Intracellular magnesium: Transport and regulation in epithelial secretory cells

The mechanisms of magnesium (Mg2+) transport, the regulation of intracellul ar Mg2+ concentrations and the relationship between Mg2+ and Ca2+ signaling during the stimulus-secretion coupling process in pancreatic acinar cells and other secretory epithelia are reviewed in this article. Our results dem onstrate the existence of a Na+ - and ATP-dependent transport system for Mg 2+ extrusion from Mg2+-loaded cells. Moreover, employing such different tec hniques as spectrofluorimetry and atomic absorbance spectroscopy to measure intracellular free magnesium concentration [Mg2+](i) from magfura-2-loaded acini and acinar cells and Mg2+ content in effluent samples from perfused pancreatic segments, respectively, confirm that secretagogues such as acety lcholine (ACh) and cholecystokinin-octapeptide (CCK-8) can evoke marked and significant extrusion of Mg2+ which is closely associated with the mobiliz ation of intracellular calcium. These effects may be modulated by different mediators including cAMP, Protein Kinase C and nitric oxide/cGMP. This red uction in [Mg2+](i) seems to be a prerequisite for optimal generation and m aintenance of the calcium signal and subsequently, the secretion of enzymes , since an increase in extracellular Mg2+ concentration, [Mg2+](o) and an i ncrease in [Mg2+](i) inhibit secretagogue-induced secretory responses, an e ffect exerted through a derangement of the calcium signaling events. In con clusion, the evidence presented in this review strongly supports an importa nt modulatory role of magnesium in the control of secretory epithelial cell s function.