Effects of calcium on permethrin-phosphatidylserine interactions in model membranes

Abstract

The effects of permethrin on lipid packing order in liposomal membranes containing phosphatidylserine (PS) have been examined in the presence and absence of calcium. Steady-state fluorescence anisotropy and phase-modulation techniques were utilized with the fluorescent probes 1,6-diphenyl-1,3,5-hexatriene (DPH), trans-parinaric acid, and cis-parinaric acid to determine alterations in lipid packing by permethrin as a function of temperature. In general, permethrin exhibited disordering effects only in gel phase lipid. In pure PS membranes, permethrin had little effect on small unilamellar vesicles (SUVs). However, substantial reductions in lipid packing order were detected by decreases in DPH fluorescence anisotropy when these SUVs underwent isothermal fluid to gel phase transition upon exposure to Ca 2+. Although Ca 2+ increased the main PS phase transition temperature ( T c ), a small residual phase transition was detectable by parinaric acid anisotropy and lifetimes within the temperature range corresponding to the SUV phase transition. Permethrin smoothed this residual transition, suggesting that this compound altered a process which was preventing a portion of PS from undergoing the Ca 2+-mediated isothermal transition to the solid phase. In mixed liposomes containing dipalmitoyl phosphatidylcholine (DPPC) at a PS:DPPC ratio of 3:2, the T c of the SUVs was increased while that of the Ca 2+-treated vesicles was decreased. As in the pure PS system, permethrin had a greater disordering effect upon the lipid packing in the Ca 2+-treated membranes. There was a slight disordering effect by permethrin in PS:DPPC SUVs at temperatures below the T c , indicating the existence of DPPC enriched domains in the gel phase lipid which were sensitive to permethrin. The results demonstrate that Ca 2+-mediated changes in membrane characteristics, such as isothermal fluid to gel phase transition, can influence membrane sensitivity to the lipid disordering effects of permethrin.