INTRODUCTION

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ANIMAL STUDIES designed to evaluate the arterial baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) over a wide blood pressure (BP) range indicate that normal pregnancy is associated with a moderate impairment of the ability of arterial baroreflexes to buffer acute perturbations in BP (7, 11, 30, 36, 41). The gestational impairment in baroreflex control is a likely contributing factor to the reduction in cardiovascular stability during hemorrhage in the gravid state (8).

The mechanisms responsible for gestational depression of arterial baroreflexes are not well understood. One candidate is the hormone angiotensin II (ANG II). In normal pregnancy, plasma ANG II is elevated in humans (19) and in rabbits (10). Circulating ANG II can modulate central control of HR and sympathetic outflow via the area postrema, a circumventricular organ (29, 33, 32). The sensitivity of the sympathetic baroreflex is reported as unchanged (27, 32) or depressed (34, 35) during acute elevations in peripheral ANG II. Chronic infusion of ANG II in conscious rabbits decreased the gain of the cardiac baroreflex and reset the reflex control of HR and norepinephrine (6) in a manner that was, in part, independent of the ANG II-mediated hypertensive response. In pacing-induced heart failure in rabbits, plasma ANG II levels are elevated and reflex control of HR and RSNA are depressed (29, 34). Systemic AT₁ receptor antagonism with losartan enhanced baroreflex sensitivity in the heart failure rabbits, an effect blocked by lesion of the area postrema (29).

Endogenous ANG II may affect baroreflex control through local action of the brain renin-angiotensin system. AT₁ receptors are located at the nucleus tractus solitarius, ventrolateral medulla (VLM) and other sites important in reflex control (reviewed in Refs. 1 and 2). ANG II modulates both sympathoinhibitory and sympathoexcitatory pathways in the brain stem (2, 5, 32, 38), although the net effect of endogenous central ANG II in conscious rabbits appears to be inhibition of sympathetic and cardiac baroreflexes (5, 18). In the rat model of heart failure, depressed reflex control of HR and RSNA was ameliorated by blockade of central AT₁ receptors with losartan (12) suggesting that chronically increased activity of the renin-angiotensin system contributed to abnormal baroreflex control. The response of the brain renin-angiotensin system to normal pregnancy is largely unknown. In the rat, the pressor response to intracerebroventricularly administered ANG II is augmented during pregnancy (23), which could indicate an enhanced sympathoexcitatory role for central ANG II.

In this study, we evaluated the hypothesis that endogenous ANG II contributes to the blunted arterial baroreflex control of RSNA. Crandall and Heesch (11) reported that the angiotensin-converting enzyme inhibitor captopril had no effect on the RSNA baroreflex in pregnant rats. However, this study was performed under anesthesia, which can affect central baroreflex control. We compared RSNA baroreflex curves generated in conscious term-pregnant and nonpregnant rabbits before and after sequential intracerebroventricular and intravenous infusions of the AT₁ receptor antagonist losartan. Intravenously infused losartan blocks the effects of circulating ANG II at the area postrema (16) and crosses the blood-brain barrier (40). The use of intracerebroventricular losartan aids in separating the effects of brain ANG II from circulating ANG II on baroreflex control.

	METHODS

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The experimental and animal care protocols were reviewed and approved by the Institutional Review Board for the Use and Care of Animals of Midwestern University. Twenty-one female New Zealand White rabbits of breeding age (6 mo) were used for this study. There were three experimental groups: nonpregnant, pregnant, and time control. The arterial baroreflex was studied in the pregnant and nonpregnant groups before and after sequential treatment with intracerebroventricular and intravenous losartan (kindly supplied by Merck), which is a nonpeptide AT₁ receptor antagonist. Because the order of treatments was not randomized, a time-control protocol was performed in which losartan was replaced with the saline vehicle. The eight rabbits in the pregnant group delivered their litters on days 30-32 of gestation, which is normal gestation time for a New Zealand White rabbit. Of a total of 40 pups delivered, 33 (83%) were live births, and all rabbits delivered live neonates. Eight rabbits were in the nonpregnant group treated with losartan. The time-control protocol was performed in seven rabbits 10-34 days after the RSNA electrode implant surgery. Of these seven rabbits, two rabbits are also represented in the nonpregnant group. Five of the time-control rabbits were studied 3-9 days postgestation and three of these five rabbits are also represented in the pregnant group.

Surgical Preparation

General. Rabbits were anesthetized with Telazol (tiletamine hydrochloride and zolazepam hydrochloride 15 mg/kg im, Elkins-Sinn; Cherry Hill, NJ) and xylazine (xylazine hydrochloride 25 mg/kg im, Butler; Columbus, OH) and intubated with a cuffed endotracheal tube. To maintain a surgical plane of anesthesia, the rabbits were mechanically ventilated with 2.0-2.5% halothane or isoflurane in room air. Buprenorphine hydrochloride (0.03 mg/kg, Reckitt and Colman; Richmond, VA) was given immediately and at 5-8 h postoperatively for pain management.

Implantation of intracerebroventricular cannula. A right lateral intracerebroventricular cannula and an aortic perivascular occluder were implanted during the first surgery. The intracerebroventricular cannula consisted of a 23-mm length of 22-gauge stainless steel tubing with a removable stainless steel stylet (Small Parts; Miami Lakes, FL). Coordinates were 1 mm caudal to bregma, 3 mm lateral to midline, and 9 mm from the surface of the skull. The cannula was initially fixed in place with the use of three skull screws and small amounts of denture repair material (Accelar 20 Light; Patterson Dental; St. Paul, MN). A protective plastic cap with a removable screw top was then placed over the cannula, and additional dental cement was used to fix the cap in place. To fabricate the protective cap, the top 10 mm (including the threads for the screw top) were cut from a 2-ml microcentrifuge tube. Two 18-mm wires, cut from a straightened paperclip, were placed parallel (5-6 mm apart) though the bottom portion of the cap. The wires provided a scaffold for anchoring the cap in the dental cement. Removal of the screw top provided access to the cannula.

Implantation of aortic and vena caval perivascular occluders. To manipulate BP for evaluation of the arterial baroreflex, perivascular occluders were implanted on the thoracic descending aorta and on the thoracic inferior vena cava in separate surgeries. The occluders were fabricated in the laboratory. The aortic occluder was implanted in the same surgery as the intracerebroventricular cannula. At least 10 days separated the implantation of the aortic and vena caval occluders. A left thoracotomy was performed at the third intercostal space for placement of the aortic occluder. The vena caval occluder was implanted through a right thoracotomy at the fourth intercostal space. For both procedures, the occluder tubing exited the thorax through an adjacent intercostal space, and the free end of the appliance was secured in a subcutaneous pouch. The second thoracotomy and renal nerve implant surgeries were separated by a median of 31 (14-55) days in the nonpregnant group, 37 (33-50) days in the pregnant group and 48 (14-55) days in the time-control group.

Implantation of renal nerve recording electrodes. In nonpregnant and pregnant rabbits, chronic recording electrodes were implanted on the left renal nerves. This surgery was performed in pregnant rabbits on day 25 of gestation. Gestation in the rabbit is 30-32 days. Via a retroperitoneal approach the left kidney was exposed, and one or two renal nerves were dissected away from the renal artery. The recording electrodes were two 30-cm lengths of Teflon-coated multistranded stainless steel wire (0.009-in. diameter, 316SS7/44T, Medwire; Mt. Vernon, NY). One end of each electrode was stripped and curled into a J-shaped hook. The renal nerves were placed in the hooks and suspended above the renal artery. The bare end of a third length of wire was embedded in perirenal fat to serve as a ground. The entire nerve-electrode-ground complex was then embedded in silicone gel (Silgel 604, Wacker Chemie). The three electrode leads were secured to muscle at the incision site, routed subcutaneously to the dorsal aspect of the neck, and externalized. Small gold pins were crimped onto the exposed ends of the leads. The leads were wrapped in a strip of cloth adhesive tape for protection, wound into a small bundle, and secured to the skin for storage. Rabbits were studied on the second or third day postsurgery. This time corresponded to day 28 of gestation (term pregnancy) in the pregnant rabbits.

Experimental Procedures

Instrumentation. On the day of the experiment, the rabbit was placed in a wooden box (15 × 40 cm) with a wire mesh floor and a lid with a grid that allowed instrumentation leads and catheters to exit the box. The skin overlying the central ear artery and marginal vein were anesthetized with topical EMLA cream (2.5% lidocaine and 2.5% prilocaine, Astra; Westborough, MA). Arterial BP was obtained from a small Teflon catheter (Angiocath 24-gauge, outer diameter = 0.7 mm, Deseret; Sandy, UT) placed into the central ear artery by percutaneous placement or via a cutdown under an additional local anesthetic block with 2% lidocaine hydrochloride. HR was derived from the arterial pressure pulse with the use of a Grass tachograph. Venous access was obtained by percutaneous placement of a 24-gauge Teflon catheter into the contralateral ear vein. The free ends of the aortic and vena caval occluders were retrieved from their respective subcutaneous pouches under a local anesthetic block (2% lidocaine). Extension tubing connected the occluders to water-filled 1-ml glass syringes. The lateral ventricle cannula was connected via PE-50 tubing to a 50-µl Hamilton syringe mounted on an infusion pump for intracerebroventricular drug infusions (see Intracerebroventricular drug infusion).

Generation of baroreflex curves. The BP-RSNA relationship was generated by sequential inflations of the vena caval and aortic occluders (13). Inflation of the vena caval occluder decreased BP, whereas inflation of the aortic occluder increased BP. Before data collection, two to three sets (aortic plus caval) of ramp inflations were performed and discarded because a previous study by Bell et al. (4) showed that the first two sets of ramp inflations in an experimental session result in an exaggerated reflex response. A 30-s rest period preceded a slow, manual ramp inflation of the occluder at a target rate of 2-3 mmHg/s. At least 2 min separated the aortic and vena caval occluder inflations to allow return of BP, HR, and RSNA to basal values. Three to four sets of occluder inflations were completed in a sequential manner in each animal.

Intracerebroventricular drug infusion. Five intracerebroventricular infusions were performed during the experimental protocol: 1) isotonic saline (25 µl), which was the vehicle for ANG II and losartan; 2) ANG II (125 ng/25 µl, Sigma); 3) losartan (80 µg/20 µl); 4) ANG II after intracerebroventricular losartan; and 5) ANG II after combined intracerebroventricular and intravenous losartan treatment. All drug infusions were given at 10 µl/min. During the time-control protocol, saline was substituted for intracerebroventricular ANG II or losartan. During the saline and ANG II infusions, data were recorded for analytic purposes during, and for 10 min after, the infusion. Infusion of ANG II into the lateral ventricles in the rabbit results in a pressor response (see RESULTS). Blockade of the pressor response to intracerebroventricular ANG II after central infusion of losartan was considered evidence of central AT₁ receptor block.

Experimental Protocols

Effect of AT₁ receptor block on arterial baroreflex. Pregnant and nonpregnant groups were treated similarly. After instrumentation, and at least 30 min of rest, the hemodynamic and RSNA responses to a bolus intravenous infusion of ANG II (50 ng/kg) was measured.

Data Analysis

Baroreflex control of RSNA. Resting BP, HR, and RSNA for each baroreflex curve was derived from the average of the two 30-s periods preceding the ramp inflations. The data from one set of aortic and vena caval ramp inflations were combined into a single data file for determination of a BP-RSNA baroreflex curve. With the use of an iterative least squares regression, BP-RSNA data were fit with a four-parameter sigmoid logistic function (25): RSNA = A/{1 + exp[B (BP  C)]} + D, where A is the range between the upper and lower plateaus, B is the range-independent slope coefficient, C is the BP at the midpoint of the RSNA range (BP₅₀), and D is the lower plateau. The upper plateau is equal to A + D. The gain of the BP-RSNA baroreflex curve at BP₅₀ (maximum gain) was calculated as B · A/4.

Intracerebroventricular ANG II infusions. The data were initially partitioned into 30-s averages. The 30-s average before the infusion represented baseline data. The 30-s interval at which the pressor response reached steady-state was identified for the intracerebroventricular ANG II infusion performed in the intact condition. From this point to the end of the collection period, data were averaged into a single value that represented the steady-state response. Steady-state values for the saline and two ANG II infusions performed after losartan block were calculated using the same interval as the initial ANG II infusion. A three-way ANOVA with repeated measures (group × icv infusion × time) was utilized to compare responses.

Intravenous ANG II infusions. Baseline data are represented by a 30-s average. For the initial ANG II infusion, a 6-s average was taken at the time of the peak pressor response. The same interval was used to calculate the response to the two ANG II infusions performed after intravenous losartan. Responses were compared by using a three-way ANOVA with repeated measures (group × intravenous infusion × time).

	RESULTS

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The pregnant rabbits (4.1 ± 0.2 kg) were heavier than the nonpregnant rabbits (3.6 ± 0.1 kg; P < 0.05). The average body weight of the time-control rabbits was 3.9 ± 0.1 kg.

Effect of losartan on rest hemodynamics. In the time-control group of rabbits, resting BP and HR did not change during the experiment. Term pregnancy was associated with a lower resting BP (P < 0.05, Fig. 1) compared with the nonpregnant rabbits. Intracerebroventricular losartan had no effect on resting BP or HR in nonpregnant rabbits, whereas the addition of intravenous losartan decreased resting BP by 10 ± 2 mmHg. In the pregnant rabbits, intracerebroventricular losartan decreased resting BP by 9 ± 3 mmHg without affecting rest HR. The addition of intravenous losartan further decreased resting BP by 13 ± 2 mmHg. The combination of intracerebroventricular and intravenous losartan decreased resting BP to a greater extent in pregnant (by 21 ± 2 mmHg) than nonpregnant rabbits (by 13 ± 3 mmHg; P < 0.05). RSNA was expressed as a percentage of the RSNA recorded during intracerebroventricular saline treatment. RSNA responded similarly in the losartan-treated and time-control groups, and thus the apparent increases in RSNA in response to intracerebroventricular losartan and intracerebroventricular plus intravenous losartan are unlikely to be a drug effect. Averaged over the three groups, rest RSNA increased to 130 ± 10% of saline RSNA and 147 ± 11% of saline RSNA, equivalent to the intracerebroventricular losartan and intracerebroventricular plus intravenous losartan time periods, respectively (P < 0.001).

View larger version (24K): [in this window] [in a new window]   Fig. 1.   Mean values for blood pressure (BP), heart rate (HR), and renal sympathetic nerve activity (RSNA) at rest in nonpregnant, pregnant, and time-control groups of rabbits. Data were recorded before central losartan administration [intracerebroventricular (icv) saline], after icv losartan (Los) was administered into the right lateral ventricle and after losartan was administered intravenously, which resulted in a combined central and peripheral AT1 block (Los icv + iv). The time-control rabbits received icv or iv saline in place of losartan. RSNA is expressed in arbitrary units (au). RSNA during the icv saline treatment was normalized to 100% for data analysis. *P < 0.05 vs. icv saline; #P < 0.05 vs. nonpregnant group.

Responses to intracerebroventricular and intravenous ANG II. The time-control group received saline intracerebroventricularly in place of ANG II intracerebroventricularly. The infusion was performed four times and corresponded in time with the intracerebroventricular saline and three ANG II infusions received by the pregnant and nonpregnant groups. There was no significant effect of repeated saline intracerebroventricular infusions on BP (average change 2 ± 0.8 mmHg), HR (average decrease of 0.4 ± 2 beats/min) or RSNA (average increase 8 ± 5%) during the experiment.

View larger version (42K): [in this window] [in a new window]   Fig. 2.   Original data recordings of BP, RSNA, and HR in a pregnant rabbit in response to ANG II infused into the right lateral ventricle before and after central losartan administration. *Artifact in RSNA recording. Artifacts were removed from the data file before analysis.

View larger version (18K): [in this window] [in a new window]   Fig. 3.   Mean changes in BP and HR and percent changes in RSNA in nonpregnant and pregnant rabbits in response to icv saline, icv ANG II, and icv ANG II after central losartan administration (icv ANG II plus icv Los) and the combined central and peripheral losartan treatment (ANG II + Los icv + Los iv). *P < 0.05 nonpregnant vs. pregnant.

Effect of losartan on arterial baroreflex. The BP-RSNA relationship under control conditions (intracerebroventricular saline) for the pregnant and nonpregnant rabbits is shown in Fig. 4. The BP₅₀ of the baroreflex curves was similar among groups (Table 1). The maximum RSNA response to a progressive decrease in BP was lower in the pregnant group (248 ± 20 vs. 357 ± 41% of rest RSNA, P < 0.05), resulting in a lower RSNA range (30% decrease). The slope coefficient was similar among groups. As a result, the maximum gain (gain at BP₅₀) of the baroreflex control of RSNA in the pregnant group was depressed relative to the nonpregnant group (44% decrease).

View larger version (24K): [in this window] [in a new window]   Fig. 4.   Mean RSNA baroreflex curves during the icv saline treatment for the nonpregnant and pregnant rabbits. RSNA is expressed relative to rest RSNA (100%) for both groups of rabbits. For each curve, the rest value for BP and RSNA is identified by the open circle.

View larger version (27K): [in this window] [in a new window]   Fig. 5.   Mean RSNA baroreflex curves for the nonpregnant rabbits during the three consecutive treatments: icv saline, icv losartan, and icv plus iv. RSNA losartan is expressed relative to rest RSNA (100%) during the icv saline treatment. For each curve, the rest value for BP and RSNA is identified by the open circle.

View larger version (25K): [in this window] [in a new window]   Fig. 6.   Mean RSNA baroreflex curves for the pregnant rabbits during the three consecutive treatments: saline icv, losartan icv, and losartan icv plus iv. RSNA is expressed relative to rest RSNA (100%) during the icv saline treatment. For each curve, the rest value for BP and RSNA is identified by the open circle.

	DISCUSSION

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Reduced arterial baroreflex control of RSNA at term pregnancy has been demonstrated in conscious rabbits and rats (7, 30, 36). The observations that normal pregnancy was associated with an activation of peripheral renin-angiotensin system (10, 19) and that ANG II was linked to abnormal baroreflex control in heart failure (12, 29, 34) led to the hypothesis that ANG II contributed to the reduction in the arterial baroreflex control of RSNA in conscious pregnant rabbits. We found that central AT₁ receptor block with intracerebroventricular losartan caused a depressor response in pregnant but not nonpregnant rabbits. However, neither central nor subsequent systemic treatment with losartan reversed the gestational depression in the maximal gain of the BP-RSNA relationship. Taken together, these results suggest than in normal pregnancy, ANG II has an enhanced role in the tonic support of BP but does not mediate the reduced arterial baroreflex control of RSNA.

The pregnant rabbits demonstrated a slight but significant resting hypotension at term. The BP₅₀ of the RSNA baroreflex curve was similar between the pregnant and nonpregnant rabbits, which differed from our previous results (36). A robust leftward shift in the BP₅₀ of cardiac or RSNA baroreflexes is not always observed in pregnant animals (7, 30). The sensitivity (gain) of the RSNA baroreflex is dependent on the RSNA range and the slope coefficient, which describes the rate of change in the sigmoid curve (25). In conscious rats, the impairment in the RSNA baroreflex is manifested as a reduced sympathoexcitatory response to hypotension (30). Previously, we (36) reported in rabbits that the reduction in maximal gain during pregnancy was primarily due to a decrease in the slope coefficient. There was a more robust difference in RSNA range versus slope coefficient between the nonpregnant and pregnant rabbits in the current study, although the maximal reflex gains in both groups were identical across the two studies (36). Thus the gestational depression in the maximal gain of the RSNA baroreflex is a reproducible observation in conscious rabbits that may be manifested to varying degrees as a reduced maximal sympathoexcitation and rate of change in the sigmoid BP-RSNA relationship. Additionally, Brooks and colleagues (7) concluded that the decreased sensitivity of the cardiac baroreflex in rabbit pregnancy is primarily related to reduced sympathetic responsiveness to changes in arterial pressure.

As observed in the nonpregnant rabbits, intraventricular losartan in conscious, normal animals typically has nonhypotensive effects (5, 12, 14, 18). The decrease in BP associated with intracerebroventricular losartan in the pregnant group suggests an enhanced role for central AT₁ receptors in support of resting BP in pregnancy. Endogenous ANG II and related angiotensin peptides provide a tonic excitation of sympathoexcitatory rostral VLM neurons as well as sympathoinhibitory caudal VLM neurons (reviewed in Ref. 2). Thus one potential explanation for the hypotensive effect of central losartan in the pregnant group is an augmented tonic sympathoexcitatory role for AT₁ receptors in the rostral VLM. If AT₁ receptors facilitate resting sympathetic drive in normal pregnancy, it may not be manifested as an increase in absolute sympathetic outflow. Resting plasma catecholamines are not elevated in normal human pregnancy (3). Schobel et al. (39) found that muscle sympathetic nerve activity was similar between normotensive pregnant and nonpregnant females, although Greenwood et al. (17) found that muscle sympathetic nerve activity decreased postpartum in normotensive patients followed longitudinally. With the limitations of multifiber nerve recordings in mind, resting RSNA appears normal in conscious rabbits (36) but elevated in conscious rats (30).

The addition of intravenous losartan decreased BP in both pregnant and nonpregnant rabbits. Compared with the unblocked condition, combined intracerebroventricular and intravenous losartan had a greater hypotensive effect in the pregnant group due primarily to the hypotension associated with intracerebroventricular losartan. An augmented hypotensive effect of peripheral renin-angiotensin system antagonism in pregnancy has been observed in conscious rabbits (41) and in anesthetized but not in conscious rats (11, 37). Losartan crosses the blood-brain barrier (40), so it cannot be ruled out that the additional hypotensive effect of systemic losartan was mediated by block of central AT₁ receptors inaccessible to intracerebroventricular losartan, in addition to AT₁ receptor block at the vasculature and area postrema (15, 31). Overall, our data with intracerebroventricular and intravenous losartan suggest than endogenous ANG II has an enhanced role in the tonic support of BP in normal pregnancy, and that this effect is primarily mediated by central AT₁ receptors accessible to intracerebroventricular losartan administration.

Losartan infused intracerebroventricularly had no effect on the gain or range of the RSNA baroreflex in normal or pregnant rabbits. There have been conflicting reports on the effect of acute administration of angiotensin receptor antagonists on sympathetic baroreflexes in normal animals. The BP-RSNA relationship was unaffected by systemic (34) or central (14) angiotensin receptor antagonism in conscious rabbits or by losartan intracerebroventricular in conscious rats (12). In contrast, Bendle et al. (5) found that in the conscious rabbit, losartan administered into the fourth ventricle increased the maximal sympathetic response to hypotension and chemoreceptor stimulation.

One possibility in the current study is that the losartan infused into the lateral ventricle did not reach a high enough concentration in the fourth ventricle to block AT₁ receptors in the brain stem involved in modulation of reflex sympathetic responses. This is unlikely to entirely account for the lack of response. First, the dose of losartan (80 µg iv) was adequate to block the pressor response to 125 ng ANG II, which in the rabbit is believed to originate primarily from activation of AT₁ receptors in the brain stem (21). As seen in Fig. 2, the onset of the pressor response to ANG II infused into the lateral ventricle was delayed by ~2.5 min, which supports a brain stem, rather than forebrain site of action of ANG II. Second, intracerebroventricular administration of losartan in heart-failure rats but not in normal rats enhanced the RSNA baroreflex (12). Third, the hypotensive response to intracerebroventricular losartan observed in the pregnant rabbits shows that the concentration of losartan in the fourth ventricle was at least adequate to block AT₁ receptors involved in tonic regulation of sympathetic outflow. Thus the results from the intracerebroventricular losartan protocol provide initial evidence that central AT₁ receptors do not mediate inhibition of the BP-RSNA relationship in conscious, pregnant rabbits.

In this study, we followed intracerebroventricular losartan with a systemic infusion and repeated the baroreflex assessment. There is experimental evidence in rats that losartan infused intracerebroventricularly does not block the pressor effects of ANG II microinjected into the area postrema (16), which leaves open the possibility that circulating ANG II continued to exert an inhibitory effect on the RSNA baroreflex during the central AT₁ receptor block. Alternatively, the additional effect of intravenous losartan on the RSNA baroreflex could be due to block of brain stem AT₁ receptors not accessible to intracerebroventricular losartan. It is not possible to distinguish between these two possibilities in the present study.

We found that the combination of intravenous and intracerebroventricular losartan did not normalize the maximal gain of the BP-RSNA relationship in the conscious, pregnant rabbits, which provides strong evidence that neither circulating nor brain ANG II are importantly involved in the gestational depression of the sensitivity of the RSNA baroreflex. Similarly, Welch et al. (41) found that intravenous losartan did not ameliorate the depressed cardiac baroreflex gain in pregnant rabbits. These two studies in conscious animals support the earlier observations of Crandall and Heesch (11) on the neutral effect of captopril on the RSNA baroreflex in the anesthetized rat. The apparent lack of a significant role for ANG II in modulation of arterial baroreflexes in normal pregnancy is surprising for several reasons. Plasma ANG II levels are elevated in rabbit pregnancy (10), and it is well established that ANG II can depress cardiac and/or sympathetic baroreflexes though an action at the area postrema (29, 31, 33-35). In models of heart failure in both rabbits (29, 34) and rats (12), the peripheral renin-angiotensin system is activated and either systemic or intraventricular AT₁ receptor block improves the depressed baroreflex control of RSNA and HR. Finally, Hines and Porter (23) found that the pressor response to intracerebroventricular ANG II was elevated in the anesthetized pregnant rat, suggesting that the central sympathetic response to ANG II was enhanced.

It is possible that other factors present at term pregnancy modulate the role of ANG II on sympathetic reflexes. The elevated estrogen levels in pregnancy may play a role in modulating ANG II responses. Estradiol binds to catecholaminergic neurons of the nucleus tractus solitarius, caudal VLM (A1 cell group), and the A5 cell group (22), which participate in the sympathetic baroreflex pathway (2) and may be required for ANG II suppression of sympathetic baroreflexes (15). Estradiol decreases AT₁ receptor binding and mRNA expression in the subfornical organ and pituitary (26) of the rat, areas that are importantly involved in ANG II stimulation of thirst. Functionally, exogenous estradiol infused intracerebroventricularly attenuated ANG II intracerebroventricular-induced pressor effects in ovariectomized rats (24). However, we did not find a difference in the steady-state pressor, HR, or sympathoexcitatory responses to intracerebroventricular ANG II between the pregnant and nonpregnant rabbits. Further work is necessary to determine whether the chronic elevation in estrogen in the pregnant state affects ANG II modulation of sympathetic baroreflexes.

The mechanisms underlying the gestational depression in sympathetic and cardiac baroreflexes have remained elusive. Plasma levels of estrogens and progesterone are elevated during pregnancy. Current evidence suggests that it is unlikely that 17-estradiol acts independently to depress baroreflex function. Chronic administration of estradiol to conscious ovariectomized rats (20) or conscious rabbits (9) has no effect on sympathetic baroreflexes. Rather than potentiate, estradiol may antagonize the central modulatory effects of vasopressin (20) and ANG II (see above) on cardiovascular control. In contrast, Masilamani and Heesch (30) have shown that a metabolite of progesterone, 3--OH-dihydroprogesterone, infused exogenously into female rats can produce acute alterations in baroreflex control of RSNA that resemble the depression in RSNA control during gestation (30). Interestingly, the acute effect of 3--OH-dihydroprogesterone is blunted in males or ovariectomized females, which suggests that estrogens or other ovarian hormones may prime the central nervous system to respond to the nongenomic actions of progesterone (28).

In summary, term pregnancy in rabbits is characterized by a reduction in the maximal gain of the BP-RSNA relationship. The pregnant rabbits had an enhanced hypotensive response to central AT₁ block, but the gestational depression in the RSNA baroreflex was not affected by central or systemic losartan treatment. Taken together, these results suggest than in normal rabbit pregnancy, ANG II has an enhanced role in the tonic support of BP but does not mediate the reduced arterial baroreflex control of RSNA.