Receptors are a specific functional protein molecule, usually located in cell membrane and are the means by which most drugs produce their effects. Each cell generally only expresses certain types of receptors, which depend on the function/purpose if that cell receptors are responsible for the selectivity of drug action.
The binding of a drug to a receptor is reversible

Four types of receptors for chemical messengers:
1) Ionotropic
• not necessarily a true receptor
• linked to ion channels situated in cell membranes
• mainly involved in fast actions – the delay between ligand bonding and the opening of the channel occurs in milliseconds

2) Metabotropic
• linked via G-proteins to membrane enzymes and intracellular processes
• G-protein linked receptors – usually result in a rapid transduction – up to a few seconds
• G-protein linked receptors are a single polypeptide chain – they interact with either ion channels or second messengers

3) Linked to tyrosine kinases
• time response of these enzyme initiated transduction is slow – up to several minutes
• activation of tyrosine kinase receptors  autophosphorylation of tyrosine  activation of pathways involving protein kinases
• eg used by insulin

4) DNA linked receptors
• linked to DNA interactions
• slow process – up to several hours
• receptors are intracellular – drug must pass through cell membrane to be reach receptor

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