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Departments of 1 Physiology, 2 Pathology, and 4 Medicine and 3 Center For Cardiovascular and Muscle Research, State University of New York, Health Science Center at Brooklyn, Brooklyn, New York 11203
In
vitro extracellular Mg2+ concentration
([Mg2+]0) produces
endothelium-dependent and endothelium-independent relaxations in rat
aorta in a concentration-dependent manner. These relaxant effects of
Mg2+ on intact rat aortic rings, but not denuded rat aortic
rings, were suppressed by either
NG-monomethyl-L-arginine
(L-NMMA),
N
-nitro-L-arginine methyl
ester (L-NAME), or methylene blue. The inhibitory effects of L-NMMA and L-NAME could
be reversed partly by L-arginine.
[Mg2+]0-induced dilatation in vivo
in rat mesenteric arterioles and venules was almost completely
inhibited by NG-nitro-L-arginine and
L-NMMA. Removal of extracellular Ca2+
concentration ([Ca2+]0) or
buffering intracellular Ca2+ concentration in endothelial
cells, with 10 µM
1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM, markedly attenuated the relaxant effects of Mg2+.
Mg2+ produced nitric oxide (NO) release from the intact
aortic rings in a concentration-dependent manner. Removal of
[Ca2+]0 diminished the increased NO
release induced by elevated levels of
[Mg2+]0. In vivo infusion of
increasing doses (1-30 µM/min) of MgSO4, directly
into the femoral veins of anesthetized rats, elicited significant
concentration-dependent sustained increases in serum total Mg and
concomitant decreases in arterial blood pressure. Before and after
employment of various doses of MgSO4, intravenous administration of either L-NMMA (10 mg/kg) or
L-NAME (10 mg/kg) increased (i.e., reversed) the
MgSO4-lowered blood pressure markedly, and intravenous
injection of L-arginine restored partially the increased
blood pressure effects of both L-NMMA and
L-NAME. Our results suggest that 1) small blood
vessels are very dependent on NO release for Mg2+
dilatations and 2) the endothelium-dependent relaxation induced by extracellular Mg2+ is mediated by release of
endothelium-derived relaxing factor-NO from the endothelium, and
requires Ca2+ and formation of guanosine
3',5'-cyclic monophosphate.
rat aorta; microvessels; calcium; endothelium-derived relaxing factor; vasodilatation; guanosine 3',5'-cyclic monophosphate
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