Hormonal Control of Calcium and Phosphate

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Calcium Homeostasis:

• Calcium is an extracellular ion.

• Calcium in plasma: 40% attached to protein; 15% associated with anions; 45% is ionized/free.

• Calcium in bone: 99% total calcium.

Calcium and Phosphate:

• Calcium and phosphate are in equilibrium.

Hydroxyapatite:

• Bone = hydroxyapatite (complex of calcium and phosphate) laid down in protein (osteoid) matrix.

• Hydroxyapatite = complex of calcium and phosphate.

Bone Deposition vs Resorption:

• Product of [calcium] and [PO4] greater than solubility product = bone deposition.

• Product of [calcium] and [PO4] less than solubility product = bone resorption.

Parathyroid Hormone:

• Peptide hormone.

• Released from parathyroid in response to low calcium.

• "Phosphate thrashing hormone."

• Increases calcium reabsorption and decreases phosphate reabsorption in kidney.

• Increases formation of osteoclasts and 1,25 di-OH D3 (active vitamin D).

Calcitonin:

• Peptide hormone released from parafollicular cells (C cells) of thyroid gland.

• Released in response to elevated calcium.

• Decreases activity of osteoclasts (cause bone reabsorption).

Sedentary Lifestyle and Weightlessness:

• PTH increases, urinary calcium increases, plasma levels of calcium increase.

Pseudohypoparathyroidism:

• Tissue resistance to PTH; Gs protein deficiency; mental retardation, short, stocky, missing parts of fingers.

Primary Hyperparathyroidism:

• Increased PTH secretion by parathyroid tumor or ectopic parathyroid tissue.

• Calcium (increased) and phosphate (decreased) concentrations are in opposite directions.

• Muscle weakness and fatigability.

Primary Hypoparathyroidism:

• Inadequate PTH secretion.

• Tetany present.

• Calcium (decreased) and phosphate (increased) concentrations are in opposite directions.

Secondary Hyperparathyroidism:

• Vitamin D deficiency, increased PTH.

• Calcium and phosphate concentrations are in the same direction (decrease).

Secondary Hypoparathyroidism:

• Excessive vitamin D; decreased PTH.

• Calcium and phosphate concentrations are in the same direction (increase).

Vitamin D3 (cholecalciferol):

• 1,25 di-OH D3.

• Secreted by kidney's proximal tubule.

• Synthesis occurs in skin (UV exposure) to liver to kidney.

• Stored in fat tissue.

• Estimated by serum levels of 25 OH vitamin D.

• Tanned skin decreases synthesis by preventing UV penetration.

Actions of 1,25 di-OH D3:

• Increases plasma calcium and phosphate.

• At abnormally high levels, increases bone resorption and release of calcium and phosphate from bone.

Osteomalacia:

• Deformable, undermineralized bones.

Osteoporosis:

• Thin, fragile bones.

1,25 di-OH D3 deficiency:

• Rickets (kids) and osteomalacia (adults).

1,25 di-OH D3 excess:

• Osteoporosis.

Additional Readings:

Basic Endocrinology

1. Introduction to Endocrinology
2. Hypothalamic-pituitary system
3. Adrenal Hormones
4. Antidiuretic Hormone (ADH) and ECF Regulation
5. Endocrine Pancreas
6. Growth Hormone
7. Adrenal Medulla
8. Hormonal Control of Calcium and Phosphate
9. Thyroid Hormones
10. Hormones of Male Reproduction
11. Hormones of Female Reproduction
12. Fluid Compartments of the Body
13. Notes on Hypothalamus Anterior Pituitary and Thyroid
14. Additional Notes on Female Reproduction
15. Hormonal Signaling Pathways
16. FAQ on Adrenal Hormones
17. FAQ on Male Reproduction
18. Synthesis and Deficiencies of Adrenal Hormones
19. Significance of Glycosylated Hemoblogin (HbA1c)
20. Significance of Measuring Albumin while with Calcium Levels
21. Stepwise Approach to Treatment of Ascites
22. How to differentiate between Diabetes Insipidus vs Psychogenic Polydipsia

Related Topics

1. Histology of the Endocrine System
2. Histology of the Male Reproductive System
3. Histology of the Female Reproductive System

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