Local Anesthetics Xiaoping Du Room E417 MSB Department of Pharmacology Phone (312)355 0237; Email: [email protected] Summary: Local anesthetics are drugs used to prevent or relieve pain in the specific regions of the body. Currently used local anesthetics binds to voltage-gated Na+ channels in peripheral nerves, blocks sodium movement through sodium channel, and thus blocks nerve conduction. Membrane potential and neurotransmission: Neuron transmits information mainly by two mechanisms: chemical and electrical signals. Information within a neuron is mainly transmitted by electrical signals. Electrical signals are propagated by the mechanism called action potential. Resting neurons maintain an intracellular negative membrane potential. The mechanism for this resting membrane potential is as follows: Na+/K+ ATPase (sodium pump) transports intracellular Na+ to extracellular in exchange of entry of K+ into cells. This creates a concentration gradients of Na+ and K+ (Figure 1). The resting neuron cell membrane contains much more open K+ channels than open Na+ and Cl- channels or channels for other ions. K+ flows to the outside down the concentration gradient, resulting in a negative potential inside the cell. Figure 1. Mechanisms of resting and action membrane potentials. When stimulated (electrically or chemically), a depolarization of the membrane potential in the neuron (axon) membrane opens voltage-gated Na+ channels. This leads to a burst of flow of Na+ into the cell down the concentration gradient, causing a reverse of the membrane potential (from negative inside to positive inside). Eventually, the influx of Na+ is stopped when Na+ concentration gradient is balanced by the reversed potentialgradient. Na+ channels are closed by the voltage-sensitive regulatory domain and become temporarily insensitive to depolarization. Subsequently, voltage-gated K+ channels open, allowing accelerated outflow of K+. The membrane potential returns to resting state. This process is called action potential, and takes only a few milliseconds to complete. Action potential at one site of the neuron causes partial depolarization of neighboring region, activates voltage-gated Na+ channels in the neighboring region and thus causes propagation of the action potential (electrical signals) along the axon to synapses. Local anesthetics bind to and block the function of voltage-gated sodium channel and thus block conduction of action potenials. Chemistry of local anesthetics: The most widely used local anesthetics today include lidocaine, bupivacaine, procaine, and tetracaine. Currently used local anesthetics are structural mimetic of cocaine. Most of the drugs consist of a hydrophobic groups (often an aromatic ring) connected by an intermediate chain (containing an ester or amide bond) to an ionizable group (usually a tertiary amine group). Local anesthetics are weak bases. pKa for most local anesthetics is in the range of 8.0-9.0. A balance of charged and uncharged forms is present in the body. The ratio between the cationic and uncharged forms of these drugs are determined by the Henderson-Hasselbalch equation (Log(cationic form/uncharged form)=pKa-pH). The uncharged form is more lipophilic and thus more rapidly diffuse through the membrane. However, the charged form has higher affinity for the receptor site of the sodium channel. Mechanism of action: Local anesthetics reversibly bind to voltage-gated sodium channel, block Na+ movement through the channels and thus block action potential and neural conduction. At adequate dosage, these drugs should reversibly inhibit conduction of all neurons. Both the pharmacological effects and most toxic effects of local anesthetics arise from this mechanism.Na+ channels are heterotrimeric transmembrane proteins, consisting of α (Mr~260 kDa), β1 (36 kDa) and β2 (33 kDa) subunits. α subunit contains four homologous domains (I-IV); each domain contains 6 α-helical transmembrane segments (S1-S6). The S5 and S6 segments of the four domains form a pore that allows the passage of Na+ ions. Voltage sensor is located in the 4th transmembrane segments of each domain which is rich in positively charged residues. The loop between domain III and IV serves as an inactivation gate which folds to block the pore shortly after opening of the channel. The binding site for local anesthetics is located in the S6 transmembrane domain of segment IV close to the intracellular side of the membrane. Figure 2. A schematic of the voltage-dependent sodium channel. Characteristics of local anesthetic action: 1. Local anesthetics preferably block small nerve fiber. This is because distance of passive propagation of impulses in the small fibers is shorter. In general, small unmyelinated C fibers (pain signal) and small myelinated Aδ fibers (pain and temperature) are blocked before larger myelinated Aγ, Aβ and Aα fibers (postural, touch, pressure and motor signals) (Table 1). 2. Nerves with higher firing frequency and more positive membrane potential are more sensitive to local anesthetic block (use-dependent block). This is because the charged local anesthetic molecules are more likely to access to the binding sites in the open Na+ channel, and less likely to dissociate from its binding sites in the open or inactivated channels in comparison with the resting Na+ channels. Sensory fibers, especially pain fibers, have a high firing rate and a relatively long action potential duration than motor fibers, and thus are more sensitive to low concentration of local anesthetics. Frequency- and voltage-dependence is also the mechanism of anti-arrhythmic effects of local anesthetics on cardiac cells. 3. In nerve bundles, fibers that are located circumferentially are affected first by local anesthetics. In large nerve trunks, motor nerves are usually located circumferentially and may be affected before the sensoryfibers. In the extremities, proximal sensory fibers are located more circumferentially than distal sensory fibers. Thus, loss of sense may spread from proximal to distal part of the limb. 4. Effectiveness of local anesthetics is affected by the pH of the application site. As mentioned above, uncharged form of these drugs is more likely to penetrate the membrane but the charged form is more active in blocking Na+ channel. Under high pH, most drug molecules are uncharged forms, but have a lower affinity for the
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