Endogenous modulation of the blunted adrenergic response in resistance-sized mesenteric arteries from the pregnant rat

OBJECTIVE: We tested the hypothesis that during pregnancy the endothelium mediates the blunted response to adrenergic vasoconstriction. STUDY DESIGN: Mesenteric resistance arteries from late pregnant (n = 6) and age-matched virgin control (n = 6) Sprague-Dawley rats were studied in a myograph. RESULTS: Arteries from pregnant rats were 35% less sensitive to phenylephrine vasoconstriction than were those from nonpregnant rats (mean effective concentration that produced a 50% response 2.26 vs 1.48 μmol/L, pregnant vs nonpregnant, p \lt 0.01 ). Meclofenamate had no effect on the vasoconstrictor response in arteries from either group. Inhibition of endothelium-derived relaxing factor with Nº-nitro-L-arginine methyl ester or endothelial cell removal had a similar twofold increase in phenylephrine sensitivity in arteries from both the pregnant and nonpregnant rats (mean effective concentration that produced a 50% response 2.26 vs 1.11 μmol/L for pregnant rats and 1.48 vs 0. 72 μmol/L for nonpregnant rats, p \lt 0.01). However, methacholine relaxation response was potentiated in pregnant versus nonpregnant rats (mean effective concentration that produced a 50% response 0.030 vs 0.049 μmol/L, p \lt 0.01). CONCLUSION: Although the potential for endothelium-dependent relaxation is augmented in mesenteric arteries of the pregnant rat, the decreased sensitivity to phenylephrine during pregnancy is not modulated acutely by endothelium-derived relaxing factor or by prostaglandin products of the cyclooxygenase pathway. (AM J OBSTET GYNECOL 1992;167:1691-8.)