The structural domain of LAs responsible for their lipophilicity is the aromatic group. The lipid solubility or partition coefficient of a LA determines its ability to pass through biological membranes and reach their receptor sites.14,18-25 Consequently, LAs must be able to partition into, diffuse across, and finally dissociate from neuronal plasma membranes. As the lipid solubility increases, the partition of a LA through neuronal membranes also increases.

LAs with higher partition coefficients require the use of lower doses to achieve the same degree of neuronal blockade as that achieved by agents with lower lipid solubilities. Therefore, the primary determinant of a LA’s potency is its partition coefficient (Table 2).14,18-25 At some point this relationship reverses and a highly lipid soluble LA would become trapped in the neuronal membrane. The relative potencies of LAs are reflected by their concentrations in aqueous solutions.

Table 2. Partition Coefficients, Relative Potencies, and Percent Concentrations of LAs.14,18-25
Partition coefficient (lipid solubility)43212517346
Relative potency≈2≈1≈1≈1≈8
Percent concentration22 or 3440.5