Paratharmone function and metabolism

PARATHORMONE

Parathormone secreted by parathyroid gland is essential for the maintenance of blood calcium level within a very narrow critical level. Maintenance of blood calcium level is necessary because calcium is an important inorganic ion for many physiological functions.

Source of Secretion

Parathormone (PTH) is secreted by the chief cells of the parathyroid glands.

Chemistry

Parathormone is protein in nature, having 84 amino acids. Its molecular weight is 9,500.

Half-life and Plasma Level

Parathormone has a half-life of 10 minutes. Normal plasma level of PTH is about 1.5 to 5.5 ng/dL.

Synthesis

Parathormone is synthesized from the precursor called prepro-PTH containing 115 amino acids. First, the prepro-PTH enters the endoplasmic reticulum of chief cells of parathyroid glands. There it is converted into a prohormone called pro-PTH, which contains 96 amino acids. Pro-PTH enters the Golgi apparatus, where it is converted into PTH.

Metabolism

Sixty to seventy percent of PTH is degraded by Kupffer cells of liver, by means of proteolysis. Degradation of about 20% to 30% PTH occurs in kidneys and to a lesser extent in other organs.

ACTIONS OF PARATHORMONE

PTH plays an important role in maintaining blood calcium level. It also controls blood phosphate level.

ACTIONS OF PARATHORMONE ON BLOOD CALCIUM LEVEL

Primary action of PTH is to maintain the blood calcium level within the critical range of 9 to 11 mg/dL. The blood calcium level has to be maintained critically because, it is very important for many of the activities in the body. PTH maintains blood calcium level by acting on:

Paratharmone


1. Bones

2. Kidney

3. Gastrointestinal tract.

1. On Bone

Parathormone enhances the resorption of calcium from the bones (osteoclastic activity) by acting on

osteoblasts and osteoclasts of the bone. Resorption of calcium from bones occurs in two

phases:

i. Rapid phase

ii. Slow phase.

Rapid phase

Rapid phase occurs within minutes after the release of PTH from parathyroid glands. Immediately after reaching the bone, PTH gets attached with the receptors on the cell membrane of osteoblasts and osteocytes. The hormone-receptor complex increases the permeability of membranes of these cells for calcium ions. It accelerates the calcium pump mechanism, so that calcium ions move out of these bone cells and enter the blood at a faster rate.

2. On Kidney

PTH increases the reabsorption of calcium from the renal tubules along with magnesium ions and hydrogen ions. It increases calcium reabsorption mainly from the distal convoluted tubule and proximal part of the collecting duct. PTH also increases the formation of 1,25- dihydroxycholecalciferol (activated form of vitamin D) from 25-hydroxycholecalciferol in kidneys.

3. On Gastrointestinal Tract

PTH increases the absorption of calcium ions from the GI tract indirectly. It increases the formation of 1,25- dihydroxycholecalciferol in the kidneys. This vitamin, in turn increases the absorption of calcium from GI tract. Thus, the activated vitamin D is very essential for the absorption of calcium from the GI tract. And PTH is essential for the formation of activated vitamin D.

ACTIONS OF PARATHORMONE ON BLOOD PHOSPHATE LEVEL

PTH decreases blood level of phosphate by increasing its urinary excretion. It also acts on bone and GI tract.

1. On Bone

Along with calcium resorption, PTH also increases phosphate absorption from the bones.

2. On Kidney

Phosphaturic action It is the effect of PTH by which phosphate is excreted through urine. PTH increases phosphate excretion by inhibiting reabsorption of phosphate from renal tubules. It acts mainly on proximal convoluted tubule.

3. On Gastrointestinal Tract

Parathormone increases the absorption of phosphate from GI tract through calcitriol.

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