Calcium displacement by local anesthetics. Dependence on pH and anesthetic charge

Ca²+ and local anesthetics act either antagonistically or agonistically on an extraordinary variety of both neuronal and non-neuronal processes. In order to evaluate the molecular basis of these relationships, an examination of the effect of local anesthetics on ⁴⁵Ca²⁺ binding to the cytoplasmic surface of the erythrocyte membrane was conducted. It was found that all anesthetics, regardless of charge or molecular geometry, depleted the red cell membrane to the same level of residual ⁴⁵Ca²⁺ Thus, the same number of ⁴⁵Ca²⁺ binding sites are accessible to such diverse compounds as lidocaine (+), t-butyl alcohol (O), and sodium pentobarbital (-). The concentration of each anesthetic required for ⁴⁵Ca²⁺ displacement was found to correlate with the concentration of anesthetic observed to perturb several biological processes. The function describing the relationship between ⁴⁵Ca²⁺ displacement and anesthetic concentration was shown to depend on the charge and degree of substitution of the anesthetic. Cationic anesthetics were found to displace ⁴⁵Ca²⁺ with a hyperbolic dependence on anesthetic concentration, while uncharged anesthetics displaced ⁴⁵Ca²⁺ invariably in a cooperative manner. Furthermore, tertiary amine anesthetics converted from largely hyperbolic to sigmoidal behavior as the solution pH was increased. A peculiar rise in membrane-bound ⁴⁵Ca²⁺ was observed in the presence of low concentrations of straight chain alcohols, but this increase converted to the usual ⁴⁵Ca²⁺ displacement at higher alcohol concentrations. The tendency to enhance ⁴⁵Ca²⁺ binding at low alcohol concentration decreased as the degree of substitution of the carbinol carbon increased. The above observations are all predicted in a model of calcium displacement which ascribes the capacity of an anesthetic to displace calcium to its ability to insert between a two-point calcium attachment site on the membrane surface and to increase the Ca²⁺-ligand bond distance beyond the optimal bond length of 9.6Å.