DETERMINATION OF CYTOSOLIC MG2-1 CELLS WITH MAG-FURA-2( ACTIVITY AND BUFFERING IN BC3H)

The magnesium buffer coefficient (B-Mg) was calculated for BC3H-1 cell s from the rise in cytosolic Mg2+ activity observed when magnesium was released from ATP after iodoacetate (IAA) and NaCN treatment. The bas al cytosolic Mg2+ activity (0.54 +/- 0.1 mM) measured with mag-fura-2 doubled when 4.54 mM magnesium was liberated from ATP: B-Mg was 12.9 i ndicating that a 1 mM increase in Mg2+ activity requires an addition o f about 13 mM magnesium. The accuracy of this value depends on these a ssumptions: (a) all of the magnesium released from ATP stayed in the c ells; (b) the rise in Mg2+ was not secondary to pH-induced changes in B-Mg; (c) mag-fura-2 measured Mg2+ and not Ca2+; and (d) the accuracy of the mag-fura-2 calibration. Total magnesium did not change in respo nse to IAA/CN treatment, thus the change in Mg2+ activity reflected a redistribution of cell magnesium. pH changes induced by NH4Cl pulse an d removal had little effect on Mg2+ activity and the changes were slow er than and opposite to pH-induced changes in Ca2+ activity measured b y fura-2. Ca2+ responses were temporally uncoupled from Mg2+ responses when the cells were treated with IAA only and in no cases did Ca2+ le vels rise above 1 mu M, showing that the mag-fura-2 is responding to M g2+. Additional studies demonstrated that approximate to 90% of the ma g-fura-2 signal was cytosolic in origin. The remaining non-diffusible mag-fura-2 either was bound to cytosolic membranes or sequestered in o rganelles with the fluorescence characteristics of the Mg2+-complexed form, even when cytosolic free Mg2+ activity was approximately 0.5 mM. This bound mag-fura-2 would appear to increase the K-d and thus clear ly limits the accuracy of our estimmate for B-Mg. Despite this limitat ion, we demonstrate that Mg2+ is tightly regulated in face of large ch anges in extracellular Mg2+, and that the interplay observed between p H, Ca2+ and Mg2+ activities strongly supports the hypothesis that thes e factors interact through a shared buffer capacity of the cell.