The effects of magnesium(Mg2+) on contraction produced by norepinephrine(NE) were studied in isolated thoracic aorta of chronic ethanol-fed rats.
Male Wistar rats were randomly divided into control and ethanol-fed groups. Six-week-old rats received ethanol in their drinking water at a concentration of 5%(v/v) for the first week, 10% for the next two weeks, and 20% for the last four
weeks.
Under pentobarbital sodium (35mg/kg i.p) anesthesia, the thoracic aorta was isolated and cut into rings(4-5mm long). and isometric force were recorded continuously. After the aortic rings with or without endothelium were precontracted with
NE(5×10-7
M), the bathing medium was replaced with Mg2+ deficient Krebs-Ringer solution containing the same concentration of NE. Mg2+ was then added in a cumulative dose manner (0.6, 1.2, 2.4, 3.6, and 4.8 mM). To test the effects of acute ethanol on
Mg2+
-induced changes in vascular reactivity, various amounts of ethanol(25, 100, and 175 mM) were added 15 minutes before precontraction with NE.
Chronic ingestion of ethanol increased the NE-induced contraction of aortae. With intact endothelium, withdrawal of Mg2+ from the bathing medium produced a vasoconstriction and readdition of Mg2+ led to a dosedependent vasorelaxation.
However
removal of endothelium or treatment with methylene blue, a known inhibitor of endothelium-derived relaxing factor(EDRF), abolished the relaxation produced by Mg2+, except at the highest concentration Mg2+ -deficiency also led to a
profound inability of the precontracted tissues to relax when challenged with acetylcholine, Both acute and chronic treatment with ethanol inhibited the vasoconstriction induced by withdrawal of Mg2+ as well as the vasorelaxation
induced by
readdition of Mg2+.
The results obtained in this study show that contractility of rat aortic vascular smooth muscle cells appears to be sensitive to alterations in extracellular Mg2+, and acute or chronic ethanol treatment attenuates the responses to Mg2+.
Endothelial cells may have a modulatory role on the responses of the vascular tone to Mg2+ and Mg2+ is also an important cofactor for ACh-induced endothelium-dependent relaxation in isolated rat thoracic aortae.
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