lecture 26 - The pancreas, the hypothalamus and pituitary, growth hormone

Question 1

Exocrine gland

Answer 1

secrete substances out and external to the body (the gut is consider the outside of the body)

e.g. cells of the pancreatic acini secrete digestive enzymes

Question 2

Endocrine gland

Answer 2

secrete their products, hormones, directly into the blood.

Question 3

Beta cells

Answer 3

Produces and secretes the hormone insulin. Insulin lowers blood glucose levels by increasing the rate of glucose uptake and use by most body cells, and by increasing glycogen synthesis in skeletal muscles and liver

Question 4

Alpha cells

Answer 4

Produces and secretes the hormone glucagon. Glucagon raises blood glucose levels by increasing the rates of glycogen breakdown and glucose release by the liver

Question 5

Pancreas

Answer 5

The pancreatic islets secrete hormones regulating the rate of glucose uptake and utilisation by body tissues

Question 6

Pancreatic islets

Answer 6

Endocrine gland

Question 7

Blood glucose concentration

Answer 7

Must be maintained within a narrow range at all times for normal functions. If it is too high for too long, diabetes develops. If too low, hypoglycaemia occurs. The brain must be supplied with glucose at all times, as glucose is the only fuel that the brain uses but blood glucose concentration changes throughout the day as we use fuel continuously but we eat intermittently

Question 8

Fed state

Answer 8

Cellular uptake of nutrients and anabolic metabolism (synthesis of glycogen, protein and fat)
Mediated by insulin

Question 9

Fasting state

Answer 9

Mobilisation of nutrients and catabolic metabolism (breakdown of glycogen, protein and fat)
Mediated by glucagon

Question 10

Two metabolic states during the day

Answer 10

Fed state and fasting state

Question 11

How is blood glucose regulated?

Answer 11

Hormonal regulation - insulin and glucagon maintain blood glucose concentration between 70-110 mg dL^-1 (reference range)

Insulin secretion is increased when blood glucose concentration increases and glucagon secretion increases when blood glucose concentration decreases.

Question 12

Insulin

Answer 12

Insulin is a peptide hormone therefore target cells have membrane receptors

The increase in blood glucose concentration stimulated the pancreatic islet beta cells to secrete insulin into the bloodstream (beta cells are the sensor and control centre)

This hormone in the target cells …. In muscle and adipose cells it stimulates the uptake of glucose (storage of glucose), increases amino acid uptake, glycogen and protein synthesis and adipose tissue is going to utilise the glucose to form fat. In the liver cells the glucose output stops and this stimulus stimulates the increase in net glucose uptake which causes glycogen and fat synthesis

Overall this leads to decrease in blood glucose concentration and this stimulus acts as negative feedback to inhibit the loop from continuing despite the stimulus of high blood glucose concentration now being gone (response reduces the stimulus)

Note that insulin is the only hormone that can decrease the blood glucose concentration

Question 13

Insulin overall causes

Answer 13

Insulin overall increases fuel storage (glycogen)

Question 14

Glucagon

Answer 14

Glucagon is a peptide hormone therefore target cells have membrane receptors

The decrease in blood glucose concentration stimulates the pancreatic islet alpha cells to secrete glucagon into the bloodstream. (alphas cells are the sensors and the control centre)

This hormone in the liver cell (target cell) stimulates the increase breakdown of glycogen (glycogenolysis), increase in glucose synthesis (gluconeogensis) and increase in ketone synthesis(ketones are used by some cells to create energy)

Overall this leads to an increase in blood glucose concentration and an increasing in blood ketone concentration (this happens so that the blood glucose can be directed primarily towards the brain) . This stimulus acts as a negative feedback to inhibit the loop from continuing since the stimulus of low blood glucose is gone (response reduces the stimulus)

Not that growth hormone, adrenaline and cortisol can also increase blood glucose concentration

Question 15

Glucagon overall causes

Answer 15

Increased fuel release

Question 16

Glycogenolysis

Answer 16

Breakdown of glycogen

Question 17

Gluconeogenesis

Answer 17

Glucose synthesis

Question 18

Glycogen

Answer 18

Stored form of glucose

Question 19

Pituitary gland

Answer 19

Located at the base of the brain and is attached to the hypothalamus. The hypothalamus controls the secretion of the pituitary hormones (when activated by neural input, the hypothalamus stimulates the pituitary glad to secrete hormones

Some pituitary hormones stimulate target cells and some stimulate the secretion of hormones by other endocrine glands

Has two parts - anterior lobe and posterior lobe and each part has a distinctive mechanism for hormone secretion

Question 20

Hypothalamus

Answer 20

Controls the secretion of the pituitary glands

Question 21

Anterior lobe of the pituitary gland

Answer 21

Connected to the hypothalamus by blood vessels
Neural input to hypothalamus triggers the release of inhibiting or releasing hormones into blood vessels
Hormones travel via blood vessels to anterior pituitary
Hormones bind to receptors in anterior pituitary
Triggers release of another hormone into blood circulation

Question 22

Posterior lobe of the pituitary gland

Answer 22

Connected to the hypothalamus by neurons and secretes hormones into the blood

Connected to the hypothalamus directly through neurons
Cell bodies in hypothalamus and axon terminals in posterior pituitary
When action potentials reach the axon terminal, they do not synapse onto another neuron but instead release their own hormones into the blood

Question 23

How does the hypothalamus communicate with the posterior pituitary?

Answer 23

The hypothalamus is connected to the posterior lobe by neurons - cell bodies in the hypothalamus and axons terminate in the posterior lobe.

Posterior lobe hormones are made in the hypothalamus (cell body of neuron), travel down the axon and are stored at the axon endings until requires (peptide hormones)

The hypothalamus uses neural communication with the posterior lobe to release hormones into the blood

Question 24

Hormones released by the posterior pituitary?

Answer 24

ADH (antidiuretic hormone) promotes water retention in kidneys (stimulates the kidneys to reabsorb water, the kidneys conserve water when the body dehydrates)
Oxytocin stimulates uterine muscle contraction during childbirth (an example of positive feedback) and milk release

Both of these hormones are made in the hypothalamus and stored in the posterior pituitary until required

Question 25

How does the hypothalamus communicate with the anterior pituitary?

Answer 25

Stimulus is the neural input within the hypothalamus.

The hypothalamus signals the anterior pituitary to release hormones by hormonal stimulation, secreting stored releasing hormones (or inhibiting hormones)

Hormone binds to receptor of membrane of a specific cell type and a specific peptide hormone is secreted e.g. prolactin, growth hormone

Question 26

Feedback regulation of the anterior pituitary

Answer 26

In the hypothalamus the release of a releasing hormone stimulates the anterior pituitary gland to release the pituitary hormone which stimulated the target organ to release its hormone

There are negative feedback loads which prevents too much releasing hormone being release (either by the pituitary hormone or the releasing hormone itself in some cases)

Question 27

Anterior pituitary hormones

Answer 27

Growth hormone, Thyroid stimulating hormone, prolactin, ACTH

Question 28

Growth hormone

Answer 28

Growth hormone (GH), also called somatotropin or human growth hormone, peptide hormone secreted by the anterior lobe of the pituitary gland. It stimulates the growth of essentially all tissues of the body, including bone.

Question 29

Direct and indirect effects of growth hormone

Answer 29

Direct effects
Muscle - stimulates protein synthesis and inhibits cellular uptake of glucose (long term)
Liver - stimulates glucose synthesis (short term)
Fat- increases triglyceride breakdown in adipose tissue (short term)

Indirect effects of GH
Promotes the growth of bones, muscles and other tissues by causes the release of somatomedin C, which promotes cell division (long term)

Summary - long term effects on growth and short term effects on metabolism

Question 30

Feedback regulation of growth hormone

Answer 30

The hypothalamus has somatostatin neurons and GHRH neurons.

GHRH stimulates the release of GH from the anterior pituitary
Somatostatin inhibits the release of GH from the anterior pituitary.
GH feeds back to the hypothalamus preventing the release of more GH (negative feedback)
Somatomedin C/IGF1 produced by the liver feeds back to inhibit GHRH and promote somatostatin release from the hypothalamus

GH has effects one the muscle, liver and fat as well as indirect effects.

Question 31

GH concentrations over a day and a lifetime

Answer 31

Have peaks and troughs throughout the day and GH concentrations are highest during sleep

GH concentrations are higher in children than in adults, with a peak period during puberty. GH secretion declines with ageing therefore worsening wound repair for adults.

Question 32

Somatomedin C

Answer 32

A hormonal growth factor

Effects- feedsback to the hypothalamus to inhibit further release of GHRH, interestingly also feedback to the somatostatin neurons which releases GHIH

Question 33

Posterior lobe versus anterior lobe of the pituitary

Answer 33

The hypothalamus is connected to the posterior lobe by axons and to the anterior lobe by blood vessels

Posterior pituitary hormones are made in the hypothalamus but stored in the posterior pituitary. Action potentials cause the release of posterior pituitary hormones.

Anterior pituitary hormones are made in the anterior pituitary by specific cells (different cells produce different hormones). The hypothalamus communicated with the anterior lobe using releasing hormones and inhibiting hormones.

Pituitary hormones bind to receptors on the membrane of specific cells and a specific response is exhibited by those cells (this may be a direct cellular response or the release of a further hormone)

Question 34

Too much GH

Answer 34

Likely to see more SS and less GHRH

Likely to get additional signalling to the liver, muscle and fat cells. Get additional formation of somatomedin C

Question 35

Too little GH

Answer 35

Not enough somatomedin C (or signalling to liver, muscle and fat cells)