Effects of Local Anesthetics on the Rate of Rotational Mobility of Phospholipid Liposomes

Using fluorescence probes, 2-(9-anthroyloxy) stearic acid (2- AS) and 12-(9-anthroyloxy) stearic acid (12-AS), we determined the differential effects of local anesthetics (tetracaine-HCI, bupivacaine-HCI, lidocaine-HCI, prilocaine-HCI and procaine-HCI) on the differential rotational rate between the surface (in carbon number 2 and its surroundings including the head group) and the hydrocarbon interior (in carbon number 12 and its surroundings) of the outer monolayer of the total phospholipid fraction liposome that is extracted from synaptosomal plasma membrane vesicles. The anisotropy (r) values for the hydrocarbon interior and the surface region of the liposome outer monolayer were$0.051{\pm}0.001$ and $0.096{\pm}0.001,$ respectively. This means that the rate of rotational mobility in the hydrocarbon interior is faster than that of the surface region. Local anesthetics in a dosedependent manner decreased the anisotropy of 12-AS in the hydrocarbon interior of the liposome outer monolayer, but increased the anisotropy of 2-AS in the surface region of the monolayer. These results indicate that local anesthetics have significant disordering effects on the hydrocarbon interior, but have significant ordering effects on the surface region of the liposome outer monolayer.