PARATHYROID GLANDS
1. What is parathyroid hormone? Where is it secreted? What are the stimuli to PTH secretion? Is plasma Ca2+ concentration increased after PTH secretion?
2. What are the mechanisms by which PTH increases absorption of Ca2+ from bone?
3.
How does PTH influence the kidney so that the plasma Ca2+ increase from bone absorption is not lost by kidney excretion?4. What is the active form of vitamin D? Where is it formed? Is PTH involved in its formation?
5. Where does the active form of vitamin D exert its influence? What is its effect?
The parathyroid glands are located near or embedded within the thyroid gland. In domestic animals they consist of one (pig) or two (dog, cat, ruminant, horse) pairs of bean-like organs. The parathyroids are sometimes so close to the thyroid gland that differentiation is difficult.
Parathyroid Hormone and Calcium Ion Regulation
Parathyroid hormone (PTH) is a polypeptide with a molecular weight of 9,500 that contains a chain of 84 amino acids.
A low plasma Ca2+ concentration (hypocalcemia) stimulates secretion of PTH from the parathyroid gland, whereas hypercalcemia inhibits PTH secretion. A less effective stimulator of PTH secretion is hypomagnesemia.
Calcium and phosphorus are absorbed from bone under the influence of PTH by two processes. The most rapid means by which PTH increases the plasma Ca2+ concentration is called osteolysis; it involves osteoblasts and osteocytes (see Chapter 7). These cells are normally involved in calcium and phosphorus deposition, but in osteolysis they are involved in absorption. PTH inhibits osteoblast synthesis of new bone but increases osteoblast-initiated recruitment of osteocytes to transport calcium and phosphorus from the bone fluid to the extracellular fluid. In this instance absorption of Ca2+ and phosphorus occurs without loss of bone matrix.
However, PTH also increases osteoblast- initiated recruitment of osteoclasts (bone dissolvers). In contrast to osteolysis, osteoclastic activity causes loss of bone matrix and, over a period of time, excavations are visible. Osteolysis is considered to be the rapid phase of calcium and phosphate absorption, and activation of osteoclasts is considered to be the slow phase of bone absorption and calcium phosphate release.Action of PTH on the Kidneys
PTH increases plasma Ca2+ concentration, but its action would be ineffective if a change did not occur in the kidneys,to increase Ca2+ absorption from tubular fluid. This change is brought about by PTH. At the same time, phosphate reabsorption by the kidney diminishes. This change is also affected by PTH and the calcium to phosphorus ratio of approximately 2:1 in plasma is maintained.
PTH and 1,25-Dihydroxycholecalciferol Formation
Parathyroid hormone greatly enhances both calcium and phosphate absorption from the intestines by increasing the rate of formation of 1,25-dihydroxycholecalciferol, known as calcitriol, the active form of vitamin D. The original forms of vitamin D, whether from the diet or by action of ultraviolet light on skin precursors, are converted through a succession of reactions in the liver and kidney. The first conversions occur in the liver and the final conversion to calcitriol occurs in the kidney under the influence of PTH. In the intestinal epithelium, calcitriol causes the formation of a calcium-binding protein that functions at the brush border to transport calcium into the cell cytoplasm. Calcium-binding protein remains in the cells for several weeks, providing a prolonged effect on calcium.absorption.
Enhancement of intestinal phosphate absorption might result from the direct effect of calcitriol, but it could be a secondary result of the hormone’s action on calcium absorption, in which calcium acts as a transport mediator for phosphate.
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