Comprehensive Echocardiographic Assessment of Right Ventricle Function in a Rat Model of Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease caused by vasoconstriction and remodeling of the small arteries in the lungs. This remodeling leads to increased pulmonary vascular resistance, worsened right ventricular function, and premature death. Currently approved therapies for PAH largely target pulmonary vasodilator pathways; however, recent emerging therapeutic modalities are focused on other novel pathways involved in the pathogenesis of the disease, including right ventricle (RV) remodeling. Imaging techniques that allow longitudinal assessment of novel therapeutics are very useful for determining the efficacy of new drugs in preclinical studies. Noninvasive trans-thoracic echocardiography remains the standard approach to evaluating heart function and is widely used in rodent models. However, echocardiographic evaluation of the RV can be challenging due to its anatomical position and structure. In addition, standardized guidelines are lacking for echocardiography in preclinical rodent models, making it difficult to carry out a uniform assessment of RV function across studies in different laboratories. In preclinical studies, the monocrotaline (MCT) injury model in rats is widely used to evaluate drug efficacy for treating PAH. This protocol describes the echocardiographic evaluation of the RV in naïve and MCT-induced PAH rats.

Local intra-uterine Ang-(1-7) infusion attenuates PGE2 and 6-keto PGF1α in decidualized uterus of pseudopregnant rats

BACKGROUND

Cyclooxygenase (COX)-derived prostanoids (PGE_2, PGI₂) are important contributors to the process of decidualization. Previous studies showed the presence of Ang-(1-7) in the primary and secondary decidualized zones of the implantation site at early pregnancy. Decreased concentrations of Ang-(1-7) were found in the decidualized uterus compared to the non-decidualized uterus of pseudopregnant rats, suggesting that low levels of Ang-(1-7) are required for successful decidualization at early pregnancy.

METHODS

To understand the role of Ang-(1-7) in prostaglandin production in a decidualized uterus, induced by a bolus injection of sesame oil, Ang-(1-7) (24 μg/kg/h) or vehicle was then infused directly into the decidualized uterine horn using an osmotic minipump. The right horns were not injected or infused and served as non-decidualized uterine horns in both groups of animals.

RESULTS

Decidualization increased PGE₂ concentration in the uterus (0.53 ± 0.05 vs. 12.0 ± 3.2 pmol/mg protein, p < 0.001, non-decidualized vs. decidualized horns); Ang-(1-7) infusion attenuated the increase of PGE₂ (12.0 ± 3.2 vs. 5.1 ± 1.3 pmol/mg protein, p < 0.01 control vs. Ang-(1-7) treated decidualized horns). The stable metabolite of PGI₂ (6-keto PGF_1α) was increased with decidualization (0.79 ± 0.17 vs. 3.5 ± 0.82 pmol/mg protein, p < 0.001, non-decidualized vs. decidualized horns). Ang-(1-7) infusion attenuated the increase in 6-keto PGF_1α in the decidualized horn (3.5 ± 0.82 vs 1.8 ± 0.37 pmol/mg protein, p < 0.05 control vs. Ang-(1-7) treated decidualized horns). The circulating levels of 6-keto-PGF_1a and TXB₂ were decreased by Ang-(1-7) infusion, while no difference was observed in circulating PGE₂. Although the global assessment of cleaved caspase 3 immunostaining, a marker of apoptosis, was unchanged within the Ang-(1-7) decidualized horn, there were localized decreases in cleaved caspase 3 staining in the luminal region in the decidualized uterus of Ang-(1-7)-treated rats.

CONCLUSIONS

These studies show that increased local uterine Ang-(1-7) alters the uterine prostaglandin environment, possibly leading to disruptions of early events of decidualization.

Decidualized pseudopregnant rat uterus shows marked reduction in Ang II and Ang-(1-7) levels

Previous studies showed that angiotensin (Ang) II and Ang-(1-7) concentrations were reduced in the implantation site at day 7 of pregnancy in Sprague-Dawley rats as compared to the site immediately adjacent to it, which does not have the embryo attached, clearly showing the importance of the blastocyst in the regulation of renin-angiotensin system (RAS).

OBJECTIVE

The objective of this study was to evaluate the regulation of the RAS in the decidualized uterus in the pseudopregnant rat, a model without the presence of a conceptus.

METHODS

Ovariectomized, adult female rats were sensitized for the decidual cell reaction with steroid treatments; decidualization was induced by oil-injection of the right horn; the left horn served as a control. The uterine content of Ang I, Ang II, and Ang-(1-7) was examined in the decidualized and non-decidualized uteri.

RESULTS

Both Ang-(1-7) and Ang II and ACE and ACE2 mRNA were significantly reduced in the decidualized horn as compared to the non-decidualized horn. Immunocytochemical characterization of Ang II, Ang-(1-7), ACE and ACE2 demonstrated that Ang-(1-7), Ang II, and ACE2 polarize to the anti-mesometrial pole with decidualization.

CONCLUSION

The decidualization process elicits marked reduction in uterine Ang II and Ang-(1-7) content as compared to the non-decidualized horn. The differential immunocytochemical expression of Ang II and Ang-(1-7) with ACE2, but not ACE in the anti-mesometrial pole of the decidualized horn may favor the formation and action of Ang-(1-7) in the anti-mesometrial pole, an area which plays a role in triggering the decidualization process.

The uterine placental bed Renin-Angiotensin system in normal and preeclamptic pregnancy

Previously, we demonstrated activation of the renin-angiotensin system in the fetal placental chorionic villi, but it is unknown whether the immediately adjacent area of the maternal uterine placental bed is regulated similarly. This study measured angiotensin peptides, renin-angiotensin system component mRNAs, and receptor binding in the fundus from nonpregnant subjects (n = 19) and in the uterine placental bed from normal (n = 20) and preeclamptic (n = 14) subjects. In the uterine placental bed from normal pregnant women, angiotensin II peptide levels and angiotensinogen, angiotensin-converting enzyme, angiotensin receptor type 1 (AT(1)), AT(2), and Mas mRNA expression were lower as compared with the nonpregnant subjects. In preeclamptic uterine placental bed, angiotensin II peptide levels and renin and angiotensin-converting enzyme mRNA expression were significantly higher than normal pregnant subjects. The AT(2) receptor was the predominant receptor subtype in the nonpregnant fundus, whereas all angiotensin receptor binding was undetectable in normal and preeclamptic pregnant uterine placental bed compared with nonpregnant fundus. These findings suggest that the maternal uterine placental bed may play an endocrine role by producing angiotensin II, which acts in the adjacent placenta to vasoconstrict fetal chorionic villi vessels where we have shown previously that AT(1) receptors predominate. This would lead to decreased maternal-fetal oxygen exchange and fetal nutrition, a known characteristic of preeclampsia.

Vascular perfused segments of human intestine as a tool for drug absorption

Blood-based vascular perfusion of isolated segments of human jejunum was developed as a tool for drug absorption studies before clinical trials. Acceptance criteria for viable human gut preparations included stable blood flow, arterial pressure, glucose utilization, active peristalsis, oxygen uptake, less than 3% absorption of a 70,000 mol. wt. dextran, and a ratio of first-order absorption rate constants (k(a)) of antipyrine to terbutaline of > or =1.4. Mannitol absorption was less than that of antipyrine but larger than that of terbutaline and could not be used as a negative control in absorption studies with human intestine. In separate perfusions (n = 3) a cassette of nine drugs was administered into the gut lumen, and the net absorption of each drug into the circulation was measured over 75 min. Using the mean values of k(a), the test compounds could be ranked into four groups: group 1: sulfasalazine and furosemide, k(a) = 3.9 to 4.0 x 10(-3) min(-1); group 2: cimetidine, timolol, nadolol, and ranitidine, k(a) = 6.4 to 8.3 x 10(-3) min(-1); group 3: atenolol and metoprolol, k(a) = 9.6 x 10(-3) min(-1); and group 4: theophylline, k(a) = 17.5 x 10(-3) min(-1). The rationale for evaluating yet another oral absorption system was as follows: first, a human gut segment with an intact vascular system is the closest system available to a clinical trial without performing one; and second, the data generated would be a direct measure of net drug transport from the gut lumen into the vascular circulation under near physiological conditions, which is not possible in models lacking a blood supply.

Activation of local chorionic villi angiotensin II levels but not angiotensin (1-7) in preeclampsia

The chorionic villi in the placenta are responsible for the regulation of fetal oxygen and nutrient transport. Although the peripheral renin-angiotensin system is activated during normal pregnancy, the regulation of the local chorionic villi renin-angiotensin system remains unknown. Therefore, placental chorionic villous tissue was collected from nulliparous third-trimester normotensive or preeclamptic subjects and was analyzed for angiotensin peptide content, angiotensinogen, renin, angiotensin-converting enzyme (ACE), ACE2, neprilysin, angiotensin II type 1 (AT(1)), angiotensin II type 2, Mas receptor mRNAs, and angiotensin receptor density and subtype. Angiotensin II in chorionic villi was significantly higher in preeclamptic subjects, whereas angiotensin (1-7) was not different. Angiotensinogen and AT(1) receptor gene expression was significantly higher in preeclamptic subjects. No differences were observed in renin, ACE, ACE2, or neprilysin gene expression. Mas receptor mRNA in preeclamptic subjects was decreased. The AT(1) receptor was the predominant receptor subtype in normal and preeclamptic chorionic villi. There was no difference in the density of the AT(1,) angiotensin II type 2, and angiotensin (1-7) receptors. These results indicate that enhanced chorionic villous expression of angiotensin II may result from increased angiotensinogen. Elevated angiotensin II, acting through the AT(1) receptor, may favor vasoconstriction in placental chorionic villi and contribute to impaired fetal blood flow and decreased fetal nutrition observed during preeclampsia.

ACE2 and ANG-(1-7) in the rat uterus during early and late gestation

The present study was designed to determine ANG peptide content [ANG I, ANG II, ANG-(1-7)], ACE2 mRNA, and the immunocytochemical distribution of ANG-(1-7) and ACE2 in the uteroembryonic unit during early and late gestation in Sprague-Dawley rats and in a rat model of pregnancy-induced hypertension, the reduced uterine perfusion pressure (RUPP) model. At early pregnancy ANG-(1-7) and ACE2 staining were localized in the primary and secondary decidual zone and luminal and glandular epithelial cells. During late gestation, ANG-(1-7) and ACE2 staining was visualized in the labyrinth placenta and amniotic and yolk sac epithelium. Uterine ANG II concentration at early pregnancy was significantly decreased by 21-55% in the implantation and interimplantation sites compared with virgin rats, whereas ANG-(1-7) levels were maintained at prepregnancy levels. At late gestation, uterine concentrations of ANG I and ANG II were significantly increased (30% and 25%, respectively). In RUPP animals, ANG-(1-7) concentration is significantly reduced in the uterus (181 +/- 16 vs. 372 +/- 74 fmol/g of tissue) and placenta (143 +/- 26 vs. 197 +/- 20 fmol/g of tissue). ACE2 mRNA increased in the uterus of early pregnant compared with virgin rats, yet within the implantation site it was downregulated. At late pregnancy, ACE2 mRNA is elevated by 58% in the uterus and decreased by 59% in RUPP animals. The regulation of ANG-(1-7) and ACE2 in early and late pregnancy supports the hypothesis that ANG-(1-7) and ACE2 may act as a local autocrine/paracrine regulator throughout pregnancy, participating in the early (angiogenesis, apoptosis, and growth) and late (uteroplacental blood flow) events of pregnancy.

Angiotensin-(1-7) inhibits in vitro endothelial cell tube formation in human umbilical vein endothelial cells through the AT(1-7) receptor

Angiotensin-(1-7) is increased in the circulation during human pregnancy, but its functional role is unknown. Recent studies suggested that it opposes angiotensin II mediated vascular growth. Because angiogenesis is critical to normal embryonic development during human pregnancy, this study assessed the in vitro effects of angiotensin-(1-7) on human umbilical vein endothelial cell tube formation. The blocking effects of the angiotensin-(1-7) receptor antagonist, D-[Alanine7]-Ang-(1-7), and angiotensin II receptor AT1 and AT2 antagonists, losartan and PD123319, on tube formation were measured by counting tube branch points. Human umbilical vein endothelial cells were cultured in EGM-2 medium and treated with angiotensin-(1-7) (0.17 nM-17 microM) for 18 h. Angiotensin-(1-7) inhibited tube formation by 24% (P < 0.01) at all doses tested. Treatment with 1.7 microM angiotensin-(1-7) plus 17 microM D-[Alanine7]-Ang-(1-7) resulted in the reversal of angiotensin-(1-7) mediated inhibition of tube formation (P < 0.05). Losartan (17 microM) also reversed the angiotensin-(1-7) mediated inhibition of tube formation (P < 0.05). Tube formation was unaffected by PD123319. These results suggest that angiotensin-(1-7) has an anti-angiogenic effect on human umbilical vein endothelial cells through a unique AT(1-7) receptor that is sensitive to losartan, indicating that angiotensin-(1-7) may play an important role in the regulation of vascular growth in the placenta during pregnancy.

Temporal-spatial expression of ANG-(1-7) and angiotensin-converting enzyme 2 in the kidney of normal and hypertensive pregnant rats

We recently demonstrated that renin-angiotensin system (RAS) overactivity during late gestation in rats is associated with increased kidney and urine levels of ANG-(1-7) and enhanced kidney immunostaining of ANG-(1-7) and angiotensin-converting enzyme 2 (ACE2). To understand the temporal-spatial changes in normal and hypertensive pregnancies, the renal distribution of ANG-(1-7) and ACE2 in association with kidney angiotensin peptides and ACE2 activity was examined in virgin, normal pregnant (NP; gestational days 5, 15, and 19) and reduced uterine perfusion pressure (RUPP at day 19) pregnant Sprague-Dawley rats. ANG-(1-7) and ACE2 immunocytochemical staining increased 1.8- and 1.9-fold and 1.7- and 1.8-fold, respectively, at days 15 and 19 of NP, compared with virgin rats. ANG-(1-7) and ANG II concentrations were increased in the kidney at 19 days of gestation. ACE2 activity measured using a fluorescent substrate was increased 1.9- and 1.9-fold in the cortex and 1.9- and 1.8-fold in the medulla at days 15 and 19 of NP. In the RUPP animals, ANG-(1-7) immunostaining and concentration were significantly decreased compared with 19-day NP rats. ACE2 activity was unchanged in the cortex and medulla of RUPP rats. In conclusion, during NP, the concurrent changes of ACE2 and ANG-(1-7) suggest that ACE2 plays an important role in regulating the renal levels of ANG-(1-7) at mid to late gestation. However, the decrease in renal ANG-(1-7) content in the absence of a concomitant decrease in ACE2 implicates the participation of other ANG-(1-7) forming or degrading enzymes during hypertensive pregnancy.

Effect of estrogen on neprilysin expression in uterus and kidney of Sprague-Dawley normotensive and heterozygous (mRen2)27-transgenic hypertensive rats

The present study was designed to determine whether estrogen modulates the angiotensin processing enzymes in membrane homogenates obtained from uterus and kidney cortex and medulla of Sprague-Dawley (SD) and heterozygous (mRen2)27-transgenic hypertensive (Tg(+)) female rats treated with or without 17beta-estradiol (E2). We evaluated estrogen's influence on neprilysin (NEP), an endopeptidase that forms angiotensin-(1-7) [Ang-(1-7)] and on aminopeptidase (AMP), which degrades Ang-(1-7). Renal tissue from normotensive and hypertensive male rats was also evaluated. E2 up-regulated NEP mRNA in the uterus of both SD and Tg(+) and this was associated with increased NEP activity in the uterus of SD (0.31+/-0.03 nmol/min/mg versus 0.18+/-0.04 nmol/min/mg of protein, p<0.05) and Tg(+) (0.26+/-0.04 nmol/min/mg versus 0.13+/-0.02 nmol/min/mg of protein, p<0.05) female). E2 had no significant effect on NEP activity in cortex and medulla of hypertensive and normotensive female. In female animals, cortical NEP activity is two-fold higher than medullary; in males there is a four-fold higher cortical NEP activity as compared to medulla. In male animals, medullary NEP was significantly lower than females with or without E2 treatment; no gender specific effect was found in cortex. E2 treatment also caused a two-fold increase in AMP activity in the uterus and 1.6-fold decrease in kidney cortex of SD and Tg(+) female (p<0.05). Our studies indicate that NEP may be a primary candidate for increased Ang-(1-7) processing in the uterus with estrogen treatment; kidney NEP, on the other hand, showed no modulation by estrogen, suggesting that down regulation of other processing enzymes, like AMP and ACE, may come into play in the kidney with estrogen replacement. In addition, these studies showed that there is tissue-specific regulation of NEP with estrogen treatment that is strain independent.

Distribution of angiotensin-(1-7) and ACE2 in human placentas of normal and pathological pregnancies

This work was designed to study the expression of the vasodilator peptide angiotensin-(1-7) [Ang-(1-7)] and its generating enzyme (ACE2) in the uteroplacental interface. Placentas were obtained from 11 early pregnancy failures (5 miscarriages and 6 ectopic pregnancies), 15 normotensive, and 10 preeclamptic gestations. In placental villi, the main sites of immunocytochemical expression of Ang-(1-7) and ACE2 were the syncytiotrophoblast, cytotrophoblast, endothelium and vascular smooth muscle of primary and secondary villi. Syncitial Ang-(1-7) expression in samples obtained from miscarriages and ectopic pregnancies was increased compared to normal term pregnancy [2.0 (2.0-2.25 for the 25 and 75% interquartile range) vs 1.3 (1.0-1.9), p<0.01]. In the maternal stroma, Ang-(1-7) and ACE2 were expressed in the invading and intravascular trophoblast and in decidual cells in all 3 groups. Ang-(1-7) and ACE2 staining was also found in arterial and venous endothelium and smooth muscle of the umbilical cord. The expression of Ang-(1-7) and ACE2 was similar in samples obtained from normal term or preeclamptic pregnancies, except for increased expression of ACE2 in umbilical arterial endothelium in preeclampsia [0.5 (0.5-0.8) vs 0.0 (0.0-0.0), p<0.01]. The uteroplacental location of Ang-(1-7) and ACE2 in pregnancy suggests an autocrine function of Ang-(1-7) in the vasoactive regulation that characterizes placentation and established pregnancy.

Enhanced expression of Ang-(1-7) during pregnancy

Pregnancy is a physiological condition characterized by a progressive increase of the different components of the renin-angiotensin system (RAS). The physiological consequences of the stimulated RAS in normal pregnancy are incompletely understood, and even less understood is the question of how this system may be altered and contribute to the hypertensive disorders of pregnancy. Findings from our group have provided novel insights into how the RAS may contribute to the physiological condition of pregnancy by showing that pregnancy increases the expression of both the vasodilator heptapeptide of the RAS, angiotensin-(1-7) [Ang-(1-7)], and of a newly cloned angiotensin converting enzyme (ACE) homolog, ACE2, that shows high catalytic efficiency for Ang II metabolism to Ang-(1-7). The discovery of ACE2 adds a new dimension to the complexity of the RAS by providing a new arm that may counter-regulate the activity of the vasoconstrictor component, while amplifying the vasodilator component. The studies reviewed in this article demonstrate that Ang-(1-7) increases in plasma and urine of normal pregnant women. In preeclamptic subjects we showed that plasma Ang-(1-7) was suppressed as compared to the levels found in normal pregnancy. In addition, kidney and urinary levels of Ang-(1-7) were increased in pregnant rats coinciding with the enhanced detection and expression of ACE2. These findings support the concept that in normal pregnancy enhanced ACE2 may counteract the elevation in tissue and circulating Ang II by increasing the rate of conversion to Ang-(1-7). These findings provide a basis for the physiological role of Ang-(1-7) and ACE2 during pregnancy.

Vascular responses to Angiotensin-(1-7) during the estrous cycle

Mesenteric arteries (230-290 microm) were isolated from virgin female rats at diestrous and proestrous phases of the estrous cycle and from ovariectomized (OVX) rats with or without estrogen (E2) replacement (17beta-estradiol, 7.5 + 5 mg pellets, 21 d release). Arteries were mounted in a pressurized myograph system. Angiotensin-(1-7) [Ang-(1-7)] concentration-dependent responses (10(-10)-10(-5) M) were determined in arteries preconstricted with endothelin-1 (10(-7) M). Mesenteric arteries were pretreated with the specific Ang-(1-7) antagonist, D-[Ala7]-Ang-(1-7) (10(-7) M) to assess the Ang-(1-7) receptor-mediated dilator effect. Ang-(1-7) did not dilate mesenteric arteries from virgin rats at diestrus and placebo-treated OVX female rats as compared to the time control; however, Ang-(1-7) elicited a modest dilation at proestrus as compared to diestrus, which reached statistical significance at 10(-8) M concentrations. Ang-(1-7) caused a concentration-dependent vasodilation in mesenteric arteries of females with E2 replacement, with an EC50 of 21 nM. D-[Ala7]-Ang-(1-7) blocked the vasodilator effect of Ang-(1-7). Our results demonstrate that during proestrus Ang-(1-7) elicits modest vasodilation as compared to diestrus, but lacks vasodilatory properties in vessels from diestrous and ovariectomized rats. Estrogen replacement restores a significant dilator response to Ang-(1-7) in OVX rats that is mediated by a D-[Ala7]-Ang-(1-7) sensitive site.

Enhanced renal immunocytochemical expression of ANG-(1-7) and ACE2 during pregnancy

Previously we demonstrated that kidney concentration and urinary excretion of angiotensin-(1-7) are increased during normal pregnancy in rats. Since this finding may reflect local kidney production of angiotensin-(1-7), we determined the immunocytochemical distribution of angiotensin-(1-7) and its newly described processing enzyme, ACE2, in kidneys of virgin and 19-day-pregnant Sprague-Dawley rats. Sprague-Dawley rats were killed at the 19th day of pregnancy, and tissues were prepared for immunocytochemical by using a polyclonal antibody to angiotensin- (1-7) or a monoclonal antibody to ACE2. Angiotensin-(1-7) immunostaining was predominantly localized to the renal tubules traversing both the inner cortex and outer medulla. ACE2 immunostaining was localized throughout the cortex and outer medulla and was visualized in the renal tubules of both virgin and pregnant rats. The quantification of angiotensin-(1-7) and ACE2 immunocytochemical staining showed that in pregnant animals, the intensity of the staining increased by 56% and 117%, respectively (P<0.05). This first demonstration of the immunocytochemical distribution of angiotensin-(1-7) and ACE2 in kidneys of pregnant rats shows that pregnancy increases angiotensin-(1-7) immunocytochemical expression in association with increased ACE2 intensity of staining. The findings suggest that ACE2 may contribute to the local production and overexpression of angiotensin-(1-7) in the kidney during pregnancy.

Pregnancy enhances the angiotensin (Ang)-(1-7) vasodilator response in mesenteric arteries and increases the renal concentration and urinary excretion of Ang-(1-7)

The vasoactive effect of angiotensin (Ang)-(1-7) in mesenteric resistance arteries together with its plasma and kidney concentration and urinary excretion was assessed in pregnant and virgin rats. Mesenteric arteries (230-290 microm) were mounted in a pressurized myograph system and Ang-(1-7) concentration-dependent response curves (10(-10)-10(-5) M) were determined in arteries preconstricted with endothelin-1 (10(-7) M). The Ang-(1-7) response was investigated in vessels with and without pretreatment with the Ang-(1-7) antagonist [D-[Ala(7)]-Ang-(1-7)] (10(-7) M). Ang-(1-7) caused a significantly enhanced, concentration-dependent dilation of mesenteric vessels (EC(50) = 2.7 nM) from pregnant compared with virgin female rats. D-[Ala(7)]-Ang-(1-7) eliminated the vasodilator effect of Ang-(1-7). There was no significant change in plasma concentration of Ang-(1-7) in pregnant animals. On the other hand, 24 h urinary excretion and kidney concentration of Ang-(1-7) were significantly higher in pregnant animals. The increased mesenteric dilation to Ang-(1-7) with enhanced kidney concentration and 24 h urinary excretion rate of Ang-(1-7) suggests an important role for this peptide in cardiovascular regulation during pregnancy.

Characterization of angiotensin-(1-7) receptor subtype in mesenteric arteries

Mesenteric arteries from male Sprague-Dawley rats were mounted in a pressurized myograph system. Ang-(1-7) concentration-dependent responses were determined in arteries preconstricted with endothelin-1 (10(-7)M). The receptor(s) mediating the Ang-(1-7) evoked dilation were investigated by pretreating the mesenteric arteries with specific antagonists of Ang-(1-7), AT(1) or AT(2) receptors. The effects of Ang-(3-8) and Ang-(3-7) were also determined. Ang-(1-7) caused a concentration-dependent dilation (EC(50): 0.95 nM) that was blocked by the selective Ang-(1-7) receptor antagonist D-[Ala(7)]-Ang-(1-7). Administration of a specific antagonist to the AT(2) receptor (PD123319) had no effect. On the other hand, losartan and CV-11974 attenuated the Ang-(1-7) effect. These results demonstrate that Ang-(1-7) elicits potent dilation of mesenteric resistance vessels mediated by a D-[Ala(7)]-Ang-(1-7) sensitive site that is also sensitive to losartan and CV-11974.