The effects of phagocytosis on plasma membrane microviscosity were studied by fluorescence depolarization techniques. It was shown that lipophilic probes are accumulated in intracellular vesicles to a significant degree in fibroblasts and neutrophils. Microviscosity was thus determined from the behavior of probes in isolated membranes. Phagocytosis of oil emulsions or polystyrene beads by rabbit polymorphonuclear leukocytes induces a marked decrease in plasma membrane microviscosity that parallels the extent of phagocytosis. Liposomes made from extracts of membrane lipid show qualitatively the same changes, indicating that the alteration of microviscosity results at least in part from changes in lipid composition. The decrease in microviscosity is abolished when colchicine is present during phagocytosis. Addition of-colchicine to membranes previously isolated from control or phagocytic cells has no effect on their microviscosity. The results suggest that phagocytosis is accompanied by a microtubule-dependent reorganization of membrane lipids. During phagocytosis, particles are enveloped by plasma membrane and subsequently internalized. In polymorphonuclear'leukocytes (PMN), this process is accompanied by striking changes in the composition of the residual plasma membrane. For example, certain lectin receptors are selectively removed from the surface (1) while transport proteins are selectively spared (2). Treatment of cells with low doses of colchicine, an inhibitor of microtubule assembly, prevents the induced topographical changes without affecting the rate or amount of ingestion (1, 3). The existence of these colchicine-sensitive phenomena indicates a role for microtubules in the control of membrane topography. Further, the large and rapid membrane reorganization accompanying phagocytosis provides a unique system for analyzing the consequences of microtubule-membrane interaction at the molecular level.We report here studies of the microviscosity of the plasma membrane of phagocytic cells. Microviscosity was inferred from the fluorescence depolarization of lipophilic probes embedded in isolated plasma membranes and in liposomes made from extracts of the membranes. Large changes in microviscosity are observed which parallel the degree of phagocytosis. Colchicine has no direct effect on membrane microviscosity, but blocks changes induced by phagocytosis. METHODSThe fluorescence polarization method used for the study of membrane microviscosity has been described extensively (4) and has been used with artificial liposomes (4, 5,), membranes (6), and whole cells (7,8). The polarization of-fluorescence is determined by the rotation of the probe during the lifetime of the excited state. The rotation of the probe in turn is dependent on the viscosity of its environment ("microviscosity")-rapid Abbreviations: DPIH, 1,6-diphenyl-1,3,5-hexatriene; PMN, polymorphonuclear leukocytes; BSA, bovine serum albumin. rotation leading to depolarization. By determining the polarization of the probe in media of known viscosi...

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