Penetration of furosemide into phospholipid monolayers

Furosemide is a surface-active anion and it tends to displace lipid monolayers from the surface at positive polarizations lowering their potential stability range. The efficiency of the penetration and the displacement increases with decreasing surface pressure of the monolayer. Lower capacitance at a wider potential range corresponds to higher surface pressure. Monolayers with higher capacitances are indeed more readily penetrated and displaced as demonstrated by further increase in their capacitance and increase in their proton conductance. Furosemide raises the capacitance of the monolayer in the stable region due to intercalation between the head groups thus reducing the thickness of the hydrocarbon layer. In pure PC monolayer about 10% increase in capacitance is observed in the presence of 6 X 10(-4)M furosemide. The effect of furosemide becomes more pronounced with increasing sphingomyelin content in the mixed monolayers. The monolayer of PE is more condensed and its capacitance is lower (approximately 1.45 microF/cm2) and is stable in a wider potential range than that of PC. It is less affected by furosemide and concentrations higher than 10(-3) M are required to narrow the stability range and to increase the capacitance.