Three hormones secreted from three different endocrine glands and acts at three different types of cells/organs to regulate the level of calcium in the body. The hormones involved are:
- PTH (Parathyroid Hormone) – from parathyroid glands.
- Calcitonin (CT) – from C- cells of Thyroid gland
- Cholecalciferol - (a vitamin D metabolite).
Some other hormones also affect the level of calcium in the body like adrenal corticosteroids, estrogens, thyroxin, somatotropin, glucagon etc. but to much lesser extent.
Parathyroid gland: There are two pairs of this gland present in anterior cervical region lateral to the trachea on either side. Parathyroid gland secretes Parathyroid Hormone (PTH) which is synthesized originally in the form of pre-pro PTH having 115 amino acids. It is then degraded to Pro-PTH of 90 amino acids molecule by cleavage. Further a hexapeptide is cleaved off to release 84 amino acid long active PTH with a molecular weight of 9500 Dalton. First two are stored form of PTH. Functions of parathyroid Hormone;-
- Elevates the blood calcium con concentration.
- Deceases the blood phosphorus concentration as well as blood K+
- Increase the urinary excretion of phosphorus by decreases tubular re-absorption at proximal convoluted tubules
- Increase the tubular reabsorption of calcium, so decreased loss of calcium in urine (ascending limb of loop of Henle, distal convoluted tubule and collecting duct)
- Increase the calcium resorption from bones (mobilization of bone calcium).
- Increase the formation of active Vit. D. (1,25,dihydroxy cholecalciferol).
Secretion of PTH: The PTH secretion is under the direct control of serum Ca concentration. Low blood calcium stimulates release of PTH hormone e.g. during rickets, pregnancy and lactation, calcium decreases thereby increasing the PTH secretion. With increase in PTH output and on sustained production for a long period the gland becomes hypertrophied.
Calcitonin/ Thyrocalcitonin (CT)
It is a 32 a. a. chain length secreted from C-cells or para-follicular cells of thyroid (in mammals).
Functions of calcitonin :
- Causes decrease in blood calcium and phosphorus i.e. opposite to PTH.
- CT is called an ‘emergency hormone’ since it is quickly released as a result ofhigh blood calcium may be due dietary intake or excess PTH action which may beserious. This is possible only due to presence of pre-formed CT in stored form & its release.
Regulation of CT secretion: Concentration of Ca in plasma & ECF is the principal stimulus for secretion of CT. As calcium concentration increases, CT secretion also increases. Receptor for CT are present in ascending high of loop of Henle & DCT and it causes diuresis of sodium, calcium and chloride where as PTH retains calcium and hydrogen ions.
Physiological significance of PTH & CT
Calcitonin & PTH provide dual negative feed backs mechanism to maintain concentration of calcium in ECF. PTH is more related to minute to minute modification so that individuals living in low Ca high P environment do not develop hypocalcemia. CT acts as an emergency hormone for averting any hypercalcemia that may develop due to post-prandial increase of Ca or excessive loss of Ca in pregnancy or lactation. CT is having short term effect where PTH has long term effect. Vitamin D
Vitamin D occurs in two from Vit. D3 (cholecalciferol) & irradiated ergosterol (Vit D2).
Hormonal metabolite is chemically known as 1, 25 dihydroxy cholecalciferol [1,25(OH)2D3] .
Why Vit .D as can be called as a hormone?
- Chemical structure similar to steroid hormone.
- Very small quantity needed.
- Synthesized by one organ from provitamin.
- Transported by blood in bound from to target organ (i.e. intestine & Bone).
- Enhances rate of existing reaction in target cells.
- Mechanism of action Similar to Steroid molecules.
- Toxic in excessive amounts
Vit. D3 is absorbed from diet and bound to a -2 globulin for traveling through the blood.
Functions of Vit. D;-
- Vit. D & its active metabolites increase the absorption of Ca & P from intestine. So, Vit. D makes available Ca & P for proper mineralization of bones. The target tissue is intestine.
- Where as PTH maintains Ca & P ratio and also small quantity of Vit. D is needed for PTH to have its effect of resorption of Ca from bone.
- At kidneys 1,25 dihydroxy Cholecalciferol increases calcium resorption from PCT.
- 1, 25 dihydroxy Cholecalciferol have negative feed beck on parathyroid gland to PTH output.