Ion modulation of membrane permeability: Effect of cations on intact cells and on cells and phospholipid bilayers treated with pore-forming agents

Abstract

Leakage of ions (Na⁺, K⁺) and phosphorylated metabolites (phosphorylcholine, 2-deoxyglucose 6-phosphate) through membrane lesions in intact cells or in cells modified by ‘pore-forming’ agent has been studied. Leakage from intact cells isinduced by protons and by divalent cations such as Cu²⁺, Cd²⁺ or Zn²⁺. Leakage from agent-modified cells—or across phospholipid bilayers modified by agent—isprevented by low concentrations of the same cations and by higher concentrations of Ca²⁺, Mn²⁺ or Ba²⁺; Mg²⁺, dimethonium, spermine, or spermidine are virtually ineffective. The relative efficacy of a particular cation (e.g. Ca²⁺) depends more on cell type than on the nature of the pore-forming agent. The predominant effect is on binding of cation to specific sites, not on surface charge. Surface charge, on the other hand, does affect leakage from agent-modified cells in that suspension in nonionic media reduces leakage, which can be restored by increasing the ionic strength: univalent (Na⁺, K⁺, Rb⁺, NH ₄ ⁺ ) and divalent (Mg²⁺, dimethonium) cations are equally effective; addition of protons or divalent cations such as Zn²⁺ to this system inhibits leakage. From this and other evidence here presented it is concluded that leakage across membranes is modulated by the presence of endogenous anionic components: when these are in the ionized state, leakage is favored; when unionized (as a result of protonation) or chelated (by binding to divalent cation), leakage is prevented. It is suggested that such groups are exposed at the extracellular face of the plasma membrane.