Uterine blood flow responses to ICI 182 780 in ovariectomized oestradiol-17beta-treated, intact follicular and pregnant sheep

Abstract

Oestrogen dramatically increases uterine blood flow (UBF) in ovariectomized (Ovx) ewes. Both the follicular phase and pregnancy are normal physiological states with elevated levels of circulating oestrogen. ICI 182 780 is a pure steroidal oestrogen receptor (ER) antagonist that blocks oestrogenic actions in oestrogen-responsive tissue. We hypothesized that an ER-mediated mechanism is responsible for in vivo rises in UBF in physiological states of high oestrogen. The purpose of the study was to examine the effect of an ER antagonist on exogenous and endogenous oestradiol-17beta (E2beta)-mediated elevations in UBF. Sheep were surgically instrumented with bilateral uterine artery blood flow transducers, and uterine and femoral artery catheters. Ovx animals (n = 8) were infused with vehicle (35% ethanol) or ICI 182 780 (0.1-3.0 microg min(-1)) into one uterine artery for 10 min before and 50 min after E2beta was given (1 microg kg(-1) I.V. bolus) and UBF was recorded for an additional hour. Intact, cycling sheep were synchronized to the follicular phase using progesterone, prostaglandin F2alpha(PGF2alpha) and pregnant mare serum gonadotrophin (PMSG). When peri-ovulatory rises in UBF reached near peak levels, ICI 182 780 (1 or 2 microg (ml uterine blood flow)-1) was infused unilaterally (n = 4 sheep). Ewes in the last stages of pregnancy (late pregnant ewes) were also given ICI 182 780 (0.23-2.0 microg (ml uterine blood flow)-1; 60 min infusion) into one uterine artery (n = 8 sheep). In Ovx sheep, local infusion of ICI 182 780 did not alter systemic cardiovascular parameters, such as mean arterial blood pressure or heart rate; however, it maximally decreased ipsilateral, but not contralateral, UBF vasodilatory responses to exogenous E2beta by approximately 55-60% (P < 0.01). In two models of elevated endogenous E2beta, local ICI 182 780 infusion inhibited the elevated UBF seen in follicular phase and late pregnant ewes in a time-dependent manner by approximately 60% and 37%, respectively; ipsilateral >> contralateral effects (P < 0.01). In late pregnant sheep ICI 182 780 also mildly and acutely (for 5-30 min) elevated mean arterial pressure and heart rate (P < 0.05). We conclude that exogenous E2beta-induced increases in UBF in the Ovx animal and endogenous E2beta-mediated elevations of UBF during the follicular phase and late pregnancy are partially mediated by ER-dependent mechanisms.

Figure 1. The effects of local infusion of ICI 182 780 (0.1–3.0 μg min⁻¹) on MAP (A) and HR (B) in Ovx E₂β-treated ewes (n = 8)

ICI 182 780 was infused from time −10 min to 50 min, and E₂β (1 μg kg⁻¹i.v.) was given at time zero. ICI 182 780 had no effect on MAP or HR at any dose. Values are means ± s.e.m.

Figure 2. Relative changes in ipsilateral versus contralateral UVR (A) and UBF (B) as a function of ICI 182 780 dose in E2β-treated Ovx ewes (n = 8)

These data were calculated relative to control UVR and UBF responses averaged (5-min intervals, 90–120 min) across the steady-state 90–120 min plateau in uterine vasodilatory responses to E2β. Ipsilateral ICI 182 780-related inhibition was dose-dependent (+P < 0.05 versus zero dose) and exceeded (*P < 0.05) the contralateral response to E2β but was greatest reaching a plateau at 2.5–3.0 μg min⁻¹ (56 ± 10%). Values are means ± s.e.m.

Figure 3. Effects of unilateral uterine artery infusion of ICI 182 780 on ipsilateral versus contralateral UBF in follicular phase ewes (n = 4)

The studies that were repeated during the unilateral infusion of ICI 182 780 (1–2 μg ml⁻¹ ipsilateral uterine blood concentrations) were averaged within an animal and then meaned. Ipsilateral UBF was decreased (≥ 70 min; P > 0.05) by a maximum average of 60 ± 8% (P < 0.01) and became significantly different (*P < 0.01) from contralateral UBF at approximately 60 min. Values are means ± s.e.m.

Figure 4. The systemic cardiovascular effects of unilateral uterine artery infusion of ICI 182 780 in late pregnant sheep (n = 7)

A, with the higher doses of ICI 182 780, MAP began to increase by 5 min reaching significance versus time 0 control at 15 and 30 min (*P < 0.05), but returned to control values by 45 min (P > 0.05). When compared both to the low doses of ICI 182 780 and vehicle, MAP was elevated from 5 to 30 min (+P < 0.05). B, when compared to the zero control, HR was unaltered throughout the study in all groups (P > 0.05). However, HR values were elevated during infusion of high dose ICI 182 780 compared to low dose ICI 182 780 and vehicle (*P < 0.05). Values are means ± s.e.m.

Figure 5. Time course of UVR responses to unilateral uterine artery infusion of ICI 182 780 (for 60 min) in late pregnant ewes (n = 8)

A, the higher doses of 1.0–2.0 μg ml⁻¹ resulted in significantly higher UVR almost immediately compared to the lower doses (0.23–0.9 μg ml⁻¹) as well as vehicle and contralateral controls. B, all ewes (n = 8) exhibited significant ipsilateral decreases in UBF in response to the higher doses of unilateral ICI 182 780 infusions. The average baseline (control) ipsilateral UBF was 607 ± 39 ml min⁻¹. We were able to inhibit ipsilateral UBF by an overall average of 37 ± 4% (P < 0.01). Values are means ± s.e.m.