Disproportionate decrease in α- compared with β-adrenergic sensitivity in the dorsal hand vein in pregnancy favors vasodilation

Background - Altered vascular responses to adrenergic agonists during pregnancy are thought to play an important role in the regulation of blood pressure and placental blood flow. Because α₁-adrenergic and β₂-adrenergic sensitivity act in opposing directions to determine vascular tone, we simultaneously evaluated α-adrenergic-mediated vasoconstriction and β-adrenergic-mediated vasodilation in dorsal hand veins during and after pregnancy. Methods and Results - Twenty healthy pregnant women were studied at 32 to 37 weeks of gestation and again 12 weeks after delivery. Vascular response to phenylephrine (PE) and isoproterenol (ISO) was measured in a dorsal hand vein using the linear variable differential transformer technique. The dose of PE resulting in 50% constriction (CD₅₀) was determined. The response to ISO was measured after the PE preconstriction. Pregnant and postpartum values, expressed as geometric mean (95% CI), were compared by paired t test. α-Adrenergic sensitivity during pregnancy (CD₅₀, 2.7 μg/min [95% CI, 1.5 to 5.0]) was markedly decreased, ≈7-fold, compared with postpartum (0.4 μg/min [95% CI, 0.3 to 0.7] [P<0.01]). β-Adrenergic vasodilation was also attenuated during pregnancy. The ED₅₀ of ISO (dose of ISO resulting in 50% of the maximal response, E_max) was greater during pregnancy (20 ng/min [95% CI, 11 to 35]) than postpartum (8 ng/min [95% CI, 5 to 12]) (P<0.05). ISO E_max was also significantly less during pregnancy (81% [95% CI, 65 to 97] compared with postpartum (105% [95% CI, 97 to 113]) (P<0.01). Conclusions - Normal pregnancy is characterized by decreased venous sensitivity to both α₁-adrenoceptor-mediated vasoconstriction and β₂-adrenoceptor-mediated vasodilation. The greater decrease in α₁ compared with β₂ response may contribute to the vasodilated state characteristic of human pregnancy.