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1)Cytokines peptides secreted by cells into the ECF act as
autocrines, paracrines, or endocrine hormones as leptin produced
by adipocytes are
sometimes called adipokines
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Endocrine system
The endocrinesystem
broadcasts its hormonalmessages to essentially
all cells by secretion into
blood and extracellular
fluid.
Like a radio broadcast,
it requires a receiverto
get the message
In the case of endocrine
messages, cells must
bear a receptorfor the
hormone being broadcast
in order to respond.
Nervous system
The nervous
systemexerts point-to-
point control
through nerves,
similar tosending
messages by
conventional
telephone.
Nervous control
is electricalin
nature and fast.
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Endocrineglands or specialized
cells release hormones
into the circulating
blood that influencethe function of cells at
another location in the
body.
Paracrine
in which cells secret substances
that diffuse into the extracellular
fluid and affect neighboring
cells.
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Three Stages of Signal Transduction
1. Reception of extracellular signal by cell
2. Transductionof signal from outside of cell to inside of cell
often multi-stepped
Note not necessarily transduction of ligand
3. Cellular Response
Response is inititiated and/or occurs entirely within receiving
cell
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2a. Transduction
2b. Transduction
2c. Transduction
2d. Transduction
1. Reception
3. ResponseResponses usually involve increasing or decreasing some
Proteins unction
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1.Proteins and Polypeptides, including hormones secreted by the anterio
and posterior pituitary gland, the pancreas (insulin and glucagon), the
parathyroid gland (parathyroid hormone), and many others.
2.Derivatives of the amino acid
Tyrosine, secreted by the thyroid (thyroxine and triiodothyronine)
and the adrenal medullae (epinephrine and norepinephrine)
Tryptophane-derived hormones as Serotonin and Melatonin
Glutamic acid-derived hormone as Histamine
3. Steroids hormones:-are derived mainly from cholesterol secreted
by the adrenal cortex (cortisol and aldosterone), the ovaries
(estrogen and progesterone), the testes (testosterone), and the
placenta (estrogen and progesterone)
Eicosanoids: small molecules synthesized from fatty acid substrates
(e.g. arachidonic acid) its receptors located within cell
membranes
ass cat on o ormones
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Lipid-soluble Hormones Steroids
Lipids derived from cholesterol in
SER
Different functional groups attachedto core of structure provideuniqueness
Interact with specific intracellularreceptors (within the cell) to turnspecific genes on or off effective forhours or days
Thyroid hormones
Tyrosine ring plus attached iodinesare lipid-soluble
Activate enzymes involved in thecatabolism of fats and glucose
Help set our basal metabolic rate
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Six steps of cell-cell communication:
1. Synthesis of signal.
2. Release of the signaling molecule by the signalingcell: exocytosis, diffusion, cell-cell contact.
3. Transport of the signal to the target cell.
4. Detection of the signal by a specific receptorprotein
5. A change in cellular metabolism, function ordevelopment triggered by the receptor-signalcomplex
6. Removal of the signal
R t
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Receptors
Cell membrane
ReceptorsCytoplasmic
Receptors
Nuclear
ReceptorsFor protein, some a.aderivatives
As GH, insul in and
catecholamines
Estrogen,
testosterone
and cortisole
Thyroid
hormones
Classif ied according to the subcellular second
messenger that mediate the information from the cell
membrane receptors to inside the cell
1. G-Protein Coupled Receptors (GRCR)2. I on channel receptors3. Receptors-linked-Kinase
4. Receptor- kinase
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Cell Surface Receptor-mediated
Signaling
1.G-Protein Coupled Receptors (GRCR)
2.Ion channel receptors
3. Receptors-linked-Kinase
4. Receptor- kinase
a. Voltage gated ion channel
b. L igand gated ion channel
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GPCRs are involved in a range of signaling pathways
including light detection odorant detection and detection
of certain hormones and neurotransmitters
Many different mammalian cell-surface receptors
including GPCRs are coupled to a trimeric signal-
transducing
G protein
Ligand binding activates the receptor which activates the
G protein which activates an effector enzymeto generate
an
intracellular second messenger
e.g.
adenylyl cyclase
converts ATP to
c MP
depending on regulation at the effector protein
this
pathway can be either activated or inhibited
Gsproteins result in stimulation of the effector enzyme
Gi proteins result in inhibition of the effector enzyme
1-G-Protein-Linked Receptors
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G-Protein-Linked Receptors
Fig. 11.7
I I h l t
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I-Ion-channel receptors
ligand binding changes the confirmation of the receptor so that
specific ions flow through it
-the resultant ion movement alters the electric potential across the
plasma membrane
found in high numbers on neuronal plasma membranes
e.g. ligand-gated channels for sodium and potassium
also found on the plasma membrane of muscle cells
binding of acetylcholine results in ion movement and eventual
contraction of muscle
They are of two types;-
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2A-Voltage-gated ion channel
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2B-Ligand-gated ion channel
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3 Tyrosine kinase l inked receptors
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Lack intrinsic catalytic activity
Binding of the ligand results in the formation of a receptor
dimer (2 receptors)
This dimer then activates a class of protein called tyrosinekinases
This activation results in the phosphorylation of downstream
targets by these tyrosine kinases
E.g Receptors for cytokines such as erythropoietin receptor
3-Tyrosine kinase-l inked receptors
4 Tyros ne K nase Receptors
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-Also called receptor tyrosine kinases OR ligand-tr iggeredprotein kinases -Similar to tyrosine-linked receptors -
ligand binding results in formation of a dimer
-BUT: they differ from tyrosine-linked receptorsintrinsic
catalytic activity
4-Tyros ne-K nase Receptors
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