Hormonal Regulation Of Growth

Question 1

What is human GH made of? Where is it located?

Answer 1

191-amino acid single-chain polypeptide that is the most abundant hormone in the adenohypophysis

Question 2

What secondary messenger system does GH activate?

Answer 2

It binds to GH-R which is is a membrane bound receptor that activates JAK/STAT

Question 3

What does GH do?

Answer 3

Has a wide spectrum of biological activities.

It directs growth of bone, soft tissue, and viscera

It acts as a metabolic switch to burn fat and store protein = CHO. (Increase in protein synthesis, amino acid transport, increases lipolysis, reduced glucose transport and metabolism.

Increases IGF production in the liver and other organs

Question 4

Where is GHRH produced?

Answer 4

In neurons of the hypothalamus. It is then released from the anterior pituitary via the endocrine cells called somatotropic cells.

Question 5

When is GH release at its highest in the circadian cycle?

Answer 5

At night (opposite to cortisol)

Question 6

What is the negative regulator of GH in the hypothalamus + pituitary?

Answer 6

Somatostatin which inhibits GH release from the anterior pituitary gland.

Question 7

What factors act on the hypothalamus to increase and decrease GH release?

Answer 7

Exercise (increases)

Sleep

Starvation (increases)

Question 8

Why is GH released in response to starvation?

Answer 8

GH helps us survive prolonged starvation by switching metabolism away from proteins as a fuel source

Question 9

Does the increase in GH during starvation mean growth can take place in response?

Answer 9

No, Many of the growth effects of GH are stimulated indirectly through the increased production of IGF from the liver (~75%) and other tissues (~25%).

Question 10

What is IGF and what does it do?

Answer 10

Insulin-like Growth Factor (resembles insulin)

IGFs stimulate amino acid uptake and activate protein and DNA synthesis. They are strongly mitogenic and hypertrophic.

This hormone can act in a paracrine and autocrine manner

Question 11

What are the types of IGF in adults and foetuses?

Answer 11

Adult form: IGF-1

Foetal form: IGF-II

Question 12

How are IGF and GH different and similar?

Answer 12

They both stimulate growth pathways via different secondary messenger systems.

GH doesn’t really stimulate proliferation whereas IGF does.

Question 13

How is IGF-1 increased? How does GH affect this?

Answer 13

In response to feasting.

GH potentiates GH effects but not GH release.

GH cannot increase IGF levels in the absence of insulin

Question 14

What is necessary for IGF release?

Answer 14

GH and insulin must both be present to increase IGF-1 levels.

No insulin = No IGF

Question 15

Why does excess GH cause gigantism?

Answer 15

Because GH is important for growth before adulthood and higher circulating GH results in faster growth.

Excess GH results in gigantism and deficiencies cause dwarfism.

Question 16

What happens if GH is produced in excess after puberty?

Answer 16

Thickening of bones of the hands, feet and jaw causing a disease called acromegaly

Question 17

What are the effects of agromegaly?

Answer 17

Cartilage enlarges increasing the size of nose and ears this growth can impinge on certain nerves.

Visual-field losses due to pressure on optic nerves

Bell’s palsy due to pressure on facial nerve

Carpal tunnel syndrome

Spreading teeth and bite difficulties

Joint and bone aches

Soft tissues enlarge

Hypertension also common

Question 18

What factors can cause under-secretion of GH?

Answer 18

Tumours of the hypothalamus over-secrete somatostatin

Infections can affect hypothalamic GHRH or somatostatin and alter GH release

GH secretion is susceptible to irradiation

Question 19

What causes the baka pygmy people to be smaller in size than normal?

Answer 19

Lack of IGF production (they have normal GH levels)

Question 20

What causes Laron-type dwarfism?

Answer 20

Reduced GH receptor expression

Question 21

What other hormones act with GH for growth?

Answer 21

Steroid hormones (androgens, estrogen)

Thyroid hormones (adequate levels of TH are required for normal growth. Thyroid hormones act primarily through stimulation of GH secretion and production. TH can also increase responsiveness of target cell to GH)

Question 22

What does thyroid hormone do to GH activity?

Answer 22

They potentiate GH at several levels.

GHRH receptors are more sensitive

Pituitary cells produce more GH

Target cells are more responsive to GH

Question 23

What is GH treatment used for?

Answer 23

Recombinant hGh developed to treat GH deficiencies in children as well as cachexia and muscle wasting disorders.

Hormone replacement therapy uses recombinant hGH to reverse the reduced lean muscle and bone mass associated with aging

Question 24

What do GHRH peptides do?

Answer 24

Act on pituitary glands to stimulate release of GH