Pharmacology

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Drug Receptors and Receptor Mechanisms

Ligand-gated Ion channels: 

  • Fastest response; used in neurotransmission.
  • Binding of neurotransmitter causes opening of the channel's pore to ions; the cell depolarises (excitation) or hyperpolarises (inhibition).
  • Cellular response results from the change in membrane potential, or from the change in intracellular ionic concentration.
  • EXAMPLES: Nicotinic superfamily, glutamate superfamily, purinergic superfamily (ATP).

G-Protein Coupled Receptors:  

  • These are slower than the ligand-gated receptors and allow slower neurotransmission.
  • Process of signal transduction: 
    • The G-protein trimer is bound to GDP.
    • The agonist occupied receptor binds to the G-protein and GDP is exchanged for GTP. The trimer dissociates to α (which is bound to the GTP) and βγ.
    • The α and βγ subunits reach and activate or inhibit effectors. 
    • GTP hydrolyses and the trimer forms again. The GTP hydrolysis rate determintes the length of action. 
  • EXAMPLES: Muscarinic ACh receptor, α and β adrenoceptors, GABA & glutamate metabotropic receptors. 
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Drug Receptors and Receptor Mechanisms

Kinase Linked Receptors:

  • These are agonist stimulated enzymes. Part of the receptor molecule has intrinsic enzymatic activity that is activated when the agonist binds to the receptor. 
  • Process of signal transduction: 
    • An agonist binds to the monomeric receptor, when the agonist binds, the receptor undergoes a conformational change and dimerises. This dimerisation process results in the autophosphorylation of the receptor. The receptor can now recognise an intracellular protein. The complex formed between the receptor and the intracellular protein results in the effect (enzyme activation, gene transcription etc.)
  • EXAMPLES: Tyrosine kinase receptors for insulin, guanylate cyclase receptor for ANP and NO 

Nuclear Receptors: 

  • Two classes of nuclear receptors - Steroid receptors (when inactive are found in the cytoplasm bound to proteins) and PPAP (generally reside in the nucleus and detect lipids that are generally already present).
  • Nuclear receptors respond to lipid-soluble hormones (e.g. steroids & thryoid hormones) as they are able to cross the membrane.
  • Process of action:
    • The agonist binds to the receptor and the receptor undergoes homodimerisation or heterodimerisation and binds to DNA. Transcription of specific genes is then activated or inhibited. 
  • The response is very slow, needing hours to see the effects. 
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