Effects of magnesium on nitric oxide synthase activity in
endothelial cells.
Howard, Adam B., R. Wayne Alexander, and W. Robert Taylor.
Department of Medicine, Division of Cardiology, Emory University,
Atlanta, Georgia, 30322 and Veterans Affairs Medical Center, Decatur,
Georgia, 30033
APStracts 2:0135C, 1995.
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 minute exposure to
magnesium concentrations varying from 0.010 to 10.0 mM. In contrast,
when A23187-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 to cells exposed to 1.0 mM MgCl2
(p<0.01). These data suggested that a change in the intracellular
magnesium concentration had an effect upon 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's and exposed to MgCl2. NOS activity was measured
via the conversion of [3H]-L-arginine to [3H]-L-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
non-specific chloride effect. In addition, increasing the
concentration of NaCl to 15 mM had no effect upon NOS activity,
ruling out a non-specific osmotic effect (101.6+10.5% of control
activity, p=ns). Finally, in cell monolayers exposed to A23187, 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 mM MgCl2 and 1.87
mM CaCl2, p=ns), suggesting that magnesium antagonizes calcium
-dependent NO release by endothelial cells. CONCLUSIONS: 1)
extracellular magnesium does not acutely affect NO release by PAEC's,
2) magnesium inhibits PAEC nitric oxide synthase 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.
Received 28 December 1994; accepted in final form 6 March 1995.
APS Manuscript Number C749-4.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 21 March 1995.