Gentamicin alters membrane structure as shown by freeze-fracture of liposomes

Freeze-fracture has been used to examine the effects of gentamicin on membrane structure in liposomes of different anionic phospholipids combined with a neutral phospholipid, phosphatidylcholine. The molar ratios of neutral: anionic lipid were 1:1 (high anionic lipid ratio) and 4:1 (low anionic lipid) and the liposomes were incubated with 0.1 mM (low) and 1 mM (high) gentamicin. With the anionic phospholipid phosphatidylinositol bisphosphate, an identifiable disruption of the membrane bilayer was observed as well as aggregation of liposomes leading to membrane fusion. These effects occurred both at low gentamicin concentration and low anionic lipid content of the liposomes; these responses were not inhibited by 1 mM Ca2+. With the other anionic lipids tested (phosphatidylserine, phosphatidylinositol and phosphatidylinositol monophosphate), only aggregation and fusion of liposomes was observed and this effect only occurred at high gentamicin concentration and high anionic lipid content. Further, 1 mM Ca2+ inhibited the responses of these other anionic lipids to gentamicin. The results demonstrate the unique character of the interaction between gentamicin and phosphatidylinositol bisphosphate and provide further support for the hypothesis that a specific binding to this lipid is a key step in the ototoxic action of aminoglycoside antibiotics. They also suggest that such an interaction in vivo might cause alterations to the structure and properties of cell membranes in the inner ear.