Ionic changes in the mitotic apparatus at the metaphase/anaphase transition

We have employed a series of permeant, nontoxic, fluorescent probes to detect changes in ionic conditions within the mitotic apparatus of living endosperm cells of Haemanthus during the transition from metaphase to anaphase. Fluorescence emission intensity measurements from the spindle for chlorotetracycline (CTC) decline before the onset of anaphase, indicating a reduction in the amount of membrane- associated Ca2+ and suggesting an efflux of Ca2+ from membrane compartments into the spindle. Subsequent to the onset of anaphase, we observe increases in fluorescence with both 8-anilino-1-naphthalene sulfonate (ANS) and 3,3'-dipentyl 2,2'-dioxacarbocyanine (diO-C5(3)), sensitive to cationic and anionic charges at membrane surfaces, respectively. The increases with ANS and diO-C5(3) suggest that redistributions of ions within the spindle accompany anaphase motion. During the metaphase/anaphase transition, spindle membrane content remains constant, as evidenced by unchanging fluorescence with the hydrophobic probe, N-phenyl-1-naphthylamine (NPN). Shifts in emission intensity from the nonspindle cytoplasm or from the spindle poles do not accompany the changes in fluorescence we observe in the spindle, suggesting that any ionic fluxes responsible for the changes in fluorescence are restricted to the spindle domain.