Wk 6

Question 1

What are the stimuli of Growth Hormone (GH)?

Answer 1

• increased amino acid levels in blood
• decreased glucose in blood
• decreased fatty acids
• exercise
• health stressors

Question 2

What is the GH receptor?

Answer 2

JAK STAT

Question 3

What is the GH receptor pathway?

Answer 3

1. GH binds to a tyrosine kinase like receptor
2. phosphorylation of amino acids
3. activates JAK enzyme (phosphorylates)
4. produces signal transducer activator of transcription (STAT)
5. binds to gene sequence of DNA
6. transcription of mRNA produces IGF-1 protein

Question 4

What is the effect of GH on protein metabolism?

Answer 4

• increased uptake of amino acids and stimulates synthesis of DNA, RNA and protein
• protein anabolism of GH is mediated by IGF-1
• increases lean body mass and organ size

Question 5

What is the effect of GH on fat metabolism?

Answer 5

• enhances lipolysis and fatty acid oxidation (GH = lipolytic hormone)
• increases utilisation of fats for energy (breakdown of fats)
• increased mobilization of fatty acids from adipose tissue

Question 6

What is the effect of GH on carbohydrate metabolism?

Answer 6

• causes insulin resistance, decreased glucose uptake and utilisation by target
• need normal levels for correct pancreatic function and insulin secretion
• GH produces insulin insensitivity (diabetogenic hormone)
• increases blood glucose

Question 7

What is the effect of GH on bone and cartilage?

Answer 7

• increases metabolism in cartilage-forming cells and proliferation of chondrocytes (stem cells turn into IGF-1 responding cells)
• stimulates skeleton elongation by chondrocytes in the epiphyseal growth plates

Question 8

What is the effect of GH on insulin like growth factors?

Answer 8

• stimulates IGF-1 and IGF-2 production
• stimulates growth
• mitogenic (stimulates mitosis)
• produced in the liver and cartilage

Question 9

What is IGF-1?

Answer 9

• protein hormone
• secreted from the liver
• GH stimulates production of IGF-1
• mostly bound to IGFBP-3

Question 10

What is the negative feedback loop of GH?

Answer 10

1. High IGF-1
2. Decreases secretion of GH via suppressing somatotropes
3. Stimulates release of GHIH from hypothalamus

Question 11

What is IGF-2?

Answer 11

• Expressed in embryonic and fetal development
• Secreted mainly from placenta and fetal liver

Question 12

What are the direct actions of GH?

Answer 12

Signal transducer and activator of transcription (STAT).

Question 13

What are the indirect actions of GH?

Answer 13

• growth of bones and visceral organs
• increases skeletal and visceral growth
• increased cartilage growth, long-bone length and periosteal growth
  *mostly mediated by IGF-1

Question 14

What is an inhibitor of GH?

Answer 14

Estrogen
*Girls secrete lots during puberty, therefore usually are shorter than boys.

Question 15

What are the two disorders of GH?

Answer 15

1. Insufficient GH secretion (hyposecretion)
2. Excess growth hormone secretion
   (secretory tumour - adenoma)

Question 16

What is insufficient GH secretion?

Answer 16

• occurs before puberty, severely impaired growth
• normal intelligence
• caused by reduced GH secretion, IGF production reduced and deficient IGF action

Question 17

What are types of insufficient GH secretion?

Answer 17

Dwarfism:
- Laron dwarfism (GH receptors are unresponsive)
- African Pygmies (do not exhibit normal rise in IGF - no puberty growth spurt)

Question 18

What is excess growth hormone secretion?

Answer 18

• caused by adenoma of pituitary somatotroph cells
• causes exaggeration of physiological features

Question 19

What are the types of excess growth hormone secretion?

Answer 19

1. Gigantism (increased GH before epiphyseal plates are fused - height increases; occurs before puberty)
2. Acromegaly (increased GH after epiphyseal plates are fused - enlargement of membrane bones; jaw, forehead)