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Questions and Answers
What is a key function of vitamin D in relation to calcium?
What is a key function of vitamin D in relation to calcium?
- It solely regulates bone resorption.
- It enhances intestinal calcium uptake. (correct)
- It decreases intestinal calcium uptake.
- It supports renal calcium reabsorption only.
Which condition is characterized by muscle spasms and potential cardiac complications?
Which condition is characterized by muscle spasms and potential cardiac complications?
- Osteoporosis
- Hypercalcemia
- Hypocalcemia (correct)
- Secondary hyperparathyroidism
What results from primary hyperparathyroidism?
What results from primary hyperparathyroidism?
- Decreased PTH secretion and increased bone density.
- Excessive PTH secretion and hypercalcemia. (correct)
- Normal calcium levels with no symptoms.
- Increased calcium reabsorption in the kidneys.
How do calcimimetics affect PTH secretion?
How do calcimimetics affect PTH secretion?
What is a primary cause of postmenopausal osteoporosis?
What is a primary cause of postmenopausal osteoporosis?
What is the primary function of parathyroid hormone (PTH) in calcium homeostasis?
What is the primary function of parathyroid hormone (PTH) in calcium homeostasis?
How does parathyroid hormone (PTH) affect the kidneys?
How does parathyroid hormone (PTH) affect the kidneys?
What role does vitamin D play in calcium absorption as influenced by parathyroid hormone?
What role does vitamin D play in calcium absorption as influenced by parathyroid hormone?
Where is the majority of calcium located in the human body?
Where is the majority of calcium located in the human body?
Which of the following actions is NOT associated with parathyroid hormone (PTH)?
Which of the following actions is NOT associated with parathyroid hormone (PTH)?
What is the effect of low serum calcium levels on parathyroid hormone secretion?
What is the effect of low serum calcium levels on parathyroid hormone secretion?
What are the primary sources of calcium in humans?
What are the primary sources of calcium in humans?
What can occur if calcium concentration deviates from the normal range?
What can occur if calcium concentration deviates from the normal range?
What role does vitamin D play in calcium absorption?
What role does vitamin D play in calcium absorption?
What is the effect of parathyroid hormone (PTH) on calcium levels?
What is the effect of parathyroid hormone (PTH) on calcium levels?
What can result from high calcium and phosphate levels in the body?
What can result from high calcium and phosphate levels in the body?
What is the primary storage form of calcium in bones?
What is the primary storage form of calcium in bones?
How does calcitonin function in the body?
How does calcitonin function in the body?
What is the usual dietary absorption efficiency of calcium?
What is the usual dietary absorption efficiency of calcium?
What effect does sustained elevation of PTH have on bones?
What effect does sustained elevation of PTH have on bones?
Which channel is enhanced by vitamin D for intestinal calcium absorption?
Which channel is enhanced by vitamin D for intestinal calcium absorption?
What is the role of the calcium-sensing receptor (CaR) in calcium homeostasis?
What is the role of the calcium-sensing receptor (CaR) in calcium homeostasis?
Which process predominantly activates vitamin D to its final form?
Which process predominantly activates vitamin D to its final form?
How does parathyroid hormone (PTH) affect serum calcium levels?
How does parathyroid hormone (PTH) affect serum calcium levels?
What effect does 1,25(OH)₂D₃ have on the intestines?
What effect does 1,25(OH)₂D₃ have on the intestines?
What is the transport mechanism of vitamin D in the bloodstream?
What is the transport mechanism of vitamin D in the bloodstream?
What effect does calcitonin have on blood calcium levels?
What effect does calcitonin have on blood calcium levels?
Which statement best describes the regulation of PTH secretion?
Which statement best describes the regulation of PTH secretion?
Where are vitamin D receptors (VDRs) primarily located?
Where are vitamin D receptors (VDRs) primarily located?
What is the primary role of vitamin D receptors (VDRs) in relation to gene expression?
What is the primary role of vitamin D receptors (VDRs) in relation to gene expression?
What complex forms after the active form of vitamin D binds to the VDR?
What complex forms after the active form of vitamin D binds to the VDR?
Which proteins are crucial for intestinal calcium absorption regulated by VDRs?
Which proteins are crucial for intestinal calcium absorption regulated by VDRs?
What occurs as a result of insufficient VDR activity?
What occurs as a result of insufficient VDR activity?
What characteristic defines Vitamin D-dependent rickets type II (VDDR II)?
What characteristic defines Vitamin D-dependent rickets type II (VDDR II)?
How does VDR activity relate to geographical factors?
How does VDR activity relate to geographical factors?
What are potential therapeutic applications mentioned for VDR agonists?
What are potential therapeutic applications mentioned for VDR agonists?
What role does 1,25-dihydroxyvitamin D₃ play in calcium absorption?
What role does 1,25-dihydroxyvitamin D₃ play in calcium absorption?
Flashcards
Why is calcium important?
Why is calcium important?
Calcium is a vital mineral for numerous bodily functions including bone formation, muscle contraction, and nerve impulse transmission.
Where is most of the calcium in our bodies located?
Where is most of the calcium in our bodies located?
The majority of calcium in the body is stored in bones and teeth in an insoluble form.
What is the role of PTH in calcium regulation?
What is the role of PTH in calcium regulation?
Parathyroid Hormone (PTH) is a crucial hormone secreted by the parathyroid glands in response to low calcium levels in the blood.
How does PTH increase blood calcium?
How does PTH increase blood calcium?
PTH stimulates bone resorption, which is the breakdown of bone tissue to release calcium into the bloodstream.
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What role does PTH play in the kidneys?
What role does PTH play in the kidneys?
PTH promotes the reabsorption of calcium in the kidneys, preventing its loss in urine.
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How does PTH influence intestinal calcium absorption?
How does PTH influence intestinal calcium absorption?
PTH indirectly enhances calcium absorption in the intestines by stimulating the production of active vitamin D.
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What is hypocalcemia?
What is hypocalcemia?
Hypocalcemia is a condition of low calcium levels in the blood, often caused by PTH deficiency or impaired calcium absorption.
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What is hypercalcemia?
What is hypercalcemia?
Hypercalcemia is a condition of abnormally high calcium levels in the blood, usually due to excessive PTH production or bone disorders.
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What is the first step in vitamin D's action?
What is the first step in vitamin D's action?
The active form of vitamin D, 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), binds to VDRs in the cell nucleus.
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How does the vitamin D-VDR complex activate gene expression?
How does the vitamin D-VDR complex activate gene expression?
The VDR, bound to vitamin D, forms a complex with the retinoid X receptor (RXR) to bind to specific DNA sequences called vitamin D response elements (VDREs) on target genes.
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What is the general outcome of the VDR-RXR complex binding to DNA?
What is the general outcome of the VDR-RXR complex binding to DNA?
VDR-RXR complex initiates the transcription of vitamin D-responsive genes, leading to increased synthesis of proteins like calcium absorption, bone mineralization, and cell differentiation.
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How does VDR affect calcium absorption in your gut?
How does VDR affect calcium absorption in your gut?
VDRs in the small intestine regulate the synthesis of proteins like TRPV6, calbindin, and PMCA, facilitating active and passive calcium absorption.
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How does VDR contribute to calcium levels in the blood?
How does VDR contribute to calcium levels in the blood?
VDRs in the kidneys promote calcium reabsorption in the distal convoluted tubules, aiding in maintaining serum calcium levels.
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What happens if there's insufficient VDR activity?
What happens if there's insufficient VDR activity?
Insufficient VDR activity, either due to vitamin D deficiency or VDR mutations, can lead to bone diseases like rickets or osteomalacia due to poor calcium absorption.
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What is VDDR II and what causes it?
What is VDDR II and what causes it?
Mutations in the VDR gene can lead to conditions such as Vitamin D-dependent rickets type II (VDDR II), characterized by impaired calcium absorption and bone mineralization.
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What factors influence VDR activity?
What factors influence VDR activity?
VDR activity relies on sufficient levels of 1,25(OH)₂D₃, which requires adequate sunlight exposure (UVB) for synthesis.
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What is Parathyroid Hormone (PTH)?
What is Parathyroid Hormone (PTH)?
A hormone produced by the parathyroid glands, responsible for regulating calcium levels in the blood.
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How does PTH secretion respond to changes in blood calcium?
How does PTH secretion respond to changes in blood calcium?
PTH secretion is inversely proportional to plasma calcium levels. Low calcium in the blood leads to increased PTH secretion, while high calcium reduces PTH release.
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What role does the Calcium-Sensing Receptor (CaR) play in calcium regulation?
What role does the Calcium-Sensing Receptor (CaR) play in calcium regulation?
The calcium-sensing receptor (CaR) is a protein that detects calcium levels in the blood. It sits on the surface of cells in the parathyroid glands and kidneys. When blood calcium is high, CaR activates and reduces PTH release.
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What is vitamin D and how does it relate to calcium?
What is vitamin D and how does it relate to calcium?
Vitamin D is a steroid hormone produced in the skin upon exposure to sunlight. It gets activated in the liver and kidneys and then helps the body absorb calcium from food.
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What is the active form of vitamin D and what does it do?
What is the active form of vitamin D and what does it do?
The active form of vitamin D, 1,25(OH)2D3, is produced in the kidneys. It promotes calcium absorption in the intestines, bone resorption, and renal reabsorption of calcium.
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What is calcitonin and what does it do to blood calcium?
What is calcitonin and what does it do to blood calcium?
Calcitonin is a hormone produced by the thyroid gland that helps to decrease blood calcium levels. It mainly works by inhibiting bone resorption, meaning it slows down the breakdown of bones.
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What are VDRs and where are they located?
What are VDRs and where are they located?
Vitamin D receptors (VDRs) are proteins located inside the nuclei of cells in target organs like the intestines, kidneys, and bones. They bind to active vitamin D and then turn on genes that help control calcium absorption and bone metabolism.
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How is calcium regulated in the body?
How is calcium regulated in the body?
The mechanism of calcium regulation involves a complex interplay of hormones like PTH, vitamin D, and calcitonin. These hormones work together to keep blood calcium levels within a narrow range, ensuring proper bone health, muscle function, and nerve transmission.
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What is the Calcium Receptor (CaR)?
What is the Calcium Receptor (CaR)?
A G-protein coupled receptor that regulates PTH secretion in response to extracellular calcium levels.
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How do mutations affect the CaR?
How do mutations affect the CaR?
Mutations in the CaR can lead to either hypercalcemia or hypocalcemia, depending on whether the receptor is less or more sensitive to calcium.
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What are calcimimetics and calcilytics?
What are calcimimetics and calcilytics?
Calcimimetics are drugs that activate the CaR, leading to suppressed PTH secretion and lower blood calcium levels. Calcilytics, on the other hand, decrease the sensitivity of the CaR, resulting in increased PTH and higher blood calcium.
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What causes postmenopausal osteoporosis?
What causes postmenopausal osteoporosis?
Postmenopausal osteoporosis is characterized by increased bone breakdown (osteoclast activity) and reduced bone formation (osteoblast function) due to estrogen deficiency. This leads to weakened bones and a higher risk of fractures.
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How is osteoporosis treated?
How is osteoporosis treated?
Treatments for osteoporosis include anti-resorptive agents (bisphosphonates, SERMs) that inhibit bone breakdown and anabolic therapies (teriparatide) that promote bone formation. Calcium and vitamin D supplementation are also crucial for maintaining bone health.
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Calcium Transport: Passive Diffusion
Calcium Transport: Passive Diffusion
Calcium's movement across cell membranes, usually from areas of high concentration to low, without requiring energy.
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Renal Calcium Reabsorption
Renal Calcium Reabsorption
The process of reabsorbing calcium back into the bloodstream from the filtered fluid in the kidneys.
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Parathyroid Hormone (PTH)
Parathyroid Hormone (PTH)
The hormone that regulates calcium levels in the blood by increasing its absorption, release from bones, and reabsorption in the kidneys.
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Hypocalcemia
Hypocalcemia
A condition where calcium levels in the blood are lower than normal, often leading to muscle spasms and seizures.
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Hypercalcemia
Hypercalcemia
A condition where calcium levels in the blood are higher than normal, often causing fatigue, weakness, and kidney stones.
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Calciphylaxis
Calciphylaxis
A serious complication caused by high calcium and phosphate levels, leading to calcification and hardening of blood vessels.
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Bone Resorption
Bone Resorption
The process of degrading bone tissue to release calcium into the bloodstream, stimulated by parathyroid hormone.
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Bone Formation
Bone Formation
The process of incorporating calcium into bone tissue, helping to build and strengthen bones.
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Calcium Homeostasis
- Calcium is crucial for bone formation, muscle function, and cell signalling
- 99% of calcium is stored in bones and teeth, 1% in blood and extracellular fluid
- Intracellular calcium levels are tightly regulated within organelles like ER and mitochondria
- Normal calcium levels are vital for healthy neuromuscular function and cardiac function
- Deviations from the normal range can cause serious neuromuscular and cardiac issues
Primary Hormones in Calcium Homeostasis
-
Parathyroid Hormone (PTH):
- Secreted by parathyroid glands in response to low blood calcium
- Stimulates osteoclast activity increasing bone resorption and calcium release into the bloodstream
- Enhances calcium reabsorption in the kidneys, reducing phosphate excretion
- Stimulates vitamin D conversion to calcitriol, increasing intestinal calcium absorption
- Overall, PTH acts synergistically to elevate serum calcium levels, counteracting hypocalcemia
-
Vitamin D (1,25(OH)2D3):
- Synthesized in skin (UVB) or obtained from diet, requires liver and kidney activation
- Increases serum calcium by promoting intestinal calcium absorption and bone resorption
- 1,25(OH)2D3 is essential for calcium uptake and absorption
Calcium Regulation Mechanisms
- Ca2+-sensing receptor (CaR): Monitors blood calcium levels and regulates PTH release. High calcium inhibits PTH; low calcium causes PTH release
- Vitamin D activation pathway: UVB converts 7-dehydrocholesterol to pre-vitamin D3, converted in the liver to 25(OH)D3, then to active 1,25(OH)2D3 in the kidneys
Effects of PTH
- Increases blood calcium levels
- Stimulates osteoclast activity
- Enhances renal calcium reabsorption
Mechanism of Action and Function of Vitamin D:
- Vitamin D is necessary for calcium absorption; its active form (1,25(OH)2D3) binds to VDRs, activates gene expression, and ensures calcium absorption, bone mineralization, and cell differentiation
- Vitamin D receptor (VDR) plays a role in calcium absorption in intestines, and renal calcium reabsorption
Intestinal Calcium Absorption
- 90% of dietary calcium is absorbed in the duodenum
- Both paracellular (passive) & transcellular (active) absorption
- Active transport via TRPV6, calbindin & PMCA requires 1,25(OH)2D3
Clinical Relevance of Calcium Imbalances
- Hypocalcemia: Neuromuscular excitability (e.g., tetany, Trousseau's or Chvostek's sign)
- Hypercalcemia: Reduced neuromuscular excitability, lethargy, cardiac arrhythmias, risk of death
Disorders of Calcium Homeostasis
- Hypocalcemia: Muscle spasm, tetany
- Hypercalcemia: Lethargy, arrhythmias, death
Role of PTH in Calcium Metabolism
- Key regulator of calcium homeostasis and is crucial for maintaining normal serum calcium levels
Effects of Calcium regulation
- PTH: PTH regulates blood calcium levels
- Vitamin D: Vitamin D enhances intestinal calcium absorption
- Calcitonin: Lowers blood calcium by suppressing osteoclast activity
Osteoporosis
- Postmenopausal osteoporosis disrupts bone remodelling, increasing osteoclast activity and decreasing osteoblast function
- Results in weakened bones prone to fractures (especially in the hip, spine, wrist)
- Treatment includes anti-resorptive agents (bisphosphonates) and anabolic therapies (teriparatide). Calcium and vitamin D supplementation are important in maintaining bone health.
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