MOLECULAR MECHANISMS OF LOCAL-ANESTHESIA - A REVIEW

Impulse block by LA occurs through the inhibition of voltage-gated Na+ channels. Both protonated and neutral LAs can inhibit Na+ channels though interference with the conformational changes that underly the activation process (the sequence of events that occurs as channels progress from the closed resting state to the open conducting state). The occlusion of open channels contributes little to the overall inhibition. Local anesthetic inhibition of Na+ currents increases with repetitive depolarizations in a process called phasic block. Phasic block represents increased LA binding, either because more channels become accessible during depolarization or because the channel conformations favored by depolarization bind LA with higher affinity. The details of phasic block are dependent on LA chemistry: certain LAs bind and dissociate quite rapidly, others act more slowly; some LAs interact effectively with closed states that occur intermediately between resting and open states, others favor the open channel, and still others have a higher affinity for inactivated states. Channel activation accelerates LA binding, and LAs may bind more tightly to activated and inactivated than to resting channels. In this regard, both the modulated receptor and the guarded receptor hypotheses are valid. In binding to activated and inactivated channels, LAs prevent the conformational changes of activation and antagonize the binding of activator agents that poise channels in activated, open states. These reciprocal actions are one aspect of the concerted conformational rearrangements that occur throughout Na+ channels during gating. The LA binding site may exist in the channel's pore, at the membrane-protein interface, or within the protein subunits of the channel. Judging from its susceptibility to intracellular proteases and its accessibility to LAs with limited membrane permeability (i.e., quaternary LAs in the cytoplasm), the site lies nearer to the cytoplasmic than the external surface of the membrane. Nevertheless, protons in the external medium influence the dissociation of LA from the closed channel. Binding of LAs at the inhibitory site is weak and loose. If one accounts for the membrane-concentrating effects of LA hydrophobicity that are expressed as membrane: buffer partition coefficients equal to 102-104, then the apparent LA affinities are low. The equilibrium dissociation constants calculated on the basis of free drug in the membrane are 1-10 mM, with a correspondingly weak binding to the inhibitory LA site. The stereospecificity of LA action is also relatively nonselective, suggesting a loose fit between ligand and binding site. We speculate that a LA molecule acts by fitting into an amphipathic pocket located on or between membrane-spanning regions of the Na+ channel. In proposed models for the tertiary and quaternary structure of the Na+ channel, each of four homologous regions of the one large polypeptide contain six to eight membrane-spanning α helices (a-h). Several of these helices are composed of acidic or basic amino acid side chains that are aligned along one edge of the helix, forming ion pairs with each other. Each ion pair constitutes an electric dipole, and when the membrane is depolarized these dipoles shift locally, the electrically coupled helices slide in or out of the membrane (accounting for gating current), and the pore-forming regions of the channel (designated as the helices marked c) change conformation, permitting ion passage, thus opening the channel. We proposed that LAs bind at the dipole-containing helices such as to inhibit their rearrangement in response to membrane depolarization. The site's availability to LA depends on the channel's conformation and, reciprocally, LA binding limits the Na+ channel to certain conformations, Smaller LAs enter and leave the site relatively quickly, larger ones enter and leave more slowly and may not be able to reach the deeper recesses. Intermediate-sized LAs enter relatively rapidly, can reach deeply, and leave at rates that are dependent on the drug's overall shape and structural flexibility. This model is speculative, but it provides an alternative to the widely promulgated 'open-channel blocking' diagrams and, we believe, explains the experimental results equally well.