Diffusion of injected macromolecules within the cytoplasm of living cells.

The diffusion of macromolecules introduced into the cytoplasm of human fibroblasts 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 approximately 10(-8) cm2/sec at 22 degrees C, consistent with the kinetics of spreading of the fluorescent probe following microinjection and approximately 1/70 the values in aqueous buffer. The diffusion of labeled bovine serum albumin was shown to be strongly dependent on temperature and, in fact, similar to that expected in a 61% aqueous sucrose solution. However, the marked reduction in diffusion at 5 degrees C could be fully reversed by incubation with 0.1 mM colchicine. These findings suggest that 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.