The Endochrine System Flashcards
Explain the difference between the endocrine, paracrine and autocrine mechanism
Hormones are produced by glands in various areas of the body. These glands are surrounded by lots of blood vessels allowing the hormones to enter the circulation. Hormones then act upon receptors anywhere in the body. This is known as the endocrine mechanism. In the paracrine mechanism, transport to the site is the same as the endocrine mechanism but instead of stimulating receptors on cells, the hormone is excreted into the extracellular space and act on nearby cells. In the autocrine mechanism, hormones work on the cell that excreted them
Explain how peptide hormones are synthesised and how their by-products can be used in healthcare
Peptide hormones are synthesised via transcription and translation. The initial product is a pre-prohormone which is then cleaved at the initial N-terminal to produce a prohormone. The prohormone is then cleaved further to produce a mature hormone and peptides. These peptides have a longer half life than the hormone itself and, therefore, can be used an indicator of hormone levels in the body.
List some key characteristics of peptide hormones
They do not require a carrier protein, they have short half lives and they mostly work by activating protein kinases
Give an example of an amine hormone and how the work in the body.
Thyroxine (T4) and Tri-iodothryronine (T3) are two amine hormones that fall under the class of thyroid hormones. The act as ligands for transcription factors. If the thyroid hormone receptor (THR) is unliganded, it recruits a co-repressor which inhibts expression of target genes. If thyroid hormones bind to THR, the co-repressor can no longer bind and a co-activator binds instead.
Give an explanation of how steroid hormones are synthesised.
Steroid hormones are all derived from cholesterol which can be taken in through the diet or made within the body. Dietary cholesterol is associated with LDL. When taken up into the cell, the LDL particle releases free cholesterol. The cholesterol is then converted into steroid hormone precursor - prenenolone which is then converted into the appropriate hormone.
What happens when a hormone binds to a carrier protein? Give examples of bound and unbound hormones and give a reason for each.
Binding to a carrier protein serves two purposes, it increases the half life of the hormone and it provides a ‘reservoir’ of hormone in the blood. This reservoir exists in equilibrium, meaning that if the unbound supply of a hormone is used up, more will dissociate from their carrier proteins. Examples of bound hormones are steroid and thyroid hormones. Because these hormones act as ligands for transcription factors, there needs to be a consistent supply of them in the body. An unbound hormone would be, for example, adrenaline or noradrenaline. Because binding to a carrier protein causes a longer half life and adrenaline and noradrenaline are linked with the sympathetic nervous system, if these hormones were to bind, it would result in a longer recovery period from stressful situations.
Explain how ADH release is regulated
ADH release is triggered by activation of osmoreceptors. These receptors react to the osmolarity of the blood as when they shrink, they activate ADH synthesising neurons. Osmoreceptor activation also stimulates the feeling of thirst
Explain how ADH has an effect on water balance
ADH is a V2 receptor agonist. These receptors are found in the kidneys and are Gs linked. The increase in cAMP causes aquaporins to open, allowing more water to be reabsorbed. ADH also constricts blood vessels in order to raise blood pressure.
Explain how aldosterone has an effect on water balance
Aldosterone is a ligand for transcription factors - specifically transcription factors that control the expression of genes involved in sodium transport such as sodium channels. Aldosterone upregulated expression of these genes with - as a result - leads to greater sodium reuptake in the kidney. As water reuptake follows osmosis, there is greater water reuptake as a result
Explain how aldosterone release is regulated
When blood pressure (BP) drops, release of an enzyme called renin is stimulated. Renin cleaves angiotensinogen to give Angiotensin 1. Angiotensin 1 is then converted to Angiotensin 2 by a angiotensin converting enzyme (ACE). Angiotensin 2 then stimulates aldosterone release from the adrenal cortex
Explain how Atrial Natiuretic Peptide (ANP) regulates water balance
ANP reacts in response to increased blood receptors which is measured by stretch receptors in the heart. Release of ANP causes an increase in blood flow to the kidney medulla. As there is a limit to how much water can be reabsorbed in any given time, more of the water is lost. This is compounded by decreased sodium reuptake. ANP inhibits the phosphorylation of ENaC channels. ANP also inhibits renin and aldosterone which increase water reuptake.
