Hypomagnesemia inhibits nitric oxide release from coronary endothelium: Protective role of magnesium infusion after cardiac operations

Background. Postoperative hypomagnesemia is common in patients who have undergone cardiac operations and is associated with clinically significant morbidity resulting from atrial and ventricular dysrhythmias. Magnesium supplementation may increase the cardiac index in the early postoperative period. Methods. The action of the magnesium cation on coronary vascular reactivity was studied. Segments of canine epicardial coronary artery were suspended in organ chambers to measure isometric force (95% O₂/5% CO₂, 37°C). Results. In coronary segments constricted with prostaglandin F_2α, (2 × 10^-6 mol/L), acetylcholine and adenosine diphosphate (10^-9 to 10^-4 mol/L) induced vasodilation in arteries with endothelium (n = 10, each group; p < 0.05). Acetylcholine-mediated vasodilation was blocked by N^G-monomethyl-L-arginine (10^-4 mol/L) and N^G-nitro-L-arginine (10^-4 mol/L), two inhibitors of nitric oxide synthesis from L-arginine (n = 10, p < 0.05). The removal of magnesium from the organ chamber solution impaired vasodilation in response to acetylcholine and adenosine diphosphate. However, normal endothelium-dependent vasodilation could be restored by return of magnesium to the bathing solution. Vascular relaxation in response to bradykinin (10^-9 to 10^-6 mol/L), which was found to induce endothelium-dependent vasodilation independent of nitric oxide production, was unaffected by magnesium removal (n = 10). Conclusions. Hypomagnesemia selectively impaired the release of nitric oxide from the coronary endothelium. Because nitric oxide is a potent endogenous nitrovasodilator and inhibitor of platelet aggregation and adhesion, hypomagnesemia could promote vasoconstriction and coronary thrombosis in the early postoperative period.