Adsorption of local anesthetics on phospholipid membranes.

In order to elucidate various types of adsorption modes of local anesthetics in membranes, a study of local anesthetic adsorption on lipid membranes was made by measuring electrophoretic mobility of phospholipid vesicles in the presence of local anesthetics of various concentrations in the vesicle suspension solution. The amounts of local anesthetics to be adsorbed on the membrane surface were deduced from the electrophoretic mobility of a phosphatidylcholine vesicle at various concentrations of the cationic form of local anesthetics. The order of surface adsorption of local anesthetic was dibucaine greater than tetracaine greater than procaine. A surface partition coefficient, Ks = 1/ACs, was introduced, where A is the membrane surface area per local anesthetic molecule adsorbed and Cs the surface concentration of local anesthetics. The amounts of local anesthetic adsorbed on phosphatidylserine membrane were much greater than that of the phosphatidylcholine membrane. It was deduced that the major factor for this large adsorption was due to the enhancement of cationic forms of local anesthetic concentrations at the charged membrane surface. Divalent cations inhibited such surface adsorption of local anesthetics by reducing surface concentrations of local anesthetics where the surface potential of the negatively charged membrane surface was influenced by the presence of divalent cations in the solution as well as by the reduction of fixed surface charges due to divalent cation binding. Some association modes of local anesthetics on nerve membranes are discussed with the results obtained in the above adsorption study.