Endocrine System

Year 1 - Semester 2

How many different types of endocrine cell are found in the islets of Langerhans?

4

Which endocrine cell types are found in the islets of Langerhans?

alpha, beta, delta, F

What do alpha cells secrete?

glucagon

What do beta cells secrete?

insulin

What do delta cells secrete?

somatostatin

What do F cells secrete?

pancreatic polypeptide

basic cell signalling arc

1. source cell provides a signal

2. hormone/cell mediator is released as a signal

3. target cell with specific receptor receives the signal

What are the 3 main types of signalling cell?

autocrine, paracrine, endocrine

How do autocrine cells communicate?

they secrete signalling molecules which then bind to complementary receptors on their own surface membranes so they respond to their own signals

How do paracrine cells communicate?

one cell secretes a signalling molecule which binds to a complementary receptor on a nearby target cell

How do endocrine cells communicate?

one cell releases a hormone which enters the bloodstream and travels through to a complementary receptor on a target cell further away

Where are source endocrine cells typically found?

in the brain

Where is the right lobe of the pancreas found?

within the mesoduodenum

Where is the left lobe of the pancreas found?

within the deep leaf of the greater omentum

How are the exocrine cells of the pancreas found?

in acini

How are endocrine cells of the pancreas found?

in islets of Langerhans

features of active insulin

half-life of 5-8 mins, peptide hormone, water soluble

What type of receptor does insulin bind to?

tyrosine kinase-coupled membrane receptor

What different things can stimulate insulin secretion?

increased blood glucose concentration, increased amino acid concentration in blood, increased GI hormone production, increased parasympathetic stimulation of beta cells, decreased sympathetic stimulation of beta cells, local paracrine inhibition by glucagon

What is the overall effect of insulin?

increased stores of glycogen, fat and protein

functions of insulin

to increase glucose oxidation, glycogenesis, lipogenesis and protein synthesis

process of insulin release in beta cells

1. glucose enters the cell via GLUT2 transporter

2. glucose is respired to form ATP inside cell

3. ATP causes ATP-sensitive potassium channels to close which causes a build up of potassium ions inside the cell

4. the potassium ions depolarise the cell membrane

5. voltage-gated calcium channels open

6. influx of calcium ions triggers exocytosis of insulin vesicles

Which GI hormones trigger the release of insulin?

GIP and GLP-1

How does glucose move into cells in the pancreas?

via GLUT2 transporters

How does glucose move into muscle and fat cells?

via GLUT4 transporters

Which cells release GIP and GLP-1 in response to food?

intestinal epithelial cells

What is the effect of insulin binding to receptors on liver cells?

hexokinase is upregulated which converts glucose to glucose-6-phosphate, which decreases the concentration of glucose inside the cells so creates a concentration gradient

What is the effect of insulin binding to receptors on fat and muscle cells?

more GLUT4 transporters are presented on cell membranes

Which tissues do not respond to insulin?

CNS, mammary gland, GI tract, kidney tubules

How does glucose enter CNS cells?

GLUT3 transporters

How does glucose enter cells of mammary glands?

secondary active transport with sodium ions

How does glucose enter cells of the GI tract?

secondary active transport with sodium ions

How does glucose exit kidney tubules?

secondary active transport with sodium ions

How long is the half life of active glucagon?

4-6 mins

How does glucagon activate cells?

binds to a specific G protein-coupled membrane receptor which activates adenylate cyclase

What stimulates glucagon secretion?

decrease in blood glucose concentration, increased sympathetic and parasympathetic activity to alpha cells, increased amino acid blood concentration, local paracrine inhibition by insulin

functions of glucagon

stimulates glycogenolysis and gluconeogenesis, inhibits glycogenesis, stimulates hormone-sensitive lipase

diabetes mellitus

a disease in which the glucose concentration of plasma is elevated due to reduced insulin secretion or decreased sensitivity of target cells to insulin

How is the sudden absorption of glucose after a meal prevented?

insulin is released in anticipation of food

How does glucose enter/exit the liver?

via GLUT2 transporters working in both directions

Why is it important to strictly control the concentration of glucose in the CNS?

neurones rely on glucose for metabolism and excess glucose increases osmolarity of CSF which draws water out of neurones

Which type of diabetes is more common in dogs?

type 1

Which type of diabetes is more common in cats?

type 2

What are the clinical signs of diabetes?

hyperglycaemia, polydipsia, polyuria, ketoacidosis

problems caused by diabetes

production of ketones

glucose toxicity of nerves

high blood pressure

diabetic cataracts and blindness

kidney problems

connective tissue degradation

systemic inflammation

weight loss

How does hyperglycaemia cause polyuria?

the ability of the kidneys to reabsorb filtered glucose is exceeded which results in glucose in the urine which increases urine volume due to solvent drag of water

How does diabetes cause weight loss?

the lack of glucose in cells causes decreased protein synthesis and so muscles break down protein for ATP synthesis

A

infundibulum

B

sphenoid bone

C

posterior pituitary gland

D

anterior pituitary gland

E

intermediate lobe of the pituitary

What is another name for the pituitary gland?

hypophysis

Where is the pituitary gland found?

in the pocket of the sphenoid bone

Where are the anterior pituitary and intermediate pituitary lobe derived from?

endoderm of the buccal cavity

Where is the posterior pituitary derived from?

neural ectoderm

Which hormones are released from the posterior pituitary gland?

oxytocin and ADH/vasopressin

Which system is the posterior pituitary gland a part of?

central nervous system

Is the posterior pituitary gland an endocrine gland?

no

Is the anterior pituitary gland an endocrine gland?

yes

Where is oxytocin and ADH produced?

hypothalamus

What connects the hypothalamus to the anterior and intermediate pituitary?

hypophyseal portal system

tropic hormone

a hormone which is used to cause another hormone to be released via a secondary messenger system

How are hormones released from the posterior pituitary gland?

the paraventricular and supraoptic nuclei in the hypothalamus produce hormones which travel to the posterior pituitary gland where they are released into the bloodstream

How are hormones released from the anterior pituitary gland?

neuroendocrine cells in the hypothalamus produce tropic hormones which travel to the anterior pituitary via a hypothalamic-pituitary portal system, at the pituitary the tropic hormones are converted to secondary hormones which are released into the bloodstream

Which hormone is released from the intermediate lobe of the pituitary gland?

melanocyte-stimulating hormone

Which hormones are released from the anterior pituitary gland?

FSH, LH, adrenocorticotropic hormone, thyrotropin, prolactin, growth hormone

Which neuro-hormones are produced by the hypothalamus?

ACTH-releasing hormone, TSH-releasing hormone, GH-releasing hormone, GHIH, gonadotropin-releasing hormone, dopamine

What is the effect of GH-inhibitory hormone on the anterior pituitary gland?

inhibits secretion of growth hormone and TSH

What is the effect of dopamine on the anterior pituitary gland?

inhibits secretion of prolactin

How is secretion of hormones from the pituitary gland regulated?

target organs are stimulated to secrete another hormone which travels to the hypothalamus through the bloodstream and inhibits it from secreting more hormones via a negative feedback loop

Where is the thyroid gland found?

in two lobes either side of the trachea just below the larynx

X

colloid

Y

C-cell

Z

follicular cell

A

parafollicular C cell

B

follicular cell

C

thyroid follicle

D

thyroglobulin (in colloid)

E

basement membrane

How is TGB-bound T3 and T4 synthesised?

iodination of tyrosine residues on thyroglobulin within the lumen of thyroid follicles after follicular cells have oxidised iodide and transported it into the colloid

What is the only known use of iodine in the body?

synthesis of thyroid hormone

Which thyroid hormone is the main product of the thyroid gland?

T4

Which thyroid hormone is more potent?

T3

Which type of cell produces thyroid hormone?

follicular cell

How are free T3 and T4 produced?

colloid droplets re-enter follicular cells where they bind to lysosomes which cleave T3 and T4 molecules from TGB

What happens to T4 within target cells?

it is de-iodinated to T3 or inactivated to rT3

How does T3 and T4 leave follicular cells?

simple diffusion across the basolateral membrane into surrounding capillaries

How are most thyroid hormones transported in the blood?

bound to thyroxine-binding globulin or albumin

How is a very small amount of thyroid hormones transported in the blood?

free in plasma

What regulates thyroid hormone synthesis and secretion?

thyrotropin-releasing hormone, thyroid-stimulating hormone, plasma iodide

What causes the secretion of thyroid hormones to increase gradually?

prolonged exposure to cold

What largely drives secretion of TRH from the hypothalamus?

central nervous system

What causes thyroid follicular cells to be inactive?

absence of TSH

What causes follicular hyperplasia?

prolonged high concentrations of TSH

What is the effect of thyroid hormone?

increases the metabolic rate in all tissues except gonads, brain and spleen

What are the 2 ways that thyroid hormone affects growth?

direct action on cells and by influencing growth hormone and potentiating its effects on target tissues

Where in the body are the adrenal glands found in the dog?

beneath the peritoneum, craniomedial to the kidneys

What are the 3 zones of the adrenal cortex?

zona reticularis, zona fasciculata, zona glomerulosa

What are the 2 main sections of the adrenal gland?

medulla and cortex