Short-term reduction in dietary intake of magnesium causes deficits in brain intracellular free Mg2+ and [H+](i) but not high-energy phosphates as observed by in vivo P-31-NMR

P-31-NMR spectroscopic studies were performed in vivo on brains of rats fed 30-35% normal dietary Mg intake for 6 weeks. Within 2 weeks of the moderately restricted Mg diet serum Mg fell 50%, and brain intracellular free [Mg2+](i) fell 15%; within 3 weeks of restricted diet, brain [Mg2+](i) fell 40% and remained at this level for the additional 3 weeks. Intracellular pH (pH(i)) progressively rose in a reciprocal manner for 4 weeks. At no interval of time did brain phosphocreatine (PCr), [ATP], or inorganic phosphate change despite the fall in brain [Mg2+](i), brain pH(i) and serum Mg. The Mg-deficiency-induced cytosolic loss of protons (resulting in an alkaline cytosol) could be a compensatory mechanism to stabilize [PCr], [ATP] and [ADP] levels via creatine kinase, thus maintaining cytosolic phosphorylation potential. The rise in pH(i) associated with Mg-deficiency would also account for increased cerebral vascular muscle contractility under these conditions. Lastly, these studies indicate that brain [Mg2+](i) may change without a concomitant change in cell [ATP], and that brain [Mg2+](i) may be a useful marker for total body Mg2+ status. (C) 1997 Elsevier Science B.V.