Parathyroid hormone
Parathyroid hormone (PTH) is a single chain polypeptide containing 84 amino acids that secreted from Parathyroid gland. PTH is synthesized as part of a large molecule containing 115 amino acids called pre pro-parathyroid hormone (pre‑pro PTH) by the ribosome.
This large molecule is converted to pro-PTH composed of 90 amino acids in the rough endoplasmic reticulum. It moves to the Golgi apparatus and is enzymatically cleaved to produce the biologically active PTH. Active Parathyroid hormone may be secreted directly or packaged into secretory granules.
Regulation of PTH secretion
Parathyroid hormone has a unique feedback control by the concentration of calcium and to a lesser extent magnesium ion in serum.
Low blood level of ionized Ca2+ stimulates rapid and pronounced release of parathyroid hormone Calcium has a negative feedback effect on PTH release.
Magnesium ion has an effect on parathyroid secretory rate similar to that of calcium, but its effects is not equipotent to that of calcium. The concentration of blood phosphates has no direct regulatory influence on the synthesis and secretion of PTH. PTH is a essential hormone for life.
Biological effects of Parathyroid hormone
Parathyroid hormone is the principal hormone involved in the minute‑to-minute and fine regulation of blood calcium level in mammals. It exerts its biological action by directly influencing the function of target cells, primarily the bone and kidney and indirectly in the intestine to maintain plasma Ca level.
Effects of Parathyroid hormone on bone
Osteoclastic osteolysis: PTH activates osteoclasts to cause the breakdown of bone and releases Ca into the blood. Only osteoblasts but not osteoclasts have PTH receptors and hence the osteoblasts bind with PTH. Osteoclasts are stimulated by PTH indirectly through osteoblast-binding with PTH.
The initial binding of PTH to osteoblasts lining the bone surface exposes the bony matrix followed by attraction of osteoclasts to the resorptive bony surface. The mature osteoclasts are attached to the resorptive bony surface and they cause the dissolution of the bony matrix, thus the bone resorption.
The plasma membrane of the active osteoclasts is modified into numerous projections known as ruffled or brush border. The osteoclasts form a sealed chamber within a restricted area at the place of attachment.
At this restricted area, low pH is created by the accumulation of lactic, citric acids and H+ released from carbonic acid by carbonic anhydrase.
Low pH and increased lysosomal enzymatic actions of acid phosphatase and collaginase favour hydrolysis of bony substances, and thus release Calcium ions along with organic component, hydroxyproline. These products are transported to plasma by pump mechanism.
PTH also stimulates conversion of osteoprogenitor cells to osteoclasts, and sustains osteoclastic activity and inhibits osteoblastic activity.
Effects of Parathyroid hormone on kidney
Parathyroid hormone increases tubular reabsorption of calcium, whereas increases urinary excretion of phosphates, (phosphaturia), K+, bicarbonate, Na+ and amino acids and urinary excretion of hydroxyproline. It also causes renal retention of H+ ions.
PTH activates 1 α – hydroxylase to convert 25 hydroxy cholecaliferol to 1, 25 dihydroxycholecaliferol (vitamin D3) in the kidney.
Effects of Parathyroid hormone on intestine
Parathyroid hormone promotes absorption of Calcium and PO4 from GI tract by activating 1, 25 DHCC.
Effects of Parathyroid hormone on Blood
Parathyroid hormone elevates the blood Calcium concentration and decreases the blood PO4 concentration.