Introduction to endocrinology Flashcards
an overview of diabetes and endocrinology
What is an endocrine system?
- Integrates and controls organ function
- Via chemicals (hormones)
- Which are carried through the blood to target organs
- Distal from the hormone site of synthesis
- Response may be fast or slow
What is a hormone?
- Endocrine hormone: target organs are distal to the site of synthesis
- Paracrine hormone: act local to the site of synthesis
- Autocrine hormone: act on/ in the same cell it was produced
- Exocrine hormones: released from exocrine glands via ducts to the external environment
What is a neurohormone?
- Endocrine and nervous system combine
- Nerves release hormones which travel in the blood and travel to their target cells
- e.g. Adrenalin
What is endocrinology?
The branch of physiology and medicine concerned with endocrine glands and hormones
Describe endocrine communication
- Hormones travel in the blood to their target organs/ tissue
- Tissues detect hormones through the presence of a specific receptor for that chemical in/on the cell
- No receptor= no response
Describe neural communication
- Neurotransmitters released from presynaptic neurons travel across the synaptic cleft to the postsynaptic cell to influence its activity
- Neurotransmitter is a chemical released by a neuron but only acts within the synaptic cleft
What is the “life history” of a typical hormone?
- Secreted by a cell or group of cells
- Secreted from those cells into the blood
- Transported via the blood to distant targets
- Exert their effects at very low concentrations (act in the range 10^-9 - 10^-12M)
- Act by binding to receptors on target tissues
- Have their action terminated (often through negative feedback loops)
Describe peptide or protein hormones
- composed of chains of amino acids
- Common
- Synthesised in advance of the need and then stored in vesicles until they are required
- An example of a peptide hormone is insulin, TRH or FSH
- Water soluble and so dissolve easily in plasma making transport via the blood simple and easy
- It cannot cross cell membranes, however, as they are water soluble
- They bind to membrane bound receptors on target cells
- Once bound these receptors generally create relatively fast biological responses (seconds-minutes)
- Most work by modulating the GPCR or the tyrosine kinase linked signalling pathways
- These pathways phosphorylate existing pathways in the cell and modify their function
- e.g. open/close ion channels, activate or inactivate enzymes
- The tyrosine kinase pathway may also alter gene expression (not for insulin though)
How are peptide or protein hormones produced?
- Initial protein produced by ribosomes is large and inactive (preprohormone)
- Preprohormones contain one or more copies of the active hormone in their amino acid sequence
- Preprohormones are cleaved into smaller units in the RER (prohormones)
- Prohormones are smaller but still inactive
- Prohormones are packaged into vesicles in the Golgi apparatus along with proteolytic enzymes which break the prohormone down into the active hormone and other fragments
- Hormones and fragments are stored in the endocrine cell until release is triggered then all vesicle contents are released into the plasma (co-secretion)
How do steroid hormones interact with their receptors and what happens when they do?
• Steroids cross the plasma easily as they are lipophilic
• Their receptors are located inside the cells
• They trigger either activation or inhibition of gene function within the nucleus (genomic effect)
- Genes control the synthesis of protein so these hormones either increase or decrease protein synthesis
• This is a relatively slow process (hours-days)
• Effects persist for longer
• Some evidence shows that steroids sometimes bind to cell surface receptors which would mean a quick response
Describe the production and excretion of steroid hormones
• All derived from cholesterol
- Which steroid is produced depends on the cell as they all have different enzymes and so synthesise different derivative of cholesterol
• Synthesised directly when needed
• They cannot be retained within the lipid membranes
• Once synthesised they diffuse across the membrane into the ISF and the blood
How are steroid hormones transported in the blood?
- They are transported bound to carrier proteins such as albumin
- This stabilises their transport through the plasma and protects them from enzymatic degradation which greatly increases their life span
What are steroid hormones produced by?
• Steroid hormones are produced by:
- Gonads (testes and ovaries): sex steroids
- Placenta: hCG, sex steroids
- Kidney: vitamin D3
- Adrenal cortex: Corticosteroids
What are amine hormones derived from?
- All derived from one of two amino acids (tryptophan and tyrosine)
- The only amine hormone not derived from tyrosine is melanin which is derived from tryptophan
What are catecholamines?
- Dopamine from the brain
- Norepinephrine from neurons
- Epinephrine from the adrenal medulla
- Similar mechanisms of actions to peptide hormones (hydrophilic)
What are thyroid hormones?
- Similar mechanisms of action to steroid hormones (lipophilic)
- Only free steroid/thyroid hormone in the plasma can diffuse across capillary walls to the target cells
- The hormone-protein complex ratio is much in favour to the bound (complexed) hormone
- The extent of protein binding can have important effects on the hormone’s actions
- In health levels of free (active) hormone remain constant
- As free hormone leaves the plasma more hormone is released from the carriers
- Typically only minute quantities of hormone are required for physiological functions
- Free hormone + complexed hormone = total plasma hormone
Give two examples of an amine hormone
- Catecholamines
- Thyroid hormone
What (in logical sequence) are the factors which determine the availability of a hormone or a neurohormone to its target cell?
• Prolonged exposure to low hormones in the plasma can result in upregulation of receptors on target tissues
• Prolonged exposure to high hormones in the plasma can result in down-regulation of receptors on target tissues
• This may not only affect the hormones own receptors but also the receptors of other hormones
• Permissive effect
- The presence of one hormone enhances the effect of another
- e.g. epinephrine causes only modest lipolysis, but when thyroid hormones are also present, greatly increased lipolysis occurs
What are the major categories of physiological function that are governed by endocrine pathways and mechanism?
• Most endocrine pathways will respond to negative feedback reflexes e.g. parathyroid hormone
• Some endocrine pathways also respond to neural feedback loops e.g. adrenalin
• The secretion of some hormones can be subjected to multiple control mechanisms e.g. insulin
- Autonomic nerve activity
- Presence of food in the gut
- Additional hormones such as glucagon
What are the main routes and types of signal that determine the rate of hormone synthesis and secretion by a typical endocrine cell?
- Hormone in the blood depends on the rate of secretion and the rate of removal
- Removal is by secretion or metabolic transformation and mainly occurs in the liver or kidneys
- Generally catecholamine and peptide hormones are secreted easily so have a short half-life in plasma
- Steroid and thyroid hormones take hours and days to excrete or metabolise, because they are protein bound
