Effects of Halothane, Thiopental, and Lidocaine on Fluidity of Synaptic Plasma Membranes and Artificial Phospholipid Membranes

Abstract

The effects of halothane, thiopental and lidocaine were studied with spin-labeling methods in rat synaptic plasma membranes (order parameter) and artificial phospholipid membranes (lateral diffusion). Halothane had a biphasic action, low concentrations (0.64 mM) ordering and high concentrations (2.9 mM) fluidizing both types of membranes. A biphasic effect in phospholipid membranes was also seen with thiopental, 0.1 mM ordering and 10 mM fluidizing; in synaptic plasma membranes both low and high concentrations caused an increased order in the lipid bilayer region. At high thiopental concentrations, a considerable number of molecules may have reacted with membrane proteins or accumulated in the highly fluidic hydrophobic interior region of the membrane without affecting the rotational movement of the labeled fatty acid. Lidocaine alone, or together with CaCl2, at various concentrations to 10 mM had no significant effect; a fluidizing effect of 1 mM CaCl2 was possibly a result of interaction of CaCl2 with the label. The 3 lipid-soluble anesthetics interacted differently with the lipid part of membranes. Lidocaine did not seem to affect bilayer lipids, while thiopental and halothane in phospholipid vesicles and halothane alone in synaptic membranes caused a dose-dependent biphasic effect.