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Endocrine System > Intro to the Endocrine System > Flashcards

Intro to the Endocrine System Flashcards

1
Q

What is the endocrine system?

A

A system that integrates and controls organ function via the secretion of chemicals (hormones) from cells, tissues or glands which are then carried in the blood to target organs, distal from the site of hormone synthesis, where they influence the activity of that target organ.

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2
Q

What other system does the endocrine system work with to maintain homeostasis?

A

Nervous system

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3
Q

What is the speed of response dependent on?

A

Response may be fast (within seconds) e.g. increased heart rate in response to adrenalin, or slow (over days) e.g. increased protein synthesis in response to growth hormone.

Dependent on hormone type and signal transduction pathway associated with that hormone

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4
Q

Name three other chemicals which endocrine hormones should not be confused with

A
  • Paracrine
  • Autocrine
  • Exocrine
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5
Q

Describe the action of paracrine chemicals

A

Act local to the site of synthesis, do not travel to distant sites e.g. histamine

Chemicals diffuse in ECF to affect neighbouring cells

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6
Q

Describe the communication of autocrine chemicals

A

Act on/in the same cell that synthesises the hormone e.g. cytokines

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7
Q

Describe the action of exocrine chemicals

A

Released from exocrine glands via ducts to the external environment including the GI tract e.g. saliva, sweat, bile

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8
Q

Describe the endocrine communication

A

Hormones travel in the blood to their target organs/tissues

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9
Q

How do tissues detect hormones?

A

Through the presence of specific receptors for that chemical on/in the cells

No receptor = no response

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10
Q

Where are the receptors found on the cells?

A

Either inside (must be lipophilic hormone) or on the outside of the cell (must be lipophobic hormone)

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11
Q

Describe neural communication

A

Neurotransmitters released from presynaptic neurons travel across the synaptic cleft to the postsynaptic cell to influence its activity.
Neurotransmitter is the chemical released by the neuron but, in contrast to hormones, acts locally within the synaptic cleft.

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12
Q

Describe neuroendocrine communication

A

Endocrine and nervous systems combine. Nerves release hormones which enter blood and travel to their target cells e.g. hypothalamic – posterior pituitary axis.

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13
Q

Why don’t hormones elicit a response in every type of tissue?

A

All hormones circulate throughout the body in the blood, but the response to any one hormone is highly specific because only target cells have receptors for the hormone.

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14
Q

Can a hormone have more than one effect?

A

Yes, while the response of a target cell to any one hormone is highly specific, the same hormone can have different effects in different target cells.

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15
Q

Why can a hormone have different responses in different tissues?

A

Due to the hormone having different receptor types in different tissues

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16
Q

Give an example of a hormone that has more than one action in different tissues

A

Insulin
• Skeletal muscle/adipose tissue - increase in Glc uptake
• Liver - Increase glycogenesis + decreased gluconeogenesis

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17
Q

What is the function of hormones (and neurotransmitters)?

A

Bring about changes in the activity of their target cells and tissues (increase/decrease a particular activity)

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18
Q

Name four glands involved in the endocrine system

A

Thyroid gland, adrenal gland, pituitary gland, hypothalamus

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19
Q

Name six features of an endocrine hormone

A
  1. Produced by a cell or group of cells
  2. Secreted from those cells into the blood
  3. Transported via the blood to distant targets
  4. Exert their effects at very low concentrations (act in the range 10^-9 - 10^-12 M)
  5. Act by binding to receptors on target tissues
  6. Have their action terminated, often via negative feedback loops
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20
Q

Why is only a small amount of endocrine hormone released into the blood?

A

As the receptors are high sensitive to the hormone

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21
Q

What are the three classifications of endocrine hormones?

A
  • Peptide or protein hormones - composed of amino acids
  • Amine hormones - all derived from one of two amino acids (tryptophan or tyrosine)
  • Steroid hormones - all derived from cholesterol
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22
Q

Describe basic synthesis of peptide hormones

A

Synthesised as preprohormone in advance of need then cleaved into prohormone and stored in vesicles until required

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23
Q

What are the stages in the synthesis of peptide hormones?

A
  1. mRNA on the ribosomes binds to AA to form preprohormone
  2. Taken to the ER, enzymes remove signal sequence creating inactive prohormone
  3. Prohormone is packaged into secretory vesicles in the Golgi app.
  4. Proteolytic enzymes in the vesicles chop prohormone into active hormone + peptide fragments
  5. Vesicle stored until release is stimulated
  6. Release contents by exocytosis into ECF
  7. Hormone moves into circulation
24
Q

What is the clinical use peptide hormone?

A

By measuring inactive fragments in plasma i.e. C peptide in diabetes

25
Q

What is C-peptide?

A

Inactive fragment cleaved from the insulin prohormone.

26
Q

Give an example of an inactive fragment of a peptide hormone which can be measured

A

C-peptide
• Levels of C-peptide in plasma or urine are often measured to indicate endogenous insulin production from the pancreas (produced in equal amounts)
• However, because insulin is metabolised faster, levels of C-peptide are typically about 5x higher than endogenous insulin

27
Q

Why would C-peptide be measured and not insulin directly?

A

Because C-peptide gives an indication of how much insulin is produced by the pancreas, and so is independent of synthetic insulin administration (if diabetic)

28
Q

Describe the mechanism of action of peptide hormones

A
  • Water soluble so dissolves in the plasma, but means it cannot cross cell membrane
  • Binds to membrane bound receptors and creates fast responses
  • Modulating the GPCR or tyrosine kinase linked signalling pathways
  • These pathways phosphorylate existing proteins in the cell and modify their function i.e. open or close ion channels, activate or inactivate
29
Q

What are the two pathway of peptide hormone signal transduction?

A

Hydrophillic/lipophobic -> binds to a cell surface receptor which is either:
• G Protein Couple Receptor:
• Tyrosine Kinase Linked Receptor

30
Q

Describe G protein couple receptor pathway

A

Activate 2nd messenger system and/or ion channels leading to modification of existing proteins - rapid response

31
Q

Describe Tyrosine Kinase Linked Receptor

A

Alters gene expression

32
Q

What are the two different types of amine hormones derived from tyrosine?

A

Catecholamines and thyroid hormones

33
Q

Name three different types of catecholamines

A
  • Dopamine (CNS neurotransmitter)
  • Norepinephrine (neurotransmitter)
  • Epinephrine (hormone released by adrenal medulla)
34
Q

Describe the actions of catecholamines and thyroid hormones

A
  • Catecholamines - similar mechanism of action to peptide hormones (hydrophilic)
  • Similar mechanism of action to steroid hormones (lipophilic)
35
Q

Name an amine hormone not derived from tyrosine?

A

Melatonin (derived from tryptophan)

36
Q

What is the action of melatonin?

A

Regulates circadian rhythm

37
Q

What are the features of steroid hormones?

A
  • Synthesised directly as needed (instead of being stored)
  • Highly lipophilic
  • Diffuse across membrane into ISF and blood
38
Q

How are steroid hormones transported?

A

Bound to carrier proteins such as albumin as they’re hydrophobic

This stabilises their transport through plasma and protects them from enzymatic degradation, increasing their half life

39
Q

Name four structures which produce steroid hormones

A
  1. Gonads (testes and ovary) - sex steroids
  2. Placenta - hCG, sex steroids
  3. Kidney - vitamin D3
  4. Adrenal cortex - corticosteroids
40
Q

What are steroid hormones derived from?

A

Cholesterol

41
Q

How are different steroid hormones formed from cholesterol?

A

Depending on what tissue it is, determines what enzymes are presents, which determines what steroid hormone is produced through metabolism of cholesterol

42
Q

Where are the receptors for steroid hormones found?

A

Inside cells (cytoplasmic or nuclear receptors) are they are lipophilic and so cross plasma membrane easily

43
Q

What is the action of steroid hormones?

A

Binding to receptors trigger either activation or repression of gene function within the nucleus = genomic effect

44
Q

Why is there a delay in the hormone release and biological effect?

A

Genes control the synthesis of protein so these hormones either increase or decrease protein synthesis

This is a slow process so there is a delay, but effect persists for around the same time.

Some may bind to membrane receptors - rapid response

45
Q

What are free hormones?

A

Small amount of unbound free steroid/thyroid hormone in the plasma which is important as only free hormones can diffuse across capillary walls to target cells.

46
Q

Why do the levels of free (active) hormone remain constant?

A

The Law of Mass Action:

As free hormone leaves the plasma (taken up by cells) more hormone is released from the carriers

47
Q

How do you work out total plasma?

A

Free hormone + complexes hormone = total plasma (hormones)

48
Q

What are the different carrier proteins in the plasma that steroid/thyroid hormones are transported by?

A
  • Can be specific (corticosteroid-binding globulin)

* Non-specific (albumin)

49
Q

Why do steroid/thyroid hormones need transport?

A

As they have poor solubility in plasma

50
Q

What are the two effects of carrier proteins on hormones?

A
  • Increases solubility

* Protects from degradation -> increased has life

51
Q

How is the bound:unbound ratio maintained?

A
  • As steroid taken up; more is released from carrier
  • Allows for a reservoir of hormone ready for action
  • Prolonged activity
52
Q

What is the concentration of hormone determined by?

A

The balance between secretion and degradation/excretion

53
Q

What is secretion responsive to?

A
  • Negative feedback (i.e. parathyroid hormone)
  • Neural feedback loops (i.e. adrenaline)
  • Multiple control mechanisms (i.e. glucose)
54
Q

What are the homeostatic responses when normal hormone concentration is disrupted?

A
  • Prolonged low [hormone]plasma –> up-regulation of receptor number (increases tissue sensitivity to hormone)
  • Prolonged high [hormone]plasma –> down-regulation of receptor number (decreases tissue sensitivity to hormone)
55
Q

Describe permissive effects of hormones

A

The presence of one hormone enhances the effect of another i.e. epinephrine causes only modest lipolysis in adipose tissue, but when thyroid hormones are also present, greatly increased lipolysis occurs.

TH –> increases synthesis of receptors for epinephrine on adipocytes. TH itself has no effect on lipolysis but is permissive to epinephrine.

56
Q

Describe antagonistic effects of hormones

A

The presence of one hormone reduces the effect of another e.g. growth hormone impairs the response to insulin by decreasing the number of insulin receptors on tissues.

Endocrine System (29 decks)

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