Biol. Pharm. Bull. 28(12) 2185—2188 (2005)

a fluid condition (characterized by loosely packed, freely diffusing, kinked, unsaturated glycerophospholipids) called the liquid-disordered (ld) phase, in which discrete domains are embedded in a liquid-like, more ordered state called the liquid-ordered (lo) phase. 1) It could be predicted that thermotropic transitions, if existing, either of ld or of lo phase are not detectable in cell membranes: the first is commonly occurring below °C, owing to the high proportion of very lowmelting (poly)unsaturated lipids; the second is abolished by the presence of high proportions of cholesterol. Likely, these are the main reasons why true lipid phase transitions in mammalian somatic cells are an event uncommonly detected, while are usual and deeply investigated in model membranes. Starting from these premises, we hypothesized that cholesterol depletion from cell membranes may bring about the conditions leading to the emergence of detectable lipid phase transitions. In order to test this hypothesis, we analyzed by differential scanning calorimetry (DSC) membrane fractions prepared from cultured A431 cells treated with methyl-b-cyclodextrin (MbCD), a compound widely used in cell biology, for manipulation of cell membrane cholesterol content, and in pharmaceutical applications for numerous purposes, including improving the bioavailability of drugs.