Endocrine System Flashcards
Name the endocrine glands.
Thyroid Parathyroid Pineal Pituitary Adrenal Pancreas Ovaries Testes
Name and describe the 4 main hormones that regulate metabolism.
- Insulin – produced from the pancreas and secreted in the fed state
- Glucagon – produced by the pancreas and signals the starved state
- Adrenaline – produced by the adrenal glands and is secreted in starvation and stress
- Glucocorticoids – produced by adrenal glands and are secreted in starvation and stress
What is hormone signalling?
Hormones are released by cells in these endocrine glands and enter the bloodstream, where they travel to and affect target cells.
Many cells that do not sit within endocrine glands also produce molecules which as hormones.
What are the 4 types of hormones?
- Amines – small molecules derived from amino acids, such as thyroid hormone, the catecholamines, adrenaline and noradrenaline.
- Peptides/polypeptides – larger molecules, products of genes, such as insulin, glucagon and growth hormone.
- Steroids – derived from cholesterol, such as vitamin D, glucocorticoids and the sex hormones.
- Fatty acid derivatives – prostaglandins, leukotrienes and thromboxanes.
Define signal transduction.
The process by which cells can respond to changes in their environment.
What are some general features of hormone signalling?
- Extracellular signalling molecules bind to specific protein receptors and initiate a cascade of signalling events inside the cell.
- An important factor in cell signalling is amplification. This is so that a small amount of hormone can bring about larger changes in the body, such as changes in metabolism, gene expression or changes in cell shape or movement.
- Cells must be able to ‘switch off’ the intracellular signalling events once the signal concentration falls.
How do hydrophilic and hydrophobic hormones bind to receptors to initiate signal transduction?
Hydrophilic/polar hormones cannot cross the plasma membrane s must bind to cell surface receptors that are embedded in the cell membrane. They have a specific binding region to which the hormone binds.
Hydrophobic molecules such as steroid hormones are not water soluble and tend to move around in the circulation, bound to a carrier protein. At their target tissues, they can diffuse across the cell membrane and interact with intracellular receptors.
What are short and long term cellular responses?
Short term – normally brought about by changes to existing cellular proteins that alter their function.
Long term – take minutes or hour to become apparent. Generally, changes in gene expression are involved here. These types of cell signalling responses change the amount of particular proteins inside the cell.
Describe the action of insulin on glucose homeostasis.
- Secreted in response to high blood glucose by the beta cells of the pancreas
- Signals the fed state
- Promotes glucose uptake into tissues and fuel storage
Describe the action of glucagon on glucose homeostasis.
- Secreted in response to low blood glucose by alpha cells of the pancreas
- Signals the fasting state
- Promotes fuel metabolism and glucose release by the liver
Describe the action of adrenaline on glucose homeostasis.
- Secreted in response to low blood glucose and fear via the adrenal medulla
- Stimulates fuel mobilisation, especially in muscle and adipose tissue
Describe the action of glucocorticoids on glucose homeostasis.
- Secreted largely in response to stress but also starvation
- Long term effects on the expression of enzymes involved in fat and carbohydrate metabolism
What range must glucose be regulated within?
3.5 to 6.5 mM
Describe the consequences of hypoglycaemia.
Many tissues are obligate or preferential glucose oxidisers. So anything leading to a precipitous fall in glucose is likely to be life-threatening. This is probably why animals have a number of hormones that work together (glucagon, adrenaline, glucocorticoids) to prevent hypoglycaemia.
Changes to brain function caused, including:
- Palpitations
- Nausea
- Shivering
- Drowsiness
- Unsteadiness
- Confusion
- Coma and death
Describe the consequences of hyperglycaemia.
More damaging in the long term.
- Osmotic diuresis/loss of fluid and electrolytes: dehydration, excessive thirst and diarrhoea.
- Glucose is a chemically active molecule that can covalently modify proteins causing glycosylation. So can manifest itself as microangiopathy/damage to the small blood vessels, such as in the nephron, macroangiopathy/damage to large blood vessels, leading to hypertension.
- Can also cause cataracts, from glucose metabolising to sorbitol that accumulates in the lens, and neuropathy.
How are hormone levels measured?
- Hormones are typically only present in normal animals in the blood at low concentrations.
- Hormone assays have to be very sensitive and specific and they commonly use antibodies made against the hormone of interest.
- ELISAs are frequently used and can also be used to detect pathogens or quantities of drugs.
Describe the brief histology of thyroid tissue.
Follicles containing colloid - a protein-rich viscous liquid. Follicles are surrounded by follicular/cuboidal epithelium.
Where are thyroid hormones derived?
From the amino acid, tyrosine.
Each follicle contains lots of thyroglobulin, a precursor polymer containing tyrosine. Cleaving the backbone of this molecule, and with the addition of iodines, produces the thyroid hormones.
Describe the process of thyroid hormone synthesis.
- Follicular cell synthesises enzymes and thyroglobulin for colloid.
- Iodide ions are co-transported into the cell with sodium ions and transported into the colloid.
- Enzymes add iodine to thyroglobulin to make T3 with 3 iodines and T4 with 4 iodines.
- Thyroglobulin is taken back into the cell.
- Intracellular enzymes separate T3 and T4 from the protein.
- Free T3 and T4 enter the circulation.
Describe the properties of T3.
- 10% proportion of total hormones released from the thyroid glands
- 5 times more active than T4
- Circulates loosely bound to a protein
- Half life = 1 day
Describe the properties of T4.
- 90% proportion of total hormones released from the thyroid gland
- 5 times less active than T3
- Circulates the blood tightly bound to a protein
- Half life = 6 days
What happens to T4 at tissues?
Most of the physiological effects of thyroid hormones are due to T3.
So, in target tissues, most T4 is deiodinated to T3.
T4 might be ‘pool or reserve’ for T3, so regulation of conversion of T4 to T3 is important.
Describe the action of thyroid hormones at target cells.
- Move around the body until they reach target cells.
- T3/4 enters the target cell.
- Most of T4 is converted to T3 by now.
- T3 enters the nucleus and binds to the thyroid hormone receptor, THR.
- Binding of THR to promotor elements activates gene transcription, which may have many effects of TH in different target cells.
Describe the hypothalamus-pituitary axis.
- Hypothalamus produces thyrotropin releasing hormone, TRH.
- TRH is transported along the hypophyseal portal circulation to the anterior pituitary.
- TRH binds to thyrotroph, cells that release thyroid-stimulating hormone.
- Thyrotrophs release TSH.
- TSH is transported in the bloodstream to the thyroid gland.
- Binds to the follicle cells in the thyroid gland.
- This increases T3 and T4 synthesis, by increasing the activity of the iodine pump, and so also increases the iodination of tyrosine. Increases the size, number and activity of the thyroid cells. This means there is an increase in re-uptake of the colloidal thyroglobulin.