IN-VITRO EFFECTS OF MAGNESIUM-SULFATE IN ISOLATED INTRAPULMONARY AND MESENTERIC-ARTERIES OF PIGLETS

Magnesium sulfate (MgSO4) has been proposed to be an efficient treatme nt in persistent pulmonary hypertension of the newborn. We compared th e ability of MgSO4 to inhibit the responses to several vasoconstrictor s in isolated intrapulmonary and mesenteric arteries from 10-17-d-old piglets. MgSO4 (3-100 mM) produced a slight vasodilator effect in pulm onary arteries precontracted with the thromboxane A(2) mimetic U46619 (10(-6) M), noradrenaline (10(-5) M), and KCI (80 mM) (15.1 +/- 3.7%; 20 +/- 3.33%; 10.4 +/- 0.9% at 100 mM MgSO4, respectively). In contras t, in mesenteric arteries MgSO4 produced a marked vasodilation (80.4 /- 4.0%, 93.1 +/- 3.46%, and 87.5 +/- 1.93% at 100 mM MgSO4, respectiv ely, p < 0.01 versus pulmonary arteries). The vasodilator effect of Mg SO4 was endothelium-independent and reversed by increasing the extrace llular Ca2+ concentration. After incubation for 1 h of pulmonary arter ies with three different MgSO4 concentrations (0, 1.2, and 4.8 mM) the re were no differences in the contractile responses to U46619 nor in t he vasodilator effects of acetylcholine or sodium nitroprusside. Rapid removal of Mg2+ from bath medium produced a transient vasodilation wh ich was more marked in pulmonary than in mesenteric arteries and was g reatly reduced by the removal of endothelium or by the nitric oxide sy nthase inhibitor L-NAME (10(-4) M). We conclude that MgSO4 is a poor v asodilator of pulmonary arteries in vitro and at physiologic concentra tions appears to inhibit nitric oxide release from the pulmonary endot helium. Thus, the possible beneficial clinical effects of MgSO4 in per sistent pulmonary hypertension of the newborn do not seem to be relate d to a direct effect on pulmonary vascular smooth muscle.