1. PTH Calcitonin 10mg% Vitamin D Lecture 52 Ca++ Homeostasis
Effect on serum calcium
PTH
Calcitonin mg%
Vitamin D
Normal
serum calcium

2. Dietary Ca++
Ca++
ECF
Ca++
Bone
Intestine
Ca++
Kidney

3. PTH Chief cells of Parathyroid gland 84 a.a- 9,500 MW
1st Biological Activity
Transcription
Translation
Pre Pro PTH Pro PTH PTH
PTH
Serum Ca++

4. Mode of Action of PTH Bone Kidney Osteoclast number
Receptor for PTH on osteoblasts
Secrete Factors
Osteoclast differentiating Factor
PTH
(RANKL)
Stimulate Osteoclasts
(In the presence of preosteoclasts)
osteoblasts
Kidney
Inhibits phosphate reabsorption (Proximal and distal tubule).
Stimulates Calcium reabsorption (distal).

5. Parathyroid hormone gene related peptide
PTHrP
Isolated from
Lung tumors
Kidney tumors
Responsible for the
hypercalcemia of malignancy

6. Calcitonin Main effect on Bone Osteoclasts shrink in size 32 aa
3,000 mw
C cells of thyroid.
Main effect on
Bone
Inhibits osteoclastic bone resorption
Osteoclasts shrink in size

7. Vitamin D Metabolism *
24(OH)ase
24,25(OH)2D3
(inactive
metabolite)

8. Mechanisms of Action of Steroids

9. 1,25(OH)2D3 Intestine calcium binding protein calbindin
epithelial calcium channel TRPV6
intestinal calcium absorption
(also phosphate absorption)
Bone stimulates osteoclastic activity
intracellular receptor for 1,25(OH)2D3 in osteoblasts
1,25(OH)2D3 stimulates synthesis of RANKL
Stimulates osteoclasts
(in the presence of preosteoclasts
Kidney enhances the action of PTH at the distal tubule

10. Summary BONE KIDNEY INTESTINE PTH Calcitonin Vitamin D #osteoclasts
Inhibits phosphate reabsorption (proximal and distal tubule)
Increases calcium reabsorption (distal tubule)
No direct effect
PTH
1,25(OH)2D3
which
calcium
absorption (intestine)
Calcitonin
SHRINKS
osteoclasts
Urinary calcium excretion (only at supra-physiological concentration)
No effect
(inhibits gastric secretion from stomach)
Vitamin D
Calcium absorption
#osteoclasts
Enhances action of PTH on calcium at the distal tubule
(also
phosphate absorption)