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Unformatted text preview: Change nature of the cell
(Longer-lasting effect) Modulation of signal effect
• Priming (upregulation)
Signal bindsÆ more receptors synthesized
more hormone can bind cell
• Desensitization (downregulation)
Prolonged exposure to high signal molecule
levels can reduce receptor expression.
* Downregulation may be avoided by pulsatile
secretion of the messenger. • Receptor-mediated endocytosis
Receptor-ligand complex internalized on vesicle
to enhance duration of effect. Electrochemical communication Mechanisms of Messenger Action
• Hydrophilic signal molecules — most amino acid class
– Bind to membrane receptors on cell surface
– Primary effect: turn enzymes on/off Æ D activity of cell.
– Secondary effect: enzymes may produce or activate transcription
factors Æ turn genes on/off.
• Lipophilic signal molecules — steroids & thyroid hormones
– Bind to intracellular receptors in cytoplasm or nucleoplasm
– Primary effect: turn genes on/off Æ D nature of cell.
– Secondary effect: gene expression may produce or...
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- Fall '10