APK 2105C Lecture Notes - Lecture 33: Phenoxybenzamine, Vasoconstriction, Alpha-Adrenergic Agonist
5 Jun 2018
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Chapter 5, Lecture 3
Chemical Messengers
• Signal transduction mechanisms
o Binding of messenger molecule to target cell receptor
▪ Location of receptor depends on whether the messenger is lipophilic or
lipophobic
• Membrane receptor (water-soluble)
• Cytosolic receptor (fat-soluble)
• Nuclear receptor (fat-soluble)
o Results in changing the activity of proteins OR stimulation of protein synthesis (or
inhibition) in the target cell
• Properties of target cell receptors
o Receptors are very specific
▪ Different affinities for specific chemical messengers within a class
• Might be able to bind multiple catecholamines but might want to
bind with one type over the other more
▪ Multiple cells will be exposed but only a very will actually be affected
because of this
o Target cells possess many different receptors
▪ Multiple types of messengers can act on any given cell
o Receptor binding and magnitude of target cell—
response to a messenger
▪ Graph looks like enzyme activity
▪ Increases and then levels off
• Number of receptors that are bound
increases with more concentration of
messenger (ligand receptor binding)
• Leveling off is when every single
receptor is bound to a messenger
▪ Chronically low levels of binding will cause
an up-regulation in the number of receptors created
• This will increase the chances of receptor binding
▪ Chronically high levels of chemical messengers will cause a down-
regulation in receptors—lower target cell response
• Ex: chronic drug users
o Cells are bombarded with too much chemical
o Cells down-regulate receptors
o Not as much of an effect on receptors—creates a tolerance
for that drug
o Tries to increase dose of drug to get more effect
▪ Target cell response is also affected by its affinity for the messenger
• Will have more binding if there is more affinity
• Higher concentration = higher percentage of receptors bound up
to 100%
• Higher affinity = faster rate of binding
o Receptor agonists vs. antagonists
▪ Not all receptor binding results in a response
▪ Agonists = ligands that cause cellular response when receptor binding
occurs
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