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Endocrine > Hormones in Ca Regulation > Flashcards

Hormones in Ca Regulation Flashcards

1
Q

Where and in what form is most Ca in the body stored?

A

bones as hydroxyapatite salt

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2
Q

What are the three major fractions of Ca in the plasma?

A
  • Ionized Ca
  • Protein bound calcium to albumin
  • Ca complexed to citrate and phosphate forming soluble complexes
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3
Q

What is the role of Ca in the coagulation cascade?

A

Initial platelet plug formation and most steps of blood coagulation

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4
Q

What is the role of Ca in the CNS/PNS?

A

Important for neuromuscular excitability

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5
Q

What are the two major intracellular effects of Ca?

A
  • Second messenger (Gq proteins)

- Cofactor for various enzymes

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6
Q

What are the three major hormones that play a role in Ca metabolism?

A
  • PTH
  • Vitamin D
  • Calcitonin
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7
Q

Where are the parathyroid glands located?

A

the top and bottom of the lateral lobes of

thyroid gland

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8
Q

What are the cells that are in the parathyroid that secretion PTH?

A

Chief cells

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9
Q

What are the roles (generally) of PTH?

A

Plays a major role in bone remodeling, and extracellular Ca homeostasis

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10
Q

What is the role of PTH at the level of the kidney?

A

Regulates the renal excretion of phosphate and the activation of vitamin D

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11
Q

How much PTH is stored by the chief cells, as opposed to synthesized on demand?

A

Relatively small amount are stored

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12
Q

At what Ca levels (relative) is PTH secreted?

A

Low Ca levels

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13
Q

What is the precursor form of PTH? What organelle of the chief cells secrete this?

A

preproPTH

rER

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14
Q

What is the “leader sequence” found on preproPTH?

A

A sequence of about 25 amino acids in its N terminus that serves as a place for cleavage, occurring within minutes of pre-proPTH synthesis

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15
Q

Where does the cleavage of preproPTH happen, and where does proPTH go after this occurs?

A

As it exits the rER, with proPTH then exiting to the Golgi

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16
Q

What happens to proPTH in the Golgi?

A

Trypsin like enzymes cleave a hexapeptide from the N terminal end of proPTH, converting proPTH into active PTH

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17
Q

Where is PTH stored in chief cells? What causes its release?

A

Into secretory granules in the Golgi complex

Low extracellular Ca causes deactivation of the calcium-sensing receptors (CaR)

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18
Q

What is the sensor on chief cells that activates the synthesis of PTH when low Ca levels are present in the serum? What type of sensor is this? How long does it take for PTH to be secreted?

A

CaR
Gq
PTH is released within seconds

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19
Q

What does it mean that both PTH synthesis and release occurs in a negative feedback manner?

A

High calcium suppresses PTH secretion, while low calcium stimulates hormone release

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20
Q

What is the effect of vitamin D on PTH? How?

A

Reduces PTH gene expression by suppressing PTH mRNA transcription and stability

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21
Q

What is the effect of phosphate on PTH?

A

Activates CaR, and thus decreases PTH release

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22
Q

Where are CaR sensors present? (3)

A

Parathyroid gland
Thyroidal C cells
Kidney tubules

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23
Q

What are the intracellular steps that occur with CaR activation? What specific part of this pathway causes PTH degradation?

A

IP3 and DAG lead to the activation of PLC and Ca release from the sER

Leukotriene generation triggers the degradation of PTH

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24
Q

Persistent hypercalcemia can cause what percent of PTH degradation within the cell?

A

90%

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25
Q

What happens to the CaR when there is hypocalcemia?

A

The Ca2+ sensor is in a relaxed conformational state and cannot activate the second
messengers involved in the degradation of PTH.

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26
Q

What is the major receptor on target cells for PTH?

A

PTHR1

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27
Q

What are the two intracellular pathways that are activated with PTHR1 activation?

A

Gs and Gq

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28
Q

What happens intracellularly with Gs activation?

A

Increase in cAMP and activation of PKA. This phosphorylates and activates enzymes

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29
Q

What happens intracellularly with Gq activation?

A

PLC activation results in the formation of DAG and IP3, which in turn leads to the activation of PKC and the release of intracellular Ca

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30
Q

What allows target cells of PTH to switch between Gs and Gq pathways?

A

The cell is able to switch from one pathway to the other by using Na+/H+ exchanger regulatory factor [NHERF1]

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31
Q

What are the two primary targets of PTH? What

A

Kidney

Bone Osteoblasts

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32
Q

What other hormone, besides PTH, can activate bone osteoblasts and the kidney? How does it do this?

A

PTH-related protein (PTHrP)

Shares 13 amino acid sequence with PTH (although is a product of a separate gene)

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33
Q

What is the effect of PTH and PTHrP on the kidney and osteoblasts?

A

Increasing Ca reabsorption in the kidney, and increase Ca mobilization from bone

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34
Q

What is the effect of PTH on the intestines?

A

Increases intestinal reabsorption of Ca (via Vitamin D)

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35
Q

What, besides increasing renal reabsorption of Ca, does PTH do in the kidney? (2)

A

Stimulates phosphate excretion and the activity of 1α-hydroxylase (for Vit D synthesis)

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36
Q

How does PTH binding to receptors in the kidney lead to increased Ca uptake?

A

PTH stimulates the insertion and opening of a Ca2+ channel on the apical
membrane, facilitating the entry of Ca2+ into the cell

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37
Q

What is the proteins that binds Ca once it enters the cytosol of kidney epithelial cells from the kidney? What does this protein do? What vitamin is this protein dependent on?

A

Calbindin-D28K

cytosolic diffusion of Ca2+ from the apical influx to the basolateral efflux sites

Vit D

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38
Q

What are the two proteins that secrete Ca into the bloodstream from kidney epithelial cells?

A

Na+/Ca2+ exchanger and a Ca2+- adenosine triphosphatase (ATPase)

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39
Q

Vitamin D activates what protein on the kidney epithelial cells to stimulate Ca release into the circulation?

A

Ca2+ATPase

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40
Q

How does PTH decrease renal phosphate reabsorption?

A

decreasing the expression of type II Na+/PO42− cotransporter

41
Q

What are the four types of cells found in bone?

A
  • Osteogenic (osteoprogenitor) cells
  • osteoblasts
  • Osteocytes
  • Osteoclasts
42
Q

What is the role of Osteogenic (osteoprogenitor) cells?

A

are undifferentiated cells with a high mitotic activity

and can develop into osteoblasts

43
Q

What are the only cells in the bone that maintain their ability to divide?

A

Osteogenic (osteoprogenitor) cells

44
Q

What is the role of osteoblasts in bone? Do they express PTH receptors?

A

They are involved in bone formation and mineralization. They express
PTH receptors

45
Q

What is the role of osteocytes in bone?

A

Osteocytes are trapped osteoblasts in bone matrix that function to maintain the bone matrix

46
Q

What are the two main components of the bone matrix?

A 
Collagen
and hydroxyapatite (calcium rich crystals)
47
Q

What is the role of osteoclasts in bone? What cells are they derived from?

A

cells developed from macrophages/monocytes that secrete acids and enzymes to dissolve the bone matrix

48
Q

True or false: bone is constantly undergoing remodeling

A

True

49
Q

What cells in bone have PTHR1 receptors? What does this trigger?

A

Osteoblasts

This triggers synthesis of RANKL (receptor activator of nuclear factor NF-kB ligand)

50
Q

What is RANKL? Where is this found? What does this bind to?

A

receptor activator of nuclear factor NF-kB ligand

Found within osteoblasts

binds to RANK receptor on osteoclastic precursors

51
Q

What stimulates the maturation of osteoclasts?

A

RANKL binds to RANK receptor in osteoclasts precursors,

52
Q

What is osteoprotegerin and what is its role?

A

A protein secreted by osteoblasts that prevents the binding between RANK and RANKL, thereby inhibiting the osteoclastic bone resorption

53
Q

What is the effect of PTH and glucocorticoids on the bone?

A

PTH and glucocorticoids decrease the production of osteoprotegerin

54
Q

What is the effect of estrogen on the bone?

A

Increases the expression of osteoprotegerin, thereby inhibiting osteoclast maturation

55
Q

What is the protein found on the surface of bone that osteoclasts attach to? What does this form?

A

β-integrins
This forms an isolated extracellular space between themselves and the bone
surface that functions as a lysosome

56
Q

What is the transporter on the osteoclast cells that allows for the breakdown of bone? What does this do to encourage bone breakdown?

A

H+-ATPases

Generates an acidic environment (pH =4)

57
Q

Where does the Ca and phosphate go once it is broken down by osteoclasts?

A

Undergoes transcytosis to be secreted into the blood

58
Q

What is the clinical marker for bone breakdown? Why?

A

Alk phos, since this is secreted along with Ca and phosphate by osteoclasts during the breakdown of bone

59
Q

What can result in hyperparathyroidism?

A
  • hyperplasia
  • adenoma/carcinoma
  • Chronic renal failure
60
Q

How can chronic renal failure lead to the hyperparathyroidism?

A

Reduces vit D and Ca reabsorption, making the parathyroid gland resistant to both the vitamin D and Ca negative feedback of PTH release

61
Q

What happens to [Ca] in the serum and the urine with hyperparathyroidism? What about plasma [phosphate]?

A

Hypercalcemia
Hypercalciuria
Hypophosphatemia

62
Q

What is the sequelae of hyperparathyroidism on the kidneys?

A

Renal calculi

63
Q

What is the only vitamin that is not required in the diet?

A

Vitamin D

64
Q

What are the two forms of Vitamin D?

A
  • vitamin D2 (ergocalciferol)

- vitamin D3 (cholecalciferol).

65
Q

UV irradiation of what two molecules leads to the development of Vit D2 and D3 respectively?

A
  • ergosterol (D2)

- 7-dehydrocholesterol (D3)

66
Q

What is the name of the active form of vitamin D?

A

Calcitriol (1,25 D3)

67
Q

What are the steps of calcitriol synthesis from 7-dehydrocholesterol?

A
  1. UV light irradiates the skin to form provit D
  2. Provit D3 isomerized to D3 and transported to the liver
  3. Liver converts to 25-hydroxyvit D3
  4. Kidney converts to 1,25-D3
68
Q

What is the protein that transports Vit D3 from the skin to the liver?

A

Vitamin D binding protein

69
Q

What happens to Vit D3 in the liver?

A

Converted to 25-hydroxyvitamin D3

70
Q

What is the major circulating form of Vit D?

A

25-hydroxyvitamin D3

71
Q

What is the enzyme in the kidney that converts 25-hydroxyvit D3 to 1,25 D3?

A

1α-hydroxylase

72
Q

How is the production of calcitriol regulated?

A

Negative feedback regulation by plasma Ca–An increase in plasma Ca2+ levels inhibits the hydroxylation at C-1 and
favors hydroxylation at C-24, leading to the synthesis of an inactive form of vitamin
D

73
Q

How does PTH stimulate the formation of Vitamin D?

A

Simulates that activity of 1α-

hydroxylase

74
Q

True or false: Calcitriol can be also synthesized from vitamin D2 (ergosterol) when obtained
from the diet.

A

True

75
Q

What are the four target organs of Vit D?

A

Intestines
Bone
Kidney
Parathyroid gland

76
Q

What is the effect of Vit D on the kidneys?

A

Facilitate PTH-mediated calcium reabsorption in the distal kidney tubules

77
Q

What is the effect of Vit D on the parathyroid gland?

A

Suppress the synthesis

and release of PTH

78
Q

What is the effect of Vit D on bones?

A

Regulate bone resorption and formation

79
Q

Does calcitriol exert its effect through cell membrane receptors, nuclear receptors, or both? What is the significance of this?

A

Both, meaning it can exerts its effects through both increased gene transcription, and nongenomic mechanisms

80
Q

How does calcitriol exert its effects in the intestines?

A

Regulates the transcription of TRPV6 and the Na/Ca exchanger on intestinal epithelial cells

81
Q

What is the channel that allows calcium into the intestinal epithelial cells from the lumen of the intestines?

A

Transient receptor potential vanilloid channel type 6 (TRPV6)

82
Q

What is the channel that allows Ca out of the basolateral side of the intestinal epithelium into the bloodstream?

A

Plasma membrane Calcium ATPase and the Na/Ca exchanger

83
Q

What is the effect of Vitamin D on bone?

A

It stimulates bone resorption (osteoclast formation) causing mobilization of Ca

84
Q

What are the three effects of excess vitamin D?

A
  • calcinosis of soft tissues
  • Deposition of Ca and phosphate in the kidney
  • Hypercalcemia
85
Q

What are the genetic disorders that can lead to vitamin D deficiency?

A

mutations in 1α-hydroxylase

86
Q

What are the two forms of vitamin D deficiency in children/adults?

A

Rickets in children

Osteomalacia in adults

87
Q

What are the classic findings in Rickets?

A

Bowing of the weight bearing bones
Dental defects
Weakness

88
Q

What type of hormone is calcitonin?

A

Peptide hormone

89
Q

Where is calcitonin produced? What is it initially produced as?

A

C cells of the thyroid gland as procalcitonin

90
Q

What are the receptors that trigger calcitonin release?

A

CaR

91
Q

What is the MOA of calcitonin in the bones?

A

inhibits bone resorption by decreasing osteoclast motility, and inhibits Carbonic anhydrase activity

92
Q

What is the MOA of calcitonin in the kidneys?

A

Increases urinary Ca excretion by inhibiting the renal tubular Ca reabsorption via G proteins

93
Q

True or false: There are no significant clinical correlations associated with calcitonin excess or deficiency

A

True–calcitonin does not appear critical for the regulation of calcium homeostasis

94
Q

What is calcistat?

A

The multi organ regulation of calcium homeostasis exists as a big systemic regulatory
network referred to as calcistat.

95
Q

What is the normal range of Ca serum levels (in mM)? Above what level is Ca resorption slowed, and excretion increased?

A

1.1-1.3 mM

Above 1.3 mM

96
Q

What is the plant version of vitamin D?

A

D2 (ergocalciferol)

97
Q

What is the circulating form of vitamin D? Active form?

A 
Circulating = 25, hydroxy
Active = 1,25 dihyrdroxy
98
Q

Does Ca bound to serum proteins activate CaR?

A

Nope

Endocrine (32 decks)

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