The effect of pregnancy on the angiotensin II pressor response in the rabbit

Effects of L-arginine oral supplements in pregnant spontaneously hypertensive rats

PURPOSE: To evaluate the effects of L-arginine oral supplementation in spontaneously hypertensive pregnant rats (SHR). METHODS: Thirty SHR and ten Wistar-EPM-1 virgin female rats were used in the study. Before randomization, females were caged with males of the same strain (3:1). Pregnancy was confirmed by sperm-positive vaginal smear (Day 0). Wistar-EPM-1 rats served as counterpart control (C-1). SHR rats were randomized in 4 groups (n=10): Group Control 2, non-treated rats; Group L-Arginine treated with L-arginine 2%; Group Alpha-methyldopa treated with Alpha-methyldopa 33mg/Kg; Group L-Arginine+Alpha-methyldopa treated with L-arginine 2%+Alpha-methyldopa 33mg/Kg. L-arginine 2% solution was offered ad libitum in drinking water and Alpha-methyldopa was administered by gavage twice a day during the length of pregnancy (20 days). Blood pressure was measured by tailcuff plethysmography on days 0 and 20. Body weight was measured on days 0, 10 and 20. Results were expressed as mean ± SD (Standard Deviation). One-Way ANOVA/Tukey (or Kruskal-Wallis/Dunn, as appropriate) was used for group comparisons. Statistical significance was accepted as p<0.05. RESULTS: There was no significant weight gain in isolated L-arginine treated SHR. Mean blood pressure decreased in L-arginine-treated SLR compared with untreated-SHR rats. CONCLUSION: L-arginine oral supplementation reduces blood pressure in spontaneously hypertensive rats during pregnancy.

Mechanisms regulating angiotensin II responsiveness by the uteroplacental circulation

Pregnancy is associated with increases in cardiac output and uterine blood flow (UBF) and a fall in systemic vascular resistance. In ovine pregnancy, UBF rises from approximately 3% of cardiac output to approximately 25% at term gestation, reflecting a >30-fold rise in UBF by term. This increase in UBF supports exponential fetal growth during the last trimester and maintains fetal well-being by providing excess oxygen and nutrient delivery. These hemodynamic changes are associated with numerous hormonal changes, including increases in placental steroid hormones and enhanced activation of the renin-angiotensin and sympathetic nervous systems, all of which are believed to modulate systemic and uterine vascular adaptation and vascular reactivity. Systemic pressor responses to infused ANG II are attenuated in normotensive pregnancies and the uteroplacental vasculature is even less sensitive, suggesting development of mechanisms to maintain basal UBF and permit the rise in UBF necessary for fetal growth and well-being. The effects of ANG II on the uteroplacental vasculature are reviewed, and the mechanisms that may account for attenuated vascular sensitivity are examined, including ANG II metabolism, vascular production of antagonists, ANG II-receptor subtype expression, and the role of indirect mechanisms.

Effects of hypertension on maternal adaptations to pregnancy: experimental study on spontaneously hypertensive rats

CONTEXT

Animal models for essential hypertension have been used for understanding the human pathological conditions observed in pregnant hypertensive women.

OBJECTIVE

To study the possible effects of pregnancy on hypertension and of hypertension on pregnancy in spontaneously hypertensive rats (SHR), and in their normotensive Wistar-Kyoto (WKY) counterparts.

TYPE OF STUDY

Comparative study using laboratory animals.

SETTING

Animal Research Laboratory of Clinical Medicine at the Medical School of Botucatu, São Paulo State University, Brazil.

SAMPLE

Ten to twelve-week-old virgin female normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). The animals were separated into four groups: 15 pregnant spontaneously hypertensive rats (SHR-P), 10 non-pregnant spontaneously hypertensive rats (SHR-NP), 15 pregnant normotensive rats (WKY-P), and 10 non-pregnant normotensive rats (WKY-NP).

MAIN MEASUREMENTS

The blood pressure was evaluated by the tail cuff method, in rats either with or without prior training for the handling necessary for tail cuff measurements. The maternal volume expansion was indirectly evaluated by weight gain, and by systemic parameters as hematocrit, hemoglobin, total protein, albumin and sodium retention. The perinatal outcome of pregnancy was evaluated by analysis of resorptions, litter size, rate of low weight and number of stillbirths.

RESULTS

The late fall in blood pressure in the pregnant SHR strain and in the normotensive WKY strain can only be detected in rats previously trained to accept the handling necessary for the tail cuff measurement. During pregnancy the body weight gain was significantly higher in WKY than in SHR rats. Systemic parameters were significantly lower in pregnant WKY rats than in non-pregnant WKY rats, while no differences were observed between pregnant and non-pregnant SHR groups. In pregnant WKY rats the sodium retention was higher from the 13th day onwards, while in SHR rats this occurred only on the 21st day. The characteristics of reproductive function such as number and weight of fetus, perinatal mortality and the resorption rate were significantly affected in the SHR strain.

CONCLUSION

The SHR strain may be considered as a model for chronic hypovolemic maternal hypertension, with the fetal growth retardation being determined by this hypovolemic state.

Decreased angiotensin II receptors mediate decreased vascular response in hepatocellular cancer

OBJECTIVE

The authors' objective was to determine the origin of the diminished pressor responsiveness of angiotensin II infusion in hepatoma by evaluating angiotensin II receptor status in normal liver, hepatoma tumor, and cultured hepatocytes and H4IIE cells.

SUMMARY BACKGROUND DATA

Hepatocellular cancer is a highly vascular tumor, where the neovasculature is unique in that it arises only from the hepatic arterial circulation, whereas normal liver has both hepatic arterial and portal venous blood supply. The tumor neovasculature is also characterized by an abnormal vascular reactivity to vasoconstrictors, including the response to angiotensin II. The altered response of tumor vasculature to angiotensin II offers a potential therapeutic opportunity for modulation of tumor blood flow. However, the origin of the decreased vascular response is unknown.

METHODS

The authors evaluated the hepatic vascular response to angiotensin II infusion by determining hepatic arterial blood flow to normal liver and to tumor by means of radioactive microspheres. The angiotensin II receptor status in the normal liver, hepatoma tumor, and cultured hepatocytes and H4IIE cells was determined br radioligand binding analysis and in cryostat sections derived from normal liver and hepatoma tumor by means of in situ binding analysis with biotinylated angiotensin II.

RESULTS

Angiotensin II infusion decreased the hepatic arterial flow to normal liver and increased hepatoma to liver flow ratio. The number of angiotensin II receptors in normal liver was significantly higher than that in hepatoma (239 +/- 20 fmol/mg protein in normal liver vs. 162 +/- 15 fmol/mg protein in hepatoma) without a change in the affinity (4.4 +/- 0.8 nM in normal liver vs. 4.7 +/- 1.2 nM in hepatoma). H4IIE cells and primary hepatocytes had low receptor density. In situ binding analysis revealed that angiotensin II receptors were mainly on the smooth muscle cells of the neovasculature.

CONCLUSIONS

The data suggests that the diminished vascular response to angiotensin II hepatoma may relate a loss of angiotensin II receptor on tumor neovasculature.

Effects of prostaglandin inhibition on the renal function curve during ovine pregnancy

OBJECTIVE

This study was designed to assess the influence of prostaglandins on the steady-state relationship of mean arterial pressure and urinary sodium excretion.

STUDY DESIGN

Ten nonpregnant and nine pregnant ewes were chronically prepared with indwelling catheters and maintained in metabolism cages. All the ewes received a continuous intravenous infusion of indomethacin (5 mg/kg/day) for 24 days. Step increases in sodium intake (20, 100, 400, and 1200 mmol per day), each maintained for 6 days, were administered as a fixed dietary (20 mmol) component with the balance infused intravenously. Mean arterial pressure was continuously monitored. The renal function curves were constructed from the steady-state mean arterial pressure-urinary sodium excretion relationships after sodium balance was attained.

RESULTS

Indomethacin inhibited urinary excretion of prostaglandin E2 by 62% +/- 4% in nonpregnant and 47% +/- 6% in pregnant ewes, and 6 keto-prostaglandin F1 alpha, the stable metabolite of prostaglandin I2, by 55% +/- 4% and 65% +/- 6% in nonpregnant and pregnant ewes, respectively. Blood volume was not affected by indomethacin or salt in either group. Plasma angiotensin II was suppressed less in pregnant ewes (60% vs 80%) by increases in salt intake. Chronic treatment with indomethacin did not alter the renal function curves of either nonpregnant or pregnant ewes.

CONCLUSIONS

These results suggest that prostaglandins do not influence the steady-state relationship between mean arterial pressure and urinary sodium excretion in either nonpregnant or pregnant sheep.

Altered angiotensin-II receptors in human hepatocellular and hepatic metastatic colon cancers

OBJECTIVE

To characterize angiotensin-II receptor density and affinity in normal and cirrhotic livers and in hepatocellular and metastatic colorectal cancer.

SUMMARY BACKGROUND DATA

Several studies have indicated a possible beneficial effect of angiotensin-II as a biologic response modifier in the treatment of hepatic or metastatic colon cancer. This is based on evidence that angiotensin-II will cause a selective increase in arterial vasoconstriction in normal liver compared with tumor.

METHODS

Human hepatoma (5), metastatic colon (10), or cirrhotic (3) liver was obtained. Non-tumor-bearing regions served as normal liver. Angiotensin-II receptor binding was determined on membranes with 125I-angiotensin-II and in situ studies were performed using the biotin-avidin detection system.

RESULTS

Angiotensin-II receptor density was markedly down-regulated in tumor compared with normal or cirrhotic liver.

CONCLUSIONS

A loss of angiotensin-II receptors occurs on the neovasculature of hepatic tumors.

Effects of endothelial cell removal on constrictor and dilator responses of aortae of pregnant rats

1. The possibility has been examined that changes in sensitivity of the aorta occurring during pregnancy in the rat are due to changes in output of endothelium-derived relaxing factor (EDRF). 2. Concentration-isometric response curves, obtained in vitro, from thoracic aortic rings of non-pregnant rats in oestrous and 20-day pregnant rats to noradrenaline (NA) and phenylephrine (PE) were shallower with lower maxima when compared to those obtained to the thromboxane A2-mimetic U46619. 3. Removal of endothelium from aortae of non-pregnant animals caused significant shifts to the left of the curves obtained using all three agonists. In the case of NA or PE, EC50 values were reduced approximately fourfold, with maximum responses also increasing significantly. Endothelial cell removal caused an approximately eightfold increase in sensitivity to U46619 but no change in maximum response. The presence of oxyhaemoglobin (Hb) had a similar effect to endothelial removal on responses to NA and PE. 4. Rings obtained from 20-day pregnant rats showed significantly increased maximum responses to NA and PE when compared with those from non-pregnant animals. Removal of the endothelium or the presence of Hb caused significant shifts of the dose-response curves to the left and further increases in maxima. 5. In contrast, no differences in maximum responses to U46619 could be detected between the aortic rings of pregnant and non-pregnant animals. Removal of the endothelium from either caused increases in sensitivity which did not differ significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of acute haemorrhagic hypotension during pregnancy and lactation in conscious goats

Effects of acute, short-lasting haemorrhage have been studied in six goats. The experiments were made 55 +/- 3 days (mid-pregnancy = MP), and 24 +/- 2 days (late pregnancy = LP) before parturition, in lactation weeks 3-8, and in the non-pregnant, non-lactating (= control) period. The room temperature was kept at 20 +/- 1 degrees C, except during one experimental series in lactating goats, which was performed at +30 degrees C. Approximately the same volume of blood had been removed in all periods, when the mean arterial blood pressure (MAP) fell. At this moment bleeding was stopped. The volume loss, calculated as per cent of total blood volume, was significantly smaller during pregnancy and lactation than in the control period. The fastest heart rate, both at rest and during haemorrhage, was observed in LP goats. The respiratory frequency of the LP goats increased markedly in response to haemorrhage, but the most rapid respiratory rate was observed in lactating goats bled at the room temperature +30 degrees C. The increase in plasma vasopressin (AVP) concentration coincided with the fall in MAP. The AVP rise seen during late pregnancy was significantly higher than in all the other periods. The plasma noradrenaline (NA) concentration increased by about five times in LP goats, but did not change significantly in the other periods. Plasma renin activity did not rise, with the exception of a small increase in lactating goats bled at +30 degrees C. The plasma aldosterone concentration rose, but there was no difference between periods.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of highly purified eicosapentaenoic acid on vascular reactivity to angiotensin II and norepinephrine in the rabbit

There is disagreement about whether supplementation of the diet with fish oil, which is rich in eicosapentaenoic acid (EPA), lowers blood pressure. We gave highly purified EPA in a soft capsule (90% ethyl ester form of EPA; EPA-E), to female rabbits (100 mg/kg/day) for 4 weeks. Vascular response to vasoconstrictor agents was assessed serially by measuring the systolic blood pressure using a Grand-Rothschild capsule in the ear. There was no change in systolic blood pressure of rabbits treated with EPA-E, but rabbits given EPA-E for one week or longer were significantly less responsive to the pressor effects of angiotensin II than the controls. Responses to norepinephrine did not change. Rabbits given EPA-E for four weeks had significantly more EPA in the serum, but there were no differences in serum levels of triglycerides, total cholesterol, or high-density lipoprotein cholesterol. These results suggest that vascular responses to exogenous angiotensin II can be selectively depressed by short-term treatment with EPA-E in rabbits without changing systolic blood pressure.

In vitro prostacyclin production by ovine uterine and systemic arteries. Effects of angiotensin II

Normal pregnancy is associated with reduced systemic pressor responses to infused angiotensin II (ANG II); furthermore, the uterine vascular bed is even less responsive to vasoconstriction by ANG II than the systemic vasculature overall. The mechanism(s) for this refractoriness remains unknown. To determine if vessel production of prostacyclin may be responsible, uterine and omental artery segments were obtained from four groups of sheep, nonpregnant (NP), pregnant (P; 131 +/- 4 d), early postpartum (2.2 +/- 0.4 d), and late postpartum (16 +/- 2 d), and incubated in Krebs-Henseleit alone or with ANG II in the absence or presence of Saralasin. Prostacyclin was measured as 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha). Synthesis of 6-keto-PGF1 alpha was de novo, since aspirin inhibited its formation. P and early uterine arteries produced more 6-keto-PGF1 alpha than NP and late vessels (P less than 0.05): 386 +/- 60 (X +/- SE) and 175 +/- 23 vs. 32 +/- 5 and 18 +/- 4 pg/mg X h, respectively. A similar relationship was observed for omental arteries: 101 +/- 14 and 74 +/- 14 vs. 36 +/- 10 and 22 +/- 4 pg/mg X h, respectively. Furthermore, synthesis by arteries from P and early animals was greater in uterine than omental vessels (P less than 0.05); this was not observed in NP or late vessels. ANG II increased 6-keto-PGF1 alpha production 107 +/- 20% and 92 +/- 16% in P and early uterine arteries only; the threshold dose was between 5 X 10(-11) and 5 X 10(-9) M ANG II. This ANG II-induced increase in 6-keto-PGF1 alpha by uterine arteries was inhibited by Saralasin, which by itself had no effect. During pregnancy, the reduced systemic pressor response to ANG II and the even greater refractoriness of the uterine vascular bed may be reflective of vessel production of the potent vasodilator, prostacyclin. Furthermore, in the uterine vasculature, this antagonism may be potentiated by specific ANG II receptor-mediated increases in prostacyclin.

Effect of pregnancy in the rabbit on the pressor response to angiotensin and noradrenaline

The pressor responses to angiotensin II injections of 29, 117 and 468 pmol/kg and infusion at 54 pmol/kg per min were compared in near-term pregnant and matched non-pregnant rabbits. The responses to noradrenaline injections of 652, 2608 and 10 432 pmol/kg and infusion at 1185 pmol/kg per min were similarly compared. At all doses of angiotensin and noradrenaline, whether by injection or infusion, the response of the pregnant animals was significantly greater. However the control arterial pressures of the pregnant rabbits were lower and fell in a range where the baroreceptor reflex might be less effective. Thus less efficient buffering could contribute to the greater response to the vasoconstrictor agents. To test the effect of pregnancy on the vascular response to angiotensin and noradrenaline when reflex and central effects were excluded, hexamethonium was used to produce autonomic ganglion blockade. The response to both vasoconstrictors was now less in the pregnant rabbits than in the non-pregnant. Thus in assessing the effects of pregnancy on the responses of the rabbit to angiotensin and noradrenaline the effect of pregnancy on mean blood pressure must be taken into account, since this can influence the extent to which the arterial baroreceptors may modulate the response.

Metabolic clearance of angiotensin II in pregnant and nonpregnant sheep

Reduced vascular responsiveness to infused angiotensin II (ANG II) has been observed during pregnancy. It has been proposed that infusions produce lower circulating concentrations of ANG II in pregnancy, due to an increase in the metabolic clearance rate of ANG II (MCRangii). We have evaluated the MCRangii and the arterial plasma concentrations of ANG II during constant infusions of 1.15 micrograms ANG II/min into chronically instrumented pregnant (n = 6) and nonpregnant (n = 9) sheep. Although the pressor responses were significantly less in the pregnant than in the nonpregnant sheep (17.5 +/- 0.5 vs. 34.9 +/- 3.2 mmHg, P less than 0.001), the values for MCRangii were not different: 56.2 +/- 6.3 ml X min-1 X kg-1 in nonpregnant and 55.9 +/- 4.3 ml X min-1 X kg-1 in pregnant sheep. The steady-state plasma ANG II concentrations during the infusions were slightly less in pregnant than in nonpregnant sheep (388 +/- 36 vs. 454 +/- 36 pg/ml); however, this difference would be responsible for only a 2-mmHg reduction in the pressor response. We conclude that the reduced pressor response to infused ANG II in pregnancy is not due to an increase in MCRangii nor to lower plasma ANG II concentrations.

The pressor response to angiotensin II: the roles of peripheral and cardiac responses in pregnant and nonpregnant sheep

A reduced pressor response to infused angiotensin II has been observed in pregnancy. Although this has been considered to reflect reduced sensitivity of the peripheral vasculature to angiotensin II, it has also been suggested that the reduced pressor response is due to a fall in cardiac output during infusion of angiotensin II in pregnancy. In the present study we investigated the hemodynamic responses to infused angiotensin II in chronically instrumented, pregnant and nonpregnant sheep. We measured changes in cardiac output and systemic vascular resistance and related these changes to the increase in mean arterial pressure. The response in systemic vascular resistance to angiotensin II was substantially reduced in pregnant sheep. During angiotensin II infusion cardiac output fell in both groups, but the effect of the fall in cardiac output in reducing the pressor response was greater in nonpregnant animals. Thus, it appears that the reduced pressor response to angiotensin II in pregnant sheep is due to reduced systemic vascular responsiveness and not to differences in cardiac output responses.

Dietary sodium manipulation and vascular responsiveness during pregnancy in the rabbit

The interrelationship of sodium intake and blood pressure regulation during pregnancy is not clear. The effects of dietary sodium loading and restriction on plasma levels of catecholamines, mean arterial pressure, and vascular response to two pressor agents, Levophed and angiotensin II, were investigated in 49 chronically prepared primigravid rabbits. Sodium loading increased mean arterial pressure (p less than 0.005), but did not alter the response to either pressor agent. Sodium restriction did not alter mean arterial pressure, but did increase plasma norepinephrine (p less than 0.05) and epinephrine (p less than 0.02). Negative correlations between plasma levels of norepinephrine and vascular response to infusions of both pressor agents were observed during sodium restriction, -0.61 (p less than 0.05) for angiotensin II, and -0.74 (p less than 0.05) for norepinephrine. A similar correlation of -0.81 (p less than 0.05) was observed for angiotensin II in control animals. Norepinephrine appears to play a significant role in blood pressure maintenance and vascular response in pregnancy. This role is enhanced during sodium restriction.

Differential sensitivity to angiotensin II in pregnant rabbits

Threshold concentrations for angiotensin II and barium chloride were determined in the perfused hind limb of the nonpregnant, pseudopregnant, and pregnant rabbit. The threshold concentration for these vasoconstrictors was defined as the amount of drug per milliliter of hind limb blood flow which would increase hind limb blood pressure by 4 mm Hg. There was no difference in the mean angiotensin II threshold concentration between the nonpregnant and pseudopregnant rabbits. The angiotensin II dose for the pregnant rabbits was double that of the first two groups. There was no significant difference in the mean barium chloride threshold concentration between the three groups. This difference in the pregnant angiotensin II threshold concentration with no difference in barium chloride threshold concentration indicates a change in vascular sensitivity which is dependent upon a specific angiotensin II-receptor interaction and is not inherent in the contractile response of the vessel, nor is it mimicked by pseudopregnancy.

The conscious instrumented rabbit: a model for the study of mechanisms of blood pressure regulation during pregnancy

Conscious pregnant and nonpregnant rabbits were used to further evaluate the role of prostaglandin (PG) and plasma renin activity (PRA) in the systemic hemodynamics of pregnancy. Pregnant rabbits had high peripheral blood levels of both PGE2 and PRA. Systemic blood pressure was not affected in either pregnant or nonpregnant by the administration of an inhibitor of prostaglandin synthesis. Pregnant rabbits, however, had a much larger decrease in blood pressure than nonpregnant animals when given the angiotensin I (AI)-converting-enzyme inhibitor, captopril. Pregnant rabbits were more resistant to the pressor effect of exogenous AII than nonpregnant animals. The pressor effect of AII increased in pregnant rabbits after the administration of meclofenamate and parturition but was not changed by volume expansion. In contrast, the sensitivity of nonpregnant rabbits to AII increased after volume expansion, but not after treatment with inhibitors of prostaglandin synthesis. These studies demonstrate that a remarkable similarity exists between pregnant rabbits and pregnant women in the pressor response to AII. This study is the first to correlate the vasopressor response to AII with PRA and the level of a circulating vasopressor prostaglandin in pregnant animals. The results strongly suggest that this model will be fruitful in further attempts to define the factors controlling systemic hemodynamics during pregnancy.

Uterine and peripheral blood concentrations of vasodilator prostaglandins in conscious pregnant rabbits

The concentrations of prostaglandin E2 (PGE2) and prostacyclin (PGI2) were estimated in blood obtained from the uterine venous effluent and a peripheral artery of conscious pregnant and nonpregnant rabbits. The blood was analyzed both with a superfusion bioassay and radioimmunoassays. PGE2, but not PGI2, was detected in a very high concentration in the uterine venous effluent of the pregnant rabbit. The peripheral blood of pregnant animals also revealed increased levels of only PGE2. The results of both assay techniques correlated closely. These studies suggest that the uteroplacental unit is the source of the increased peripheral level of the potent vasodilator PGE2. In contrast, these data fail to support the notion that PGI2 is the primary systemic-acting vasodepressor prostaglandin in rabbit pregnancy.

The rabbit: a suitable model for investigation of vascular responsiveness during pregnancy

The decreased vascular response to angiotensin II that characterizes normal pregnancy is lost in pregnancy induced hypertension and associated with an increase in response to norepinephrine. These alterations in vascular responsiveness have not been thoroughly investigated and are poorly understood. Suitable models manifesting physiologic conditions known to exist in human pregnancy have not been satisfactorily demonstrated. We investigated the chronically catheterized and unmedicated rabbit for similarities of four conditions known to occur during human pregnancy. We compared mean arterial pressure and vascular response to angiotensin II and to norepinephrine in 19 non-pregnant and 16 pregnant animals. Plasma levels of angiotensin II, norepinephrine, epinephrine, and dopamine levels were measured and compared. We found striking similarities for conditions investigated in our rabbit model when compared with data reported in the literature for human pregnancy. We conclude the rabbit is a suitable model for investigating alterations of vascular response during pregnancy.

The chronically instrumental ewe: a model for studying vascular reactivity to angiotensin II in pregnancy

Vascular refractoriness to the systemic pressor effects of angiotension II (AII) develops normally during human pregnancy. To ascertain if the ewe might provide a suitable animal model to study the mechanisms responsible for this response (unique to pregnancy) we studied this phenomenon in unanesthetized, chronically instrumented nonpregnant and pregnant sheep, 68-143 d gestation. In these studies dose-response curves were established for changes in both mean arterial pressure and uterine blood flow. The pressor response to continuous infusions of AII increases as a function of the dose of AII in both nonpregnant and pregnant animals (P less than 0.001), R = 0.943 and 0.879, respectively. However, the pregnant animals were refractory to the pressor effects of AII, requiring 0.016 microgram of AII/min per kg to elicit a 20 mm HG rise in mean arterial pressure, in contrast to 0.009 for nonpregnant animals. The slope and intercept for the regression lines are different at P less than 0.001. In pregnant animals the dose-response curve for uterine blood flow was also determined. Increases in uterine blood flow were observed at doses of AII less than 0.016 microgram/min per kg, while larger doses resulted in a progressively greater reduction in blood flow. It appears likely that the ewe may serve as an animal model suitable for the further study of the unique pregnancy-modified systemic and uteroplacental vascular responses elicited by AII.