Diffusion of injected macromolecules within the cytoplasm of living cells.

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Abstract
The diffusion of macromolecules introduced into the cytoplasm of human fibroblasts [foreskin diploid BG-9 cell] by erythrocyte-mediated microinjection was measured by the fluorescence recovery after photobleaching technique. The apparent diffusion coefficients for fluorescein-labeled IgG and fluorescein-labeled bovine serum albumin were .apprxeq. 10-8 cm2/s at 22.degree. C, consistent with the kinetics of spreading of the fluorescent probe following microinjection and .apprxeq. 1/70 the values in aqueous buffer. The diffusion of labeled bovine serum albumin was strongly dependent on temperature and similar to that expected in a 61% aqueous sucrose solution. The marked reduction in diffusion at 5.degree. C could be fully reversed by incubation with 0.1 mM colchicine. Apparently, cytoplasmic diffusion rates are reduced relative to rates in aqueous media as a result of increased aqueous phase viscosity or the impedence provided by structural elements. Several simple models to account for the data are presented.