5.4: Hormonal communication

What are hormones?

A chemical messenger secreted into the blood that affects target cells.

Where are hormones produced?

Endocrine glands

What is an endocrine system?

Communication via hormones transported in the blood to target cells.

What are endocrine glands?

contain cells that produce a hormone and release it straight into the blood. They are ductless

What are exocrine glands?

they produce a hormone which is not released directly into the blood. They have a duct which carries the secretion to another place

What are the two types of hormones?

peptide/protein hormones and steroid/lipid based hormones

What are peptide hormones?

hormones that are not soluble in the membrane so do not enter the cell. They use the first messenger second messenger model e.g adrenaline

What are steroid hormones?

they can pass through the membrane and enter the cell. They act upon the DNA in the nucleus e.g oestrogen

Name the key glands in the body, where they are located and what they do?

• pituitary gland - controls other endocrine glands

• thyroid gland - releases thyroxine to regulate metabolic rate

• thymus - releases thymosin to stimulate the release of t cells

• adrenal gland - release adrenaline to prepare body for ‘fight or flight’

• pancreas - releases insulin and glucagon to control blood glucose conc

• ovaries - release oestrogen to control menstrual cycle and pregnancy

• testes - release testosterone to stimulate sperm production and male characteristics

What is a target cell?

A cell with receptors specific to a hormone

Why is endocrine communication slower than nervous communication?

Hormones travel in blood rather than electrical impulses

Why are endocrine effects longer lasting?

Hormones remain in circulation longer

What is a first messenger?

A hormone that travels in the blood and binds to a receptor on a target cell

Where does the first messenger bind?

To receptors on the cell surface membrane

What happens when a hormone binds to its receptor?

The receptor changes shape (conformational change) - complementary

What is activated immediately after the receptor changes shape?

A G protein

What does the G protein activate?

adenyl cyclase

What does adenylate cyclase do?

Converts ATP into cAMP (cyclic AMP)

What is a second messenger?

A molecule inside the cell that relays and amplifies the signal from the first messenger - cAMP in this case

What does cAMP do?

Activates protein kinases, triggering an cascade of enzyme-controlled reactions that alter the activity of the cell

Where are adrenal glands located?

Above the kidney

What are the two regions of the adrenal gland?

Cortex (outer) and medulla (inner)

What are the layers of the adrenal cortex?

1. zona glomerulosa

2. zona fasciculata

3. zona reticularis

What is zona glomerulosa?

the outermost layer which secretes mineralocorticoids such as aldosterone

What is zona fasciculata?

the middle layer, which secretes glucocorticoids such as cortisol

What is the function of cortisol?

Regulates metabolism and increases blood glucose

How does cortisol increase blood glucose?

Stimulates gluconeogenesis

What is zona reticularis?

the innermost layer, which is thought to secrete precursor molecules that are used to make sex hormones

How does the adrenal cortex produce a range of hormones?

using cholesterol

What are hormones produced by the adrenal cortex?

steroid based and are able to enter cells directly by dissolving into the cell surface membrane. The steroid hormones enter the nucleus and have a direct effect on the DNA to cause protein synthesis

Describe the action of steroid hormones?

1. passes through cell membrane of target cell

2. binds with specific receptor (complementary shape) in the cytoplasm

3. receptor-steroid hormone complex enters the nucleus and binds to another specific receptor on the chromosomal material

4. binding stimulates the production of mRNA, which code for the production of proteins

What is the role of mineralocorticoids?

Maintain blood pressure by balancing salt (sodium and potassium) and water in the blood and body fluids.

What does aldosterone do specifically as a mineralocorticoid ?

acts on cells on the distal tubules and collecting ducts in the kidney. It increases absorption of sodium ions and increases water reabsorption to help maintain blood pressure

What is the role of glucocorticoids?

Regulate metabolism by controlling the conversion of fats, proteins, and carbohydrates to energy

What does cortisol do specifically as a glucocorticoid?

released in response to stress or as a result of low blood glucose concentration. It stimulates the production of glucose from stored compounds (e.g glycogen) in the liver

What is the role of androgens?

taken up by ovaries and converted to sex hormones (testosterone and oestrogen). They then help the development of secondary sexual characteristics and regulate the production of gametes

Which hormones does the adrenal medulla secrete?

adrenaline (epinephrine) and noradrenaline (norepinephrine). both involved in the ‘fight or flight response’

What kind of hormone is adrenaline?

a peptide hormone so it can’t diffuse through the membrane and must use the first messenger second messenger model

What are some of the effects of adrenaline?

• increases heart rate

• increases ventilation rate - more o2 enters blood

• dilates bronchioles - improves airflow to lungs

• stimulates glycogenolysis (glycogen → glucose)

• dilates the pupils

• inhibiting gut action

What type of gland is the pancreas?

A mixed gland meaning it has both exocrine and endocrine functions

What is the endocrine function of the pancreas?

hormones which are secreted from islets of Langerhans into the blood

What is the exocrine function of the pancreas?

pancreatic juices containing enzymes which are secreted into the small intestine via the pancreatic duct

What are clusters of exocrine cells called?

Acinus cells

How do acinar cells appear in stained sections?

Darkly stained

How do islets appear under a microscope?

Pale-staining clusters

What is the normal blood glucose range?

About 4–6 mmol dm⁻³

Why must blood glucose be regulated?

To prevent damage and ensure sufficient respiration. High bgc lowers water potential meaning cells could shrink and lead to cells shrinking- tiredness and thirst

What factors can increase blood glucose concentration?

• eating a carbohydrate-rich diet

• glycogenolysis

• gluconeogenesis

What factors can decrease blood glucose concentration?

• exercise

• respiration

• glycogenesis

Where are glucose regulation hormones released?

pancreas - islets of langerhans - alpha and beta cells

What do alpha cells release?

glucagon, raises blood glucose concentration

What do beta cells release?

insulin, lowers blood glucose concentration

What triggers insulin secretion in beta cells?

An increase in blood glucose concentration

How does glucose enter beta cells?

By facilitated diffusion, using a channel protein down the glucose concentration gradient

What happens to glucose once inside the beta cell?

It is metabolised in respiration releasing energy in the form of ATP

What effect does ATP have on potassium (K⁺) channels?

it binds to them causing them to close

What happens to potassium ions when K⁺ channels close?

Potassium ions cannot leave the cell

What is the effect of K⁺ ions remaining in the cell?

The membrane becomes depolarised (potential becomes less negative)

What does depolarisation of the membrane cause?

Voltage-gated calcium (Ca²⁺) channels to open

What happens when calcium channels open?

Calcium ions diffuse into the cell

What does the influx of Ca²⁺ ions trigger?

Exocytosis of insulin-containing vesicles into the blood stream

What is hypoglycaemia?

abnormally low levels of glucose in the blood; it results in inadequate delivery of glucose to the body tissues in the blood. Can lead to tiredness and even death

What is hyperglycaemia?

where blood glucose concentrations remain high for longer than normal periods, which can lead to organ damage

What cells do glucagon and insulin act on?

liver cells (hepatocytes) which can store glucose in the form of glycogen

What happens once insulin is released if blood glucose is too high?

1. insulin binds to complementary receptors on hepatocytes

2. it activates intracellular enzymes

3. causes vesicles containing glucose channels to move to membrane so more glucose can enter the cell

4. glycogenesis enzymes activated

5. more glucose converted to fats and used in respiration

What is glycogenesis?

the conversion of glucose to glycogen for storage

What happens once glucagon is released if blood glucose is too low?

1. glucagon binds to complementary receptors on hepatocytes

2. receptor activates adenyl cyclase which converts ATP into cAMP

3. cAMP (second messenger) activates a series of enzyme controlled reaction

4. glycogenolysis

5. gluconeogenesis

What is glycogenolysis?

glycogen converted to glucose

What is gluconeogenesis?

process where amino acids and fats are converted into additional glucose

Why are glucagon and insulin antagonistic?

they have opposite effects on blood glucose concentration

Why is controlling blood glucose concentration as example of negative feedback?

because it works to bring blood glucose concentration back to the optimum

What is diabetes mellitus?

a condition in which blood glucose concentrations can’t be controlled effectively - it can lead to hyperglycaemia or hypoglycaemia

What is type 1 diabetes?

Autoimmune destruction of beta cells (may also result from a viral attack)

What happens to insulin production in Type 1 diabetes?

Little or none is produced meaning the body can’t store excess glucose and glycogen

When does type 1 diabetes usually develop?

in childhood

What is type 2 diabetes (non-insulin-dependent)?

body cells become resistant to insulin and/or insufficient insulin is produced

Why do body cells become less resistant to insulin?

because the specific receptors on the surface of the liver and muscle cells become less responsive

When does type 2 diabetes usually develop?

late in life due to different factors like obesity (also family history, lack of regular exercise etc)

What are some treatments for type 1 diabetes?

• regular insulin injections

• insulin pump therapy - device constantly pumps insulin into blood through a permanent needle in skin

• islet cell transplant (specifically of beta cells)

• complete pancreas transplant

• exercise and less reducing sugar consumption

• stem cells to grow new islets of Langerhans

What are some treatments for type 2 diabetes?

• lifestyle - diet, exercise, weight loss

• medications to increase cells’ sensitivity to insulin

• medication to stimulate more insulin production

• some cases, insulin therapy

How else can humans get insulin?

from genetically modified bacteria

What are some of the benefits of using GM bacteria?

• avoids ethical and religious concerns associated with animal-derived products

• exact copy of human insulin - faster acting + more effective

• lower risk of infection

• less chance of rejection due to immune response

• capability to produce insulin in large quantities.

• cheaper to manufacture