Physiology 3120 Lecture 51: Phys 3120 - Lecture 51
2 May 2018
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Human Physiology Lecture 51
Introduction to Endocrinology
Definitions
- Endocrine System
o Tissues and cells capable of secreting and responding to hormones
▪ Hormones = signalling molecule
o Communication system
o The two components communicate via chemical messengers called hormones.
▪ Two different target organs respond to each other via hormones (chemical
messengers)
▪ Organ/Tissue A release hormone, travels in blood, and targets tissue B to elicit
endocrine response, to elicit a change in physiology to tissue B
- Neural: functions mediated by electro-chemical conduction along nerves
o Nervous system allows for rapid communication between brain and other components of the
nervous system
- Endocrine: functions are mediated by chemical messengers called hormones.
o Hormone must go through the blood stream from tissue A and target a target receptor in
tissue B
- Hormone
o A chemical substance, formed in one organ or part of the body and carried in the blood to
another organ or part of the body
o Depending on the specificity of their effects, hormones can alter the functional activity of just
one organ or of various numbers of them (GnRH versus T3).
▪ HORMONE INFLUENCE MANY ORGANS OR ONE ORGAN
▪ GnRh is produced and targets the pituitary
▪ T3 targets many organs to influence basal metabolic rate
Hormones
- Hormones are:
o Regulators of physiologic events
o Effective in minute quantities
▪ Hormones are in pictograms or nanograms (small amounts)
o Synthesized by cells/endocrine glands
▪ Or particular gland
o Greek hormon, to rouse or set in motion.
▪ Hormone influence physiological response – needs to adapt to a particular stressful
situation or particular need of the body
- Endocrine hormone: chemical messengers synthesized in specialized (endocrine) cells and then
released into the circulation where they are available for uptake by and action on remote tissues
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Not all hormones are the same!
- ENDOCRINE: chemical mediators produced in one part of the body which act on a distant part.
Remote control
o Goes through the blood system to get to the target receptor
o Secreted throughout the body to target distant organs
- PARACRINE: chemical mediators produced in one cell that acts on a neighbouring cell or tissue
Neighbourhood watch
o Para = near
o One cell influencing another cell
- AUTOCRINE: chemical mediator produced in one cell and acts on that same cell that produces it
Self control
o Hormone produced by one cell and acts upon itself, on the receptor of the same cell
Nervous vs Endocrine System
- Physical form of information transfer
o Nervous = Action potentials (requires this to elicit the response)
o Endocrine = chemicals (hormones)
- Speed of information transfer
o Nervous = fractions of seconds
o Endocrine = minutes, hours, days
▪ Depending on their properties
▪ E.g. insulin after we eat a meal it acts in minutes, whereas thyroid hormones takes
days, weeks or months to elicit long term response
▪ Endocrine system is slower than nervous system
- Mechanism of gradation (how do we influence or stimulate a response in the communication system)
o Nervous = frequency (increase the firing of AP)
o Endocrine: amplitude modulation
▪ If you want to up regulate a certain endocrine response: hormone hits target
receptor, have amplification of message due to the fact that the receptor, upon
activation can signal down stream effects which can amplify a certain response
▪ E.g. Insulin binds its receptor, increase transcription + translation of multiple copies
of the particular target gene
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- Mechanism to achieve specificity
o Nervous = "wiring"
▪ The nerves themselves and where they go to
o Endocrine = receptors
▪ How specific the receptor is to the hormone that is circulating the body – how
specific is it to receive the hormone and elicit a downstream response
• Specific of the receptor matters
▪ Not the hormone that influences the endocrine disorder, the inactivity of the
receptor of the hormone leads to pathological situations (diseases such as Type 2
diabetes)
Hormone Types
- PEPTIDE/POLYPEPTIDE
o String of amino acids
o Small monomers e.g. TRH; 3 a.a.
o Large multimeric proteins with several subunitse.g. TSH, FSH, LH; 200+ a.a.
o Water soluble
▪ Easily dissolve into the blood to travel from tissue A to tissue B
▪ May or may not be associated with carrier/binding proteins
o Note: large protein hormones subject to post-translational modification (proteolytic
processing, glycosylation) to produce a functional hormone
- STEROID
o Derived from cholesterol metabolism, 4 hydrocarbon rings with various side chains
▪ Originally cholesterol, but due to intracellular enzymes you get various metabolites
leading to different forms of steroid hormones
▪ Large steroid structures or rings that make them water INSOLUBLE
o Lipid soluble (requires binding protein in serum)
▪ NOT water soluble – need assistance (carrier proteins) to go from tissue A to tissue
B
▪ Bind serum proteins to transport them
o E.g. testosterone, estrogen, vitamin D
o Serum carrier proteins help to regulate steroid bioactivity - only free steroid is available to
the cell
- AMINO ACID DERIVATIVES
o Derived from enzymatic modifications of an amino acid
▪ Chatecholamines: derived from metabolism of phenylalanine and tyrosine
▪ T3+T4 produced from the iodination of tyrosine residues in thyroglobulin – coupled
and cleaved from the parent globulin
o e.g. epinephrine, thyroxine (T4)
▪ T4 derived from tyrosine - large aromatic structures therefore have difficulty
dissolving in water and are lipid soluble
o Require carrier proteins that take them from tissue A to tissue B (not water soluble)
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