title: Low viscosity in the aqueous domain of cell cytoplasm measured by picosecond polarization microfluorimetry.
abstract: Information about the rheological characteristics of the aqueous cytoplasm can be provided by analysis of the rotational motion of small polar molecules introduced into the cell. To determine fluid-phase cytoplasmic viscosity in intact cells, a polarization microscope was constructed for measurement of picosecond anisotropy decay of fluorescent probes in the cell cytoplasm. We found that the rotational correlation time (tc) of the probes, 2,7-bis-(2-carboxyethyl)-5-(and-6-)carboxyfluorescein (BCECF), 6-carboxyfluorescein, and 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) provided a direct measure of fluid-phase cytoplasmic viscosity that was independent of probe binding. In quiescent Swiss 3T3 fibroblasts, tc values were 20-40% longer than those in water, indicating that the fluid-phase cytoplasm is only 1.2-1.4 times as viscous as water. The activation energy of fluid-phase cytoplasmic viscosity was 4 kcal/mol, which is similar to that of water. Fluid-phase cytoplasmic viscosity was altered by less than 10% upon addition of sucrose to decrease cell volume, cytochalasin B to disrupt cell cytoskeleton, and vasopressin to activate phospholipase C. Nucleoplasmic and peripheral cytoplasmic viscosities were not different. Our results establish a novel method to measure fluid-phase cytoplasmic viscosity, and indicate that fluid-phase cytoplasmic viscosity in fibroblasts is similar to that of free water.
authors.affiliation: Department of Medicine, University of California, San Francisco 94143-0532.
authors.forename: K
authors.initials: K
authors.lastname: Fushimi
authors.forename: A S
authors.initials: AS
authors.lastname: Verkman
mesh_terms: D000818:Animals; D002455:Cell Division; D002467:Cell Nucleus / Q000502:physiology; D002478:Cells, Cultured; D003592:Cytophotometry / Q000379:methods; D003593:Cytoplasm / Q000502:physiology*; D003599:Cytoskeleton / Q000187:drug effects; D004789:Enzyme Activation; D005454:Fluorescence Polarization / Q000379:methods; D005456:Fluorescent Dyes; D013696:Temperature; D010738:Type C Phospholipases / Q000378:metabolism; D014783:Viscosity
chemical_list: D005456:Fluorescent Dyes; D010738:Type C Phospholipases