Endocrine physiology - Growth

Question 1

Factors that affect growth.

Answer 1

• growth hormone
• insulin
• sex steroids
• availability of nutrients
• stress
• genetics

Question 2

Other name for growth hormone.

Answer 2

Somatotropin

Question 3

How is the release of growth hormone controlled?

Answer 3

Growth hormone is released from the anterior pituitary. Controlled via the release of two hypothalamic neurohormones with oposing action:

• GHIH (growth hormone inhibiting hormone) (somatostatin)
• GHRH ( growth hormone releasing hormone)

Question 4

Actions of growth hormone.

Answer 4

• growth and development (indirect action)
• regulation of metabolism (direct action)

Question 5

Control of growth in early life.

Answer 5

Growth in the foetal period and the first 8-10 months of life is largely controlled by nutritional intake, but thereafter GH becomes the dominant influence on the rate at which children grow.

Question 6

Growth hormone requires the permissive action of:

Answer 6

Thyroid hormone and insulin. Children with untreated hypothyroidism and poorly controlled diabetes, have stunted growth despite normal GH levels.

Question 7

What is the role of growth hormone in adults?

Answer 7

Maintainance and repair of tissue.

Question 8

How is the effect of growth hormone achieved?

Answer 8

The effect of growth hormone is almost entirely indirect, being acheived through the action of an intermediate known as insulin like growth factor (IGF-I) aka somatomedin.

Question 9

From where is IGF-1 secreted?

Answer 9

Secreted by the liver.

Question 10

How is growth hormone controlled by negative feedback?

Answer 10

IGF-I contols GH release through a negative feedback loop. (IGF-I inhibits GHRH and stimulates GHIH)

Question 11

IGF-II

Answer 11

Functional importance appears to be limited to the foetus and the neonate.

Question 12

How are GH and IGF-I transported and what effect does this have?

Answer 12

Transported in the blood bound to carrier proteins. Around 50% is in the bound form. This provides a reservoir of GH in the blood which helps to:

• smooth out the effects of the erratic pattern of secretion
• extends half life by protecting from excretion in the urine

Question 13

Why do epiphyseal plates close during adolescence?

Answer 13

They close under the infulence of sex steroid hormones, then no further longitudinal growth is possible.

Question 14

How does growth hormone affect metabolism?

Answer 14

• mobilises glucose stores, to increase blood [glucose].
• inhibits the action of insulin (by reducing the number of insulin receptors in muscle and adipose tissue) thus augmenting the incresased blood [glucose].
• promotes lipolysis, providing a source of energy for most cells of the body, sparing glucose and again augmenting increased blood [glucose].
• promotes amino acid uptake to cells, supporting protein synthesis.

Question 15

Stimuli that increase GHRH secretion (↑GH)

Answer 15

• actual or potential ↓ energy supply to cells
• increased amounts of amino acids in the plasma
• stressful stimuli
• delta sleep
• oestrogen and testosterone

Question 16

Stimuli that increase GHIH secretion ( ↓GH)

Answer 16

• glucose
• free fatty acids
• REM sleep
• cortisol

Question 17

Cretinism

Answer 17

Cretinism is a condition where children are hypothyroid from birth. They have retarded growth because of the loss of TH’s permissive action of GH. They retain infantile facial features = hypothyroid dwarf. GH levels are normal.

Question 18

In humans there are 2 periods of rapid growth:

Answer 18

• infancy - amazing growth spurts of 2.5cm in a few days and then nothing
• puberty - due to androgens and oestrogens produce spikes in GH secretion that ↑IGF-I → ↑ growth

Question 19

What two conditions are caused by hyperscretion of growth hormone?

Answer 19

• Gigantism - before epiphyseal plates of long bones close → excessive growth -
• Acromegaly - after epiphyseal plates have sealed.