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.

Therapeutic targets in liver transplantation: angiotensin II in nonsteatotic grafts and angiotensin-(1-7) in steatotic grafts

Numerous steatotic livers are discarded as unsuitable for transplantation because of their poor tolerance of ischemia-reperfusion(I/R). The injurious effects of angiotensin (Ang)-II and the benefits of Ang-(1-7) in various pathologies are well documented. We examined the generation of Ang II and Ang-(1-7) in steatotic and nonsteatotic liver grafts from Zucker rats following transplantation. We also studied in both liver grafts the effects of Ang-II receptors antagonists and Ang-(1-7) receptor antagonists on hepatic I/R damage associated with transplantation. Nonsteatotic grafts showed higher Ang II levels than steatotic grafts, whereas steatotic grafts showed higher Ang-(1-7) levels than nonsteatotic grafts. Ang II receptor antagonists protected only nonsteatotic grafts against damage, whereas Ang-(1-7) receptor antagonists were effective only in steatotic grafts. The protection conferred by Ang II receptor antagonists in nonsteatotic grafts was associated with ERK 1/2 overexpression, whereas the beneficial effects of Ang-(1-7) receptor antagonists in steatotic grafts may be mediated by NO inhibition. Our results show that Ang II receptor antagonists are effective only in nonsteatotic liver transplantation and point to a novel therapeutic target in liver transplantation based on Ang-(1-7), which is specific for steatotic liver grafts.

Angiotensin-(1-7) serves as an aquaretic by increasing water intake and diuresis in association with downregulation of aquaporin-1 during pregnancy in rats

We previously demonstrated that kidney and urine levels of angiotensin-(1-7) [ANG-(1-7)] were increased in pregnancy. To explore the role of ANG-(1-7) on fluid and electrolyte homeostasis during pregnancy, we evaluated the effect of the ANG-(1-7) antagonist d-alanine-[ANG-(1-7)] (A-779) on kidney function. Virgin and pregnant rats received infusion of vehicle or A-779 (48 μg·kg−1·h−1) for 8 days by osmotic minipumps. Metabolic studies were done on treatment day 7–8. Virgin and pregnant rats at day 15 and 19 were killed, and blood and kidneys were collected. Kidneys were prepared for Western blot analysis for aquaporin-1 (AQP1) and aquaporin-2. In virgin female rats, A-779 increased urine volume and decreased urinary osmolality and AQP1 with no change in water intake. In 19-day pregnant rats, A-779 significantly decreased water intake and urine volume and increased urinary osmolality and kidney AQP1 expression. Only in late gestation did A-779 treatment decrease the difference between intake and output (balance). A-779 treatment increased plasma vasopressin in late gestation but did not change vasopressin in virgins. In virgin and pregnant animals, A-779 administration had no effect on blood pressure, plasma volume, blood volume, or urinary electrolytes. These results suggest that ANG-(1-7) produces antidiuresis associated with upregulation of AQP1 in virgin rats, whereas ANG-(1-7) produces diuresis in late gestation with downregulation of AQP1. ANG-(1-7) contributes to the enhanced water intake during pregnancy, allowing maintenance of the normal volume-expanded state despite diuresis produced in part by decreased AVP and AQP1.

Effects of hormone therapy on blood pressure and the renin-angiotensin system in postmenopausal women

Observational studies have documented an association between lower rates of cardiovascular disease and hormone therapy (HT). Meanwhile, randomized clinical trials have documented increased rates of myocardial infarction and stroke in women receiving hormone therapy. These seemingly discordant findings have stimulated new research to examine estrogen's effects on the cardiovascular system, including its effects on blood pressure, regulation of the renin-angiotensin system (RAS), and the clinical consequences of hypertension. In the last 6 years several studies have better defined the mechanisms by which HT affect the RAS, blood pressure, and the clinical effects of hypertension. Recent studies documented increases in angiotensinogen synthesis and the suppression of active renin with estrogen replacement. Genotype may be a factor in determining the degree of suppression of angiotensin converting enzyme levels that occurs with estrogen therapy. Estrogen supplementation in postmenopausal women increases systemic angiotensin II, a potent vasoconstrictor. This vasopressor effect is attenuated by an estrogen-induced reduction of angiotensin II type 1 receptor expression. Renal blood flow reduction, in the absence of blood pressure changes, have been reported after estrogen replacement, and an increased risk of total stroke has been demonstrated in hypertensive women on HT compared to normotensive women on this therapy. Estrogen replacement affects many components of the RAS, but its effect on this system has little effect on blood pressure. Further studies are needed to describe the effects of estrogen replacement on abnormal vasculature and how these effects relate to myocardial infarction and stroke.

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.

Angiotensin(1-7) potentiates bradykinin-induced vasodilatation in man

BACKGROUND

It has been clearly demonstrated that angiotensin(1-7) potentiates the vasodilating effect of bradykinin in isolated vessels of animals.

OBJECTIVE

To investigate the interaction between angiotensin(1-7) Ang(1-7) and bradykinin in human forearm resistant vessels of normotensive healthy men in vivo, by the measurement of forearm blood flow using venous occlusion, strain-gauge plethysmography with intra-arterial infusions of peptides in a placebo-controlled, double-blind, cross-over design.

METHODS

In eight men, bradykinin was infused intra-arterially twice; placebo, Ang(1-7), or angiotensin II was co-infused with the second infusion. The effect of inhibition of nitric oxide synthase on the interaction between Ang(1-7) and bradykinin was also tested in eight other individuals. The effects of Ang(1-7) were analyzed by analysis of variance (ANOVA) and by the ratios of individually derived areas under the dose-response curves (AUC) of bradykinin, adjusted for changes in the AUCs by repeated infusions of bradykinin with placebo.

RESULTS

Ang(1-7) (1000 pmol/min) significantly potentiated the vasodilating effect of bradykinin compared with the effect of saline (P = 0.0471, ANOVA) and in a dose-dependent manner (adjusted AUC ratio [95% confidence interval (CI)] 2.75 (1.72 to 3.78) with 1000 pmol/min, 1.62 (1.31 to 1.93) with 100 pmol/min, and 0.98 (0.80, to 1.09) with 10 pmol/min). This effect was completely abolished by co-infusion of NG-monomethyl-l-arginine [AUC ratio 0.98 (0.90 to 1.04)]. Ang(1-7) did not affect the vasodilating effects of either acetylcholine or sodium nitroprusside.

CONCLUSIONS

Ang(1-7) potentiates the vasodilating effect of bradykinin, possibly through a mechanism(s) involving nitric oxide release, in human forearm resistance vessels.

Urinary vasodilator and vasoconstrictor angiotensins during menstrual cycle, pregnancy, and lactation

Since normal human pregnancy is characterized by normotension in the face of an increased renin-angiotensin-aldosterone system (RAAS), we evaluated the temporal pattern of urinary excretion of a novel vasodilator within this system, angiotensin-(1-7) (Ang-[1-7]), during the menstrual cycle, pregnancy, and lactation. The urinary profiles of Ang I, Ang II, human chorionic gonadotropin, 17beta-estradiol, and progesterone were also determined. During the menstrual cycle, urinary Ang-(1-7) and Ang II remained stable (mean cycle value: 94.6 +/- 11.3 and 11.4 +/- 1.1 pmol/g of creatinine, respectively) in nine females. In 10 normal pregnant women, urinary Ang-(1-7) and Ang II increased throughout gestation, averaging 1499.8 +/- 310 and 224.4 +/- 58 pmol/g of creatinine, respectively (p < 0.05) at wk 35 and falling during lactation to 394.0 +/- 95 and 65.7 +/- 20 pmol/ g of creatinine (p < 0.05), respectively. The Ang-(1-7)/Ang II ratio was unchanged in the different reproductive periods. During the menstrual cycle, Ang II and Ang-(1-7) correlated with 17beta-estradiol and progesterone using multivariate analysis (r = 0.31, p < 0.001) and r = 0.28, p < 0.02, respectively). During gestation, 17beta-estradiol and progesterone correlated with urinary Ang-(1-7) (r = 0.48, p < 0.001 and r = 0.47, p < 0.001, respectively) and Ang II (r = 0.24, p < 0.03 and r = 0.25, p < 0.03, respectively); by multiple regression, only Ang-(1-7) correlated with both steroids (r = 0.49,p < 0.001). The progressive rise of Ang-(1-7) throughout gestation, probably modulated by estrogen and progesterone, suggests a physiologic counterregulation within the RAAS.

Differential effects of E and droloxifene on C-reactive protein and other markers of inflammation in healthy postmenopausal women

Although increased levels of C-reactive protein have been linked to E therapy, the significance of this finding and whether it occurs with the selective ER modulators are unknown. Thirty-five healthy postmenopausal women were enrolled in a placebo-controlled, two-period cross-over design trial to evaluate the effects of 0.625 mg oral conjugated E and 60 mg droloxifene, a structural analog of tamoxifen, on serum levels of C-reactive protein, IL-6, and endothelial cell adhesion molecules. E treatment resulted in 65.8% higher levels of C-reactive protein (P = 0.0002) and 48.1% higher levels of IL-6 (P < 0.001), but also resulted in a 10.9% reduction in soluble E-selectin (P = 0.002) and borderline reductions in vascular cell adhesion molecule-1. In contrast, droloxifene had no effect on C-reactive protein and IL-6, but did produce a significant 11% reduction in E-selectin (P < 0.00001). However, droloxifene also resulted in an 11.6% increase in vascular cell adhesion molecule-1 (P < 0.007). These data provide additional evidence of a proinflammatory effect of E that may have adverse cardiovascular consequences. However, these changes were also accompanied by a reduction in E-selectin, suggesting an antiinflammatory effect at the level of the endothelium. The net clinical impact of these changes is not yet well established. In contrast, droloxifene had little or no proinflammatory effects on C-reactive protein and IL-6 and had mixed effects on endothelial adhesion molecules. This observation provides additional rationale for continuing to evaluate the potential cardiovascular benefits of selective ER modulators.

The angiotensin II AT1 receptor antagonist irbesartan prevents thromboxane A2-induced vasoconstriction in the rat hind-limb vascular bed in vivo

OBJECTIVE

We studied the vasoconstrictor effects of the thromboxane A2 (TxA2) analogue U46619 in the perfused hind limb of rats under constant flow before and after intravenous injection of irbesartan, an angiotensin II AT1 receptor antagonist, to test whether irbesartan interacts in vivo with the thromboxane A2/prostaglandin endoperoxidase H2 (TxA2/PGH2) receptor.

DESIGN

Male Sprague-Dawley rats (n = 15, body weight 350-420 g) were anesthetized with thiobutabarbital sodium (Inactin, 100 mg/kg intraperitoneally). Regional vascular responses to U46619 (0.5 and 1.0 microg) were investigated in the rat hind quarter under conditions of controlled flow before and after administration of irbesartan (10 mg/kg, intravenously). In addition, to test the specificity of the effect of irbesartan on U46619, phenylephrine (0.5, 1.0 microg) and another AT1 receptor antagonist, candesartan CV11974 (0.3 mg/kg, intravenously) were used.

RESULTS

The dose-dependent increases in hind-limb perfusion pressure produced by U46619 were significantly attenuated by prior injection of irbesartan, at a dose that blocked the angiotensin II (Ang II) pressor responses. The specificity for the response was shown with the demonstrations that the increase in vascular resistance produced by phenylephrine was unchanged by irbesartan and, furthermore, that the increase in vascular resistance produced by U46619 was unchanged by another AT1 receptor antagonist, candesartan.

CONCLUSION

This study demonstrates that irbesartan interacts at the TxA2/PGH2 receptor in the rat's hind limb in vivo, to modify changes in local regional vascular resistance. The dual antagonistic actions of irbesartan, acting at both AT1 and TxA2 receptors in blood vessels, may overall enhance its therapeutic profile in the treatment of hypertension.

Inhibition of platelet aggregability by losartan in essential hypertension

Most clinical events associated with hypertension have a thrombotic component. Losartan is a selective, competitive antagonist of the thromboxane A2 receptor in experiments performed in isolated vascular strips and in human and rat platelet-enriched plasma. In this study, we investigated for the first time whether losartan at therapeutic doses has an effect on platelet aggregability and indexes of fibrinolysis in essential hypertensive subjects. Changes in the dose-response curve to platelet aggregation induced by the thrombin receptor-activating peptide SFLRRN-NH2 were determined in 9 patients (56% men, 72% white; mean age 52.8 years) with stage I or II essential hypertension and in 9 untreated healthy volunteers. After a 4-week washout period, hypertensive subjects received 2 weeks of placebo followed by 4 weeks of losartan 50 mg/day. Both subjects and end points were blinded for treatment assignment. In addition, plasminogen activator inhibitor type 1 antigen and von Willebrand antigen were studied in all patients and controls. Four weeks of losartan produced a statistically significant (p <0.05) increase in the concentration of SFLRRN-NH2 required to induce a half-maximal response in platelet aggregation extent and rate 4 weeks after initiation of treatment. The decrease in platelet aggregability was independent of blood pressure control and the effects of gender and age. Losartan had no effect on plasma concentrations of plasminogen activator inhibitor-1 and von Willebrand factor in hypertensive subjects. These data demonstrate for the first time a novel antiplatelet effect of losartan at therapeutic doses, which was independent of changes in blood pressure, plasma markers of fibrinolytic activity, and endothelial perturbation.

Effects of estrogen replacement on the progression of coronary-artery atherosclerosis

BACKGROUND

Heart disease is a major cause of illness and death in women. To understand better the role of estrogen in the treatment and prevention of heart disease, more information is needed about its effects on coronary atherosclerosis and the extent to which concomitant progestin therapy may modify these effects.

METHODS

We randomly assigned a total of 309 women with angiographically verified coronary disease to receive 0.625 mg of conjugated estrogen per day, 0.625 mg of conjugated estrogen plus 2.5 mg of medroxyprogesterone acetate per day, or placebo. The women were followed for a mean (+/-SD) of 3.2+/-0.6 years. Base-line and follow-up coronary angiograms were analyzed by quantitative coronary angiography.

RESULTS

Estrogen and estrogen plus medroxyprogesterone acetate produced significant reductions in low-density lipoprotein cholesterol levels (9.4 percent and 16.5 percent, respectively) and significant increases in high-density lipoprotein cholesterol levels (18.8 percent and 14.2 percent, respectively); however, neither treatment altered the progression of coronary atherosclerosis. After adjustment for measurements at base line, the mean (+/-SE) minimal coronary-artery diameters at follow-up were 1.87+/-0.02 mm, 1.84+/-0.02 mm, and 1.87+/-0.02 mm in women assigned to estrogen, estrogen plus medroxyprogesterone acetate, and placebo, respectively. The differences between the values for the two active-treatment groups and the value for the placebo group were not significant. Analyses of several secondary angiographic outcomes and subgroups of women produced similar results. The rates of clinical cardiovascular events were also similar among the treatment groups.

CONCLUSIONS

Neither estrogen alone nor estrogen plus medroxyprogesterone acetate affected the progression of coronary atherosclerosis in women with established disease. These results suggest that such women should not use estrogen replacement with an expectation of cardiovascular benefit.

Cardiovascular effects of droloxifene, a new selective estrogen receptor modulator, in healthy postmenopausal women

Selective estrogen receptor modulators, like tamoxifen and related compounds, have mixed estrogen agonistic/antagonistic effects. Tamoxifen may confer significant cardiovascular benefits without the estrogen-associated risks of endometrial and breast cancer. Droloxifene, a structural analogue of tamoxifen, has estrogen agonistic effects on bone and antagonistic effects on endometrial and breast tissue. Its cardiovascular effects in women are unknown. We enrolled 24 healthy postmenopausal women in a randomized, double-blind, 2-period crossover trial comparing the effects of droloxifene (60 mg/d) with conjugated estrogen (0.625 mg/d). Plasma lipids, coagulation and fibrinolytic factors, and brachial flow-mediated vasodilator responses were measured at the beginning and end of each treatment period. Droloxifene and estrogen resulted in 16.6% and 12.0% reductions, respectively, in low density lipoprotein cholesterol (P<0.001) and 13.2% and 9.5% reductions, respectively, in lipoprotein(a) (P<0.05). In contrast, estrogen, but not droloxifene, increased high density lipoprotein (18.5%, P<0.001). Droloxifene also reduced fibrinogen by 17.8% versus a 7.3% reduction with estrogen (P=0.004) but produced no estrogen-like changes in plasminogen, plasminogen activator inhibitor-1, or tissue plasminogen activator. Droloxifene and estrogen produced 36.4% and 27.3% increases, respectively, in flow-mediated vasodilation (percent change from baseline, P<0.05 for both). Droloxifene has estrogen agonistic properties regarding low density lipoprotein and lipoprotein(a) metabolism, certain coagulation factors, and endothelium-dependent vasodilation but, unlike estrogen, has no effect on high density lipoprotein/triglyceride metabolism and the fibrinolytic cascade. It remains unknown whether droloxifene can confer a true cardiovascular benefit.

Novel angiotensin II AT(1) receptor antagonist irbesartan prevents thromboxane A(2)-induced vasoconstriction in canine coronary arteries and human platelet aggregation

This study was conducted to investigate whether the novel orally active nonpeptide angiotensin II (Ang II) AT(1) receptor antagonist irbesartan interacts with the thromboxane A(2)/prostaglandin endoperoxide H(2) (TxA(2)/PGH(2)) receptor in canine coronary arteries and human platelets. Coronary artery rings were isolated from male dog hearts (n = 18) and isometric tension of vascular rings was measured continuously at optimal basal tension in organ chambers. Autoradiographic binding of [(3)H]SQ29,548, a TxA(2) receptor antagonist, in canine coronary sections was determined. Blood for platelet aggregation studies was collected by venous puncture from healthy human volunteers (n = 6) who were free of aspirin-like agents for at least 2 weeks. Vascular reactivity and platelet aggregation in response to the TxA(2) analogs U46619 and autoradioagraphic receptor binding to the TxA(2) receptor antagonist [(3)H]SQ29,548 were studied with and without irbesartan. The TxA(2) analog U46619 produced dose-dependent vasoconstriction in coronary rings (EC(50) = 11.6 +/- 1.5 nM). Pretreatment with irbesartan inhibited U46619-induced vasoconstriction, and the dose-response curve was shifted to the right in a dose-dependent manner. The EC(50) of U46619 was increased 6- and 35-fold in the presence of 1 and 10 microM of irbesartan without a change of maximal contraction. At 1 microM, irbesartan is 2-fold more potent than the AT(1) receptor antagonist losartan in the inhibition of U46619-induced vasoconstriction in canine coronary arteries. In contrast, neither AT(1) receptor antagonists (CV11974 and valsartan), the AT(2) receptor antagonist PD123319, nor the angiotensin converting enzyme inhibitor lisinopril had any effect on U46619-induced coronary vasoconstriction. Irbesartan did not change potassium chloride-induced vasoconstriction; however, irbesartan did inhibit the vasoconstriction mediated by another TxA(2)/PGH(2) receptor agonist prostaglandin F(2alpha) (PGF(2alpha)). Neither the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester nor the cyclooxygenase inhibitor indomethacin had any effect on irbesartan's attenuation of U46619-induced vasoconstriction. Irbesartan specifically reversed U46619-preconstricted coronary artery rings with and without endothelium in a dose-dependent manner. Irbesartan at high concentrations significantly competed for [(3)H]SQ29,548 binding in canine coronary sections. U46619 stimulated dose-dependent human platelet aggregation of platelet-rich plasma. Preincubation with irbesartan significantly inhibited platelet aggregation in a concentration-dependent manner. In conclusion, the dual antagonistic actions of irbesartan by acting at both the AT(1) and TxA(2) receptors in blood vessels and platelets may overall enhance its therapeutic profile in the treatment of hypertension, atherosclerosis, and arterial thrombosis.

Bi-directional actions of estrogen on the renin-angiotensin system

Estrogen stimulates the renin-angiotensin system by augmenting both tissue and circulating levels of angiotensinogen and renin. We show, however, that angiotensin converting enzyme (ACE) activity in the circulation and in tissues is reduced in two animal models of postmenopausal chronic hormone replacement. We observed a reduction of ACE activity in association with a significant increase in plasma angiotensin I (Ang I) and hyperreninemia in ovariectomized monkeys treated with Premarin (conjugated equine estrogen) replacement for 30 months. Plasma angiotensin II (Ang II) levels were not increased in monkeys treated with estrogen, suggesting that the decrease in ACE curtailed the formation of the peptide. The Ang II/Ang I ratio, an in vivo index of ACE activity, was significantly reduced by estrogen treatment, further supporting the biochemical significance of estrogen's inhibition of ACE. In ovariectomized transgenic hypertensive (mRen2)27 rats submitted to estrogen replacement treatment for 3 weeks, ACE activity in plasma and tissue (aorta and kidney) and circulating Ang II levels were reduced, whereas circulating levels of angiotensin-(1-7) (Ang-(1-7)) were increased. Ang-(1-7), the N-terminal fragment of Ang II, is a novel vasodilator and antihypertensive peptide. Thus, the net balance of these effects of estrogen on the reninangiotensin vasoconstrictor/vasodilator system is to promote the anti-hypertensive effect.

Differential response of angiotensin peptides in the urine of hypertensive animals

Urinary excretion rates of angiotensin I (Ang I), angiotensin II (Ang II), and angiotensin-(1-7) [Ang-(1-7)] were determined in normotensive Sprague Dawley (SD), spontaneously hypertensive (SHR), and mRen-2 transgenic hypertensive animals before and following blockade of Ang II synthesis or activity for two weeks. This study was performed to determine for the first time whether inhibition of Ang II alters the excretion of angiotensin peptides in the urine. Rats were given either tap water or water medicated with lisinopril, losartan or both agents in combination. Blood pressure was monitored at regular intervals during the experiment by the tail-cuff method, and once again at the end of the study with a catheter implant into a carotid artery. Metabolic studies and 24 h urinary excretion variables and angiotensin peptides were determined before and during the procedures. While all three treatments normalized the blood pressure of hypertensive animals, therapy with either lisinopril or the combination of lisinopril and losartan had a greater antihypertensive effect in both SHR and [mRen-2]27 transgenic hypertensive rats. In the urine, the concentration of the angiotensins (normalized by 24-h creatinine excretion) was several-fold higher in the untreated hypertensive animals than in normotensive SD rats. In SD rats, lisinopril or lisinopril and losartan produced a sustained rise in urinary levels of Ang-(1-7) without changes in the excretion of Ang I and Ang II. In contrast, Ang I and Ang-(1-7) were significantly elevated in SHR medicated with lisinopril alone or in combination with losartan. Only losartan, however, augmented urinary levels of Ang II in the SHR. The antihypertensive effects of the three separate regimens had no effect on the urinary excretion of angiotensin peptides in [mRen-2]27 transgenic hypertensive rats. These data show that Ang I and Ang-(1-7) are excreted in large amounts in the urine of SD, SHR and [mRen-2]27 hypertensive rats. The unchanged Ang-(1-7) excretion in transgenic hypertensive (Tg+) rats after inhibition of the renin-angiotensin system agrees with the previous finding of a reduced plasma clearance of the peptide in this model of hypertension. The data suggest that this form of hypertension may be associated with increased activity of an endogenous converting enzyme inhibitor.

Estrogen regulation of angiotensin-converting enzyme mRNA

Estrogen replacement therapy is cardioprotective in postmenopausal women; however, the precise molecular mechanisms for this modulation are not fully elucidated. We previously showed that chronic estrogen replacement therapy reduced angiotensin-converting enzyme (ACE) activity in tissue extracts and serum with an associated reduction in plasma angiotensin II. A reverse transcriptase-polymerase chain reaction assay was developed to determine whether estrogen treatment regulates tissue ACE mRNA concentration. Total RNA was isolated from kidney cortex, kidney medulla, lung, and aorta of ovariectomized Sprague-Dawley rats after 21 days of chronic 17beta-estradiol replacement therapy (5 mg pellet per rat SC) or placebo. A marked decrease in densitometric intensity ratios of amplified ACE cDNA to elongation factor-1alpha control cDNA was observed in all tissues from placebo-treated rats compared with the estradiol-treated rats (renal cortex: 0.29+/-0.04 versus 0.14+/-0.02; renal medulla: 0. 37+/-0.04 versus 0.24+/-0.03; lung: 4.49+/-0.37 versus 2.49+/-0.59; and aorta: 0.41+/-0.04 versus 0.29+/-0.02; all P<0.05). A comparable reduction in ACE activity was detected in tissue extracts from kidney cortex, kidney medulla, and lung of hormone-treated animals. Incubation of purified rat lung ACE with 1 or 10 micromol/L 17beta-estradiol had no effect on enzyme activity. These results suggest that estrogen treatment regulates tissue ACE activity by reducing ACE mRNA concentrations. Thus, the beneficial cardiovascular effects of estrogen may be mediated in part by downregulation of ACE with a consequent reduction in the circulating levels of the vasoconstrictor angiotensin II, a decrease in the metabolism of the vasodilator bradykinin, and an increase in the production of the vasorelaxant angiotensin-(1-7).

Angiotensin-(1-7): a novel vasodilator of the coronary circulation

Angiotensin-(1-7) [Ang-(1-7)] possesses novel biological functions that are distinct from angiotensin II (Ang II). In coronary arteries, the octapeptide Ang II and the heptapeptide Ang-(1-7) exert opposing actions. Ang II elicits vasoconstriction and Ang-(1-7) is a vasodilator. Ang-(1-7) elicits vasodilation by an endothelium-dependent release of nitric oxide. Further, the vasorelaxant activity is markedly attenuated by the bradykinin (BK) B2 receptor antagonist icatibant and does not appear to be associated with the synthesis and release of prostaglandins. Ang-(1-7) vasodilation is mediated by a non-AT1/AT2 receptor, since [Sar1Thr8]-Ang II, but neither candesartan, an AT1 receptor antagonist, nor PD123319, an AT2 receptor antagonist, blocked the response. Specific and high affinity binding of 125I-Ang-(1-7) to the endothelial layer of canine coronary arteries was demonstrated using in vitro emulsion autoradiography. Binding was effectively competed for by either unlabeled Ang-(1-7) or the specific Ang-(1-7) antagonist [D-Ala7]-Ang-(1-7). Additionally, Ang-(1-7) potentiated synergistically BK-induced vasodilation. The EC50 of BK vasodilation (2.45 +/- 0.51 nmol/L vs 0.37 +/- 0.08 nmol/L) was shifted 6.6-fold left-ward in the presence of 2 mumol/L concentration of Ang-(1-7). The potentiated response was specific for BK, since Ang-(1-7) did not augment the vasodilation produced by either acetylcholine or sodium nitroprusside; further, it was specific for Ang-(1-7), since neither Ang I nor Ang II augmented the BK response. In contrast to the vasodilator actions of Ang-(1-7), the potentiated response was not blocked by candesartan, PD123319 or [Sar1Thr8]-Ang II. Novel studies from our group demonstrate that Ang-(1-7) is both a substrate and inhibitor for angiotensin converting enzyme (ACE). Ang-(1-7) was shown to retard the degradation of 125I-[Tyr0]-BK in coronary rings. These studies describe novel actions of Ang-(1-7) as a vasodilator and a local synergistic modulator of kinin-induced vasodilation in coronary arteries.

Losartan inhibits thromboxane A2-induced platelet aggregation and vascular constriction in spontaneously hypertensive rats

Our recent studies have shown that the nonpeptide angiotensin II (Ang II) antagonist losartan interacts with thromboxane A2/prostaglandin H2 receptors and inhibits the thromboxane A2 (TxA2) analog U46619-induced vasoconstriction in canine coronary arteries. In this study, we further investigated whether losartan prevents TxA2-induced platelet aggregation and vasoconstriction in spontaneously hypertensive rats (SHRs). Pretreatment with losartan (10 microM) significantly reduced U46619-induced, concentration-dependent washed platelet aggregation. The inhibition is specific for losartan, because another Ang II AT1-receptor antagonist, CV11974 (10 microM), an active metabolite of TCV116, did not block the platelet aggregation caused by U46619. In addition, losartan (10 microM) augmented acetylcholine (ACH)-induced nitric oxide (NO)-dependent vasodilation and abolished the ACH-induced endothelium-derived contracting factor (EDCF)-mediated vasoconstriction in the aortic rings from adult SHRs. U46619 produced dose-dependent vasoconstriction in aortic vessels of SHRs, which was demonstrated to be blocked by the potent, selective TxA2/PGH2 receptor antagonist SQ29,548. Pretreatment with losartan (10(-6)-10(-5) M) inhibited the contractile response of U46619 and shifted the concentration-response curve to the right in a dose-dependent manner. The effective concentration at half maximal contraction (EC50) of U46619 was increased 2.5- and 7.6-fold in the presence of 1 and 10 microM losartan, respectively, without changes in maximal contraction. The active metabolite of losartan, EXP3174, at 1 microM also competitively inhibited U46619-induced contractions in aortic rings of SHRs. In contrast, neither the AT1-receptor antagonist CV11974, the AT2 antagonist PD123319, nor the angiotensin-converting enzyme inhibitor lisinopril, each at concentrations of 1 microM, had any effect on the U46619-induced constriction in aortic rings. In conclusion, losartan, acting as both AT1- and TxA2/PGH2-receptor antagonists, may enhance its therapeutic profile in the treatment of hypertension and cardiovascular disease.

Role of AT1 and AT2 receptors in the plasma clearance of angiotensin II

This study assessed the role of angiotensin (Ang) AT1 and AT2 receptors as modulators of the plasma clearance of Ang II. Groups of male spontaneously hypertensive rats (SHRs; n = 25) were given an intravenous injection of either saline, losartan, PD123319, losartan in combination with PD123319, or Sar1-Thr8-Ang II. One hour later, Ang II (0.5 microg/kg) was infused for 15 min into a vein. Immediately thereafter, arterial blood samples were collected at regular intervals for the assay of plasma Ang II levels by radioimmunoassay. The infusion of Ang II significantly increased baseline mean arterial pressure (MAP) in rats pretreated with either saline or PD123319 but not in those receiving losartan, losartan combined with PD123319, or Sar1-Thr8-Ang II. The plasma clearance of Ang II was significantly greater in rats injected with either PD123319, losartan combined with PD123319, or Sar1-Thr8-Ang II compared to those injected either saline or losartan. Furthermore, the half-life of Ang II in rats pretreated with saline or losartan was significantly greater than that measured in the other three groups. These results suggest that plasma clearance of Ang II in the SHRs is independent of an AT1 receptor, but plasma levels of the peptide are unexpectedly protected by an AT2 receptor-dependent mechanism.

Characterization of angiotensin-(1-7) in the urine of normal and essential hypertensive subjects

A total of 31 healthy volunteers [39 +/- 7 (SD) years] and 18 untreated essential hypertensive subjects [43 +/- 9 years] collected urine for 24 h after a physical examination and laboratory tests. Radioimmunoassay measurements of angiotensin-(1-7) [Ang-(1-7)] in urine and plasma were done as described previously. Sitting systolic and diastolic blood pressures (+/- SD) averaged 118 +/- 11/74 +/- 7 mm Hg and 146 +/- 16/96 +/- 8 mm Hg in normal and essential hypertensive subjects, respectively (P < .001), whereas 24 h urinary volume was not different in normal and essential hypertensive subjects (P > .05). The concentration of Ang-(1-7) in the urine of normal subjects averaged 62.6 +/- 22.6 pmol/L corresponding to a urinary excretion rate of 98.9 +/- 44.7 pmol/24 h. Concurrent measurements of plasma Ang-(1-7) showed that the content of Ang-(1-7) in urine was 2.5-fold higher than that measured in the plasma. In contrast, untreated essential hypertensive subjects had lower concentrations and 24 h urinary excretion rates of Ang-(1-7) averaging 39.4 +/- 18.0 pmol/L and 60.2 +/- 14.6 pmol/24 h, respectively, (P < .001). Differences in the excretory rate of Ang-(1-7) between normal volunteers and essential hypertensive subjects were not modified by normalization of the data by urinary creatinine excretion rates. Urinary concentrations of Ang-(1-7) correlated inversely with systolic, diastolic and mean arterial pressures (r = -0.48, P < .001). Both urinary Ang-(1-7) [odds ratio of 0.92 (95% CI: 0.88-0.97)] and age were independent predictors of systolic blood pressure. These studies demonstrated the presence of Ang-(1-7) in urine and the existence of reduced levels of the heptapeptide in individuals with untreated essential hypertension. The relatively higher concentrations of Ang-(1-7) in urine compared to plasma agrees with data that showed that Ang-(1-7) may contribute to the regulation of blood pressure. The inverse association between Ang-(1-7) and arterial pressure provides a potential marker for the characterization of forms of essential hypertension associated with reduced production or activity of vasodilator hormones.

Adrenal, kidney, and heart angiotensins in female murine Ren-2 transfected hypertensive rats

We analyzed by high-performance liquid chromatography and radioimmunoassay angiotensin I (Ang I), Ang II, Ang-(1-7), and metabolites in the adrenal, kidney and heart of normotensive female Sprague-Dawley (SD) and transgenic hypertensive [TGR(mRen-2)27] rats carrying the murine Ren-2d renin gene. The monogenetic model of hypertensive rats had significant increases in adrenal Ang II; whereas in the kidney Ang II was unchanged, but Ang I and Ang-(1-7) were significantly lower. Cardiac Ang I, Ang II, and Ang-(2-10) were significantly reduced in transgenic rats, while Ang-(2-7) was increased. In SD and transgenic rats kidney and adrenal angiotensins increased primarily during estrus or proestrus. In female transgenic rats the increased adrenal Ang II and the sustained renal Ang II may contribute to the established phase of hypertension.

Estrogen protects transgenic hypertensive rats by shifting the vasoconstrictor-vasodilator balance of RAS

In pursuit of the hypothesis that estrogen shifts the vasoconstrictor-vasodilator balance of the renin-angiotensin system, we investigated the cardiovascular responses to administration of angiotensin-(1-7) [ANG-(1-7)] and angiotensin II (ANG II) in female transgenic (mRen2)27-positive [Tg(+)] and -negative [Tg(-)] rats in the presence and absence of 3 wk of estrogen replacement therapy. Fifty-three female Tg(-) and Tg(+) rats were oophorectomized and received either 17 beta-estradiol (1.5 mg/rat s.c. for 3 wk) or vehicle. At the end of 3 wk of estrogen treatment, mean blood pressure was lowered in freely moving chronically cannulated Tg(+) (159 +/- 4 vs. 145 +/- 5 mmHg, P < 0.05) and Tg(-) (119 +/- 4 vs. 108 +/- 2 mmHg, P < 0.05) rats. Moreover, the magnitude of the depressor component of the biphasic response to ANG-(1-7) was significantly enhanced in estrogen-treated Tg(+) rats, whereas the pressor component to ANG-(1-7) was attenuated in both Tg(+) and Tg(-) rats. Estrogen replacement significantly attenuated the pressor response to ANG II in both Tg(+) and Tg(-) rats. In addition, estrogen replacement therapy significantly reduced plasma ANG-converting enzyme activity in association with a reduction in circulating levels of ANG II. Tissue levels (kidney and aorta) of ANG-converting enzyme were also reduced with chronic estrogen replacement therapy. On the other hand, estrogen augmented the levels of plasma ANG-(1-7) in Tg(+) animals. Plasma renin activity was unchanged with estrogen treatment. These findings provide the first evidence demonstrating that estrogen is protective against hypertension, possibly by amplifying the vasodilator contributions of ANG-(1-7), while reducing the formation and vasoconstrictor actions of ANG II.

Counterregulatory actions of angiotensin-(1-7)

Angiotensin (Ang)-(1-7) is a bioactive component of the renin-angiotensin system that is formed endogenously from either Ang I or Ang II. The first actions described for Ang-(1-7) indicated that the peptide mimicked some of the effects of Ang II, including the release of prostanoids and vasopressin. However, Ang-(1-7) is devoid of vasoconstrictor, central pressor, or thirst-stimulating actions. In fact, new findings reveal depressor, vasodilator, and antihypertensive actions that may be more apparent in hypertensive animals or humans. Thus, the accumulating evidence suggests that Ang-(1-7) may oppose the actions of Ang II either directly or by stimulation of prostaglandins and nitric oxide. These observations are significant because they may explain the effective antihypertensive action of converting enzyme inhibitors in a variety of non-renin-dependent models of experimental and genetic hypertension as well as most forms of human hypertension. In this context, studies in humans and animals showed that the antihypertensive action of converting enzyme inhibitors correlated with increases in plasma levels of Ang-(1-7). In this review, we summarize our knowledge of the mechanisms accounting for the counterregulatory actions of Ang-(1-7) and elaborate on the emerging concept that Ang-(1-7) functions as an antihypertensive peptide within the cascade of the renin-angiotensin system.

Effects of chronic hormone replacement on the renin-angiotensin system in cynomolgus monkeys

OBJECTIVE

To characterize the effects of estrogen, estrogen combined with progestin, and no treatment in ovariectomized cynomolgus monkeys during long-term reproductive hormone replacement.

METHODS

Forty-five surgically postmenopausal cynomolgus monkeys fed a lipid-lowering diet were administered a conjugated equine estrogen (Premarin, 7.2 micrograms/day for the first 8 months, then 166 micrograms/day for the remaining 22 months), alone or in combination with 650 micrograms/day medroxyprogesterone acetate (Cycrin) for 30 months, or left with no hormone replacement therapy. Animals were anesthetized with ketamine-pentobarbital, and samples were taken for measurements of plasma renin activity, angiotensin converting enzyme activity, and angiotensin peptides, angiotensin I (Ang I), angiotensin II (Ang II), and angiotensin-(1-7) [Ang-(1-7)].

RESULTS

Chronic replacement therapy with estrogen resulted in a significant elevation of the plasma renin activity [11.7 +/- 2.0 ng/ml per h control versus 22.8 +/- 4.6 ng/ml per h with estrogen (P < 0.05) versus 32.8 +/- 4.9 ng/ml per h with combination therapy (P < 0.01)], whereas estrogen or combination therapy caused a significant reduction in angiotensin converting enzyme activity [229 +/- 8 nmol/ml per min control versus 189 +/- 10 nmol/ml per min with estrogen (P < 0.05) versus 196 +/- 11 nmol/ml per min with combination therapy (P < 0.05)]. Both of these changes in angiotensin processing enzymes observed during replacement therapy resulted in significant increases in plasma Ang I levels [46.7 +/- 12.5 pg/ml control versus 175.5 +/- 65.9 pg/ml with estrogen (P < 0.05) and 561.7 +/- 373.6 pg/ml with combination therapy (P < 0.05)]. Plasma Ang II and Ang-(1-7) levels were not significantly changed. The mean blood pressure did not change with either treatment.

CONCLUSION

These studies reveal that, although chronic estrogen replacement activates renin activity and Ang I, it causes a shift in the processing of angiotensin peptides such that the concurrent reduction in angiotensin converting enzyme activity leads to unchanged plasma Ang II levels. Thus, the potentially harmful effects of estrogen-induced hyperreninemia are balanced by its actions interfering with the formation of the vasoactive product Ang II.

Chronic estrogen treatment in female transgenic (mRen2)27 hypertensive rats augments endothelium-derived nitric oxide release

Postmenopausal estrogen replacement therapy is associated with a reduction in cardiovascular events in women, but the mechanisms for this protection are unclear, especially in hypertensive subjects. In this study we investigated the effects of 17beta-estradiol (E2) treatment on blood pressure and endothelial function of transgenic [(mRen2)27] hypertensive and normotensive rats. Thirty female transgenic negative [Tg(-)] and hypertensive positive [Tg(+)] rats were ovariectomized and received either E2 (1.5 mg/rat, subcutaneously, for 3 weeks) or placebo. Chronic 17beta-estradiol treatment lowered mean blood pressure in both Tg hypertensive (159 +/- 4 v 145 +/- 4 mm Hg, P < .05, placebo v E2) and normotensive rats (119 +/- 4 v 108 +/- 2 mm Hg, P < .05, placebo v E2). Pressor responses to intravenous injection of phenylephrine were augmented in the Tg(+) as compared with Tg(-) rats. With chronic E2 treatment the pressor responses to phenylephrine were attenuated in both groups. Isometric tension of aortic rings was measured in vitro in organ chambers. The acetylcholine (Ach)-induced endothelium-dependent vascular relaxation was less potent in Tg(+) versus Tg(-) rats. E2 treatment significantly enhanced the Ach-induced relaxation of both Tg(+) and Tg(-) groups (ED50: 55.5 +/- 11.7 v 10.3 +/- 2.6; 23.8 +/- 6.5 v 5.1 +/- 1.2 nmol/L, placebo v E2 in Tg(+) and Tg(-), respectively). After E2 treatment the ED50 response in Tg(+) rats was no different from Tg(-) rats. However, the maximum vasodilation elicited by Ach was attenuated in Tg(+) as compared with Tg(-) rats. The calcium ionophore (A23187)-induced endothelium-dependent relaxation was less potent in Tg(+) as compared to Tg(-) rats and was enhanced by E2 treatment only in Tg(+) animals. There were no differences in the vasodilator responses elicited by sodium nitroprusside. Removal of endothelium and blockade of NO production abolished the endothelium-dependent vasodilation. The selective NO synthase inhibitor, N(G)-monomethyl-L-arginine (LMMNA), was used to evaluate indirectly the basal contribution of NO in vascular rings. The response to LMMNA was attenuated in untreated Tg(+) as compared to Tg(-) rats. E2 treatment augmented the contraction response to NOS inhibition in both Tg(+) and Tg(-) rats, resulting in a response in Tg(+) rats that was no different from Tg(-) rats. These results indicate that untreated, surgically ovariectomized hypertensive rats show deficiencies in endothelial function, which can be improved by estrogen replacement.

Nonpeptide angiotensin II antagonist losartan inhibits thromboxane A2-induced contractions in canine coronary arteries

We investigated the selectivity of a nonpeptide angiotensin II AT1 receptor antagonist losartan for the vascular thromboxane A2 (TxA2)/prostaglandin endoperoxide (PGH2) receptor in canine coronary arteries. Isometric tension was measured in canine coronary artery rings suspended in organ chambers perfused with 95% O2/5% CO2. The TxA2 analog, U46619, produced dose-dependent vasoconstdction in coronary rings (EC50, 10.6 +/- 0.9 nmol/l). Pretreatment with losartan (10(-8)-10(-5) mol/l) inhibited the contractile response of U46619 and shifted the concentration-response curve to the right in dose-dependent manner. The EC50 of U46619 was increased 3- and 13-fold in the presence of both 1 and 10 micromol/l of losartan without a change in maximal contraction. The selective TxA2/PGH2 receptor antagonist SQ29548 blocked U46619-induced contraction with greater potency than losartan in isolated coronary arteries. The active metabolite of losartan EXP3174 at 1 micromol/l did competitively block U46619-induced contractions in canine coronary rings. In contrast, the contractile responses produced by U46619 were unaffected by exposure to the nonpeptide AT1 receptor antagonist CV11974, the AT2 receptor antagonist PD123319 or the nonselective peptide angiotensin II antagonist Sar1Thr8-Ang II, each at 1 micromol/l concentration. These data indicate that losartan and its active metabolite EXP3174 are antagonists to the TxA2/PGH2 receptor in canine coronary arteries. The antagonistic effect of losartan and EXP3174 on the vascular TxA2/ PGH2 receptor may contribute to the long-term blood pressure-lowering effects of angiotensin antagonists in hypertension.

Angiotensin-(1-7) augments bradykinin-induced vasodilation by competing with ACE and releasing nitric oxide

Recent studies have shown that angiotensin-(1-7) [Ang-(1-7)] interacts with kinins and augments bradykinin (BK)-induced vasodilator responses by an unknown mechanism. In this study, we evaluated whether the potentiation of the BK-induced vasodilation by Ang-(1-7) may be attributable to inhibition of BK metabolism, release of nitric oxide, or both. Isometric tension was measured in intact canine coronary artery rings suspended in organ chambers. 125I-[Tyr0]-BK metabolism was determined in vascular rings by assessing the degradation of the peptide by high-performance liquid chromatography. Ang-(1-7) augmented the vasodilation induced by BK in a concentration-dependent manner in rings preconstricted with the thromboxane analog U46619. The EC50 of BK (2.45 +/- 0.51 nmol/L versus 0.37 +/- 0.08 nmol/L) was shifted leftward by 6.6-fold in the presence of 2 mumol/L concentration of Ang-(1-7). The response was specific for BK. since Ang-(1-7) did not augment the vasodilation induced by either acetylcholine (0.05 mumol/L) or sodium nitroprusside (0.1 mumol/L). Moreover, neither angiotensin I nor angiotensin II (Ang II) duplicated the augmented BK response of Ang-(1-7). Pretreatment of vascular rings with the nitric oxide synthase inhibitor, N omega-nitro-L-arginine (L-NA; 100 mumol/L) completely abolished the effects of Ang-(1-7) on BK-induced vasodilation whereas pretreatment with indomethacin (10 mumol/L) was without effect. The potent specific BK B2 receptor antagonist, Hoe 140. nearly abolished the BK and the Ang-(1-7) potentiated responses at 2 mumol/L, whereas at a lower concentration (20 nmol/L) Hoe 140 shifted the response curve to the right for both Ang-(1-7) and vehicle; however, the augmented response to Ang-(1-7) persisted. Preincubation of vascular rings with 20 mumol/L of the AT1 (CV11974), AT2 (PD123319), or nonselective (Sar1 Thr8-Ang II) receptor antagonists had no significant effect on the Ang-(1-7)-enhanced vasodilator response to BK. Lisinopril (2 mumol/L) significantly enhanced the BK-induced vasodilator response while at the same time it abolished the synergistic action of Ang-(1-7) on BK. In addition, pretreatment with 2 mumol/L Ang-(1-7) significantly inhibited the degradation of 125I-[Tyr0]-BK and the appearance of the BK-(1-7) and BK-(1-5) metabolites in coronary vascular rings. Ang-(1-7) inhibited purified canine angiotensin converting enzyme activity with an IC50 of 0.65 mumol/L. In conclusion. Ang-(1-7) acts as a local synergistic modulator of kinin-induced vasodilation by inhibiting angiotensin converting enzyme and releasing nitric oxide.

Differential regulation of angiotensinogen transcripts after renin infusion

To investigate angiotensinogen regulation in high-renin hypertension, we infused porcine renin intravenously at either a low (4 mU/kg per hour, n = 6) or high (20 mU/kg per hour, n = 9) dose into male Sprague-Dawley rats (225 to 250 g) for 5 days using osmotic minipumps. Control rats received 0.9% NaCl. In renin-infused rats, mean arterial pressure and plasma renin activity were significantly elevated. Both low- and high-renin infusions lowered plasma angiotensinogen levels. Plasma angiotension II was elevated in rats given renin but reached statistical significance only at the higher dose. Angiotensinogen mRNA isolated from the liver, adrenal gland, kidney, and brain was measured by slot blot analysis. Both renin doses were associated with significant decreases in the levels of liver and hypothalamic angiotensinogen mRNA. In the medulla oblongata, angiotensinogen mRNA was reduced only by the higher renin dose. The lower dose increased angiotensinogen mRNA in the adrenal gland, and in kidney, angiotensinogen mRNA level was unchanged by renin infusion. Angiotensinogen mRNA visualized on Northern blots showed that the number of mRNA species in liver decreased from three in control rats to a single mRNA species after renin infusion. Tissue differences in the size of the major angiotensinogen mRNA species were also apparent. This, together with changes in the total hybridization signal of angiotensinogen mRNA in tissues, suggests that renin differentially affects the different angiotensinogen mRNA transcripts. Results of this study indicate that angiotensinogen gene expression is regulated not only by alterations in levels of circulating angiotensin II but also by other mechanisms, presently unidentified, that are activated by renin infusions.

Effects of captopril related to increased levels of prostacyclin and angiotensin-(1-7) in essential hypertension

OBJECTIVE

To evaluate the contribution of angiotensin-(1-7) [Ang-(1-7)] and prostaglandins to the acute and long-term antihypertensive actions of captopril in mild-to-moderate essential hypertensive patients.

DESIGN AND METHODS

Blood pressure, cardiac rate and the plasma concentrations of angiotensin I (Ang I), angiotensin II (Ang II), Ang-(1-7), prostaglandin E2 and 6-keto prostaglandin F1 alpha (the breakdown product of prostacyclin) were determined in the peripheral venous blood of 24 essential hypertensive subjects before and 3 h after administration of 50 mg captopril. Eleven of 24 patients completed a 6-month treatment period with captopril monotherapy (50 mg twice a day). The hemodynamic and hormonal response produced by a last 50 mg dose of captopril was determined once again in the 11 subjects who maintained blood pressure control with captopril monotherapy for 6 months.

RESULTS

The fall in blood pressure produced 3 h after drug intake was comparable for the first and the last 50 mg captopril dose. Although the first response to captopril increased plasma levels of Ang I only, the response to the last dose of the drug (6 months after) caused significantly higher levels of Ang I and Ang-(1-7). Neither acute nor chronic therapy with captopril had a significant effect on plasma concentrations of Ang II. Although plasma levels of prostaglandin E2 and 6-keto prostaglandin F1 alpha were not modified by a first exposure to captopril, the concentrations of 6-keto prostaglandin F1 alpha but not prostaglandin E2 rose significantly in subjects treated with the inhibitor for 6 months. A negative correlation was also demonstrated between diastolic blood pressure and plasma Ang-(1-7) levels in the 11 essential hypertensive subjects in whom blood pressure was controlled with captopril monotherapy.

CONCLUSIONS

Inhibition of angiotensin converting enzyme with captopril had a significant effect on blood pressure that was not directly accounted for by a suppression of plasma Ang II levels. Continuous therapy with captopril unmasked a contribution of Ang-(1-7) and prostacyclin to the antihypertensive actions of this drug.

Angiotensin-(1-7) dilates canine coronary arteries through kinins and nitric oxide

Angiotensin-(1-7) [Ang-(1-7)] was recently recognized to have novel biological functions that are distinct from those of Ang II. In these studies, we determined the vasoactive effects of Ang-(1-7) together with the endothelium-dependent mediator(s) of these responses in canine coronary arteries. Isometric tension was measured in intact canine coronary artery rings suspended in organ chambers perfused with 95% O2/5% CO2 at 37 degrees C. Ang-(1-7) caused significant concentration-dependent vascular relaxation (2.73 +/- 0.58 micromol/L, EC50) of rings precontracted with the thromboxane A2 analogue U46,619. Pretreatment with the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (1 mol/L) abolished the vasodilator response to Ang-(1-7), whereas treatment with the cyclooxygenase inhibitor indomethacin (10 micromol/L) was without effect. The vasodilator response produced by Ang-(1-7) was blocked by 75% with the bradykinin B2 receptor antagonist Hoe 140 (1 micromol/L) or by 80% with the nonselective Ang II antagonist [Sar1,Thr8]-Ang II (1 micromol/L). In contrast, the selective AT1 or AT2 Ang II antagonists CV 11974 (1 micromol/L), and PD 123319 (1 micromol/L), respectively, were ineffective in inhibiting the Ang-(1-7)-elicited vasodilation. Furthermore, pretreatment of the coronary rings with 2 micromol/L Ang-(1-7) markedly potentiated the bradykinin response. These results suggest that Ang-(1-7) elicits coronary vasodilation that is specifically mediated by the endothelium-dependent release of nitric oxide. These responses involve a B2 bradykinin receptor and a non-AT1, non-AT2, angiotensin receptor. These data suggest that increases in circulating levels of Ang-(1-7) accompanying long-term administration of converting enzyme inhibitors or Ang II receptor blockers may contribute to the cardioprotective actions of these drugs.

Angiotensin II pressor activity depends on medial and lateral anterior hypothalamic pathways

The preoptic region of hypothalamus was disconnected from caudal structures with two different-size knife cuts in rats to investigate the pathway responsible for the effects of intracerebroventricular (ICV) and intravenous (IV) angiotensin II (ang II) on blood pressure and arginine vasopressin (AVP) release. Seven days after surgery ICV ang II (125 ng) in sham-operated (sham) rats increased mean arterial pressure (MAP) (+23 +/- 3 mmHg) and decreased heart rate (HR) (-58 +/- 5 beats/minute). However, ICV ang II had no effect on MAP or HR of rats with a large (preoptic-hypothalamic disconnection) cut. Both the pressor response (+12 +/- 2 mmHg) and the bradycardia (-39 +/- 6 beats/minute) were significantly reduced by a small (medial preoptic-hypothalamic disconnection) cut. The increased plasma AVP to ICV ang II in sham rats (9.8 +/- 3.6 pg/mL) was abolished in large-cut rats and attenuated in small-cut rats (3.2 +/- 0.7 pg/mL). IV bolus injection of ang II (125 ng) in sham rats increased MAP by 43 mmHg, whereas large-cut rats showed a blunted (25%) pressor response. The pressor response to IV infusion of ang II (8 ng/20 microL/minute for 15 minutes) was diminished in large-cut rats (+4 +/- 1 mmHg) as compared with that in sham rats (+19 +/- 2 mmHg). Both cuts transected the projection between the periventricular tissue surrounding the anteroventral third ventricle and supraoptic nucleus, but the supraoptic-neurohypophyseal pathway was severed only by the large cut.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential regulation of central vasopressin in transgenic rats harboring the mouse Ren-2 gene

The transgenic (TG) rat carrying the mouse renin gene (mRen-2d) has provided a unique opportunity to explore central interactions between the brain renin-angiotensin (RAS) and vasopressin (AVP) systems. To evaluate the hypothalamic vasopressin axis in the TG rat, we measured the central nervous system concentrations of AVP and determined the effect of angiotensin II (ANG II) and its NH2-terminal heptapeptide [angiotensin-(1-7)] on blood pressure, heart rate, and AVP release using brain microdialysis. Intracerebroventricular infusion of ANG II or ANG-(1-7) in control rats increased local AVP release from the paraventricular and supraoptic nuclei. The ANG II infusion was associated with a significant increase in blood pressure not observed with ANG-(1-7). In contrast, the angiotensin peptide-induced central AVP responses and the ANG II-induced blood pressure increase were absent in the TG animal. The plasma AVP responses to ANG II and ANG-(1-7) were comparable in the control and TG rats. The TG rats exhibited a 22-fold higher level of AVP in the dorsal lower brain stem but had lower AVP levels in the posterior pituitary and the median eminence compared with control rats. These results suggest that insertion of the mouse renin gene into the rat genome leads to alterations in the AVP axis in terms of AVP peptide content and angiotensin-induced cardiovascular and AVP responses.

Estrogen augments the contribution of nitric oxide to blood pressure regulation in transgenic hypertensive rats expressing the mouse Ren-2 gene

Transgenic rats carrying the mouse Ren-2 gene (Ren-2d)27 provide a unique model to study the interplay between the renin-angiotensin system and estrogen in the pathogenesis of hypertension. In this study we measured the effects of ovariectomy and estrogen replacement on blood pressure and the contribution of vascular endothelium relaxing factor, nitric oxide, in female transgenic hypertensive rats and normotensive Sprague-Dawley (SD) rats. Both groups of animals were either ovariectomized or sham-operated at 12 weeks of age. Ovariectomized rats were treated with either 17 beta-estradiol (70 micrograms/day) or placebo for 4 weeks, whereas sham-operated rats received placebo alone. Mean arterial blood pressure measured in conscious rats directly by an arterial catheter was significantly higher in ovariectomized rats, compared with ovariectomized rats given estrogen replacement therapy for both transgenic (167 +/- 5 v 154 +/- 4 mm Hg, P < .05) and SD rats (125 +/- 4 v 113 +/- 5 mm Hg, P < .05). The contribution of endothelium-derived nitric oxide to the maintenance of blood pressure was examined by acute systemic injection of NG-monomethyl-L-arginine (L-NMMA, 10 mg/kg). L-NMMA caused a significantly greater increase in blood pressure in sham-operated transgenic as compared to SD rats (34 +/- 3 v 14 +/- 3 mm Hg, P < .05). The response in ovariectomized transgenic rats was markedly reduced (13 +/- 3 mm Hg), reaching levels that were no different from sham-operated SD rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of hypertension in transgenic rats expressing the mouse Ren-2 gene

Transgenic (TG) rats carrying the mouse Ren-2 gene (Ren-2d)27 are a newly established monogenetic model in hypertension research. To gain an insight into the mechanisms of this form of hypertension we determined the effects of a 13-day therapy with losartan (10 mg/kg) or lisinopril (20 mg/kg) on the blood pressure (BP) and plasma levels of angiotensin (ANG) peptides of mature female TG hypertensive and Sprague-Dawley (SD) rats. The contribution of endothelium-derived nitric oxide (NO) to the maintenance of their hypertension and the response to therapy was evaluated by systemic injection of either NG-monomethyl-L-arginine (L-NMMA) or endothelin-1. Hypertension in TG rats was associated with decreased plasma ANG I, no differences in plasma ANG II, and plasma ANG-(1-7) near the detectable level. Lisinopril lowered BP more than losartan in both TG hypertensive and normotensive controls. In both strains, the chronic fall in BP produced by lisinopril was accompanied by significant increases in plasma ANG I and ANG-(1-7), while losartan augmented plasma ANG I and ANG II in both strains and plasma ANG-(1-7) in TG rats. Inhibition of NO synthase reversed the fall in BP produced by either lisinopril or losartan in SD controls. In contrast, administration of L-NMMA to TG rats given the same therapy did not. The transient endothelium-mediated relaxing phase of the depressor response to systemic injections of endothelin-1 was attenuated by losartan and lisinopril in TG rats. These studies indicate that hypertension in female TG rats is mediated by the RAS.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased expression of angiotensin peptides in the brain of transgenic hypertensive rats

We determined the levels of angiotensin I (ANG I), angiotensin II (ANG II), and the heptapeptide angiotensin(1-7) [ANG(1-7)] in the blood and brain of female Hannover Sprague-Dawley (SD) and transgenic hypertensive rats [mRen-2]27 by radioimmunoassay and high performance liquid chromatography. Hypertension was accompanied by higher plasma concentrations of ANG II, no statistical changes in ANG(1-7), and no differences in plasma ANG I levels. In the hypothalamus of transgenic rats, concentrations of ANG II and ANG(1-7) averaged 827% and 168% above values in SD rats (p < 0.005) whereas both ANG I and ANG II increased in the medulla oblongata. The data showed that the established phase of hypertension in rats harboring the mouse Ren-2 gene is associated with overexpression of the renin-angiotensin system in brain regions participating in the endocrine regulation of blood pressure.

Angiotensin(1-7) in the spontaneously hypertensive rat

We profiled the concentrations of angiotensin I (Ang I), angiotensin II (Ang II), and angiotensin(1-7) [Ang(1-7)] by the combination of radioimmunoassay and high performance liquid chromatography in the blood of 14-week-old male Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) drinking either tap water or a solution containing ceranapril (30 mg/kg) or lisinopril (20 mg/kg) for 14 days. Differences in the chemical and pharmacokinetic properties of the two converting enzyme inhibitors ruled out class-related effects. Plasma renin activity, angiotensin converting enzyme (ACE) activity, and plasma levels of Ang I and Ang II were the same in vehicle-treated WKY and SHR. In contrast, plasma levels of both Ang(1-7) and vasopressin in SHR were 3.7-fold and 2.6-fold higher, respectively (p < 0.05). Angiotensin converting enzyme inhibition reduced the blood pressure of WKY and SHR, and augmented their intake of water and output of urine. These changes were associated with increases in renin activity and plasma levels of Ang I and Ang(1-7). In both WKY and SHR, lisinopril had a greater effect in inhibiting plasma and cerebrospinal fluid ACE, reducing levels of plasma angiotensinogen, and increasing the concentrations of authentic Ang II. The principal finding of this study is that plasma Ang(1-7) is the sole component of the circulating angiotensin system that is elevated in the established phase of genetic hypertension. The finding that chronic inhibition of ACE augments circulating levels of Ang(1-7) evidenced the existence of functional pathways for the alternate processing of Ang I.

Effects of large and small transections of the preoptic-hypothalamic region on hydromineral regulation in rats

To further elucidate the role of the preoptic-hypothalamic region in fluid and electrolyte balance we studied the effect of surgical preoptic-hypothalamic disconnection using either a large (preoptic-hypothalamic disconnection) or a small (medial preoptic-hypothalamic disconnection) microknife. Both the large and small cuts seemed to transect the posterior projection originating in the periventricular tissue surrounding the anteroventral third ventricle (AV3V) and extending to supraoptic nucleus, but the supraoptic-neurohypophysial pathway was severed only by the large cut. Seven-day metabolic studies showed a disruption in hydromineral balance only in large cut rats; they had increased water intake and urine volume on day 1, a near-recovery of function on days 2 and 3, and polydipsia and polyuria on days 4 to 7. There was no difference between small cut rats and sham-operated rats in metabolic measurements. The large cut rats also had sustained hypernatremia and hyperosmolality, which was enhanced after water restriction for 48 h but was not accompanied by an increase in plasma arginine vasopressin. Our data therefore suggest that the efferent fibers running caudally from the AV3V are not involved in mediation of the hydromineral regulation of the AV3V.

A comparison of the properties and enzymatic activities of three angiotensin processing enzymes: angiotensin converting enzyme, prolyl endopeptidase and neutral endopeptidase 24.11

The discovery of angiotensin-(1-7) [Ang-(1-7)] as a bioactive Ang II fragment of the renin-angiotensin system (RAS) alters the current understanding of the enzymatic components that comprise the RAS cascade. Two neutral endopeptidases, prolyl endopeptidase (E.C. 3.4.21.26) and neutral endopeptidase 24.11 (E.C. 3.4.24.11), are capable of forming Ang-(1-7) from Ang I and have been implicated in the in vivo processing of Ang I. This makes them putative Ang processing enzymes and part of the RAS cascade. This review summarizes the physical characteristics and distribution of angiotensin converting enzyme (E.C. 3.4.15.1), a known Ang I processing enzyme, and compares its features to what is known of prolyl endopeptidase and neutral endopeptidase 24.11.

Circulating neuropeptide Y in dog plasma consists of multiple peptide fragments

Neuropeptide Y-like immunoreactivity (NPY-LI) in dog plasma was characterized and quantified using three extraction methods (Sep-Pak:acetonitrile, HCl:ethanol, and ethanol). Sep-Pak extraction yields the best recovery and preserves the integrity of the peptide. Oxidized NPY is not generated during blood collection. Using two antisera of different specificities, at least three peptide forms in normal dog arterial and venous plasma were detected. A peptide with retention times similar to oxidized NPY or peptide YY is the major component of plasma NPY-LI under basal conditions, but NPY(1-36) predominates during sympathetic stimulation. The mature peptide in dog plasma is similar to human NPY. The antiserum ABII provides a more accurate measure of circulating NPY(1-36) and its oxidized form. The antiserum ABI is useful for detecting NPY-like fragments.

Inhibition of angiotensin converting enzyme by the metalloendopeptidase 3.4.24.15 inhibitor c-phenylpropyl-alanyl-alanyl-phenylalanyl-p-aminobenzoate

Inhibitors of metallopeptidases may represent new alternatives in the treatment of cardiovascular disease. Recent investigations have linked the hypotensive properties of the metalloendopeptidase 3.4.24.15 (MEP 24.15) inhibitor c-phenylpropyl-alanyl-alanyl-phenylalanyl-para-aminobenzoate (cFP-A-A-F-pAB) to the attenuation of bradykinin metabolism. However, since angiotensin converting enzyme (ACE) is widely recognized to contribute to the metabolic clearance of bradykinin, we characterized the specificity of cFP-A-A-F-pAB towards ACE. We also determined whether cFP-A-A-F-pAB inhibits the conversion of angiotensin I (Ang I) to Ang II by pulmonary ACE. The ACE activity toward the synthetic substrate hippuryl-histidine-leucine (Hip-His-Leu) was measured in vitro using both a purified lung preparation and pooled rat serum. The ACE activity was inhibited at increasing concentrations of the MEP 24.15 inhibitor. Kinetic analysis revealed that cFP-A-A-F-pAB competitively inhibited pulmonary ACE with a Ki of 0.19 microM. In rat serum, cFP-A-A-F-pAB also competitively inhibited ACE. The hydrolysis of Ang I into Ang II by pulmonary ACE was inhibited to a similar extent by both cFP-A-A-F-pAB and the ACE inhibitor MK 422. These findings are the first to show that the MEP 24.15 inhibitor cFP-A-A-F-pAB also inhibits ACE. We suggest that the reported hypotensive actions of cFP-A-A-F-pAB may be due to the reduction in both bradykinin metabolism and Ang II generation arising from the blockade of ACE.

Tissue renin-angiotensin systems in renal hypertension

Angiotensinogen messenger RNA (mRNA) levels were measured in the brain (hypothalamus, lower brain stem, cerebellum), liver, kidneys, and adrenal glands of rats made hypertensive by ligation of the aorta between the renal arteries. We also measured renin mRNA in the kidneys of these renal hypertensive rats. The early phase of hypertension (day 6) was associated with significant increases in plasma renin activity and levels of circulating angiotensin II. The circulating renin-angiotensin system was not activated in the later phase of hypertension (day 24). Angiotensinogen mRNA levels were elevated in the lower brain stem of hypertensive rats at both stages of hypertension. In contrast, angiotensinogen mRNA levels in the hypothalamus were increased only at day 6 after aortic ligation. Decreased levels of angiotensinogen mRNA were observed in the cerebellum in both the early and later phases of the hypertension. Angiotensinogen mRNA levels in the adrenal gland below the ligature fell in the early phases but rose in the later phases of hypertension. Renin mRNA levels of the ischemic kidney remained elevated at both the early and later phases, whereas in both ischemic and nonischemic kidneys, levels of angiotensinogen mRNA remained below sham values throughout the period of study. These results indicate differential expression of renin-angiotensin system mRNAs in tissues of renal hypertensive rats. The differential changes in the expression of angiotensinogen mRNA over the course of development and maintenance of renal hypertension suggest that factors in addition to angiotensin II are important in modulating the expression of renin-angiotensin system genes.

In vivo metabolism of angiotensin I by neutral endopeptidase (EC 3.4.24.11) in spontaneously hypertensive rats

We investigated the processing enzymes involved in the formation of circulating angiotensin-(1-7) after intravenous administration of angiotensin I to conscious spontaneously hypertensive and Wistar-Kyoto rats. Immunoreactive products, including angiotensin I, angiotensin II, and angiotensin-(1-7), were measured in arterial blood by three specific radioimmunoassays. Angiotensin I infusion (2 nmol) induced a rapid increase in immunoreactive angiotensin II and angiotensin-(1-7). Pretreatment with the angiotensin converting enzyme inhibitor enalaprilat (2 mg/kg) eliminated angiotensin II formation and augmented circulating levels of angiotensin I and angiotensin-(1-7) in spontaneously hypertensive and Wistar-Kyoto rats. The elevated levels of angiotensin-(1-7) in enalaprilat-treated rats were blocked by concurrent treatment with the neutral endopeptidase (EC 3.4.24.11) inhibitor SCH 39,370 (15 mg/kg) in both strains. Administration of SCH 39,370 alone decreased angiotensin-(1-7) levels in spontaneously hypertensive rats, whereas angiotensin II levels increased in both strains (p less than 0.01). Comparisons of the metabolism of angiotensin I in the two rat strains showed increased formation of angiotensin-(1-7) in spontaneously hypertensive rats not given any of the enzyme inhibitors. In addition, levels of angiotensin I were higher after administration of SCH 39,370 in hypertensive rats. These novel findings reveal that neutral endopeptidase EC 3.4.24.11 participates in the conversion of angiotensin I to angiotensin-(1-7) and in the metabolism of angiotensin II in the circulation of both spontaneously hypertensive and Wistar-Kyoto rats. Our results suggest that neutral endopeptidase EC 3.4.24.11 is a major enzymatic constituent of the circulating renin-angiotensin system.