Introduction to Endocrine System Flashcards
What is the definition of the endocrine system?
It integrates and controls organ function via secretions of chemicals from cells, tissues, or glands
Which is carried in blood to target organs distal from site of hormone synthesis
What communicates and controls all body functions?
Endocrine system and nervous system
Describe endocrine communication
Hormones travel in blood to their target organs/ tissues
Tissues detect presence of specific receptors for that chemical
Describe neural communication
Neurotransmitters released from presynaptic neurons across the synaptic cleft to postsynaptic cell to influence activity
Local site of action
Describe neuroendocrine
Endocrine and nervous systems combine
Nerves release hormones which enter blood and travel to their target organs
Where are insulins target tissues and response?
Skeletal muscle/ adipose tissue - increase glucose uptake
Liver - increased glycogenesis and decreased gluconeogenesis
What are autocrine secretions?
Cells secrete chemicals that bind to receptors on the same cell ex. cytokines
What are paracrine secretions?
Chemicals diffuse in ECF to affect neighbouring cells ex. histamine
What is the function of hormones?
Bring about changes in activity of target cells and tissues
Embedded in physiological systems and diffuse system of glands
What are some important features about an endocrine hormone?
Produced by cell or group of cells
Secreted from cells into blood
Transported to distal targets
Exerts effects at low conc.
Bind to receptors at target tissue
Have their own action terminated
Often released in short bursts
What are the 3 classifications of endocrine hormones?
Peptide or protein hormones
Amine hormones
Steroid hormones
What is peptide hormones composed of?
Chains of amino acids
What is amine hormones derived from?
One of 2 amino acids - tryptophan (melatonin only) or tyrosine
What are steroid hormones derived from?
Cholesterol
What are some examples of peptide hormones?
TSH, FSH and insulin
What is a preprohormone?
Produced by ribosomes - initial inactive peptide hormone
These contain one or more copies of active hormone in amino acid sequence
Describe cleaving a preprohormone
Cleaved off in ER to leave smaller inactive protein called prohormone
How are prohormones packaged and broken down?
Into vesicles in Golgi apparatus, along with proteolytic enzymes which break prohormone down into active hormone and other fragments
Stored until release is triggered and contents released into plasma
What can be measured clinically in diabetes?
Inactive fragments in plasma - C -peptide
Describe C-peptide and why is it measured?
Inactive fragment cleaved from insulin prohormone
Level in plasma and urine are measured to indicate endogenous insulin production from pancreas
Levels are 5x higher than endogenous insulin as metabolised faster
Describe the mechanism of action of peptide hormones
Target GPCR or tyrosine kinase linked receptors - pathways phosphorylate existing proteins in cell and modify function
Describe peptide hormones
Water soluble so dissolve easily in plasma so transport in blood is easy
Generate fast responses - seconds/ minutes
Hydrophilic/ lipophobic
Describe a peptide hormone binding G protein couple receptor
Activates 2nd messenger system and/or ion channels leading to modification of existing proteins - Rapid response
Describe a peptide hormone binding to tyrosine kinase linked receptor
Alters gene expression - slower and more long lasting
2nd messengers may also alter gene expression
What are some examples of amine hormones?
Dopamine - CNS neurotransmitter
Norepinephrine - neurotransmitter
Epinephrine - hormone from adrenal medulla
What is the mechanism of action of amine hormones?
Catecholamines - hydrophilic so similar to peptide hormones
Thyroid hormones - lipophilic similar to steroid hormones
Describe steroid hormones
Synthesized directly when needed rather than stored and released
Highly lipophilic so cannot be retained within lipid membranes
Half life - 60-90 minutes
How are steroid hormones transported?
Bound to carrier proteins and poorly soluble in water - ex. albumin
Slow onset of action but action lasts longer
What are steroid hormones produced by?
Gonads - sex steroids
Placenta - hCG, sex steroids
Kidney - vitamin D3
Adrenal cortex - corticosteroids
What determines which specific steroid hormone is produced?
Different cells having different enzymes synthesizing different derivatives of cholesterol
Describe the mechanism of action of steroid hormones
Lipophilicity - can cross plasma membrane as receptors are located inside cells
Change of gene expression at level of nucleus - genomic effect
Increase or decrease protein synthesis
What is the time of process of steroid hormone action?
Slow process so lag time between hormone release and biological effect
Describe the physiological activity of lipophilic hormones
Small amount of unbound free steroid/thyroid hormone in plasma
Only free hormone can cross capillary walls to target cells
Free hormone < protein bound hormone (ratio)
Describe the law of mass action
Dictates that as free hormone leaves plasma then more hormone released from carriers
What is total plasma equal to?
Free hormone + complexed hormone
What are the 2 carrier protein in plasma?
Specific (corticosteroid binds to globulin)
Non specific - albumin
What does hormone carrier proteins do?
Increase solubility - required in blood mediated transport
Protects from degradation
What is secretion responsive to?
Negative feedback reflexes
Describe negative feedback of parathyroid hormone
Increased Ca conc. in plasma causes negative feedback reflex on parathyroid cell to inhibit release of hormone
What does fright, flight and fight cause?
Specialised cells in adrenal medulla release adrenaline into blood stream
What are some control mechanisms of insulin?
Plasma glucose, autonomic nerve activity, presence of food in gut and additional hormones like glucagon
