Session 7: Introduction to the Endocrine System and Pancreatic Hormones Flashcards
What does the control system in the body maintain?
Homeostasis
What variables are regulated at an optimal level in homeostasis?
Blood glucose levels, body temperature
What is established for each variable which is monitored accordingly?
Set point
What three parts does the control system of the body consist of?
1) Receptor 2) Control centre 3) Effector
What are the control system requirements?
- Receptor e.g., thermoreceptor
- Control centre e.g., hypothalamus
- Effector e.g., sweat glands
What is the circadian rhythm?
The biological clock; regular bodily rhythms that occur on a 24-hour cycle
What two hormones affect the circadian rhythm?
- Melatonin
- Cortisol
circadian rhythm is a natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours.
What can cause the set point to vary over a 24h period?
Circadian rhythm
What are the endocrine glands in the head and neck?
1) Pituitary gland 2) Pineal gland 3) Thyroid gland 4) Parathyroid glands
What is the role of the hypothalamus?
Links the nervous system to the endocrine system via the pituitary gland
What are the endocrine glands in the abdomen?
- Adrenal glands: cortex and medulla
- Pancreas
- Kidney
- Gut
What are the endocrine glands in the pelvis?
- Gonads (ovaries, testes)
- Uterus
- Placenta
What are the four groups of hormones?
1) Peptide/polypeptide hormones 2) Glycoprotein hormones 3) Amino acid derivatives 4) Steroid hormones
What type of loop does the control system use to maintain levels for homeostasis?
Feedback loop
What is the most common type of feedback loop?
Negative feedback
What is a negative feedback loop?
- The effect of the response to stimulus = is to decrease its effect
- The effector is switched off when value reaches the set point
Give an example of a negative feedback loop in the human body.
Thermoreceptors-hypothalamus-sweat glands = body temperature regulation
Describe the body temperature regulation negative feedback loop.
1) Body temperature exceeds 37 degrees Celsius
2) Thermoreceptors in the skin and brain sense this
3) Temperature regulatory centre in hypothalamus detects this
4) Sweat glands throughout body activated
5) Decrease of temperature back to optimum set point (37 degrees Celsius)
What is a positive feedback loop?
- Effect of the response to stimulus = increase its effect
- The effector is not switched off
- Control system goes out of control leading to = catastrophic change
Give examples of positive feedback loops in the body.
- Blood clotting
- Ovulation
- Lactation
What are the three routes of communication for a control system?
1) When they come into contact = cell surface chemicals
2) When they are close = local diffusion
3) When they are far apart = hormones/neurotransmitters
Hormones are chemical signals produced in what type of glands?
Endocrine glands
How do hormones travel to affect other tissues?
In the bloodstream
What is endocrine signaling?
Endocrine signaling uses the circulatory system to transport ligands
What is paracrine signaling?
Signals diffuse to and affect nearby cells (paracrine = proximity)
What is autocrine signaling?
Referring to a secreted molecule that acts on the cell that secreted it.
What is thyrotropin-releasing hormone (TRH)?
- Peptide/polypeptide hormone
- 3 amino acids long
What is glucagon?
- Peptide/polypeptide hormone
- 29 amino acids long
What is insulin?
- Peptide/polypeptide hormone
- 51 amino acids long
What is growth hormone (GH)?
- Peptide/polypeptide hormone
- 191 amino acids long
Glycoprotein hormones all have how many polypeptide chains?
Two with carbohydrate side chains (α and β chains)
Give four examples of glycoprotein hormones.
1) Thyroid stimulating hormone (TSH)
2) Follicle stimulating hormone (FSH)
3) Luteinizing hormone (LH)
4) Human chorionic gonadotropin (HCG)
Briefly describe how peptide/polypeptide and glycoprotein hormones are synthesised and secreted.
1) Synthesised as larger precursor molecules = prepro-hormones and pro-hormones
2) Pro-hormones stored in secretory vesicles before release
3) Cleaved to → active hormone and released from vesicles
4) Hydrophilic
List the three amino acid derivative hormones.
1) Thyroid hormones (T4 or Thyroxine) and T3 2) Adrenaline
All three of the amino acid derivative hormones are derived from what?
Tyrosine
Where is adrenaline stored?
In vesicles in the adrenal medulla in chromaffin cells
Is adrenaline hydrophilic or hydrophobic?
Hydrophilic
What is the name of the cells in which adrenaline hormone vesicles are stored?
Chromaffin cells
How are thyroid hormones stored?
As thyroglobulin (prohormone) extra-cellularly in follicles in the thyroid gland as colloid
Are thyroid hormones hydrophilic or hydrophobic?
Hydrophobic
Are steroid hormones stored by cells?
No
How are steroid hormones synthesised?
On demand from cholesterol esters
Are steroid hormones hydrophobic or hydrophilic?
Hydrophobic
What controls the rate of hormonal secretion?
A negative feedback loop
Explain the parathyroid hormone (PTH) secretion negative feedback loop briefly.
1) PTH secretion is stimulated by decreased blood calcium level
2) PTH acts on bone and kidney to increase blood calcium level
3) Increased blood calcium level reduces PTH’s own secretion
What stimulates PTH secretion?
A decreased calcium level in the blood
What does PTH act on to increase blood calcium level?
Bone and kidney
What is a tropic hormone?
A hormone that stimulates the secretion of another hormone
Give an example of a tropic hormone.
TSH is a tropic hormone (thyroid-stimulating hormone) which is secreted by the anterior pituitary gland stimulates the thyroid gland to secrete thyroid hormones
What does the hypothalamus stimulate or inhibit?
Pituitary gland hormone secretion via hypothalamic releasing hormones
What is hypoglycaemia?
‘Four to the floor’
What is hyperglycaemia (fasting)?
> 7
What is hyperglycaemia (random)?
> 11
Which hormones are able to travel in the blood?
- Peptide/polypeptide hormones
- Glycoprotein hormones
- Adrenaline (amino acid derivative hormone)
Which hormones must bind to a carrier to be transported in blood?
- Steroid hormone
- Thyroid hormone
Where does inactivation of hormones usually occur?
In the liver
How are peptide, polypeptide and glycoprotein hormones inactivated?
- Degraded to amino acids
- Reused for protein synthesis
How are steroid hormones and amino-acid derivative hormones inactivated?
- Small changes in their structure
- Recycled or excreted
What is the pancreas?
Organ of digestive (exocrine) and endocrine systems
How large is the pancreas in a healthy adult?
Weight ~200-300g, length ~15-20cm
How is the pancreas anatomically divided?
1) Head 2) Body 3) Tail
How is the pancreas functionally and histologically divided?
1) Endocrine = 2% (Insulin, Glucagon)
2) Exocrine = 98% (digestive enzymes)
What function does the majority of the pancreas have?
Exocrine (98%)
What function does the minority of the pancreas have?
Endocrine (2%)
What is the anatomy and vasculature of the pancreas?
Artery near duodenum = common hepatic artery
The artery inside pancreas = splenic artery
Artery exiting pancreas = superior mesenteric artery
How does the pancreas develop?
- Ventral pancreatic bud and dorsal pancreatic bud
- Rotation of the ventral duct (bud)
- Fusion of the buds
What are the functions of the pancreas?
1) Exocrine 2) Endocrine
What does the exocrine pancreas do?
Secretes digestive enzymes (amylases, lipases, proteases etc)
What does the endocrine pancreas do?
Produces polypeptide hormones
What polypeptide hormones does the endocrine pancreas produce?
1) Insulin & Glucagon = regulate blood glucose
2) Somatostatin = inhibits pancreatic hormones
3) Ghrelin and amylin = regulate appetite
What endocrine cells are found in the pancreas?
Islets of Langerhans = Alpha, Beta
What exocrine cells are found in the pancreas?
Acini
What are the cells of the islet of Langerhans in the pancreas that function to regulate glucose homeostasis?
1) Alpha-cells = glucagon-secreting
2) Beta-cells = Insulin-secreting
3) Delta-cells = Somatostatin-secreting
What cells of the islet of Langerhans in the pancreas function to regulate appetite?
- Ghrelin-secreting cells (<1%)
- Pancreatic polypeptide-secreting PP cells (>1%)
What is insulin?
Hormone produced by beta-cells that lowers blood glucose
What is glucagon?
Hormone produced by alpha-cells that increases blood glucose
What tissues are insulin-sensitive?
Adipose, skeletal muscle and liver
Compare and contrast actions and metabolic effects of insulin and glucagon.
Insulin affects metabolism of lipids, proteins and carbohydrates while glucagon affects only metabolism of lipids and carbohydrates.
Briefly describe how insulin is synthesised.
1) Preproinsulin gene transcribed to m-RNA in nucleus.
2) Preproinsulin m-RNA moves to cytoplasmic ribosomes where N-terminal peptide is formed. N-terminal peptide allows transport of ribosomes with m-RNA to RER
3) Preproinsulin m-RNA translated to preproinsulin
4) N-signal peptide is cleaved forming = proinsulin
5) Proinsulin transported to the Golgi where it is cleaved into insulin and C peptide
6) Insulin and C-peptide are packaged into secretory vesicles for storage
What is the insulin secretion process?
1) Margination = secretory vesicles move along microtubules towards plasma membrane of beta-cell
2) High blood glucose means glucose transported into beta cell by facilitated diffusion (GLUT2).
3) Influx of calcium into beta cell
4) Increased intracellular calcium leads to contraction of microfilaments
5) Vesicle membranes fuse with the plasma membrane
6) Insulin and peptide C are released from the vesicle via exocytosis
What is the difference between negative and positive feedback loop?
Negative feedback loops decrease the effect of a stimulus, while positive feedback loops increase the effect.
What is a hormone?
Chemical signaling molecules produced by the endocrine glands and secreted directly into the bloodstream.
What are the three key features of hormones?
1) Must be able to travel to all parts of the body in 30 seconds
2) Different effects in different places
3) Effect of the hormone on the target cell depends on its concentration in the blood
What does the structure of a hormone define?
1) Produced
2) Transported in the blood
3) Interacting with cell receptors
4) Inactivated
What is margination in the secretion of insulin?
Secretory vesicles move along microtubules towards the plasma membrane of the beta-cell
What is the name of the glucose transporter on beta cells?
GLUT2
How does insulin exert its effects?
Insulin receptors are tyrosine kinase receptors
What do insulin receptors have?
Alpha-units and beta-units
How is glucagon synthesised in alpha cells?
Glucagon is synthesised as a large precursor molecule known as pre-pro-glucagon
How do secretory granules of glucagon move to the cell surface of alpha-cells?
By a process known as margination
How are secretory granules of glucagon released from alpha-cells?
Via a process known as exocytosis
How does glucagon exert its effects on glucagon receptors?
Glucagon receptors are G-protein coupled receptors (GPCR)
What activates adenylate cyclase in the glucagon receptor?
The alpha-subunit of GPCR (glucagon receptor) activates adenylate cyclase which prompts the production of cyclic-AMP
What condition is associated with low blood glucose?
Diabetes mellitus
What is the clinical sign of abnormal insulin/glucagon levels?
Name hormone of which secretion is controlled by circadian rhythms: cortisol
Give an example of positive feedback in the body.
Blood clotting cascade
Is the statement true or false? Parathyroid hormone (PTH) is secreted when calcium levels in plasma fall. PTH acts on kidneys and bones to increase calcium levels in plasma which reduces secretion of PTH.
True
What type of feedback mainly involves maintenance of homeostasis in the body?
Negative feedback and an example of it is regulation of body temperature.
List the three components of the control system.
1) Receptor 2) Control centre 3) Effector
Give an example of the control centre in the control system of the body.
Hypothalamus
Which hormones need to be bound to a carrier in order to be transported in blood?
Steroids and thyroid hormones
What are thyroid hormones stored as in the thyroid gland?
Thyroid hormones are steroid hormones stored extra-cellularly in follicles in thyroid gland as a fluid called colloid. It contains pro-hormone called thyroglobulin.
What does the endocrine pancreas secrete?
Hormones
What does the exocrine pancreas secrete?
Digestive enzymes
Which ion stimulates insulin secretion from beta cells?
Ca2+
How does insulin secretion occur?
b) Exocytosis
Where does the packaging of insulin in secretory vesicles take place?
Golgi apparatus
What does preproinsulin contain?
A, B, C chains and N-terminal peptide
What type of hormones are insulin and glucagon?
They are polypeptide hormones. They are hydrophilic and because of this they are freely transported in the blood.
What is secreted by pancreatic alpha cells?
Glucagon
What is the name of the pancreatic cell which secretes digestive enzymes?
Acinar cells
What happens to the beta subunit of insulin receptor following binding?
Becomes active tyrosine kinase which undergo autophosphorylation.
Which tissue is the most affected by glucagon?
Liver
