Chronic Nitric Oxide Synthesis Inhibition Does Not Prevent Pregnancy Vasodilation in the Rat

Prolonged uterine artery nitric oxide synthase inhibition modestly alters basal uteroplacental vasodilation in the last third of ovine pregnancy

Mechanisms regulating uteroplacental blood flow (UPBF) in pregnancy remain unclear, but they likely involve several integrated signaling systems. Endothelium-derived nitric oxide (NO) is considered an important contributor, but the extent of its involvement is unclear. Bolus intra-arterial infusions of nitro-l-arginine methyl ester (l-NAME) modestly decrease ovine basal UPBF; however, the doses and duration of infusion may have been insufficient. We, therefore, examined prolonged uterine artery (UA) NO synthase inhibition with l-NAME throughout the last third of ovine pregnancy by performing either continuous 30-min UA infusion dose responses (n = 4) or 72-h UA infusions (0.01 mg/ml) at 104-108, 118-125, and 131-137 days of gestation (n = 7) while monitoring mean arterial pressure (MAP), heart rate (HR), and UPBF. Uteroplacental vascular resistance (UPVR) was calculated, and uterine cGMP synthesis was measured. Thirty-minute UA l-NAME infusions did not dose dependently decrease UPBF, increase UPVR, or decrease uterine cGMP synthesis (P > 0.1); however, MAP rose and HR fell modestly. Prolonged continuous 72-h UA l-NAME infusions decreased UPBF ∼32%, increased UPVR ∼68% (P ≤ 0.001), and decreased uterine cGMP synthesis 70% at 54-72 h (P ≤ 0.004); the noninfused uterine horn was unaffected. These findings were associated with ∼10% increases in MAP and decreases in HR that were greater at 104-108 than 118-125 and 131-137 days of gestation (P = 0.006). Although uterine and UA NO and cGMP synthesis increase severalfold during ovine pregnancy, they contribute modestly to the maintenance and rise in UPBF in the last third of gestation. Thus, local UA NO may primarily modulate vasoconstrictor responses. Notably, the systemic vasculature appears more sensitive than the uterine vasculature to NO synthase inhibition.

Responses of isolated perfused uterine vascular beds of nonpregnant and pregnant rats to endogenous and exogenous nitric oxide

The responses to endothelial vasodilators and exogenous nitric oxide (NO) were characterized in intact isolated uterine vascular beds of nonpregnant, midpregnant and late-pregnant rats perfused with Kreb's buffer (37 degrees C, 5% CO(2) in air, pH approximately 7.4) containing 2% dextran and indomethacin. Phenylephrine increased perfusion pressure in the vascular beds equally in all three groups. In the presence of phenylephrine, N(omega)-nitro-L-arginine methyl ester (L-NAME) significantly augmented perfusion pressure in the order: nonpregnant<midpregnant<late-pregnant uterine vascular bed. Acetylcholine and bradykinin-induced attenuation of perfusion pressure did not depend on gestational age. The decrease in perfusion pressure induced by acetylcholine was nonsignificantly attenuated by L-NAME in vascular beds from pregnant rats. The attenuation induced by bradykinin reached significant level in the vascular beds from midpregnant rats. The diethylamine (DEA)/NO-induced decrease in perfusion pressure was not influenced by L-NAME in any group. These data demonstrate the augmentation of basal release of NO associated with progression of pregnancy, while the responses to endothelial vasodilators do not depend on gestational age and are not abolished by inhibition of NO synthase, suggesting involvement of nonprostanoid non-NO factor in the control of uterine circulation.

Endothelin and blood pressure regulation in the female rat: studies in normal pregnancy and with nitric oxide synthase inhibition-induced hypertension

OBJECTIVE

To evaluate the role of endothelin (ET) in blood pressure regulation in normal pregnant and nonpregnant rats and with nitric oxide synthase (NOS) inhibition.

METHODS

Pregnant and nonpregnant Sprague-Dawley rats were treated for 7 days with an ET(A)-selective (A-127722 or FR-139317), ET(B)-selective (A-192621), or ET(A)/ET(B) nonselective (A-182086) endothelin receptor antagonist, and/or with the NOS inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME, 2. 5 mg/kg/h). In pregnant rats, the ET antagonists and L-NAME were administered from gestational day 14 through day 21 (term = 22 days). All rats received indwelling arterial catheters for blood pressure measurement. Mean arterial pressures were recorded on infusion days 1, 4, and 7 and these data were compared by analysis of variance among experimental groups with p < 0.05 considered significant.

RESULTS

The ET(A) receptor antagonism lowered blood pressure in both pregnant and nonpregnant rats (p < 0.05), whereas ET(B) antagonism resulted in hypertension (p < 0.001). ET(B) antagonism-induced hypertension was attenuated by pregnancy (p < 0. 001). Hypertension was induced in all rats treated with L-NAME (p < 0.001). Endothelin receptor antagonism, regardless of specificity, did not ameliorate L-NAME-induced hypertension in pregnant or nonpregnant female rats. The only observed effect of ET(A) antagonism on NOS inhibition-induced hypertension was the prevention of a continued rise at infusion day 7 in nonpregnant rats.

CONCLUSIONS

Endothelin, acting via both the ET(A) and ET(B) receptors, contributes to blood pressure homeostasis in pregnant and nonpregnant rats. Endothelin receptor antagonism does not ameliorate NOS-inhibition-induced hypertension in pregnant rats.

Endothelin-1 and relaxation of the rat aorta during pregnancy in nitroarginine-induced hypertension

In pregnant rats during hypertension induced by NO synthase inhibition, endothelin (ET) plasma levels are increased as in some preeclamptic women. Previously, the enhanced vasodepressor effect of endothelin-1 (ET-1) has been observed in this model, thus we decided to study the relaxation induced by ET-1 on the aorta. Non-pregnant or pregnant Wistar rats (n = 7 by group) were fed for 7 days (day 13-day 20) on a nitroarginine-enriched diet (L-NNA, 0.063% i.e. 30 mg/kg/day) or a control diet. Systolic blood pressure, measured by the tail cuff method on conscious rats at day 20 of gestation, was raised by the chronic L-NNA treatment (mean +/- s.e.m., mmHg, p < 0.001: pregnant L-NNA treated, 145 +/- 1.84 vs. pregnant control, 101 +/- 2.00 and non-pregnant L-NNA treated, 148 +/- 3.11 vs. non-pregnant control, 119 +/- 1.80). On day 20 ex vivo aortic ring relaxation was produced by ET-1 in vessels previously precontracted with norepinephrine only when endothelium was present. In control rats, ET-1 (10(-8) to 5 x 10(-8) M) produced a short but significant relaxation (mean value between 4 to 19%) followed by a long-lasting contracting phase, and a higher ET-1 concentration (10(-7) M) only produced contraction. Chronic L-NNA treatment decreased the level of relaxation (at least p < 0.05, in non-pregnant and pregnant rats) and with a 30 min L-NAME (10(-4) M) preincubation, relaxation was completely inhibited in non-pregnant and pregnant rats. BQ-123, an ETA receptor antagonist, did not produce any effect on ET-1 induced relaxation. BQ-788, an ETB receptor antagonist, significantly decreased it. In conclusion, in female rats, as in male rats, ET-1 induces a transient relaxation in the preconstricted aorta which involves endothelial ETB receptors. Despite a decrease in the systemic vascular reactivity during late gestation, the vasodilating and vasoconstricting properties of ET-1 on the aorta are not changed.

Animal models of preeclampsia

Some of the maternal symptoms of preeclampsia can be produced by uterine ischemia, although no quadriped spontaneously exhibits this disease. It may be that the combination of upright posture and uteroplacental ischemia are necessary for manifestation of the full syndrome. Chronic nitric oxide synthase inhibition in rats produces a pattern of change that resembles the symptoms of preeclampsia, and the preeclamptic-like response of rats with adriamycin nephropathy and hyperinsulinemia is associated with endothelial dysfunction. These models are definitely of use in preeclampsia research, but because this disease only occurs spontaneously in primates, the definitive studies on preeclampsia will, of necessity, be clinical.