Comparative vasodilator effects of magnesium salts on rat mesenteric arterioles and venules.

In vivo studies were undertaken to investigate the comparative effects of different magnesium (Mg) salts (i.e., MgCl2, MgSO4, Mg acetate, Mg aspartate HCl) on arterioles (10-20 microns i.d.) and venules (15-30 microns i.d.) of the mesenteric circulation of the pentobarbitone-anesthetized rat. Perivascular administration of these Mg salts (topical doses = 0.1-100 mumoles) produced dose-dependent vasodilatation (5-50% increases in lumen size) of arterioles and venules almost instantaneously. Arterioles were more sensitive than venules to topical MgCl2 and MgSO4 but not to either Mg aspartate HCl or Mg acetate. Intra-arterial infusion of the Mg salts (1-40 mumoles/min, i.e., 5-400 mumol/kg/min for 20 min) at each dose produced vasodilatation, i.e. 10-80% increases in lumen size of mesenteric arterioles; doses above 10 mumoles/min sometimes produced bleeding from venules. Systemic i.v. infusion (1-40 mumoles/min) of all 4 Mg salts lowered systolic and diastolic arterial blood pressure dose-dependently; however, the degree or presence of vasodilatation noted in the microvessels was not always correlated with the falls in arterial blood pressure. Intravenous administration of the Mg salts resulted in dose-dependent elevation in plasma Mg (0.3-4.7 mg/dl, over control levels); increments in plasma Mg were noted as soon as 1 min after the start of the infusion. Plasma Mg levels wre higher with inorganic salts and peaked more rapidly. Large i.v. doses of Mg salts (i.e., greater than 20 mumoles/min) induced some bleeding from the venules which varied with the anion. Administration of a variety of pharmacological receptor antagonists and a cyclo-oxygenase inhibitor did not interfere with vasodilatation of arterioles or venules induced by Mg salts. These results indicate that magnesium ions: 1) are vasodilators of intact microscopic mesenteric resistance and capacitance microvessels; and 2) bring about vasodilatation by direct actions on microvascular smooth muscle cells. In addition, our data indicate that the anion associated with Mg appears to exert significant influences on microvascular tone.