Probing the Relationship of Membrane "Fluidity" to Heat Killing of Cells

Abstract

The concept that the fluid state of a cell's membranes at the onset of heating is an important factor in its response to a hyperthermic insult was proposed by Yatvin in 1977 (1). We and others (2-4)' have shown that by systematically varying the nature and amount of unsaturated fatty acids in Escherichia coli and mammalian cell membrane lipids by diet and temperature that significant changes in "fluidity" occur, as measured by fluorescent probes and fatty acid spin labels. "Fluidity" is positively correlated with sensitivity of cells subsequently exposed to hyperthermia. Our postulate, as a mechanism to explain events leading to cell death by heat, has been challenged by others, most recently by Lepock et al. (5). Although the term "fluidity" is well established within physiological jargon and will probably be with us for awhile, it suffers from the fact that it does not adequately describe properties of anisotropic structures such as phospholipid-derived membranes. Therefore, we feel it is necessary for us to define "fluidity" as it relates to our concepts so that an open line of communication will be maintained between us and others who may use the term within a different context. We make no claim as to the correctness or incorrectness of a particular definition as used by a particular investigator. The two most widely used techniques for measuring membrane fluidity involve the insertion of "electron spin resonance" or "fluorescent polarization" probes into membranes. However, since these probes see only average properties of the whole system, uncertainties arise with respect to the following: (a) subcellular distribution, (b) lateral distribution between adjacent domains (e.g., gel and liquid crystalline phases), and (c) vertical distribution of the probes within the bilayer matrix. With respect to point (c), Jain and Wu (6) have studied the interaction of small molecules (including membrane probes) with artificial phospholipid vesicles and conclude that "the quantitative and qualitative modifications observed arise from a difference in the position of localization of solutes in different regions of the bilayer along its thickness."