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Metabolism > Parathyroid > Flashcards

Parathyroid Flashcards

0
Q

Where is most of the calcium stored in the body?

A

-Skeleton

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1
Q

What is the normal range of serum calcium and what propotion of this is biologically active?

A
  • 2.2-2.5mM total

- 1-1.3mM biologically active

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2
Q

What are the functions of calcium?

long list

A
  • Builds and maintains teeth and bones
  • Regulates heart rhythm
  • Helps regulate nutrient uptake
  • Assists in normal blood clotting
  • Helps maintain nerve and muscle function
  • Lowers blood pressure
  • Important in normal kidney function
  • Needed for activity of some enzymes and some hormone receptor binding
  • Reduces blood cholesterol
  • Important in intracellular signalling
  • Appropriate levels required for nerve transmission at NMJ
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3
Q

In what form is calcium stored in bone?

A

-Hydroxyapatite crystals

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4
Q

How much calcium is exchanged between bone and ECF per day?

A

-300-600mg

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5
Q

What is the major rapid short-term regulator of calcium?

A

-Parathyroid hormone

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6
Q

How does PTH increase calcium levels?

A
  • Stimulate Ca2+ release from bone; in part by stimulating bone resorption
  • Decreases urinary loss of Ca2+ by stimulating reabsorption from the kidney
  • Indirectly stimulates Ca2+ absorption in the small intestine by stimulating the synthesis of active vitamin D in the kidney
  • Decreases Ca2+ excretion causing increased P excretion
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7
Q

How is PTH controlled?

A

-Negative feedback of calcium and phosphate
Calcium
-High calcium levels in ECF binds to the Ca receptor on parathyroid cells
-Receptor activation leads to inhibition of PTH release
-Fall in Ca levels means that there is less Ca to bind to ca receptors on parathyroid cells
-PTH released
Phosphate
-High phosphate levels leads to increased PTH secretion in order to excrete phosphate

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8
Q

Describe the actions of PTH on bone

A
  • PTH induces osteoblastic cells to synthesis and secrete cytokines
  • Cytokines stimulate differentiation and activity of osteoclasts and also protect them from apoptosis
  • PTH also decreases osteoblast bone-synthesising activity exposing the bony surface to osteoclasts
  • Resorption of mineralised bone and release of Ca2+ into ECF
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9
Q

What is an indicator of high bone turnover?

A

-Alkaline phosphatase

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10
Q

Describe the actions of PTH on the kidney

A
  • Affects tubular cells within the kidney (predominantly DCT) causing an increase in reabsorption of Ca2+ from the ascending limb and DCT
  • Thus calcium excretion is reduced
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11
Q

Why does Phosphate need to be excreted if the levels of calcium are increased?

A

-To prevent kidney stone formation

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12
Q

What effect does PTH have on vitamin D

A

-Stimulates the synthesis of active vitamin D

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13
Q

Why is vitamin D important in calcium regulation?

A

-Active vitamin D increase Ca2+ uptake in the gut by activating transcellular uptake of vitamin D by active transport (Usually only 30% of the ca in the gut is absorbed by paracellular uptake)

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14
Q

What type of regulator is vitamin D on calcium?

A

-Long-term

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15
Q

What is the chemical name for the vitamin D group?

A

-Hydroxycholecalciferols

16
Q

What is the chemical name for active vitamin D?

A

-Dihydroxycholecalciferol or calcitriol

17
Q

How is active vitamin D synthesised?

A
  • Vitamin D undergoes two hydroxylation reactions to form calcitriol
  • The first hydroxylation is in the liver on C25 and forms 25-hydroxycholecalciferol
  • The second hydroxylation is at C1 and occurs in the kidney forming 1,25-dihydroxycholecalciferol
18
Q

Do both hydroxylation reactions of vitamin D occur at the same time?

A

-No the first hydroxylation occurs in the liver and then 25-hydroxycholecalciferol circulates in the blood for 15-20 days

19
Q

What causes the second hydroxylation of 25-hydroxycholecalciferol?

A

-Upon being filtered through the kidney, if there are high PTH levels then the 25-hydroxycholecalciferol will undergo the second hydroxylation to form active vitamin D (calcitriol)

20
Q

What type of control is the second hydroxylation of vitamin D? Explain this

A
  • Negative feedback
  • If there is low calcium, PTH is stimulated which is detected in the kidney resulting in the second hydroxylation. When calcium levels rise, PTH levels fall. This is detected by the kidney and the second hydroxylation of Vitamin D stops
21
Q

What are the effects of calcitriol?

A
  • Stimulates transcellular uptake in the gut via active transport at the apical membrane and endocytosis at the basolateral membrane
  • Stimulates bone resorption same as PTH
  • Effects the kidney by decreasing Ca2+ excretion
22
Q

Where is calcitonin secreted from and what is its function?

A
  • Parafollicular cells of the thyroid
  • Involved in Ca2+ regulation however has minimal effect in humans (extensive effect in animals)
  • May become important during pregnancy
23
Q

What is the normal response of the body to hypercalacaemia?

A
  • Increase in plasma calcium
  • Decreased PTH secretion (suppresses basal secretion)
  • Kidney-> Decreased hydroxylation of 25-VitD causing decreased uptake from the gut; decreased calcium reabsorption
  • Bone-> decreased resorption of bone and increased building of bone
  • Plasma calcium levels fall
24
Q

What is the normal response of the body to hypocalacaemia?

A
  • Decrease in plasma calcium
  • Increase in PTH secretion
  • Kidney-> increased hydroxylation of 25-VitD causing increased calciuim uptake from the gut; increased calcium reabsorption from kidney tubules
  • Bone-> increased bone resorption
  • Plasma calcium levels rise
25
Q

What are the signs and symptoms of hypocalcaemia?

A 
-Hyper-excitability of NMJ leading to:
Pins and needls
Tetany (muscle spasms)
Paralysis
Convulsions
-Death
26
Q

Why does hypocalcaemia effect bone?

A

-Serum calcium is the priority so if calcium is deficient in the diet, serum calcium is maintained at the expense of bone

27
Q

Name a metabolic bone disease associated with hypocalcaemia

A
  • Rickets in children

- Osteomalacia in adults

28
Q

What is hypoparathyroidism?

A

-Decreased PTH levels meaning that serum calcium cannot be maintained so life-threatening hypocalcaemia develops
VERY RARE

29
Q

What is the main cause of hypoparathyroidism?

A

-Surgical removal of parathyroid glands during thyroidectomy

30
Q

What are the signs and symptoms of hypercalcaemia?

A
  • Renal calculi (stones)
  • Kidney damage due to high reabsorption
  • Constipation due to high levels in the gut
  • Dehydration
  • Tiredness
  • Depression
31
Q

What is the main cause of hypercalcaemia?

A

-Hyperparathyroidism usually caused by a parathyroid adenoma

32
Q

Why can tumours not associated with the parathyroid gland cause hypercalcaemia?

A

-Tumours can secrete PTHrp which behaves like PTH

33
Q

What is the usual cause of PTHrp?

A

-Metastases

34
Q

Why does PTHrp not cause increased activation of vitamin D?

A

-Unlike PTH, PTHrp does not increase the activity of C1-hydroxylase, the enzyme responsible for the second hydroxylation of 25-VitD in the kidney

35
Q

Name a tumour which can secrete PTHrp?

A

-Squamous cell carcinoma in the lung

Metabolism (14 decks)

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