Calcium And Phosphorus Metabolism Oral Physiology Notes

Calcium And Phosphorus Metabolism

The teeth help in cutting, grinding and mixing of food. These actions are facilitated by stronger muscles of mastication and healthy teeth structure. There needs to be homeostasis of the main minerals involved in the formation of the tooth structure such as, calcium and phosphorus to have healthy teeth. Calcium salts present in the dentin of teeth, make the teeth resistant to the compressional forces during chewing. Calcium is also needed for the early forming teeth.

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Calcium:

• Total calcium present in the body in a young human adult is about 1100 g. Of this, about 99% is present in bone.
• The plasma calcium level is about 9 – 11 mg%.
• Approximately, 50% of this is in ionic form and the rest is present in bound form, bound either to plasma proteins or to citrates and bicarbonates.

Functions Of Ionic Calcium

• Maintenance of resting membrane potential
• Transmission at neuromuscular junction and excitation contraction coupling of skeletal muscle
• Maintenance of excitability and contractility of cardiac muscle (as a special case)
• Blood coagulation
• Development of bone and teeth
• Activation of certain enzymes
• Release of certain hormones.

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Sources Of Calcium

• Dairy products like milk
• Yogurt and cheese
• Leafy green vegetables
• Seafood
• Legumes
• Fruits

Site Of Absorption Of Calcium And Factors Influencing Absorption

Calcium is absorbed actively from the digestive system.

Site of absorption: Duodenum (second part)

Factors influencing calcium absorption are as follows:

• 1,25-dihydroxycholecalciferol (vitamin D₃)
• Growth hormone
• Acidic PH in duodenum
• Alkaline PH and phytic acid decrease calcium absorption

Phosphate

A phosphate is a salt containing phosphorus.

• Total body phosphate is about 500–800 mg, of this 85%–90% is present in bones.
• The total plasma phosphate level is about 2.5–4.5 mg% in adults.
• About one-third of this is in inorganic form mostly as PO₄^3- 
• (phosphates),  HPO₄^2-  (hydrogen phosphate) and H₂PO₄ (phosphoric acid)
• Since phosphorus is present in all plant and animal cells, its dietary deficiency is rarely seen.

Functions of phosphate:

• Phosphate is a component of nucleic acid, hence takes part in protein synthesis, cellular reproduction, genetic phenomenon
• It plays a major role in carbohydrate metabolism as hexose phosphate and creatine phosphate
• Involved in energy transfer in intermediary metabolism of carbohydrates, Proteins and fats
• Acts as a buffer.

Homeostasis Of Calcium And Phosphorus

Calcium and phosphorus are maintained under the influence of three hormones.

They are as follows:

1. Parathormone
2. Calcitonin
3. Vitamin D₃ (active form of vitamin D)

Parathormone:

Parathormone is secreted by the parathyroid glands. There are two pairs of parathyroid glands present on the posterior aspect of the thyroid gland.

Two different types of cells are seen in parathyroid glands:

1. Chief cells
2. Oxyphil cells

• • The chief cells are responsible for secretion of parathormone (PTH).

PTH is a Polypeptide Hormone

Polypeptide Hormone Actions: PTH is a hypercalcaemic and hypophosphataemic agent (increases the plasma calcium level and decreases plasma phosphate level).

The sites of actions of PTH:

• Bone
• Kidney
• GI tract (in an indirect way)

PTH on bone:

• PTH acts on the bone matrix mobilizing calcium and phosphate by Resorption of the osseous tissue
• PTH increases the number of osteoclasts (bone resorbing cells)

PTH on kidney: PTH

• Increases the calcium reabsorption from renal tubules (DCT – distal Convoluted tubules)
• Decreases phosphate reabsorption from renal tubules (PCT – proximal Convoluted tubules)
• PTH stimulates the activity of 1-alpha hydroxylase activity, which in turn brings about increased formation of 1,25 dihydroxycholecalciferol (active form of vitamin D) PTH on gastrointestinal tract
• The action on gastrointestinal tract is an indirect one.
• The increased formation of 1,25-dihydroxycholecalciferol (vitamin D₃) in turn facilitates more of absorption of calcium from the duodenum.
• The synthesis of vitamin D₃ is given in flow chart.

Calcitonin:

• Calcitonin is produced by the parafollicular cells (c cells) of the thyroid gland.
• It is a peptide hormone secreted by the parafollicular cells of thyroid gland

Actions of calcitonin:

• It is a hypocalcaemic agent and hence decreases plasma calcium level (calcium-lowering effect of calcitonin is brought about by its action on the bones).
• Prevents calcium resorption from bone.
• Inhibits the activity of osteoclasts.
• Increases the excretion of calcium in urine.
• Protects maternal bone during pregnancy and lactation.

Vitamin D₃:

Mineralization

Mineralization Definition: Mineralization is a process of deposition of mineral salts in and around the organic matrix, resulting in a calcified structure.

Though tissue fluid is rich in calcium and phosphate ions, unprompted precipitation of calcium phosphate does not take place due to

• Inhibitory macromolecules that inhibit crystal formation.
• Stable cluster of ions needed to form a lattice structure which needs the expenditure of energy, and an energy barrier must be overcome for crystallization. The organic matrix consists of

• Collagenous proteins – type I collagen: It acts as scaffold that accumulate the minerals in holes and pores of fibrils.
• Noncollagenous proteins: Proteoglycans, phospholipids, phosphoproteins – osteopontin, osteonectin, phosphophoryn. Noncollagenous proteins are involved in mineralization of enamel.

Mechanisms of mineralization:

Mineralization process is based on two mechanisms:

1. Booster mechanism:
   The calcium and phosphate levels are increased in some tissues due to the presence/action of enzymes so that they are precipitated.
2. Seeding mechanism:
   It refers to a presence of seeding or nucleating substance, which acts as a template on which crystals are deposited.

Theories of mineralization:

1. Robinson’s phosphate (alkaline phosphatase) theory:

• The enzyme participating in the process of calcification is alkaline phosphatase that is present in the matrix vesicle.
• Alkaline phosphatase is the enzyme which participates in the process of calcification. It resides in matrix vesicle.
• This enzyme hydrolyses the organic phosphate containing substrate and increases the local inorganic phosphate concentration.
• It also hydrolyses the inhibitor of hydroxyapatite (HA) and provides inorganic phosphate for the formation of ha crystals.
• This theory was not accepted, as alkaline phosphatase was seen in other tissues
• which do not calcify and organic phosphate is not sufficient to produce inorganic phosphate to initiate calcification process.

2. Nucleation theory (seeding theory):

• Neumann and neumann (1953) proposed This theory.
• According to this theory, a nucleus is formed probably in relation to collagen, effective in gathering calcium and phosphate ions.
• Then, ha crystals grow spontaneously.
• This theory failed to explain mineralization in enamel and cartilage.
• It also failed to explain mineralization in soft tissues though it contains collagens since collagens in soft tissues are compact which hinder phosphate ion access to nucleation sites.

3. Matrix vesicle theory: Proposed by anderson and ermanno bonucci.

According to this theory, the mineralization requires the presence of extracellular matrix vesicles (small membrane-bound structures, 25– 250 nm in diameter, lying free in the matrix) where calcification occurs. Vesicles containing apatite crystals reach each cartilage cell, which aggregate and form a matrix which is mineralized. The vesicles are rich in

• Phosphotidyl serine, a lipid with high affinity for calcium ion.
• Annexins, which forms calcium channel helping in incorporating the ion within the matrix vesicle.

Matrix vesicles accumulate calcium and their membrane contain binding sites for the nucleation of HA crystals. This is most accepted theory.

Clinical considerations

Hypoparathyroidism: Decreased decretion of PTH. It could be due to inadverent removal of parathyroid glands during thyroidectomy.

•  Features:
  • Tetany
  • Neuromuscular excitability with twitching
  • Tonic and clonic contractions of muscle fibres
  • Contraction of laryngeal muscles will give rise to asphyxia and death
• Hyperparathyroidism: Increased secretion of PTH is associated with hypercalcaemia and hypophosphataemia.
• Features:
  • Pathological fractures, osteoporosis
  • Removal of calcium from gingival margin
  • Formation of renal stones
  • Deposition of calcium in the soft tissues and along the blood vessels
• Rickets: A condition associated with softening of bones in children due to the deficiency of vitamin D. Poor nutrition, less exposure to sun, malabsorption syndromes can lead to vitamin D deficiency.
• Features:
  • Frontal bossing
  • Dental hypoplasia
  • Bowing of legs
  • Rachitic rosary

Calcium And Phosphorus Metabolism Synopsis

• Total calcium present in the body in a young human adult is about 1100 g. Of this, about 99% is present in bone.
• The plasma calcium level is about 9–11 mg%. Approximately, 50% of this is in ionic form and the rest is present in bound form, bound either to plasma proteins or to citrates and bicarbonates.
• Calcium helps in maintenance of resting membrane potential, transmission at neuromuscular junction and excitation contraction coupling of skeletal muscle, maintenance of excitability and contractility of cardiac muscle (as a special case), blood coagulation, development of bone and teeth, activation of certain enzymes, release of certain hormones.
• Calcium is absorbed actively from the digestive system from the duodenum
• Factors influencing calcium absorption are 1,25 dihydroxycholecalciferol (vitamin D₃), growth hormone, acidic ph in duodenum, alkaline ph and phytic acid decrease  calcium absorption.
• A phosphate is a salt containing phosphorus.
• Total body phosphate is about 500–800 mg, of this 85%–90% is present in bones.
• The total plasma phosphate level is about 2.5–4.5 mg% in adults.About one-third of this is in inorganic form mostly as  PO₄³ (phosphates), HPO₄^2-  (hudrogen phosphate) and H₂PO₄ (phosphoric acid)
• Since phosphorus is present in all plant and animal cells, its dietary deficiency is rarely seen.
• Phosphates take part in protein synthesis, cellular reproduction, genetic Phenomenon, play a major role in carbohydrate metabolism and are involved in energy transfer.
• Calcium and phosphorus are maintained under the influence of three hormones.
• They are as follows:
  • Parathormone, calcitonin and vitamin D₃ (active form of vitamin D)
  • PTH is hypercalcaemic and hypophosphataemic agent (increases the plasma calcium level and decreases plasma phosphate level).
  • Calcitonin is a hypocalcaemic agent and hence decreases plasma calcium Level.