Reciprocal changes in intracellular and extracellular magnesium in rat pancreatic acinar cells in response to different secretagogues.

This study employs the fluorescent bioprobe Mag-fura-2 to measure the mobilisation of magnesium (Mg2+) in rat pancreatic acinar cells at rest and during stimulation with either cholecystokinin-octapeptide (CCK-8; 10(-8) M), acetylcholine (ACh; 10(-5) M) or histamine (10(-4) M). In Mag-fura-2 loaded acini suspended in normal Krebs-Ringer-Hepes (KRH) solution containing (mMol/litre) NaCL, 130; KCI, 5; HEPES, 20; KH2PO4, 1.2; CaCl2, 1.0; MgSO4, 1.0; glucose, 10; minimum essential medium, 1.0 ml/Litre and pH 7.4, both CCK-8 and ACh evoked a transient rise in intracellular free Mg2+ concentration ([Mg2+]i) followed by a sustained and prolonged decrease. Addition of CCK-8 to acinar cell suspension previously stimulated with ACh resulted in little or no change in [Mg2+]i indicating that the two secretagogues are mobilising magnesium from the same intracellular pool. Histamine had no significant effect on [Mg2+]i or on the CCK-8 evoked response. When acini were challenged in the presence of the cell-impermeant form of Mag-fura-2 (the tetrapotassium salt) in a Ca(2+)-free KRH solution containing 1-mM EGTA, CCK-8 and ACh, stimulation resulted in a gradual release of magnesium reaching a maximum after 300-350 s following application of the secretagogues (magnesium efflux). Again histamine had no effect on magnesium release. Incubation of acinar cells in a nominally magnesium-free KRH solution had no effect on basal [Mg2+]o. When challenged with CCK-8, [Mg2+]i rose transiently, followed by a sustained decrease. In contrast, ACh stimulation resulted in a triphasic response comprising an initial rise followed by a decrease, superceded by a small rise in [Mg2+]i. Again histamine had no effect on [Mg2+]i in nominally magnesium-free solution. Addition of either ACh or CCK-8 to cells placed in KRH solution with no added Mg2+ caused a small efflux of magnesium, as measured in the presence of the tetrapotassium salt of Mag-fura-2 (cell-impermeant form). These results, employing two different forms of a fluorescent bioprobe for the measurement of magnesium, indicate that unlike histamine, CCK-8 and ACh can evoke marked changes in intracellular free magnesium concentration during the stimulus-secretion coupling process.