Mechanism for the conductivity changes caused by membrane electroporation of CHO cell-pellets

Abstract

Electric field pulses, applied to densely packed pellets of Chinese hamster ovary (CHO) cells of mean radius ā_c = 7.5 ± 0.7 μm, cause major electric conductivity changes, described by three kinetic normal modes. The first mode reflects Wien effects of ionic atmosphere perturbations and ion pair dissociations (cell surfaces). Using Maxwell’s conductivity equation, the second and third mode are converted to the respective membrane conductivity modes. Electrothermodynamic analysis in terms of structural transitions from closed (C) to porated (P) membrane states of very different lifetimes, according to the scheme (C ⇌ C₁) ⇌ (P₂ ⇌ P₃), yields the mean pore radii ₂ = 1.00 ± 0.05 nm (P₂) pores and ₃ = 1.5 ± 0.1 nm (P₃) at T = 293 K (20 °C). The relaxation time τ₂ (P₂-formation) reflects the rate limiting step (C ⇌ C₁), associated with the activation dipole moment of Δm₁ = 63 × 10⁻³⁰ C m (or 19 Debye units), suggesting orientational changes of dipolar lipid head groups, in the solution membrane interfaces preceding the actual pore formations. Besides: the field-dependencies of the pore fractions f₂ and f₃ (order of 10⁻³), the field reduction factors f_λ,i≤ 1 and the membrane voltage, we obtain the zero-field pore conductivities λ⁰_p,2 = 1.7 × 10⁻² mS cm⁻¹ (P₂) and λ⁰_p,3 = 0.10 mS cm⁻¹ (P₃) and the membrane conductivity λ_m⁰ = 3.2 μS m⁻¹. The post-field conductivity changes, due to the long-lived P₃-pores, are analyzed in terms of time- (and field-) dependent efflux coefficients. The characteristic post-field pore resealing time τ_R = τ⁰₃ = 45 ± 3 s is independent of the field strength of the causative pulse and independent of the distance between the two electrodes. These results are an essential part for the optimization of the electrical pulse parameters, also for the clinical electrotransfer of bioactive substances into aggregated biological cells (tissue).