Parathormone

PARATHORMONE( PARATHYROID HORMONE )
  • Synthesized by principal/ chief cells of parathyroid gland.
  • Its secretion is regulated by plasma Ca++ concentration — fall in Ca++ concentration increases the secretion of paratharmone.
Actions
  • On Bone:
  1. In bone, PTH receptors are located on osteoblasts but not on osteoclasts.
  2. Initially and briefly, PTH causes an increase in bone formation by a direct action on osteoblasts. (This brief action is the basis for the usefulness of intermittent PTH administration in the treatment of osteoporosis.)
  3. In a second, long-lasting action on osteoclasts, PTH causes an increase in bone resorption.
  4. This second action on osteoclasts is indirect and mediated by cytokines released from osteoblasts.
  5. Thus, the bone-forming cells, osteoblasts, are required for the bone-resorbing action of PTH on osteoclasts.
  • On Kidney:
  1. PTH increases calcium reabsorption from distal tubules of kidney.
  2. PTH decreases phophate reabsorption from proximal tubules of kidney.
  3. Major regulating factor for calcium reabsorption in kidney is parathormone.
  • On intestine:
  1. PTH has no direct effect on calcium absorption from intestine but increases it indirectly by enhancing the formation of calcitriol by activating 1 alpha-hydroxylose.
Exam Question
  • Parathormone (PTH) promotes absorption of Ca++ from intestine.
  • Increased parathormone secretion would occur in a patient whose diet has been low in calcium for 8 weeks.
  • Parathormone is useful in Osteoporosis.
  • Major regulating factor for calcium reabsorption in kidney is parathormone.
  • Parathormone secretion is stimulated by decreased serum Calcium.
  • Parathormone increases calcium absorption by acting at DCT.
  • Parathormone decreases phosphate level.
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