Download Chapter 5 Chemical Messengers

Chapter 5 Chemical Messengers
Chapter Outline
5.1 Mechanisms of Intercellular Communication
• General mechanisms
o Direct: Gap junctions (smooth muscle and cardiac muscle)
o Indirect: Chemical messengers
5.2 Chemical Messengers (Table 5.2)
The messenger is produced by the source cell. It binds to the receptor on a target cell to trigger a cell
response
o Chemical Messenger Classification by Function (Table 5.1)
o Paracrine chemical messenger signals a nearby cell. Example: Histamine, responsible for
an inflammation response
o Autocrine chemical messenger is a subclass of paracrines. Chemical that signals the same
cell that secreted it
o Neurotransmitter is the messenger produced by neurons. It is released into the ECF of the
synaptic cleft. Examples: Acetylcholine, GABA, serotonin
o Hormone is the messenger produced by endocrine cells. It is secreted into the blood via the
interstitial fluid. Examples: Insulin, estrogen, thyroxin
o Neurohormone is a special class of hormone that is produced by neurons and secreted into
the blood. Examples: Antidiuretic hormone (ADH), oxytocin
• Chemical Classification of Messengers by Chemical Properties (Table 5.2)
o Solubility properties
o Chemical class
o A lipophobic ligand is water soluble; not lipid soluble. It does not cross the cell membrane.
It binds with receptors on the cell membrane. It activates enzyme and change membrane
permeability.
o A lipophilic ligand can easily crosses the cell membrane bind with intracellular location of
receptors. General action of target response is via gene activation
• Synthesis and Release of Chemical Messengers
o Amino acids neurotransmitters are synthesized within a neuron stored in vesicles and
released by exocytosis
o Amines are derived from amino acids with exception of thyroid hormones.
o Peptide and proteins are formed as other proteins.
o Steroids are derived from the cholesterol molecule
o Eicosanoids is derived from a membrane phospholipid called arachidonic acid
• Transport of Messengers
o Paracrines, autocrines, neurotransmitters, most cytokines diffuse through interstitial fluid.
Ligand is quickly degraded
o Hormones, neurohormones, some cytokines is transported in the blood either in dissolved
form or by carrier protein
o Messenger half-life is the time for a chemical to decrease its concentration by half
 Messengers dissolved in plasma have a relatively short half-life. Example: half-life of insulin
is less than 10 minutes
 Messengers bound to plasma protein have a relatively long half-life Example: Half-life of
cortisol is 90 minutes
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5.3 Signal Transduction
• Signal transduction is the process of producing a response afer a messenger binds to a receptor in
the target
• Receptor Properties
o Specificity: One messenger may bind to many receptor types
o One target may have many types of receptors
o The number of receptors per cell varies and is dynamic
• Magnitude of target response depends on three factors:
o Concentration of the messenger (ligand)
o Number of receptors per target cell
o Receptor affinity for the messenger
 Agonists and Antagonists
o Agonist is when a ligand binds to a receptor triggers a normal response
o Antagonist is when a ligand binds to a receptor not result in a response. Its response is the
opposite of that to an agonist
 Mechanisms: Signal Transduction (Table 5.4)
o Intracellular-mediated responses
 Characteristic of lipophobic ligands (except thyroid hormones)
 Receptors are found in the cytosol or nucleus
 Cell response is via gene activation
o Membrane-bound receptor-mediated responses
 Channel-linked receptors (Chapter 8)
 Enzyme-linked receptors
 Receptor and enzyme—same protein
 Ligand binding activates the enzyme
 Action is direct
 Activated enzyme causes the target response
 Examples: Tyrosine kinases and guanylate cyclases
 G protein–linked receptors (Chapter 8)
• Signal Amplification=Small amounts of ligand can cause a huge response in the target
5.4 Long-Distance Communication (Table 5.6)
 Endocrine Communication (Chapter 6)
 Nervous Communication (Chapter 7 and 8)
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