Fig. 4. Quantitative estimation of nuclear translocation of phosphorylated ERK in RTgill-W1 cells, as determined from the ratio of staining intensity measured in the nucleus to that of a cytoplasmic region of the cells. (A) Time course of the change in nuclear/cytoplasmic ratio following the addition of EGF is shown in controls and in cells after pre-incubation with leptomycin B. Values are means ± s.e.m. of at least 20 cells from 2–3 independent cultures. (B) Nuclear/cytoplasmic fluorescence ratio of cells after exposure to hypo-osmotic shock, EGF or Cu at the time where the maximum increase of relative nuclear fluorescence was determined (5' or 15'). Values are means ± s.e.m. of at least 16 cells from 2–3 independent cultures. Asterisks indicate a statistical significant difference (P<0.05) from the value at time zero (A) or from cells in the absence of LB (B).