Calcium-magnesium interactions in pancreatic acinar cells

Although the role of calcium (Ca2+) in the signal transduction and pathobiology of the exocrine pancreas is firmly established, the role of magnesium (Mg2+) remains unclear. We have characterized the intracellular distribution of Mg2+ in response to hormone stimulation in isolated mouse pancreatic acinar cells and studied the role of Mg2+ in modulating Ca2+ signaling using microspectrofluorometry and digital imaging of Ca2+- or Mg2+-sensitive fluorescent dyes as well as Mg2+-sensitive intracellular microelectrodes. Our results indicate that an increase in intracellular Mg2+ concentrations reduced the cholecystokinin (CCK) -induced Ca2+ oscillations by inhibiting the capacitive Ca2+ influx. An intracellular Ca2+ mobilization, on the other hand, was paralleled by a decrease in [Mg2+]i, which was reversible upon hormone withdrawal independent of the electrochemical gradients for Mg2+, Ca2+, Na+, and K+, and not caused by Mg2+ efflux from acinar cells. In an attempt to characterize possible Mg2+ stores that would explain the reversible, hormone-induced intracellular Mg2+ movements, we ruled out mitochondria or ATP as potential Mg2+ buffers and found that the CCK-induced [Mg2+]i decrease was initiated at the basolateral part of the acinar cells, where most of the endoplasmic reticulum (ER) is located, and progressed from there toward the apical pole of the acinar cells in an antiparallel fashion to Ca2+ waves. These experiments represent the first characterization of intracellular Mg2+ movements in the exocrine pancreas, provide evidence for possible Mg2+ stores in the ER, and indicate that the spatial and temporal distribution of intracellular Mg concentrations profoundly affects acinar cell Ca2+ signaling.