LIFESCI 7C Lecture Notes - Lecture 2: Prothoracicotropic Hormone, Ecdysone, Positive Feedback
Week 2:
Ch 38.1: Overview of Endocrine Function
Homeostasis Review
● Homeostasis: The ability to stay stable, or maintain a stable state
○ E.g. heart rate oscillates around “set point” of 60 bpm
○ Negative feedback: tells heart rate to slow down when too fast and to speed up when too slow
● Involves many chemical processes that must be coordinated to maintain stability
○ Lies in cell’s ability to sense and send signals to be able to respond!
○ → Cell signalling
38.1: An Overview of Endocrine Function
● Hormones play diverse physiological roles in the body: animal growth/development and particularly regulating
organism’s response to environment and maintaining stable physiological condiitions within cells/whole animal
The endocrine system helps regulate an organism’s response to its environment
● Changes in environment → physiological challenges or stresses that organisms must respond to by altering
functional states of its body
○ Sensory signals→ nervous system reception → endocrine system response (slower signaling than nervous
system) → physiological changes → appropriate response to environmental cues
The endocrine system regualtes growth and development
● The release of circulating hormones from endocrine changes allow for broad changes in many organ systems,
allowing for growth and development
○ E.g. estrogen/testosterone important in determining male/female sexual characteristics both in embryo
and during puberty
○ E.g. insect life cycle; molting (sheding exoskeleton) at each larval stage and metamorphasis
● Insect brains contain neurosecretory cells → neurons in the hypothalamus that secretes hormones into the
bloodstream. Hormones act on endocrine glands/other targets within the insect
○ Stimulate production of juvenile hormone, PTTH (which then trigers ecdysone) and so one
● Small amounts of hormones released from key glands in body regulate major stages of growth and body changes
during metamorphasis
○ Hormonal regulation by endocrine system critical for coordinated changes in multiple organ systems
● Hormones also regulate growth in humans and other vertabrate animals
○ E.g. growth of skeleton/tissues
○ pituitary gland A gland beneath the brain that produces a number of hormones, including growth
hormone.
■ Tumors in pituary gland → gigantism/pituary dwarfism
The endocrine system underlies homeostasis
● homeostasis The active regulation and maintenance, in animals, organs, or cells, of a stable internal
physiological state in the face of a changing external environment.
○ Maintaining homeostasis depends on feedback from target organ to endocrine gland that secretes the
hormone; can occur over varying distances within the body, coordinating functions of severla organs at
any one time
■ In resonse to feedback, endocrine gladn modifies its own production of hormone (increase or
decrease)
■ negative feedback final product of a biochemical pathway inhibits the first step; the process;
stimulus starts pathway to inhibit it. Negative feedback is used to maintain steady conditions, or
homeostasis. (more typical in homeostasis)
● Endocrine organ (effector) releases hormone and hormone causes body response that
opposes intial stimulus to return body to set point
■ positive feedback stimulus starts pathway to enhance it/further response (reinforces). until
interrupted.
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● Stimulus causes the response in the same direction of the intial stimulus, which leads to
further response → escalation of resopnse
● Move system farther and farther away from set point
■ Set point: starting point (e.g. set point in heart rate or blood clucose level)
● Negative feedback: Blood glucose level regulation
○ Glucose level ncreases when meal is consumed, falls when glucose is taken into cells for energy
○ After a meal (hgh glucose), pancreatic β (beta) cells secrete insulin hormone, which circulates in the
blood
■ Insulin present → muscle/liver atkes up glucose from the blood to either use it or convert it to
glycogen (storage unit)
■ Insulin guards against high blood glucose level
○ Low glucose: pancreatic α (alpha) cells secretes glucagon hormone, which circulates in the blood
■ Glucagon present→ stimulates breakdown of glycogen into glucose and its release from
muscle/liver cells
■ Glucagon guards against low blood glucose level
■ diabetes mellitus A disease that results when the control of blood-glucose levels by insulin fails.
○ Both involve response feeding back into secreting cells to reduce further hormone secretion
● Positive feedback: mammals during birth; Uterine contractions → oxytocin released from pituary gland
○ Oxytocin (stimulus) released cuses uterine muscles (effector) to contract more forecefully
○ Contractions stimulate (positive feedback) pituary gland to secrete more oxytocin
● Negative feedback mechanisms return a system to a set point; positive feedback mechanisms amplify a
response.
38.2: Properties of Hormones
● Presence of receptors on cells within target organs that bind specific hormones
Hormones act specifically on cells that bind the hormone
● Bind to receptors located on surface of or inside cells
○ presence/absence of receptor for a hormone determine which cells respond or don’t
■ E.g. oxytocin received by uterine and secretory cells in breast (cells have receptors)
■ Hormone can exert its effect only on specific tissues
● Binding hormone to receptor triggers changes in target cell → cellular response
○ Specific action of a hormone depends on teh kind of response it triggers from target cell
■ E.g. can alter ion flow across the cell membrane, activate an intracellular signal transduction
cascade that leads to changes in the biochemical activity of a target cell, or initiate more
substantial changes in gene and protein expression.
Two main classes of hormone are peptide and amines, and steroid hormones
● Hydrophilic hormones (peptide and amide hormones): their receptors=on cell surface
○ peptide hormone A hormone that is a short chain of linked amino acids.
■ Sometimes large enough to be considered proteins; can evolve through sequence changes
○ amine hormone A hormone that is derived from a single aromatic amino acid, such as tyrosine.
● Hydrophobic hormones (steroid hormones their receptors=inside cell
○ steroid hormone A hormone that is derived from cholesterol.
■ Cannot evolve; evolutionary changes on their function depend on changes in: their synthetic
enzymes or receptors they bind to and cellular responses they trigger
● peptide/amide hormones are more abundant than steroid hormones; more diverse in their actions
○ Cannot diffuse across plasma membrane (thyroid hormones=exception); bind to cell surface receptors
○ Alter biochemical activity of target by initiating signaling cascades w/in target cell
■ Activated enzymes are typically protein kinases (phosphorylators)
■ Can lead to changes in gene expression or alter metabolism (by turning enzymes on/off)
■ Can trigger cell to grow, divide, change shape, release another hormone
● Act on timescales of minutes → hours
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○ E.g. insulin and glucagon, gastrin and cholecytokinin (regulate mammalain digestive system), epinephrine
and norepinephrine (adrenaline and noradrenaline → flight-or-flight response)
● Steroid hormones can diffuse freely across cell membrane to bind w/ receptors in cytoplasm or nucleus, forming a
steroid hormone-receptor complex
○ HR complexes in cytoplasm re transported into nucleus of the cell; most common act as transcription
factors that stimulate/repress gene expression (alter protein production)
○ Typcially have profound/longlasting jeffects on cells/tissues; days→ months
○ E.g. sex hormones estrogen, progesterone, testosterone, cortisole (response to stress and inhibition of
inflamation)
■ Cortisol and similar hormones used medically to reduce symptoms of inflammatory disorders but
risky bc also suppress immune system → ^ chance of infection and v wound healing
Hormone signals are amplified to strengthen their effect
● Typically released in small amounts but amplified before and after cell-receptor binding
○ 1st set of signaling: passing signal from one endocrine gland to the next (hormonal pathway)
○ 2nd set of signaling: signal transduction cascade in the target cell (cell signaling pathway)
● Hormonal pathway: between endocrine glands and tissues; called endocrine axes
○ Hormonal signals amplified along pathway called the hypothalamic- pituary axis from hypothalamus →
anterior pituary gland → target gland/tissues in body
■ Hypothalamus intiially releases trace amounts of releasing factor A peptide hormone that
signals to the anterior pituitary gland through blood vessels, leading to a much larger release of
associated hormones from that organ.
● E.g. hypothalamus secrets corticotropin releasing factor, which stimulates anterior pituary
gland to secrete larger amount of adrenocorticotropic hormone (ACTH)
■ Hormones released by anterior pituary gland bind to cell receptors in target organ
● E.g. ACTH acts on cells of adrenal cortex,stimulating secretion of cortisol homrone (acute
stress response); cuases conversion of glycogen and aminos to glucose
● yields 56000x more gluclose than intiial weight of releasing factors secreted in
hypothalamus
● Amplification in invertabrates: regulation of insect molting and metamorphosis: 2 signaling steps
○ Brain releases PTTH, PTTH signals prothoracic gland
○ Prothoracic gland releases much larger amount of ecdysone, hormone which regulates
growth/metamorphasis; Ecdysone binds to cell receptors in tnarget organ
■ Signaling cascades within these cells further amplify signal
Hormones are evolutionarily conserved molecules w/ diverse functions
● E.g. some vertabrate hormones can also be found in many invertabrate (diverged 500 million years ago); rles in
invertabrate animals have yet to be discovered
● Typically, same hormone serves different functions in vertebrates and invertabrates, may even serve different
functions within vertabrates
○ E.g. tyroid-stimulating hormones (TSH) regulates metabolism in mammals but triggers metamorphasis in
amphibians and feather molt in birds; even found in snales/invertabrates that lack thyroid gland (snails:
triggers stimulation of sperm/eggs)
● Receptors for many hormones evolved well before their hormones;
○ Even tho structure of hormone or its receptor is often largely unchanged, both can be readily selected to
take on new roles as organisms evolve new behaviors and exploit new environment (e.g. roles of TSH
across species)
● Many peptides orignally identified as hormones in various tissues have also been found to function as
neurtrasnmitters in nervous system
○ E.g. oxytocin, which stimulates uterine contraction and release of milk, also serves as neurotransmitter in
brain and is believed to influence social behavior and sexual arrousal
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Document Summary
Homeostasis: the ability to stay stable, or maintain a stable state. E. g. heart rate oscillates around set point of 60 bpm. Negative feedback: tells heart rate to slow down when too fast and to speed up when too slow. Involves many chemical processes that must be coordinated to maintain stability. Lies in cell"s ability to sense and send signals to be able to respond! Hormones play diverse physiological roles in the body: animal growth/development and particularly regulating organism"s response to environment and maintaining stable physiological condiitions within cells/whole animal. The endocrine system helps regulate an organism"s response to its environment. Changes in environment physiological challenges or stresses that organisms must respond to by altering functional states of its body. Sensory signals nervous system reception endocrine system response (slower signaling than nervous system) physiological changes appropriate response to environmental cues. The release of circulating hormones from endocrine changes allow for broad changes in many organ systems, allowing for growth and development.
