Ion selectivity of temperature-induced and electric field induced pores in dipalmitoylphosphatidylcholine vesicles

Temperature and electric field are known to alter the permeability of the bilayer membrane in phospholipid vesicles. A study of cation selectivity of these membrane pores is reported for multilamellar liposomes (MLV) and unilamellar large vesicles (ULV, 95 +/- 5 nm diameter) of dipalmitoylphosphatidylcholine (DPPC). The permeability of ULV to Rb+ was 1.0 X 10(-6) micrograms/s at 22 degrees C and increased to 1.1 X 10(-5) micrograms/s at the gel to liquid-crystalline transition temperature (Tm) of the bilayer, at 42 degrees C. The permeability of ULV to Rb+ continued to increase beyond the Tm and reached 1.0 X 10(-4) micrograms/s at 56 degrees C, a 100-fold increase over the permeability at 22 degrees C. In contrast, the permeability of ULV to Na+ showed a local maximum of 6.0 X 10(-6) micrograms/s at 42 degrees C and decreased at temperatures higher or lower than the Tm. For MLV, the permeability to both Rb+ and Na+ peaked dramatically at the phase transition temperature, 42 degrees C, and subsided at lower and higher temperatures. When ULV were exposed to an electric field, the permeability to Rb+, Na+, and sucrose surged at a field strength of 30 kV/cm; 30 kV/cm can induce a transmembrane potential of 210 mV. In ULV, the electrically perforated lipid bilayer exhibited selectivity for Rb+ over Na+ only at a narrow electric field range, between 31 and 33 kV/cm. For MLV, no well-defined breakdown voltage was recorded.(ABSTRACT TRUNCATED AT 250 WORDS)