasts.

🔸b) It has been shown that now that the second and third generation potent amino-BPNs like alendronate, zolendronate, have important metabolic effects in the mevalonate pathway for isoprenoid lipid synthesis.

They inhibit prenylation of certain GTP binding proteins involved in cytoskeletal organization, membrane ruffling and vesical movement.
The net result is inactivation of osteoclasts, impaired vesicles fusion and enhanced apoptosis.