Veterinary Physiology-III

BIOLOGICAL EFFECTS

• 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

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 Ca 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.

On kidney

• It 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.

On intestine

• Promotes absorption of Ca and PO₄ from GI tract by activating 1, 25 DHCC.

On Blood

• Elevates the blood Ca concentration and decreases the blood PO₄ concentration