Questions and Answers
What is the role of vitamin D in calcium absorption?
It is essential for the biosynthesis of calbindin protein
What happens to calcium absorption in the duodenum when calcium intake is low?
It increases
Where does most of the calcium absorption take place?
Jejunum and ileum
What inhibits calcium absorption?
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What percentage of filtered calcium is reabsorbed in the kidneys?
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Where is calcium reabsorption regulated by parathyroid hormone?
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What is critical for normal cell function, neural transmission, and bone structure?
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What percentage of total calcium absorption takes place in the large intestine?
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Which hormone increases calcium excretion in the urine?
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What is the effect of oestrogens on osteoporosis?
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What is the role of calcitonin in calcium homeostasis?
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What is the effect of parathyroid hormone on bone?
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What is similar in structure to Parathyroid Hormone (PTH) receptor?
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What is the result of excessive secretion of parathyroid hormone in primary hyperparathyroidism?
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What is the role of 1, 25-dihydroxycholecalciferol in calcium homeostasis?
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What can cause secondary hyperparathyroidism?
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What is the effect of thyroid hormones on calcium levels?
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What is the common cause of secondary hyperparathyroidism?
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Which receptor plays an important role in regulation of extracellular calcium?
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What is the effect of secondary hyperparathyroidism on bones?
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What is the effect of insulin on bone?
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What is the result of inadequate production of parathyroid hormone?
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What is the primary function of parathyroid hormone?
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What is a common cause of hypoparathyroidism?
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Where are the parathyroid glands located?
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What is the result of hypocalcemia in hypoparathyroidism?
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What is the precursor of parathyroid hormone that is formed in polysomes?
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What is the major target of parathyroid hormone?
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What is the response to low extracellular concentrations of free calcium?
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What happens to Pre-Pro PTH in the rough ER?
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What is the role of the Golgi cisternae in parathyroid hormone biosynthesis?
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What is the relationship between parathyroid hormone secretion and blood calcium levels?
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What is the primary action of parathyroid hormone when calcium ion concentrations in extracellular fluid fall below normal?
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What is the effect of parathyroid hormone on bone?
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How does parathyroid hormone enhance calcium absorption from the small intestine?
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What is the effect of parathyroid hormone on calcium excretion in urine?
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What is the effect of parathyroid hormone on phosphate ions in urine?
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How does parathyroid hormone stimulate calcium release from bone?
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What is the net effect of parathyroid hormone on calcium ion concentrations in blood?
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What is the indirect mechanism by which parathyroid hormone enhances calcium absorption from the small intestine?
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Study Notes
Calcium Absorption and Regulation
- Calcium absorption is related to calcium intake and vitamin D.
- Active transcellular calcium transport occurs in the duodenum and is increased when calcium intake is low.
- Passive paracellular calcium transport occurs in the jejunum and ileum and is the dominant process when calcium intake is adequate or high.
- Calcium absorption is inhibited by phosphates and oxalates, which form insoluble salts with calcium in the intestine.
Calcium Reabsorption in the Kidneys
- Around 99% of filtered calcium is reabsorbed in the kidneys.
- 60% of reabsorption occurs in the proximal tubules, and the remainder occurs in the ascending limb of the loop of Henle and the distal tubule.
- Distal tubule absorption is regulated by parathyroid hormone.
Regulation of Calcium Homeostasis
- The regulation of plasma Ca2+ levels is critical for normal cell function, neural transmission, membrane stability, bone structure, blood coagulation, and intracellular signaling.
- The regulation relies on interactions among parathyroid hormone (PTH), dietary vitamin D, and calcitonin.
Parathyroid Hormone (PTH)
- PTH is secreted by the parathyroid glands and is the most important endocrine regulator of calcium and phosphorus concentration in extracellular fluid.
- PTH is secreted from cells of the parathyroid glands and finds its major target cells in bone and kidney.
- Biosynthesis of PTH involves three stages: pre-Pro PTH, Pro-PTH, and PTH.
- Control of PTH secretion is regulated by low extracellular concentrations of free calcium.
Physiologic Effects of Parathyroid Hormone
- PTH has direct action to bring calcium ion concentrations in extracellular fluid back within the normal range.
- PTH accomplishes this by stimulating three processes:
- Mobilization of calcium from bone
- Enhancing absorption of calcium from the small intestine
- Suppression of calcium loss in urine
Parathyroid Hormone Receptor
- PTH receptor structure is similar to that of other G-protein coupled receptors.
Disease States
- Primary hyperparathyroidism is caused by parathyroid gland disease, leading to excessive secretion of PTH.
- Secondary hyperparathyroidism is caused by disease outside of the parathyroid gland, leading to excessive secretion of PTH.
- Hypoparathyroidism is characterized by inadequate production of PTH, leading to decreased concentrations of calcium and increased concentrations of phosphorus in blood.
Key Points in Calcium Homeostasis
- Calcium homeostasis is regulated by three hormones: parathyroid hormone, vitamin D, and calcitonin.
- Parathyroid hormone increases plasma calcium by mobilizing it from bone, increasing reabsorption from the kidney, and increasing the formation of 1, 25-dihydroxycholecalciferol.
- 1, 25-dihydroxycholecalciferol increases calcium absorption from the intestine, mobilizes calcium from the bone, and increases calcium reabsorption in the kidneys.
- Calcitonin inhibits bone resorption and increases the amount of calcium in the urine, thus reducing plasma calcium.
- The calcium-sensing receptor (CASR) plays an important role in regulation of extracellular calcium.
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