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Abstract Intracellular free Mg2+ concentration ( [Mg2+]i) has beeni mplicated in the pathogenesiso of hypertension. It has been postulated that Mg2+ through its antagonistic effects on intracellular Ca2+ concentration may affect tension and contractility of vascular smooth muscle cells. An established cell line of rat thoracic aorta cells (AlO) was cultured on glass cover slips, and [Mg2+], was determined by fluorescent techniques on single cells with the use of mag-fura-2. Basal [Mg2+]i was 0.52 t 0.02 mM (n = 15). Vascular smooth muscle cells were challenged with A23187 plus 5 mM MgC12 to rapidly elevate [ Mg2+ ]i. [Mg2+]i increased to a peak of 1.03 ,t 0.09 mM within l-2 s and then quickly declined to below basal levels, 0.30 & 0.03 mM, within 45-60 s despite the continued presence of A23187 and external Mg2+. The rapid removal of the Mg2+ challenget o belowb asall evelss uggeststh e presenceo of intracellular transport mechanisms,li kely in intracellular compartments or organelles. Spatial imaging studies indicated that Mg2+ is heterogeneously distributed within the cell with the greatest variations in the perinuclear region, the area of most cytosolic organelles. Vanadate, an inhibitor of P-type adenosinetriphosphatases, inhibited the removal rate from 10.2 t 0.9 to 6.8 t 1.0 FM/S. Inhibitors of intracellular Ca2+ mobilization, thapsigargin, dantrolene, and 3,4,5-trimethoxybenzoic acid 8- (diethylamino)octyl ester, inhibited Mg2+ sequestration. Ryanodine and caffeine had no effect on Mg2+ removal. Ruthenium red did not inhibit Mg2+ sequestration, but oligomycin B slowed its removal. These studies demonstrated that [Mg2+]i in vascular smooth muscle cells is carefully controlled by active mechanisms involving intracellular and plasma membrane transporters. Alteration of this control may play a role in aberrant vasoconstriction.