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The transport of a series of charged drugs and other organic ions through lipid bilayers was studied, using single-bilayer liposomes. Monovalent organic cations and organic anions could carry charge into the liposomes independent of the nature of their counter ion. Ion-pair formation, following the addition of an excess of I- or Br-, did not facilitate this transport. However the passage of organic cations and anions across the membrane required the presence of the organic anion tetraphenylborate in the lipid bilayer. This effect of tetraphenylborate is probably due to a lowering of an intra-membrane potential energy hill, the principal barrier for transport, and not to ion-pair formation. It is concluded that organic ions that possess sufficiently lipophilic structures to mask their charged character, can pass lipid bilayers in charged form.

In a previous article [Naunyn-Schmiedeberg's Arch Pharmacol (1981) 316:266-272] transport of a number of organic ions across lipid bilayers was investigated using single bilayer liposomes. In this investigation the translocation of structurally closely related barbiturates across lipid bilayers is studied. The anionic form of the barbiturates can pass lipid bilayers, provided that the lipophilicity of the 5,5-disubstituted side chain is high enough. This permeability of barbiturates in the ionized form explains their uncoupling action of the oxidative phosphorylation and the deviations from the pH-partition theory found in absorption processes.