Transcript Endocrine Physiology Lecture 2

Endocrine Physiology
lecture 2
Dale Buchanan Hales, PhD
Department of Physiology & Biophysics
Metabolic clearance rate
(MCR)
• Defines the quantitative removal of hormone from
plasma
• The bulk of hormone is cleared by liver and
kidneys
• Only a small fraction is removed by target tissue
– protein and amine hormones bind to receptors and are
internalized and degraded
– Steroid and thyroid hormones are degraded after
hormone-receptor complex binds to nuclear chromatin
• 99% of excreted hormone is degraded or
conjugated by Phase I and Phase II enzyme
systems
MCR of some hormones
Hormone
Half-life
Amines
2-3 min
Thyroid hormones: T4
T3
6.7 days
0.75 days
Polypeptides
4-40 min
Proteins
15-170 min
Steroids
4-120 min
Hormone-Receptor
interactions
• Definition: a protein that binds a ligand with high
affinity and low capacity. This binding must be
saturuable.
• A tissue becomes a target for a hormone by
expressing a specific receptor for it. Hormones
circulate in the blood stream but only cells with
receptors for it are targets for its action.
Agonist vs. Antagonist
• Agonists are molecules that bind the receptor and
induce all the post-receptor events that lead to a
biologic effect. In other words, they act like the
"normal" hormone, although perhaps more or less
potently
• Antagonists are molecules that bind the receptor
and block binding of the agonist, but fail to trigger
intracellular signaling events
Hormone binding study
Hormone-receptor interactions
• Hormone--receptor interaction is defined by an
equilibrium constant called the Kd, or dissociation
constant.
• The interaction is reversible and how easily the
hormone is displaced from the receptor is a
quantitation of its affinity.
• Hormone receptor interactions are very specific
and the Kd ranges from 10-9 to 10-12 Molar
Analysis of hormone interactions:
Scatchard plots
Spare receptors
• In most systems the maximum biological response
is achieved at concentrations of hormone lower
than required to occupy all of the receptors on the
cell.
• Examples:
– insulin stimulates maximum glucose oxidation in
adipocytes with only 2-3% of receptors bound
– LH stimulates maximum testosterone production in
Leydig cells when only 1% of receptors are bound
Spare Receptors
• Maximum response with 2-3% receptor
occupancy
• 97% of receptors are “spare”
• Maximum biological response is achieved when
all of the receptors are occupied on an average of
<3% of the time
• The greater the proportion of spare receptors, the
more sensitive the target cell to the hormone
• Lower concentration of hormone required to
achieve half-maximal response
Binding vs. biological response
Spare receptors
Amplification by
2nd messenger
Hormonal measurements
• Bioassay
– an assay system (animal, organ, tissue, cell or enzyme
system) is standardized with know amounts of the
hormone, a standard curve constructed, and the activity
of the unknown determined by comparison
• example: testosterone stimulates growth of prostate gland
of immature or castrate rat in a dose-dependent manner.
Androgen content of unknown sample can be determined
by comparison with testosterone.
– disadvantage: cumbersome and difficult
– advantage: measures substance with biological activity,
not just amount
Original bioassay systems
defined the endocrine system
• Remove endocrine gland and observe what
happened
• Prepare crude extract from gland, inject back into
animal and observe what happened
• In isolated organ or cell systems, add extract or
purified hormonal preparations and measure
biological response
Hormonal measurements
• Chemical methods
– chromatography
– spectrophotometery
Radioimmunoassay
• Radioactive ligand and unlabeled ligand compete for same
antibody. Competition is basis for quantitation
– saturate binding sites with radioactively labeled
hormone (ligand)
– in parallel incubate complex with unknown and
determine its concentration by comparison
– cold ligand (standard or unknown) competes with
labeled ligand for binding to antibody and displaces it
in a dose-dependent way
– amount of cold ligand is inversely proportional to
amount of radioactivity
– (cold competes with hot so the more cold that binds
antibody the more hot is displaced resulting in fewer
counts being associated with complex.
radioactivity
RIA
Increasing amount of insulin
RIA
• advantages:
– extremely sensitive due to use of radioisotope
– large numbers of samples can be processed simultaneously
– small changes in hormone concentrations can be reproducibly
quantitated
– Easily automated for high-throughput analysis
• disadvantage:
– can't determine if hormone measured has biological activity
– peptide hormones can be denatured and not active but still retain
their antigenic character
Classes of hormones
The hormones fall into two general classes
based on their solubility in water.
The water soluble hormones are the
catecholamines (epinephrine and
norepinephrine) and peptide/protein hormones.
The lipid soluble hormones include thyroid
hormone, steroid hormones and Vitamin D3
Types of receptors
 Receptors for the water soluble hormones are
found on the surface of the target cell, on the
plasma membrane.
 These types of receptors are coupled to various second
messenger systems which mediate the action of the
hormone in the target cell.
 Receptors for the lipid soluble hormones reside in
the nucleus (and sometimes the cytoplasm) of the
target cell.
 Because these hormones can diffuse through the lipid
bilayer of the plasma membrane, their receptors are
located on the interior of the target cell
Hormones and their receptors
Hormone
Class of
hormone
Location
Amine
(epinephrine)
Water-soluble
Cell surface
Amine (thyroid
hormone)
Lipid soluble
Intracellular
Peptide/protein
Water soluble
Cell surface
Steroids and
Vitamin D
Lipid Soluble
Intracellular
Second messenger systems
Receptors for the water soluble hormones
are found on the surface of the target cell,
on the plasma membrane. These types of
receptors are coupled to various second
messenger systems which mediate the
action of the hormone in the target cell
Second messengers for cellsurface receptors
 Second messenger systems include:
 Adenylate cyclase which catalyzes the conversion of
ATP to cyclic AMP;
 Guanylate cyclase which catalyzes the conversion of
GMP to cyclic GMP (cyclic AMP and cyclic GMP are
known collectively as cyclic nucleotides);
 Calcium and calmodulin; phospholipase C which
catalyzes phosphoinositide turnover producing inositol
phosphates and diacyl glycerol.
Types of receptors
Second messenger systems
 Each of these second messenger systems activates
a specific protein kinase enzyme.
 These include cyclic nucleotide-dependent protein
kinases
 Calcium/calmodulin-dependent protein kinase, and
protein kinase C which depends on diacyl glycerol
binding for activation.
 Protein kinase C activity is further increased by calcium which
is released by the action of inositol phosphates.
Second messenger systems
 The generation of second messengers and
activation of specific protein kinases results in
changes in the activity of the target cell which
characterizes the response that the hormone
evokes.
 Changes evoked by the actions of second
messengers are usually rapid
Signal transduction
mechanisms of hormones
Activation of
adenylate
cyclase
Inhibition of
adenylate
cyclase
Increased
phosphoinositide
turnover
Tyrosine kinase
activation
b-adrenergic
a2-adrenergic
a1-adgrenergic
Insulin
LH, FSH, TSH,
hCG
Opioid
Angiotensin II
Growth factors
(PDGF, EGF,
FGF, IGF-1
Glucagon
Muscarinic
cholinergic – M2
Muscarinic
cholinergic – M3
Growth hormone
Vasopressin –V1
Prolactin
Vasopressin- V2
ACTH
Cell surface receptor action
G-protein coupled receptors
Adenylate cyclase, cAMP and PKA
Amplification
via 2nd
messenger
Transmembrane kinase-linked
receptors
 Certain receptors have intrinsic kinase activity. These
include receptors for growth factors, insulin etc. Receptors
for growth factors usually have intrinsic tyrosine kinase
activity
 Other tyrosine-kinase associated receptor, such as those for
Growth Hormone, Prolactin and the cytokines, do not have
intrinsic kinase activity, but activate soluble, intracellular
kinases such as the Jak kinases.
 In addition, a newly described class of receptors have
intrinsic serine/threonine kinase activity—this class
includes receptors for inhibin, activin, TGFb, and
Mullerian Inhibitory Factor (MIF).
Protein tyrosine kinase receptors
Receptors for lipid-soluble
hormones reside within the cell
 Because these hormones can diffuse through the lipid
bilayer of the plasma membrane, their receptors are located
on the interior of the target cell.
 The lipid soluble hormone diffuses into the cell and binds
to the receptor which undergoes a conformational change.
The receptor-hormone complex is then binds to specific
DNA sequences called response elements.
 These DNA sequences are in the regulatory regions of
genes.
Receptors for lipid-soluble
hormones reside within the cell
 The receptor-hormone complex binds to the regulatory
region of the gene and changes the expression of that gene.
 In most cases binding of receptor-hormone complex to the
gene stimulating the transcription of messenger RNA.
 The messenger RNA travels to the cytoplasm where it is
translated into protein. The translated proteins that are
produced participate in the response that is evoked by the
hormone in the target cell
 Responses evoked by lipid soluble hormones are
usually SLOW, requiring transcription/translation to
evoke physiological responses.
Mechanism of lipid
soluble hormone
action
Receptor control mechanisms
• Hormonally induced negative regulation of receptors is
referred to as homologous-desensitization
• This homeostatic mechanism protects from toxic effects of
hormone excess.
• Heterologous desensitization occurs when exposure of the
cell to one agonist reduces the responsiveness of the cell
any other agonist that acts through a different receptor.
• This most commonly occurs through receptors that act
through the adenylyl cyclase system.
• Heterologous desensitization results in a broad pattern of
refractoriness with slower onset than homologous
desensitization
I fought the law, but the law
won…..
Mechanisms of endocrine
disease
• Endocrine disorders result from hormone
deficiency, hormone excess or hormone
resistance
• Almost without exception, hormone
deficiency causes disease
– One notable exception is calcitonin deficiency
Mechanisms of endocrine
disease
• Deficiency usually is due to destructive
process occurring at gland in which
hormone is produced—infection, infarction,
physical compression by tumor growth,
autoimmune attack
Type I Diabetes
Mechanisms of endocrine
disease
• Deficiency can also arise from genetic
defects in hormone production—gene
deletion or mutation, failure to cleave
precursor, specific enzymatic defect (steroid
or thyroid hormones)
Congenital Adrenal Hyperplasia
Mechanisms of endocrine
disease
• Inactivating mutations of receptors can
cause hormone deficiency
Testicular Feminization Syndrome
Mechanisms of endocrine
disease
• Hormone excess usually results in disease
• Hormone may be overproduced by gland
that normally secretes it, or by a tissue that
is not an endocrine organ.
• Endocrine gland tumors produce hormone
in an unregulated manner.
Cushing’s Syndrome
Mechanisms of endocrine
disease
• Exogenous ingestion
of hormone is the
cause of hormone
excess—for example,
glucocorticoid excess
or anabolic steroid
abuse
Mechanisms of endocrine
disease
• Activating mutations of cell surface receptors
cause aberrant stimulation of hormone production
by endocrine gland.
– McCune-Albright syndrome usually caused by
mosaicism for a mutation in a gene called GNAS1
(Guanine Nucleotide binding protein, Alpha
Stimulating activity polypeptide 1).
– The activating mutations render the GNAS1 gene
functionally constitutive, turning the gene irreversibly
on, so it is constantly active. This occurs in a mosaic
pattern, in some tissues and not others.
Mechanisms of endocrine
disease
• Malignant transformation of non-endocrine
tissue causes dedifferentiation and ectopic
production of hormones
• Anti-receptor antibodies stimulate receptor
instead of block it, as in the case of the
common form of hyperthyrodism.
Grave’s Disease
Mechanisms of endocrine
disease
• Alterations in receptor number and function
result in endocrine disorders
• Most commonly, an aberrant increase in the
level of a specific hormone will cause a
decrease in available receptors
Type II diabetes
Hypothalamus and Pituitary
Hypothalamus and Pituitary
• The hypothalamus-pituitary unit is the most
dominant portion of the entire endocrine
system.
• The output of the hypothalamus-pituitary
unit regulates the function of the thyroid,
adrenal and reproductive glands and also
controls somatic growth, lactation, milk
secretion and water metabolism.
Hypothalamus and pituitary gland
Hypothalamus and pituitary gland
Hypothalamus and Pituitary
• Pituitary function depends on the hypothalamus
and the anatomical organization of the
hypothalamus-pituitary
unit
reflects
this
relationship.
• The pituitary gland lies in a pocket of bone at the
base of the brain, just below the hypothalamus to
which it is connected by a stalk containing nerve
fibers and blood vessels.
The pituitary is
composed to two lobes-- anterior and posterior
Posterior Pituitary:
neurohypophysis
• Posterior pituitary: an outgrowth of the
hypothalamus composed of neural tissue.
• Hypothalamic neurons pass through the
neural stalk and end in the posterior
pituitary.
• The upper portion of the neural stalk
extends into the hypothalamus and is called
the median eminence.
Hypothalamus and
posterior pituitary
Midsagital view
illustrates that
magnocellular neurons
paraventricular and
supraoptic nuclei secrete
oxytocin and
vasopressin directly into
capillaries in the
posterior lobe
Anterior pituitary:
adenohypophysis
• Anterior pituitary: connected to the
hypothalamus by the superior hypophyseal artery.
• The antererior pituitary is an amalgam of hormone
producing glandular cells.
• The anterior pituitary produces six peptide
hormones: prolactin, growth hormone (GH),
thyroid stimulating hormone (TSH),
adrenocorticotropic hormone (ACTH), folliclestimulating hormone (FSH), and luteinizing
hormone (LH).
Hypothalamus
and anterior
pituitary
Midsagital view
illustrates
parvicellular
neurosecretory cells
secrete releasing
factors into capillaries
of the pituitary portal
system at the median
eminence which are
then transported to
the anterior pituitary
gland to regulate the
secretion of pituitary
hormones.
Anatomical and functional
organization
neocortex
Reituclar
activating
substance
Sleep/
wake
Thalamus
Limbic
system
pain
Emotion, fright,
rage, smell
Heat regulation
(temperature)
Water balance (blood
volume, intake--thirst,
output—urine volume)
Energy
regulation
(hunger,
BMI)
Optical
system
vision
Autonomic
regulation
(blood pressure
etc)
Regulation
of
Hypothalamus
Metabolic rate, stress
response, growth,
reproduction, lactation)
posterior
pituitary
hormones
Anterior
pituitary
hormones
Hypothalamus/Pituitary
Axis
Hypothalamic releasing factors for
anterior pituitary hormones
 Travel to adenohypophysis via hypophyseal-portal
circulation
 Travel to specific cells in anterior pituitary to
stimulate synthesis and secretion of trophic
hormones
Hypothalamic releasing hormones
Hypothalamic releasing hormone
Effect on pituitary
Corticotropin releasing hormone
(CRH)
Thyrotropin releasing hormone
(TRH)
Growth hormone releasing
hormone (GHRH)
Somatostatin
Stimulates ACTH secretion
Gonadotropin releasing hormone
(GnRH) a.k.a LHRH
Prolactin releasing hormone (PRH)
Prolactin inhibiting hormone
(dopamine)
Stimulates TSH and Prolactin
secretion
Stimulates GH secretion
Inhibits GH (and other hormone)
secretion
Stimulates LH and FSH
secretion
Stimulates PRL secretion
Inhibits PRL secretion
Characteristics of hypothalamic
releasing hormones
•
•
•
•
•
•
Secretion in pulses
Act on specific membrane receptors
Transduce signals via second messengers
Stimulate release of stored pituitary hormones
Stimulate synthesis of pituitary hormones
Stimulates hyperplasia and hypertophy of target
cells
• Regulates its own receptor
Hypothalamus
and anterior
pituitary
Anterior pituitary
• Anterior pituitary: connected to the
hypothalamus by hypothalmoanterior pituitary
portal vessels.
• The anterior pituitary produces six peptide
hormones:
–
–
–
–
–
prolactin, growth hormone (GH),
thyroid stimulating hormone (TSH),
adrenocorticotropic hormone (ACTH),
follicle-stimulating hormone (FSH),
luteinizing hormone (LH).
Anterior pituitary cells and hormones
Cell type
Pituitary
Product
population
Target
Corticotroph
15-20%
Thyrotroph
3-5%
Adrenal gland
ACTH
b-lipotropin Adipocytes
Melanocytes
TSH
Thyroid gland
Gonadotroph
10-15%
LH, FSH
Gonads
Somatotroph
40-50%
GH
Lactotroph
10-15%
PRL
All tissues, liver
Breasts
gonads
Anterior pituitary hormones