RAPID MOBILIZATION OF INTRACELLULAR MG2+ BY BOMBESIN IN SWISS 3T3 CELLS - MOBILIZATION THROUGH EXTERNAL CA2+- AND TYROSINE KINASE-DEPENDENT MECHANISMS

We examined the effects of growth factors on intracellular free Mg2+ concentrations ([Mg2+](i)) in single Swiss 3T3 fibroblasts, using microfluorometry of a Mg2+-sensitive dye mag-fura-2. We had already noted an increase in [Mg2+](i) after exposure to bombesin for 30-60 min [Ishijima, S., Sonoda, T., and Tatibana, M. (1991) Am. J. Physiol. 261 (Cell Physiol. 30), C1074-C1080]. In the present work, we found that bombesin also induced early changes in [Mg2+](i) The [Mg2+](i) reached peak values within 15 s in most cells, and the significant rise lasted only for 1-2 min. The extent of the increase varied from cell to cell (0-600 mu M above basal). On the average, the [Mg2+](i) was increased from basal 0.33 to 0.54 mM. Since the time course was similar to that of [Ca2+](i) changes, and the dye mag-fura-2 also binds Ca2+, we evaluated Ca2+ interference with measurement of [Mg2+](i). The contribution of Ca2+ binding would be below 20% of the mag-fura-2 signal. The bombesin-induced [Mg2+](i) increase was not dependent on external Mg2+, but the omission of external Ca2+ decreased by 60% the [Mg2+](i) increase, and the Ca2+ channel blocker, nicardipine inhibited by 90% the [Mg2+](i) response. This inhibition was partially reversed by raising the concentration of external Ca2+. Two structurally distinct tyrosine kinase inhibitors, genistein and lavendustin A, almost completely inhibited the [Mg2+](i) response. These results suggest that bombesin rapidly induces Mg2+ mobilization from the intracellular pool, through external Ca2+- and tyrosine kinase-dependent mechanisms.