Basic Physiology of Pituitary Disease

Question 1

What does the hypothalamus do?

Answer 1

It secretes neurohormones

Question 2

Where do the trophic hormones from the hypothalamus go? And what is their role?

Answer 2

They are secreted into capillaries travelling to the anterior pituitary to govern release of anterior pituitary hormones

Question 3

Give examples of trophic hormones released from the hypothalamus (and state what kind of hormone they are)

Answer 3

• Thyrotropin releasing hormone (TRH) (peptide)
• Corticotrophin releasing hormone (CRH) (peptide)
• Growth hormone releasing hormone (GHRH) (peptide)
• Gonadotrophin releasing hormone (GnRH) (peptide)
• Growth hormone inhibiting hormone (GHIH) = somatostatin (peptide)
• Prolactin inhibiting hormone (PIH) (dopamine)

Question 4

Where do the non-trophic hormones from the hypothalamus go?

Answer 4

They travel to posterior pituitary via neuronal axons in the infundibulum where released into blood

Question 5

Explain the pathway from the stimulus to the response (without any feedback loops)

Answer 5

Stimulus –> Hypothalamus (Integrating centre 1) –> Trophic hormone (H1) –> Anterior pituitary (IC2) –> Trophic hormone (H2) –> Endocrine gland (IC3) –> Hormone (H3) –> target tissue –> response

Question 6

Explain the short-loop negative feedback of the stimulus to response pathway

Answer 6

Trophic hormone (H2) when in excess inhibits the Hypothalamus (IC1) so it no longer produces Trophic hormone (H1)

Question 7

Explain the long-loop negative feedback of the stimulus to response pathway

Answer 7

Hormone (H3) when in excess inhibits both the hypothalamus (IC1) and the anterior pituitary (IC2)

Question 8

describe the anterior pituitary gland

Answer 8

true endocrine tissue of epithelial origin; releases peptide hormones, all of which are tropic (promote growth) except prolactin

Question 9

give examples of hormones released from the anterior pituitary gland

Answer 9

o Thyroid stimulating hormone (TSH) = thyrotropin
o Adrenocorticotrophic hormone (ACTH) = corticotropin
o Follicle stimulating hormone (FSH) and luteinizing hormone (LH) = gonadotropins
o Growth hormone (GH) = somatotropin
o Prolactin

Question 10

describe the posterior pituitary gland

Answer 10

A neuroendocrine tissue of neural origin.

It releases 2 peptide neurohormones

Question 11

what hormones are released by the posterior pituitary gland?

Answer 11

o Vasopressin = anti-diuretic hormone (ADH)

o Oxytocin

Question 12

describe the growth feedback loop

Answer 12

• GHRH secreted from hypothalamus in response to stimuli
• Triggers GH secretion from anterior pituitary which triggers insulin-like growth factor-I (IGF-1) secretion by the liver and other cell types

Question 13

describe what happens when GHRH is secreted from hypothalamus in response to stimuli in growth feedback loop

Answer 13

o Actual or potential decrease in energy to cells
o Increased amounts of amino acid in plasma
o Stressful stimuli
o Delta sleep
o Oestrogen and testosterone

Question 14

What is the function of the GH in growth feedback loop?

Answer 14

o Stimulates prechrondrocytes in epiphyseal plates differentiate into chondrocytes which begin to secrete and become responsive to IGF-1
o Anti-insulin: increases liver gluconeogenesis, reduces insulin’s ability to stimulate muscle and adipose glucose uptake and makes adipocytes more sensitive to lipolytic stimuli
o BUT increases amino acid uptake and protein synthesis (anabolic as opposed to cortisol which is catabolic)

Question 15

What is the function of IGF-1?

Answer 15

autocrine or paracrine agent to stimulate differentiating chondrocytes to undergo cell division and produce cartilage for bone growth

Question 16

Describe the negative feedback lop on the growth feedback loop

Answer 16

IGF-1 exhibits a negative feedback loop on GH and GHRH by inhibiting GHRH and stimulating somatostatin. Somatotrophs in the anterior pituitary also provide negative feedback on GH

Question 17

In response to what stimuli is Somatostatin secreted from the hypothalamus?

Answer 17

o Glucose
o Free fatty acids
o REM sleep
o Cortisol

Question 18

What are some other influences on growth?

Answer 18

o Thyroid hormones
o Sex hormones
o Nutrition
o Genetic factors

Question 19

In what ways do thyroid hormones influence growth?

Answer 19

essential for growth and effects permissive to GH and IGF-1; required for cartilage ossification and teeth maturation (hypothyroid children retain infantile proportions whereas children with GH deficiency are proportionally normal but just small)

Question 20

In what ways do sex hormones influence growth?

Answer 20

stimulate GHRH at puberty for growth spurt

Question 21

IN what way does nutrition influence growth?

Answer 21

injury and disease stunt growth due to increased cortisol and increased protein metabolism

Question 22

in what way do genetic factors influence growth?

Answer 22

determine maximum growth

Question 23

when is GH mostly released?

Answer 23

GH is mostly released during the first two hours of sleep and secretion rate has rapid spontaneous and stimuli-specific responses

Question 24

how is an erratic pattern prevented with GH and IGF-1?

Answer 24

GH and IGF-1 are peptide hormones but are bound to carrier proteins in the blood (like thyroid and steroid hormones) which provides a reservoir in the bloodstream and prevents an erratic pattern