EFFECTS OF MAGNESIUM ON NITRIC-OXIDE SYNTHASE ACTIVITY IN ENDOTHELIAL-CELLS

Magnesium modulates endothelium-dependent vasodilation in intact blood vessels. Therefore, the effects of magnesium on nitric oxide (NO) release by isolated endothelial cells and nitric oxide synthase (NOS) activity in endothelial cell homogenates were studied. Unstimulated and bradykinin-stimulated NO release by porcine aortic endothelial cell (PAEC) monolayers were unaffected by 30 min of exposure to magnesium concentrations varying from 0.010 to 10.0 mM. In contrast, when A-23187-stimulated cells were exposed to 0.01, 3.16, and 10.0 mM MgCl2, NO release was decreased by 11.3 +/- 1.8, 11.7 +/- 3.0, and 20.3 +/- 7.2%, respectively, compared with cells exposed to 1.0 mM MgCl2 (P < 0.01). These data suggested that a change in the intracellular magnesium concentration had an effect on NO release, in contrast to a change in the extracellular concentration, which did not have an effect. To further assess this possibility, crude NOS extracts were prepared from PAEC and exposed to MgCl2. NOS activity was measured via the conversion of L-[H-3]arginine to L-[H-3]citrulline. Increasing the concentration of MgCl2 by 1.0, 3.16, and 10.0 mM caused a 16.0 +/- 6.8, 17.1 +/- 1.7, and 38.6 +/- 5.3% decrease in citrulline formation, respectively (P < 0.05), suggesting a direct inhibition of NOS by MgCl2. No significant difference in the degree of inhibition of NOS activity was found between MgSO4 and MgCl2, thus ruling out a nonspecific chloride effect. In addition, increasing the concentration of NaCl to 15 mM had no effect on NOS activity, ruling out a nonspecific osmotic effect [101.6 +/- 10.5% of control activity, P = not significant (NS)]. Finally, in cell monolayers exposed to A-23187, 3.16 mM CaCl2 overcame the inhibition of NO release by 3.16 mM MgCl2 (100.7 +/- 7.5% of NO released by cells exposed to 1.0 and 1.87 mM CaCl2, P = NS), suggesting that magnesium antagonizes calcium-dependent NO release by endothelial cells. In conclusion, 1) extracellular magnesium does not acutely affect NO release by PAEC, 2) magnesium inhibits PAEC NOS activity in a dose-dependent manner, 3) the inhibitory effect of MgCl2 is specific to magnesium and not due to an effect of either increased osmotic strength or chloride anion, 4) the inhibitory effect of magnesium is probably due to competitive antagonism of intracellular calcium, and 5) intracellular magnesium concentration may be an important regulator of NOS activity.