6. The endocrine system

Question 1

What are hormones?

Answer 1

Chemical messengers released by endocrine glands that travel via the bloodstream to act on target cells.

Question 2

What are the 4 main types of hormone? Give examples of each.

Answer 2

1. Peptide/polypeptide hormones (largest group)
   - short/long chains of amino acids
   - e.g. insulin, glucagon, growth hormone
2. Glycoprotein hormones
   - large protein molecules with carbohydrate side chains
   - e.g. luteinizing hormone (LH), follicle stimulating hormone (FSH), thyroid-stimulating hormone (TSH)
3. Amino acid derivatives (amines)
   - small molecules synthesised from amino acids (most often tyrosine)
   - e.g. adrenaline, T3 and T4
4. Steroid hormones
   - derives from cholesterol
   - e.g. cortisol, aldosterone, testosterone and oestrogen

Question 3

Name the major endocrine glands.

Answer 3

• Hypothalamus, pituitary and pineal
• Thyroid and parathyroid
• Thymus
• Adrenal
• Pancreas
• Ovary/testis

Question 4

Give examples of endocrine organs and the hormones they release.

Answer 4

• Heart (ANP and BNP
• Liver (IGF1)
• Stomach (gastrin and ghrelin)
• Adipose (leptin)
• Kidney (EPO, renin and calcitriol)
• Placenta (inhibin, placental lactogen)

Question 5

How do endocrine glands store hormones?

Answer 5

i) polypeptide hormones and catecholamines stored within endocrine gland cell in discrete storage vesicles
ii) steroid-producing tissues don’t store the hormones but their precursor - cholesterol - as cholesterol esters in form of lipid droplets
iii) thyroid hormone is exception - stores hormonal products outside the cell in protein colloid

Question 6

Which hormones are water or lipid soluble and how are these transported in blood?

Answer 6

Hydrophilic - transported in blood dissolved in plasma:

• polypeptide hormones
• glycoprotein hormones
• amines released by adrenal medulla (adrenaline/NA)

Hydrophobic - transported bound to specific carrier proteins (only free form is biologically active)
- amines released by thyroid (T3/T4) - steroid hormones

Question 7

Which carrier protein transports thyroid hormones?

Answer 7

thyroxine-binding globulin (TBG)

Question 8

Name 3 roles of hormone carrier proteins.

Answer 8

i) increase solubility of hormone in plasma
ii) increase hormone 1/2 life
iii) readily accessible reserve

Question 9

Which 3 main factors determine hormone levels in blood?

Answer 9

1. rate of production (synthesis and secretion) - most highly regulated aspect of hormonal control
2. rate of delivery - higher blood flow to a particular organ will deliver more hormone
3. rate of degradation - hormones are metabolised and excreted from the body

Question 10

What is the difference between tropic and trophic hormones?

Answer 10

Tropic hormones = have other endocrine glands as their target. Mostly secreted by the anterior pituitary (e.g. TSH affects thyroid gland, ACTH affects adrenal gland).

Trophic hormone = stimulate growth in the target tissue

Question 11

Describe the mode of action of hydrophilic hormones.

Answer 11

Hydrophilic hormones bind to cell surface receptors, e.g.

1. G protein-coupled receptor (e.g. adrenaline receptor)
   Hormone binds to GPCR… dissociation of Galpha subunit… activation of effector protein (e.g. adenylyl cyclase)… formation of second messenger (e.g. cAMP)… activation of protein kinase (e.g. PKA)… phosphorylation of target proteins… cellular response.
2. Tyrosine kinase (e.g. insulin receptor)
   Hormone binds to receptor causing dimerisation (except insulin receptor which is already dimerised)… autophosphorylation of specific tyrosines… recruitment of adaptor proteins and signalling complex… activation of protein kinase (e.g. PKB)… phosphorylation of target proteins… cellular response.

Question 12

Describe the mode of action of hydrophobic hormones.

Answer 12

1. Lipid-soluble hormones diffuse/are transported across plasma membrane.
2. Type I: cytoplasmic R binds hormone and R/hormone complex enters nucleus and binds DNA.
   Type II: hormone enters nucleus and binds to pre-bound R on DNA (e.g. thyroid hormone). Binding relieves repression of gene transcription.
3. R binds to specific DNA sequence called a hormone response element (HRE) in promoter region of specific genes.
4. Expression of new protein mediates effects of hormone.