Rapid Mobilization of Intracellular Mg²⁺ by Bombesin in Swiss 3T3 Cells: Mobilization through External Ca²⁺- and Tyrosine Kinase-Dependent Mechanisms

Abstract

We examined the effects of growth factors on intracellular free Mg²⁺ concentrations ([Mg²⁺]₁) in single Swiss 3T3 fibroblasts, using microfiuorometry of a Mg²⁺ -sensitive dye mag-fura-2. We had already noted an increase in [Mg²⁺]₁ after exposure to bombesin for 30-60 min [Ishijima, S., Sonoda, T., & 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⁺]¹. The [Mg²⁺]₁ reached peak values within 15 s in most cells, and the significant rise lasted only for 1-2min. The extent of the increase varied from cell to cell (0-600 μM above basal). On the average, the [Mg²⁺], was increased from basal 0.33 to 0.54mM. Since the time course was similar to that of [Ca²⁺]₁ changes, and the dye mag-fura-2 also binds Ca²⁺, we evaluated Ca²⁺ interference with measurement of [Mg²⁺]₁. The contribution of Ca²⁺ binding would be below 20% of the mag-fura-2 signal. The bombesin-induced [Mg²⁺], increase was not dependent on external Mg²⁺, but the omission of external Ca²⁺ decreased by 60% the [Mg²⁺]₁ increase, and the Ca²⁺ channel blocker, nicardipine inhibited by 90% the [Mg²⁺]₁ 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⁺]₁ response. These results suggest that bombesin rapidly induces Mg²⁺ mobilization from the intracellular pool, through external Ca²⁺- and tyrosine kinase-dependent mechanisms.