Effects of elevated intracellular magnesium on cytoskeletal integrity

Increasing the intracellular magnesium concentration of PtK2 cells by 1 mM or more resulted in the disassembly of the interphase microtubule array over a period of 5 min after microinjection. This effect was found to be both transient and fully reversible, with the microtubule arrays reforming after further incubation. These effects were studied using immunofluorescence microscopy of fixed cells, and also in living cells using rhodamine-tubulin or rhodamine-conjugated anti-tubulin antibodies and image intensification and enhancement techniques. Simultaneously and accompanying the disassembly of the microtubule arrays the F-actin stress fibres also disappeared, usually leaving the peripheral and perinuclear F-actin microfilaments intact. In contrast, increasing intracellular magnesium appeared to have no effect on the vimentin-containing intermediate filaments of PtK2 cells. These effects on the cytoskeleton were specific to magnesium and could not be mimicked by either microinjection of injection buffer of equivalent ionic strength or sham injection. Raising the intracellular free calcium to the same extent resulted in the disassembly of the microtubule network, but appeared to have no effect on the F-actin stress fibres.