Nitric oxide contributes to estrogen-induced vasodilation of the ovine uterine circulation

Estradiol-17β (E₂β), a potent vasodilator, has its greatest effects on the uterine vasculature, blood flow (UBF) increasing ≤ 10-fold. The mechanism(s) responsible for E₂β-induced vasodilation is unclear. We determined if nitric oxide (NO)induced increases in cGMP modulate estrogen- induced increases in UBF, and if cyclooxygenase inhibition modifies E₂β responses. Nonpregnant (n = 15) and pregnant (n = 8) ewes had flow probes implanted on main uterine arteries and catheters in branches of the uterine vein and artery bilaterally for blood sampling and infusion of the NO synthase inhibitor L-nitro-arginine methyl ester (L-NAME), respectively. In nonpregnant ewes E₂β (1 μg/kg) caused parallel increases (P < 0.001) in UBF (15±3 to 130±16 ml/min) and uterine cGMP secretion (23±10 to 291±38 pmol/min); uterine venous cGMP also rose (4.98±1.4 to 9.43±3.2 pmol/ml; P < 0.001). Intra-arterial L-NAME partially inhibited increases in UBF dose- dependently (r = 0.66, n = 18, P ≤ 0.003) while completely inhibiting cGMP secretion (P = 0.025). Indomethacin, 2 mg/kg intravenously, did not alter E₂β-induced responses. After E₂β-induced increases in UBF, intraarterial L-NAME partially decreased UBF dose dependently (r = 0.73, n = 46, P < 0.001) while inhibiting cGMP secretion (178±48 to 50±24 pmol/min; n = 5, P = 0.006); both were reversed by L-arginine. In pregnant ewes, E₂β increased UBF and venous cGMP (9.1±0.96 to 13.2± 0.96 pmol/ml, P < 0.01); however, intraarterial L-NAME decreased basal cGMP secretion 66% (P = 0.02), but not UBF. Acute estrogen-induced increases in UBF are associated with NO- dependent increases in cGMP synthesis, but other mechanisms may also be involved. However, vasodilating prostanoids do not appear to be important. In ovine pregnancy NO is not essential for maintaining uteroplacental vasodilation.