Endocrine 1: intro Flashcards
L 1 & 2 - general endocrine system Also from Sjaastad textbook
What are hormones and what kind of cells are they produced by?
Chemical messengers, produced and released by non-neural endocrine cells or neurons, which regulate activities in target cells that possess receptors for that hormone:
- Exerts regulatory influences on the function of other, distant cells reached via the blood or CSF
- Is effective at very low concentrations (i.e. as little as 10-12 M)
Target cell
Cell that possesses receptor(s) for a particular hormone
What is the (functional) difference between endocrine and exocrine cells?
Endocrine cells release hormones into the surrounding interstitial fluid. Exocrine cells release their secretions onto epithelial surfaces, usually by way of ducts.
What is an endocine gland? What are some examples?
A discrete gland whose primary function is the production and secretion of hormones (e.g. steroids, proteins, amines).
E.g. Thyroid, parathyroid, pineal, adrenal
Secretory cells may also be isolated cells distributed amongst cells of other tissues (e.g. gastrin secretion by stomach; CCK secretion by small intestine)
Name an organ that has both endocrine and exocrine functions
pancreas
How is hormonal regulation achieved through negative feedback?
Endocrine cell secretion is adjusted based on information received from target cells to maintain appropriate activity in the target cells (set point).
In negative feedback, changes in target cell activity trigger secretory responses that act to counteract the original change.
E.g. if activity in target cells is too high and hormones positively stimulate activity, info about current level is sent to endocrine cells that then reduce secretion levels, which reduces activity. Conversely if activity is too low, secretions are increased, which increases target cell activity.
Negative feedback mechansims can (within limits) maintain internal body environment in a relatively stable state that promotes optimal physiological functioning, despite external fluctuations (homeostasis).
Usually mediated by an interaction with the hypothalamus and/or pituitary gland (involves nervous and endocrine systems for integrated control)
Concerned with concentration of free hormones (for those that bind to transport proteins)
What is positive feedback? Example?
In positive feedback, an initial change in the activity of target cells induces events that enhance the change. E.g. action potential generation, blood clotting, ovulation
What are the two essential features of endocrine control?
- Slow action (relative to nervous system)/can have protracted effects
- Broadcast transmission
Why does the endocrine system have slower and more protracted effects than nervious system (two features)?
- Signals (i.e. hormones) usually travel through the bloodstream to reach target tiessues
- Hormones continue to act until metabolised/inactivated or excreted
What does ‘broadcast transmission’ mean in the context of the endocrine control system?
- Hormones released into the bloodstream and cerebrospinal fluid potentially bathe all cells
- Specificity of effects depends upon receptors on target cells
Hormones are present in [high/low] concentrations so cellular receptors are generally [high/low] affinity
Hormones are present in low concentrations (10-7 to 10-12 M)
so cellular receptors are generally high affinity
What are some processes that are under endocrine control ?
- Water & electrolyte balance
- Metabolism & post-absorptive nutrient homeostasis
- Coping with hostile environments (i.e. stress!)
- Reproduction
- Growth & development
What is a local hormone?
Local hormones diffuse through the interstitial fluid to act on target calls in the immediate vicinity of the secretory cell
What is a paracrine hormone?
A local hormone that affects neighbouring/neaby cells
What is an autocrine hormone?
A local hormone that influences the functions of the same cell that secretes them
What is an introcrine?
Introcrines are peptide growth factors or hormones that function within cells in addition to any endocrine, paracrine, or autocrine roles they may have
What are the three major calsses of hormones
- Steroid hormones
- Peptide and protein hormones
- Amine hormones
What are some of the properties of steroid hormones?
- Synthesised from cholesterol
- Secreted by gonads and adrenal cortex
- Lipid soluble so can pass through cell membranes - bind to intracellular receptors
- Transported across cell membrane in some cells (e.g. estrogens)
What are some properties of peptide and protein hormones?
- Structured from chains of amino acids
- Vary enormously in molecular size (tripletides –> 200+ AA)
- Soluble in aqueous solutions
- Distinction peptide vs. protein depends mainly on size- functionally not important! Generally called ‘peptide’ hormones regardless
What are some examples of peptide/protein hormones?
- antidiuretic hormone (ADH)
- insulin
- growth hormone
- gonadotropin-releasing hormone (GnRH)
What are the general features of peptide/protein hormone synthesis?
- Synthesised at ribosomes
- Stored in vesicles
- Secreted on demand
- Syntheis of a preprohormone with posttranslational processing to produce a mature hormone is common to all peptide hormones
Describe the 7 steps involved in insulin synthesis and release.
- A signal sequence (P segment) directs the molecule into the interior of the rough ER. Enzymes there cleave this off to produce proinsulin
- Proinsulin moves through the ER and into the Golgi apparatus via shuttle vesicles
- In the Golgi apparatus, three disulfide bonds fold the proinsulin molecule
- The Golgi apparatus buds off vesicles containing proinsulin and enzymes
- Enzymes cleave the C segment from proinsulin to form mature insulin
- Insulin is stored in secretory vessels awaiting release
- Depolarisation of the cell membrane triggers release by exocytosis
How and where are steroid hormones produced generally?
- Formed from cholesterol
- some obtained from dietary animal fats
- steroidogenic endocrine cells and liver cells also produce cholesterol
- Formed via stepwise reactions catalysed by enzymes in the mitochondria and smooth ER - different enzymes in different intracellular compartments produce different products (see image)
- Produced only in the endocrine cells of the gonads, adrenal cortex, and placenta
How and when are steroid hormones secreted?
-
Not stockpiled in vesicles prior to secretion (solubility!)
- made from precursors stored in lipid droplets
- Secreted immediately
- Diffuse across cell membrane
How are lipid soluble hormones generally transported in the blood?
Lipid-soluble hormones (steroid hormones, vitamin D3-metabolites, thyroid hormones) are generally transported bound to carrier proteins (from liver)
- Bind reversibly with varying degrees of affinity
- May be specific or non-specific
- Free and bound forms are in equilibrium in the blood
- Only the free form is metabolically active (must diffuse across cell membranes)
- Carrier proteins provide a reservoir of steroid hormones in blood plasma and extend half life
Amplification of signals is an important characteristic of the endocrine system. What is an example of this?
- Epinephrine released by the adrenal glands in response to stress/threat circulates in the blood until it reaches and bathes liver cells that contain ample stored glycogen
- Interacts with g-protein coupled receptor system that uses secondary messengers in a signal transduction pathway that amplifies the signal at five different steps such that very low concentrations of epinephrine can result in a very large increase in glucose concentration due to the breakdown of stored glycogen
- Total amplification is about 10 million-fold
