INSULIN SECRETAGOGUES INDUCE CA-2+-LIKE CHANGES IN CYTOPLASMIC MG-2+ IN PANCREATIC BETA-CELLS

The effects of insulin secretagogues on the cytoplasmic Mg2+ concentration ([Mg2+]i) of pancreatic β-cells were studied in suspensions and in individual β-cells using dual-wavelength fluorometry and the indicator mag-fura-2. Average [Mg2+]i was in the 800-900 μM range in a medium containing 3 mM glucose. When the sugar concentration was raised to 20 mM, the cells reacted with an initial lowering of [Mg2+]i followed by an increase. The sugar apparently also stimulated leakage of the Mg2+ indicator. Addition of 100 μM tolbutamide or raising the K+ concentration by 25 mM caused relatively rapid increases of [Mg2+]i. Methoxyverapamil prevented the [Mg2+]i-increasing actions of glucose, K+ and tolbutamide. The greatest change in [Mg2+]i was obtained when β-cells were exposed to 100 μM carbachol. In this case there was a more than 10% lowering, which was reversed upon removal of the agonist. Measurements of [Mg2+]i are important not only for understanding fluctuations of this ion, but may also aid to elucidate the mechanisms involved in the regulation of cytoplasmic Ca2+. © 1990.