IB04e-Pharmacodynamics.txt

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Anonymous
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162977
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IB04e-Pharmacodynamics.txt
Updated:
2012-07-18 12:33:01
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Local Anesthetics Pharmacodynamics
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Pharmacodynamics
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  1. *Which local anesthetics are chiral drugs (possessing an asymmetric carbon center)?
    The pipecoloxylidide local anesthetics (mepivacaine, bupivacaine, ropivacaine) are chiral drugs because their molecules possess an asymmetric carbon atom. As such, these drugs may have a left (S) or right (R) handed configuration. Molecules that are stereospecific superimposable mirror images of each other and called enantiomers. Enantiomers are optically active - they rotate plane polarized light.
  2. *Which two local anesthetics are administered as racemic mixtures (50:50 mixture of enantiomers)?
    Mepivacaine and bupivacaine are available for clinical use as racemic mixtures of the enantiomers. The enantiomers of a chiral drug may vary in their pharmacokinetics, pharmacodynamics, and toxicity. Administering a racemic drug mixture is, in reality, the administration of two different drugs.
  3. *Identify the two local anesthetics that are pure S enantiomers. What is the advantage of these local anesthetics?
    Ropivacaine and levobupivacaine have been developed as pure S enantiomers (they are not racemic mixtures). These S enantiomers produce less neurotoxicity and cardiotoxicity than racemic mixtures or the R enantiomers of local anesthetics, perhaps reflecting decreased potency at sodium ion channels.
  4. What are the 3 states of the fast, voltage-gated sodium channels?
    • (1) Closed
    • (2) Open
    • (3) Inactivated
  5. How do local anesthetics work?
    Local anesthetics work by preventing voltage gated fast sodium channels in nerve axons from opening. Local anesthetics bind to sodium channels inside the neuron when there are in the inactivated state.
  6. Does conduction block by local anesthetics require the ionized form, unionized form, or both ionized and unionized forms?
    Both forms. The nonionized form crosses the lipid bilayer to enter the neuron, and the ionized form binds to the gated sodium channel while in the inactivated state.
  7. Identify the part of the nerve cell that is affected by local anesthetics.
    The nerve axon is the site of action of the local anesthetics. Only the axon has fast, voltage-gated sodium channels.
  8. What is the primary determinant of local anesthetic potency?
    Lipid solubility.
  9. In general, what structural characteristic of local anesthetic determines lipid solubility and hence, potency?
    The total number of carbon atoms. "In general, lipid solubility (potency) is increased by increasing the total number of carbon atoms in the molecule."
  10. State 3 specific ways, in addition to adding carbon atoms, of increasing potency of a local anesthetic.
    Lipid solubility and potency are increased by adding: (a) a halide (chloride ion, bromide ion, etc) to the aromatic (benzene) ring; (b) an ester linkage; or (c) a large alkyl group on the tertiary amide nitrogen.
  11. What term defines the lowest concentration of local anesthetic that blocks impulse conduction along a given nerve fiber?
    The Minimum Blocking Concentration (Cm) is the lowest concentration of a local anesthetic that blocks conduction.
  12. Which local anesthetic agent is the standard for comparison of potencies of local anesthetics?
    Lidocaine.

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