Cholesterol content is an important factor for membrane dynamics of living cells. With well defined protocols of depletion and enrichment the impact of cholesterol on membrane dynamics was examined by fluorescence microscopy. In addition, the intracellular cholesterol
content was determined with biochemical methods. Changes of cholesterol amounts in cell membranes have previously been related to specific disease and may have some influence on the uptake of pharmaceutical agents. A combination of conventional and total internal reflection fluorescence microscopy was applied to the fluorescence marker laurdan, a polarity-sensitive probe, whose electronic excitation energy is different in polar and non-polar environment. Once incorporated into cell membranes, the fluorescence of laurdan shows a spectral shift towards longer wavelength
when its molecules get into contact with adjacent water molecules, e.g. when a phase transition from the tightly packed gel phase to the liquid crystalline phase of membrane lipids occurs. The generalized polarization (GP, characterizing this spectral shift) as well as the
fluorescence lifetime (τ) of laurdan revealed to be appropriate measures for membrane stiffness and fluidity. GP generally decreased with increasing temperature and was always higher for the plasma membrane than for intracellular membranes. Enrichment of cholesterol
caused a pronounced increase, whereas depletion of cholesterol caused a decrease of GP. In addition, pronounced changes of the fluorescence lifetime pattern occurred in the subnanosecond range. GP, and τ were determined as integral values of single cells or small cell collectives and were also displayed as microscopic images.