DYNAMICS OF INTRACELLULAR FREE MG2+ CHANGES IN A VASCULAR SMOOTH-MUSCLE CELL-LINE

Intracellular free Mg2+ concentration ([Mg2+]i) has been implicated in the pathogenesis 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 (A10) was cultured on glass cover slips, and [Mg2+]i was determined by fluorescent techniques on single cells with the use of mag-fura-2. Basal [Mg2+]i was 0.52 +/- 0.02 mM (n = 15). Vascular smooth muscle cells were challenged with A23187 plus 5 mM MgCl2 to rapidly elevate [Mg2+]i. [Mg2+], increased to a peak of 1.03 +/- 0.09 mM within 1-2 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+ challenge to below basal levels suggests the presence of intracellular transport mechanisms, likely 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 +/- 0.9 to 6.8 +/- 1.0 muM/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.