Download Chapter 2, Introduction to the vertebrate endocrine system

CHAPTER 2, INTRODUCTION
TO THE VERTEBRATE
ENDOCRINE SYSTEM
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General characteristics

Important job of endocrine – maintain a fairly stable
internal environment

Homeostasis – maintaining a set point


Set points differ with age, sex, reproductive status, time in day or year,
stage of development – Romero’s Reactive Scope Model
Endotherms vs. ectotherms

Ectotherms: using environment for temperature saves energy, but
reliant on the environment



Downside – if it’s cold so are you
Upside – the values for homeostasis are quite flexible
Endotherms: maintaining unique set points yourself (esp.
temperature) takes lots of energy


Upside – body (specifically enzymes) can always work at best
capacity
Downside – always on the lookout for food and restrictions in such can
be severe
Physiological roles of hormones

Synthesis and secretion of cellular products
 Other






hormones
Regulating metabolic processes
Contraction, relaxation, and metabolism of muscles
Reproduction
Cellular proliferation
Excretion/retention of inorganic ions
Permissive actions - Behaviors
General Characteristics

Endocrine glands
Ductless – go directly into blood stream
 Secretory cells
 Well vascularized





May be compact, discrete mass – thyroid gland
May be small clumps of cells – Islets of Langerhans
May be individual, diffuse cells – intestinal tract
Usually permanent (exception: placenta)

Area and activity can change over life – obvious one,
gonads pre and post-puberty
General endocrine glands of humans
Plus some
we don’t
think of;
whole brain,
skeleton
General Characteristics
This can be a variety
of distances
NOTE: The
difference, no going
into general
circulation
General Characteristics

Typically see hormones secreted in low concentrations
Binding to high affinity receptors
 Release of hormones pulsatile – minutes to hours, makes for
a low daily mean
 Seasonal release of hormones – makes for a low yearly
mean


Often more than one hormone produced by a single
gland


One hormone can have many actions


Whole pituitary – 10+
Estradiol (estrogen)
One function can have multiple hormone influences

Fat metabolism
Categories of hormones

3 general types of hormone

Grouped based on structure or functional similarities


Peptide hormones – peptide, polypeptide, protein


Take home – they are made of amino acids
Special cases





Book has more classes, I do some grouping – follow what I do here
Monoamine hormones – single amino acids
Thyroid hormones
Steroid hormones - cholesterol
Lipid-based hormones – other than cholesterol
Note some hormones can do double duty as neurotransmitters, depends on
location

Some unexpected neurotransmitters too
Protein (aka, peptide) hormones

Comprised of amino acids



Storable, released by exocytosis



No carriers needed, move freely in blood
Often bound with plasma proteins (esp. albumin) for protection
from degrading enzymes
Receptors typically located in cell membrane


Ca2+ dependent exocytosis
Hydrophilic


Transcription/translation
Lots of modification!
Hydrophilic nature means they cannot pass cell membranes
Secondary messengers important for their actions
Hormone modification
Preprohormones and prohormones, very prevalent and easiest to see in the
protein hormones. Other groups have preprohormone/prohormone
characteristics.
General Characteristics
Example: Proopiomelanocortin (POMC) a
classic preprohormone
(prepropeptide)
Special cases of peptide hormones

Thyroid hormones
Initially a single tyrosine, but multiple altered tyrosines
together
 Iodinated hormone
 Fat soluble and need carrier
 Receptors found inside cell membrane


Neurohormones
Synthesized in the perikarya (cell body) moved to axon
terminals
 Released as is upon stimulation from storage vesicles


Monoamine hormones

Made from single amino acid
Special protein hormones
- monoamines
Epinephrine, produced mainly from adrenal
gland tissue, is the biggie here. Do not get
confused here, dopamine and
norepinephrine are hormones and
neurotransmitters.
These are catecholamines
Special protein hormones - monoamines
The important one here, melatonin. This
is made under specific environmental
circumstances.
This is an indoleamine
Evolution of peptide hormones

Substitutions/changes in amino acids can yield new
hormones with new functions

Oxytocin and vasopressin
Both from posterior pituitary
 Very similar in sequence, 2 amino acid differences
 Oxytocin stimulates milk let-down and uterine contractions
 Vasopressin influences water retention
 Receptors similar – this leads to a interesting wrinkle in my mind


Can lead to formation of hormone families

Similar primary sequences different activities

Insulin-like growth hormone family
Steroid hormones

Majority of hormones gonadal or adrenal in origin
Adrenal glands- glucocorticoids, mineralocorticoids, weak
androgens
 Gonads- androgens, estrogens, progestins


Steroids derived from cholesterol



Liver and dietary sources
Steroids a series of carbon rings
Groups typically identified by number of carbon atoms

Cholesterol has 27 carbons, to produce steroids carbon
atoms are removed
Steroid hormones

Steroids are hydrophobic
Need carriers to move through blood
 Carriers can protect and extend half life


Receptors for steroids located in cytoplasm or nucleus


Sex steroids actions highly conserved in vertebrates


Steroids move easily through cell membranes
Strong impact on reproduction, selection tries not to mess
with reproduction
Minor structural differences yield major functional
differences
Chart outlines conversion of steroids,
note all start with cholesterol. All
conversions must be gone through to
get the more derived forms
Steroid hormones

C21 steroids
 Have
21 carbon atoms in ring structure
 Progestins,
 Are
glucocorticoids, mineralcorticoids
considered prohormones
 Can
act as hormones themselves, but are precursors for
other hormones
Steroid hormones

C19 steroids
 Have
19 carbons in ring structure
 Androgens
 Important
in males for primary and secondary sex
characters, gamete production, and reproductive
behavior
 Are considered prohormones – estradiol (estrogen)
Steroid hormones

C18 steroids
 Have
18 carbon atoms in ring structure
 Estrogens
 Produced
from androgens by aromatase
 Important to female primary and secondary sex
characteristics, gamete formation, and behavior
 Has influences on male behavior
 Aggressive
behavior in male mice
 Song system development in male birds
Lipid-based hormones: Eicosanoids

Prostaglandins – discovered in 1930’s
 1st
finding – in seminal fluid & induce uterine
contractions

Have a 20 carbon atom fatty acid skeleton
 Arachidonic
acid
 Cyclooxygnease – enzyme for modification



Known to have direct effects on behavior in insects
Involved in reproduction, smooth muscle contraction
and relaxation, and cardiac function
Other types: thromboxanes and leukotrienes
Control of hormone release

Electrical (nerve) or chemical stimulation
 Induces


endocrine cell to produce or release product
Induced by metabolic by-product
One hormone can induce another to be released
 Hypothalamic
hormones induce pituitary hormone
release

Regulation via positive or negative feedback
General actions of hormones

Hormones will act via
receptors
Peptide – in membrane
(at right)
 Steroid – intracellular or
intra-nuclear



Secondary messengers
important
Modifications to
transcription/translation
Pathophysiology

Classic


Hypofunction



Gland destruction
Extraglandular disorder
Hyperfunction



Excess or deficient levels
Gland destruction or growth
Extraglandular sources
Reduced sensitivity



Genetic
Acquired
Abnormal levels