Effect of cholesterol on interaction of dibucaine with phospholipid vesicles: a fluorescence study

Interaction of the local anesthetic dibucaine with small unilamellar vesicl es of dimyristoylphosphatidylcholine (DMPC) and dioleoyl phosphatidylcholin e (DOPC) containing different mol percents of cholesterol has been studied by fluorescence spectroscopy. Fluorescence measurements on dibucaine in pre sence of phospholipid vesicles containing various amounts of cholesterol yi elded a pattern of variation of wavelength at emission maximum and steady-s tate anisotropy which indicated that the microenvironment of dibucaine is m ore polar and flexible in membranes that contain cholesterol than in membra nes without cholesterol. Experiments on quenching of fluorescence from memb rane-associated dibucaine by potassium iodide showed a marked increase in q uenching efficiency as the cholesterol content of the vesicles was increase d, demonstrating increased accessibility of the iodide quenchers to dibucai ne in the presence of cholesterol, when compared to that in its absence. To tal emission intensity decay profiles of dibucaine yielded two lifetime com ponents of similar to1 ns and similar to2.8-3.1 ns with mean relative contr ibutions of similar to 25 and similar to 75%, respectively. The mean lifeti me in vesicles was 20-30% smaller than in the aqueous medium and showed a m oderate variation with cholesterol content. Fluorescence measurements at tw o different temperatures in DMPC SUVs, one at 33 degreesC, above the phase transition temperature and another at 25 degreesC, around the main phase tr ansition, indicated two different mode of dibucaine localization. At 25 deg reesC dibucaine partitioned differentially in presence and absence of chole sterol. However, at 33 degreesC the apparent partition coefficients remaine d unaltered indicating differences in the microenvironment of dibucaine in presence and absence of cholesterol in the phospholipid membranes. (C) 2001 Elsevier Science B.V. All rights reserved.