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REB > 6. Calcium Homeostasis > Flashcards

6. Calcium Homeostasis Flashcards

1
Q

What are the functions of calcium?

A
  1. Muscle contraction
  2. Nerve excitability
  3. Neurotransmitter and hormone release
  4. Enzyme function
  5. Blood coagulatation (cofactor for clotting factors)
  6. Structural support for healthy bones and teeth
  7. Membrane permeability
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2
Q

What does a calcium deficit in cells result in?

A

Hyperexcitability

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3
Q

What does excess calcium in the cytosol result in?

A

Increased contractability

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4
Q

How is calcium concentration regulated?

A
  1. Intake, absorption, secretion of Ca++ and vitamin D
  2. Functioning intestine, parathyroid glands and kidneys
  3. Parathyroid hormone and calcitonin
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5
Q

Where is excess calcium stored?

A

Bone

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6
Q

What two ways can calcium be absorbed?

A

Passive: relative to Ca++ conc in lumen
Active: Needs active vitamin D

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7
Q

Describe active absorption of calcium

A
  1. Entry across apical brush border is mediated by CaT1
  2. Calbindin binds and transports Ca++
  3. Enters blood mediated by plasma membrane Ca++-ATPase
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8
Q

What happens when there is an increased concentration of calcium in the blood?

A
  1. Thyroid releases calcitonin
  2. Increased calcium deposition in bone (calcification)
  3. Decreased uptake of calcium by the intestines
  4. Decreased reabsorption from kidneys
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9
Q

What happens when there is a decreased concentration of calcium in the blood?

A
  1. Parathyroid releases PTH
  2. Increased resorption from bone
  3. Increased uptake from intestines
  4. Increased reabsorption from kidneys (stimulates active vitamin D which increases intestinal uptake)
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10
Q

What factors affect calcium metabolism?

A
  1. Calcium intake and urinary loss
  2. Parathyroid hormone and calcitonin
  3. Vitamin D
  4. Function of involved organs
  5. Plasma albumin concentration
  6. Blood pH
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11
Q

What type of hormone is PTH?

A

84 amino acid polypeptide

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12
Q

What are the target organs of PTH?

A

Bone, kidneys, intestines

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13
Q

What are the functions of PTH?

A
  1. Increases Ca++ and Mg++ reabsorption from the kidney
  2. Increases Ca++, HPO4- and Mg++ uptake from the GIT
  3. Increases osteoclast activity
  4. Formation of active vitamin D
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14
Q

What is Parathyroid Hormone-related Protein?

A

Made by many adult and fetal tissues
Similar in structure to PTH but longer
Binds to same receptors as PTH
Can also be hypersecreted from breast and lung tumours

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15
Q

What are the functions of PTHrP?

A
  1. Mammary gland development and lactation
  2. Transfer of Ca++ in placenta
  3. Early bone development
  4. Vasodilation
  5. Increases resorption from bone
  6. Decreases Ca++ loss in urine
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16
Q

What type of receptor is the type 1 PTH receptor?

A

GPCR
Binds both PTH and PTHrP
cAMP second messenger
Found in bone and kidneys

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17
Q

What type of hormone is calcitonin?

A

32 amino acid polypeptide

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18
Q

Where is calcitonin synthesised?

A

Parafollicular cells in the thyroid gland

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19
Q

What are the functions of calcitonin?

A
  1. Inhibit osteoclast activity
  2. Stimulates osteoblasts
  3. Decreases plasma Ca++ by inhibiting resorption
20
Q

Describe the gene expression for calcitonin in the thyroid gland

A

4/6 exons are expressed in mRNA
Codes for preprocalcitonin
Inhibits bone resorption

21
Q

Describe the gene expression for calcitonin in the CNS

A

5/6 exons expressed in mRNA
Codes for calcitonin-gene related peptide
Potent vasodilator

22
Q

How is vitamin D converted to its active form?

A

UV light in skin

Hydroxylation in liver and kidneys

23
Q

What are the functions of vitamin D?

A
  1. Increase Ca++ uptake in the intestine
  2. Decreases loss by kidney by increasing reabsorption
  3. Stimulates resorption of bone when blood Ca++ is low
24
Q

Describe calcium excretion by the kidneys

A

Ca++ is freely filtered through glomeruli
Majority is reabsorbed in the proximal tubule
The loop of Henle and distal tubule are sensitive to PTH

25
What is calcium excretion increased by?
1. Increased plasma/dietary Ca++ 2. Deprivation of phosphate 3. Excessive vitamin D 4. Increased Na+ excretion 5. Immobilisation 6. Corticosteroid administration 7. Acidosis 8. Hyperthyroidism
26
What is calcium excretion decreased by?
1. Decreased plasma/dietary Ca++ 2. Decreased glomerular filtration rate 3. PTH 4. Increased dietary phosphate 5. Increased Ca++ utilisation (growth, pregnancy, lactation)
27
How much calcium on average is contained in serum?
2.1-2.6mmol/L
28
What percentage of calcium is bound to albumin or globulins?
40%
29
Which small anions can calcium bind to?
Bicarbonate, phosphate, citrate, lactate
30
How does blood pH affect calcium binding?
Ca++ binds to negatively charged sites on albumin or anions Competes with H+ for these sites Binding alters levels of ionised Ca in the blood Alkalosis promotes more calcium binding (less free calcium) Acidosis decreases binding (more free calcium)
31
What is osteomalacia/rickets caused by?
Vitamin D deficiency
32
How is osteomalacia/rickets diagnosed?
X-rays of affected bones | Serum calcium, alkaline phosphatase and phosphorus
33
How is osteomalacia/rickets treated?
Cholecalciferol
34
What are the causes of osteoporosis?
1. Hyperparathyroidism 2. Vitamin D deficiency 3. Calcium deficiency 4. Chronic kidney disease
35
How is osteoporosis diagnosed?
Recognised due to fractures | DEXA scan
36
How is osteoporosis treated?
1. Calcitriol 2. Bisphosphonates 3. Calcitonin 4. Hormone Replacement Therapy
37
What is Paget's disease?
Progressive bone disorder New bone formed abnormally Areas of bone resorption Increased risk of cancer
38
How is Paget's disease diagnosed?
X-rays, MRI, CT scan | Increased serum alkaline phosphatase
39
How is Paget's disease managed?
Calcitonin | Bisphosphates
40
What is the cause of hypercalcaemia?
Parathyroid tumour
41
What serum levels indicate hypercalcaemia?
>2.6mmol/litre
42
What are the symptoms of hypercalcaemia?
``` Weakness, vomiting, confusion Polyuria, polydipsia, renal stones Bone pain Constipation, duodenal ulceration Dysarrhythmias ```
43
What are the causes of hypocalcaemia?
PTH deficiency Chronic renal failure Vitamin D deficiency
44
How can hypocalcaemia be diagnosed?
Serum levels <2.1 mmol/L Trousseau's sign Chvostek's sign
45
What are the symptoms of hypocalcaemia?
``` Numbness and tingling of extremities Muscle cramps and spasm Waddling gait Bone pain, fractures Psychiatric disturbances, memory loss, confusion Convulsions, cataracts ```

REB (29 decks)

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