Coronary vasoconstrictor influence of angiotensin II is reduced in remodeled myocardium after myocardial infarction

The renin-angiotensin system plays an important role in cardiovascular homeostasis by contributing to the regulation of blood volume, blood pressure, and vascular tone. Because AT(1) receptors have been described in the coronary microcirculation, we investigated whether ANG II contributes to the regulation of coronary vascular tone and whether its contribution is altered during exercise. Since the renin-angiotensin system is activated after myocardial infarction, resulting in an increase in circulating ANG II, we also investigated whether the contribution of ANG II to the regulation of vasomotor tone is altered after infarction. Twenty-six chronically instrumented swine were studied at rest and while running on a treadmill at 1-4 km/h. In 13 swine, myocardial infarction was induced by ligation of the left circumflex coronary artery. Blockade of AT(1) receptors (irbesartan, 1 mg/kg iv) had no effect on myocardial O(2) consumption but resulted in an increase in coronary venous O(2) tension and saturation both at rest and during exercise, reflecting coronary vasodilation. Despite increased plasma levels of ANG II after infarction and maintained coronary arteriolar AT(1) receptor levels, the vasodilation evoked by irbesartan was significantly reduced both at rest and during exercise. In conclusion, despite elevated plasma levels, the vasoconstrictor influence of ANG II on the coronary circulation in vivo is reduced after myocardial infarction. This reduction in ANG II-induced coronary vasoconstriction may serve to maintain perfusion of the remodeled myocardium.     

Prevention of angiotensin II-induced cardiac remodeling by angiotensin-(1-7)

Cardiac remodeling, which typically results from chronic hypertension or following an acute myocardial infarction, is a major risk factor for the development of heart failure and, ultimately, death. The renin-angiotensin system (RAS) has previously been established to play an important role in the progression of cardiac remodeling, and inhibition of a hyperactive RAS provides protection from cardiac remodeling and subsequent heart failure. Our previous studies have demonstrated that overexpression of angiotensin-converting enzyme 2 (ACE2) prevents cardiac remodeling and hypertrophy during chronic infusion of angiotensin II (ANG II). This, coupled with the knowledge that ACE2 is a key enzyme in the formation of ANG-(1-7), led us to hypothesize that chronic infusion of ANG-(1-7) would prevent cardiac remodeling induced by chronic infusion of ANG II. Infusion of ANG II into adult Sprague-Dawley rats resulted in significantly increased blood pressure, myocyte hypertrophy, and midmyocardial interstitial fibrosis. Coinfusion of ANG-(1-7) resulted in significant attenuations of myocyte hypertrophy and interstitial fibrosis, without significant effects on blood pressure. In a subgroup of animals also administered [d-Ala(7)]-ANG-(1-7) (A779), an antagonist to the reported receptor for ANG-(1-7), there was a tendency to attenuate the antiremodeling effects of ANG-(1-7). Chronic infusion of ANG II, with or without coinfusion of ANG-(1-7), had no effect on ANG II type 1 or type 2 receptor binding in cardiac tissue. Together, these findings indicate an antiremodeling role for ANG-(1-7) in cardiac tissue, which is not mediated through modulation of blood pressure or altered cardiac angiotensin receptor populations and may be at least partially mediated through an ANG-(1-7) receptor.     

Hypoxia and high glucose upregulate AT1 receptor expression and potentiate ANG II-induced proliferation in VSM cells

We examined the effect of hypoxia and high glucose (HG) on ANG II type 1 (AT(1)) receptor expression and proliferation in cultured vascular smooth muscle (VSM) cells. Exposure of quiescent cells to hypoxia in a serum-free DME-Ham's F-12 medium for 6-24 h induced a progressive increase in AT(1) mRNA expression. Exposure of cells to 24 h of hypoxia also resulted in a significant increase in ANG II receptor binding as assessed with (125)I-labeled ANG II. Treatment with ANG II (1 microM) for 24 h under normoxic conditions caused an approximately 1.5-fold increase in both DNA synthesis and cell number, which was enhanced to approximately 3.0-fold under hypoxic conditions. An AT(1) receptor antagonist (losartan, 10 microM) blocked the ANG II-induced increase in DNA synthesis under both normoxic and hypoxic conditions. Incubations in HG medium (25 mM) for 12-24 h under normoxic conditions induced an approximately 2.5-fold increase in AT(1) mRNA levels, which was markedly enhanced by hypoxia to approximately 5.5-fold at 12 h and approximately 8.5-fold at 24 h. ANG II under HG-normoxic conditions caused a complete downregulation of AT(1) expression, which was prevented by hypoxia. These results demonstrate an upregulation of AT(1) receptor expression by hypoxia and HG in cultured VSM cells and suggest a mechanism for enhanced ANG II-induced VSM cell proliferation and the development of atherosclerosis in diabetes.     

ANG II-induced cardiac molecular and cellular events: role of aldosterone

Chronic elevation of circulating ANG II is associated with cardiac remodeling in patients with hypertension and heart failure. The underlying mechanisms, however, are not completely defined. Herein, we studied ANG II-induced molecular and cellular events in the rat heart as well as their links to the redox state. We also addressed the potential contribution of aldosterone (ALDO) on ANG II-induced cardiac remodeling. In ANG II-treated rats, and compared with controls, we found: 1) the expression of proinflammatory/profibrogenic mediators was significantly increased in the perivascular space and at the sites of microscopic injury in both ventricles; 2) macrophages and myofibroblasts were primary repairing cells at these sites, together with increased fibrillar collagen volume; 3) apoptotic macrophages and myofibroblasts were evident at the same sites; 4) NADPH oxidase (gp91phox) was significantly enhanced at these regions and primarily expressed by macrophages, whereas superoxide dismutase and catalase levels remained unchanged; 5) plasma 8-isoprostane levels were significantly increased; and 6) blood pressure was significantly elevated. Losartan treatment completely prevented cardiac oxidative stress as well as molecular/cellular responses and normalized blood pressure. Spironolactone treatment partially suppressed the cardiac inflammatory/fibrogenic responses and redox state. Thus chronic elevation of circulating ANG II is accompanied by a proinflammatory/profibrogenic phenotype involving vascular and myocardial remodeling in both ventricles. Enhanced reactive oxygen species production at these sites and increased plasma 8-isoprostane indicate the involvement of oxidative stress in ANG II-induced cardiac injury. ALDO contributes, in part, to ANG II-induced cardiac molecular and cellular responses.     

Influences of AT1 receptor blockade on tissue metabolism in obese men

ANG II applied to the interstitial space influences carbohydrate and lipid metabolism in a tissue-specific fashion. Thus endogenous ANG II may have a tonic effect on tissue metabolism that could be reversed with ANG II type 1 (AT1) receptor blockade, particularly during adrenergic stimulation. We studied 14 obese men. They were treated for 10 days with the AT1 receptor blocker irbesartan or with placebo in a double-blind and crossover fashion. At the end of each treatment period, we assessed skeletal muscle and adipose tissue metabolism using the microdialysis technique. The ethanol dilution technique was applied to follow changes in tissue blood flow. Measurements were obtained at baseline and during application of incremental isoproterenol concentrations through the microdialysis catheter. Blood pressure decreased from 133 +/- 3/84 +/- 3 to 128 +/- 3/79 +/- 2 mmHg for systolic and diastolic blood, respectively (P = 0.02 and 0.006, respectively) with AT1 receptor blockade. Isoproterenol perfusion caused a dose-dependent increase in dialysate glycerol in adipose tissue and in skeletal muscle. Irbesartan slightly reduced the isoproterenol-induced glycerol response in adipose tissue (P < 0.05 by ANOVA). Ethanol ratio, interstitial glucose supply, and lactate production in adipose tissue and skeletal muscle were similar with placebo and irbesartan. We conclude that AT1 receptor blockade in obese men does not reveal a major tonic ANG II effect on interstitial glucose supply, lipolysis, or glycolysis in skeletal muscle, either at rest or during beta-adrenergic stimulation. Endogeneous ANG II may slightly increase adipose tissue lipolysis. The mechanism may promote the redistribution of triglycerides from adipose tissue toward other organs.     

AT1 receptor participates in the cardiac hypertrophy induced by resistance training in rats

Resistance training is accompanied by cardiac hypertrophy, but the role of the renin-angiotensin system (RAS) in this response is elusive. We evaluated this question in 36 male Wistar rats divided into six groups: control (n=6); trained (n=6); control+losartan (10 mg.kg(-1).day(-1), n=6); trained+losartan (n=6); control+high-salt diet (1%, n=6); and trained+high-salt diet (1%, n=6). High salt was used to inhibit the systemic RAS and losartan to block the AT1 receptor. The exercise protocol consisted of: 4x12 bouts, 5x/wk during 8 wk, with 65-75% of one repetition maximum. Left ventricle weight-to-body weight ratio increased only in trained and trained+high-salt diet groups (8.5% and 10.6%, P<0.05) compared with control. Also, none of the pathological cardiac hypertrophy markers, atrial natriuretic peptide, and alphaMHC (alpha-myosin heavy chain)-to-betaMHC ratio, were changed. ACE activity was analyzed by fluorometric assay (systemic and cardiac) and plasma renin activity (PRA) by RIA and remained unchanged upon resistance training, whereas PRA decreased significantly with the high-salt diet. Interestingly, using Western blot analysis and RT-PRC, no changes were observed in cardiac AT2 receptor levels, whereas the AT1 receptor gene (56%, P<0.05) and protein (31%, P<0.05) expressions were upregulated in the trained group. Also, cardiac ANG II concentration evaluated by ELISA remained unchanged (23.27+/-2.4 vs. 22.01+/-0.8 pg/mg, P>0.05). Administration of a subhypotensive dose of losartan prevented left ventricle hypertrophy in response to the resistance training. Altogether, we provide evidence that resistance training-induced cardiac hypertrophy is accompanied by induction of AT1 receptor expression with no changes in cardiac ANG II, which suggests a local activation of the RAS consistent with the hypertrophic response.     

Coupling of angiotensin II AT1 receptors to neuronal NHE activity and carrier-mediated norepinephrine release in myocardial ischemia

In ischemia, cardiac sympathetic nerve endings (cSNE) release excessive amounts of norepinephrine (NE) via the nonexocytotic Na(+)-dependent NE transporter (NET). NET, normally responsible for NE reuptake into cSNE, reverses in myocardial ischemia, releasing pathological amounts of NE. This carrier-mediated NE release can be triggered by elevated intracellular Na(+) levels in the axoplasm. The fact that ischemia activates the intracellular pH regulatory Na(+)/H(+) exchanger (NHE) in cSNE is pivotal in increasing intraneuronal Na(+) and thus activating carrier-mediated NE release. Angiotensin (ANG) II levels are also significantly elevated in the ischemic heart. However, the effects of ANG II on cSNE, which express the ANG II receptor, AT(1)R, are poorly understood. We hypothesized that ANG II-induced AT(1)R activation in cSNE may be positively coupled to NHE activity and thereby facilitate the pathological release of NE associated with myocardial ischemia. We tested this hypothesis in a cSNE model, human neuroblastoma cells stably transfected with rat recombinant AT(1A) receptor (SH-SY5Y-AT(1A)). SH-SY5Y-AT(1A) constitutively expresses amiloride-sensitive NHE and the NET. NHE activity was assayed in BCECF-loaded SH-SY5Y-AT(1A) as the rate of the Na(+)-dependent alkalinization in response to an acute acidosis. ANG II activation of AT(1)R markedly increased NHE activity in SH-SY5Y-AT(1A) via a Ca(2+)-dependent pathway and promoted carrier-mediated NE release. In addition, in guinea pig cSNE expressing native AT(1)R, ANG II elicited carrier-mediated NE release. In SH-SY5Y-AT(1A) and cSNE, amiloride inhibited the ANG II-mediated release of NE. Our results provide a link between AT(1)R and NHE in cSNE, which can exacerbate carrier-mediated NE release during protracted myocardial ischemia.     

ANG II in the paraventricular nucleus potentiates the cardiac sympathetic afferent reflex in rats with heart failure.

Chronic heart failure (CHF) is characterized by sympathoexcitation, and the cardiac sympathetic afferent reflex (CSAR) is a sympathoexcitatory reflex. Our previous studies have shown that the CSAR was enhanced in CHF. In addition, central angiotensin II (ANG II) is an important modulator of this reflex. This study was performed to determine whether the CSAR evoked by stimulation of cardiac sympathetic afferent nerves (CSAN) in rats with coronary ligation-induced CHF is enhanced by ANG II in the paraventricular nucleus (PVN). Under alpha-chloralose and urethane anesthesia, renal sympathetic nerve activity (RSNA) was recorded. The RSNA responses to electrical stimulation (5, 10, 20, and 30 Hz) of the CSAN were evaluated. Bilateral microinjection of the AT1-receptor antagonist losartan (50 nmol) into the PVN had no significant effects in the sham group, but it abolished the enhanced RSNA response to stimulation in the CHF group. Unilateral microinjection of three doses of ANG II (0.03, 0.3, and 3 nmol) into the PVN resulted in dose-related increases in the RSNA responses to stimulation. Although ANG II also potentiated the RSNA response to electrical stimulation in sham rats, the RSNA responses to stimulation after ANG II into the PVN in rats with CHF were much greater than in sham rats. The effects of ANG II were prevented by pretreatment with losartan into the PVN in CHF rats. These results suggest that the central gain of the CSAR is enhanced in rats with coronary ligation-induced CHF and that ANG II in the PVN augments the CSAR evoked by CSAN, which is mediated by the central angiotensin AT1 receptors in rats with CHF.     

Cardiac modulations of ANG II receptor expression in rats with hypoxic pulmonary hypertension

Right ventricular myocardial hypertrophy during hypoxic pulmonary hypertension is associated with local renin-angiotensin system activation. The expression of angiotensin II type 1 (AT(1)) and type 2 (AT(2)) receptors in this setting has never been investigated. We have therefore examined the chronic hypoxia pattern of AT(1) and AT(2) expression in the right and left cardiac ventricles, using in situ binding and RT-PCR assays. Hypoxia produced right, but not left, ventricular hypertrophy after 7, 14, and 21 days, respectively. Hypoxia for 2 days was associated in each ventricle with a simultaneous and transient increase (P < 0.05) in AT(1) binding and AT(1) mRNA levels in the absence of any significant change in AT(2) expression level. Only after 14 days of hypoxia, AT(2) binding increased (P < 0.05) in the two ventricles, concomitantly with a right ventricular decrease (P < 0.05) in AT(2) mRNA. Along these data, AT(1) and AT(2) binding remained unchanged in both the left and hypertrophied right ventricles from rats treated with monocrotaline for 30 days. These results indicate that chronic hypoxia induces modulations of AT(1) and AT(2) receptors in both cardiac ventricles probably through direct and indirect mechanisms, respectively, which modulations may participate in myogenic (at the level of smooth or striated myocytes) rather than in the growth response of the heart to hypoxia.     

The scar neovasculature after myocardial infarction in rats

A series of novel techniques, adapted from the field of tumor biology, were developed to quantify vascular structure and function and to explore the role of ANG II receptor AT1 in cardiac remodeling after myocardial infarction (MI). We examined the scar neovasculature at 1-4 wk post-MI in Sprague-Dawley rats with a view toward its ability to deliver and exchange oxygen. CD31 and DiOC7(3) staining was used to visualize anatomical vessels vs. those perfused. EF5/Cy3 immunohistochemical staining was used to quantify tissue hypoxia. We compared untreated controls with rats treated with losartan, an AT1 receptor antagonist. Our findings indicated that, at the infarct site, there was not only a 42-75% (1-4 wk post-MI) decrease in the number of anatomical vessels compared with controls but also a decrease in the fraction of perfused vessels from 70% in normal coronary vasculature to 48% at the infarct site. These changes were accompanied by progressive increases in diffusion distance and tissue hypoxia (100% increase in EF5/Cy3 staining at 4 wk post-MI). Losartan-treated rats exhibited a significantly less marked reduction in vascular perfusion and a significantly lesser extent of tissue hypoxia. Over the course of 4 wk post-MI, there is a reduction in coronary vasculature at the infarct site, the extent of which is attenuated by losartan. These findings implicate AT1 receptor upregulation, and perhaps angiotensin-related peptides, as being antiangiogenic.     

Microinjection of ANG II into paraventricular nucleus enhances cardiac sympathetic afferent reflex in rats

The aims of present study were to determine whether angiotensin II (ANG II) in the paraventricular nucleus (PVN) is involved in the central integration of the cardiac sympathetic afferent reflex and whether this effect is mediated by the ANG type 1 (AT(1)) receptor. While the animals were under alpha-chloralose and urethane anesthesia, mean arterial pressure, heart rate, and renal sympathetic nerve activity (RSNA) were recorded in sinoaortic-denervated and cervical-vagotomized rats. A cannula was inserted into the left PVN for microinjection of ANG II. The cardiac sympathetic afferent reflex was tested by electrical stimulation (5, 10, 20, and 30 Hz in 10 V and 1 ms) of the afferent cardiac sympathetic nerves or epicardial application of bradykinin (BK) (0.04 and 0.4 microg in 2 microl). Microinjection of ANG II (0.03, 0.3, and 3 nmol) into the PVN resulted in dose-related increases in the RSNA responses to electrical stimulation. The percent change of RSNA response to 20- and 30-Hz stimulation increased significantly at the highest dose of ANG II (3 nmol). The effects of ANG II were prevented by pretreatment with losartan (50 nmol) into the PVN. Microinjection of ANG II (0.3 nmol) into the PVN significantly enhanced the RSNA responses to epicardial application of BK, which was abolished by pretreatment with losartan (50 nmol) into the PVN. These results suggest that exogenous ANG II in the PVN augments the cardiac sympathetic afferent reflex evoked by both electrical stimulation of cardiac sympathetic afferent nerves and epicardial application of BK. These central effects of ANG II are mediated by AT(1) receptors.     

Increases in brain and cardiac AT1 receptor and ACE densities after myocardial infarct in rats

In the brain, ouabain-like compounds (OLC) and the reninangiotensin system (RAS) contribute to sympathetic hyperactivity in rats after myocardial infarction (MI). This study aimed to evaluate changes in components of the central vs. the peripheral RAS. Angiotensin-converting enzyme (ACE) and angiotensin type 1 (AT1) receptor binding densities were determined by measuring 125I-labeled 351A and 125I-labeled ANG II binding 4 and 8 wk after MI. In the brain, ACE and AT1 receptor binding increased 8-15% in the subfornical organ, 14-22% in the organum vasculosum laminae terminalis, 20-34% in the paraventricular nucleus, and 13-15% in the median preoptic nucleus. In the heart, the greatest increase in ACE and AT1 receptor binding occurred at the infarct scar (approximately 10-fold) and the least in the right ventricle (2-fold). In kidneys, ACE and AT1 receptor binding decreased 10-15%. After intracerebroventricular infusion of Fab fragments to block brain OLC from 0.5 to 4 wk after MI, increases in ACE and AT1 receptors in the subfornical organ, organum vasculosum laminae terminalis, paraventricular nucleus, and medial preoptic nucleus were markedly inhibited, and ACE and AT1 receptor densities in the heart increased less (6-fold in the infarct scar). In kidneys, decreases in ACE and AT1 receptor binding were absent after treatment with Fab fragments. These results demonstrate that ACE and AT1 receptor binding densities increase not only in the heart but also in relevant areas of the brain of rats after MI. Brain OLC appears to play a major role in activation of brain RAS in rats after MI and, to a modest degree, in activation of the cardiac RAS.     

Effect of chronic stress on the cardiac baroreflex in the post-weanling rat

There is increasing evidence that early life stressors may program blood pressure control mechanisms such that the risk for cardiovascular disease in later life is increased. In the present investigation, the effect of repeated restraint/heat stress during the two-week period immediately after weaning on baroreflex function was determined and the contribution of brain angiotensin II (ANG II) to the changes was assessed in young, conscious, freely moving Sprague Dawley rats. In rats two weeks post weaning, basal MAP was significantly higher and basal HR significantly lower than rats tested immediately after weaning. This change in the operating point of HR was not accompanied by any changes in baroreflex function. Treatment with chronic icv infusion of losartan, an AT1 receptor antagonist, during the two-week period prevented the changes in basal MAP and HR. Chronic stress during the two weeks post weaning, whether due to surgical implantation of icv cannulae or due to restraint/heat stress, significantly shifted the set-point of the baroreflex function to a higher pressure. Chronic icv infusion of losartan during the period prevented these effects (at least in the case of stress due to the presence of icv cannulae) suggesting a role for brain ANG II in the change. Changes in the expression of CRH mRNA in the paraventricular nucleus could not explain the stress-related change in baroreflex function. If the rightward shift in the baroreflex persists into adulthood, it could increase the susceptibility to cardiovascular diseases such as hypertension.     

Olmesartan medoxomil treatment potently improves cardiac myosin-induced dilated cardiomyopathy via the modulation of ACE-2 and ANG 1-7 mas receptor

Angiotensin converting enzyme-2 (ACE-2) is a monocarboxypeptidase that metabolises angiotensin (ANG)-II into angiotensin 1-7 (ANG 1-7), thereby functioning as a negative regulator of the renin-angiotensin system. We investigated whether treatment with ANG-II type 1 receptor blocker, olmesartan medoxomil is associated with the attenuation of cardiac myosin-induced dilated cardiomyopathy (DCM) through recently established new axis of ACE-2/ANG 1-7 mas receptor. DCM was elicited in Lewis rats by immunisation with cardiac myosin, and 28 days after immunisation, the surviving Lewis rats were divided into two groups and treated with either olmesartan medoxomil (10 mg/kg/day) or vehicle. Myocardial protein and mRNA levels of ACE-2, ANG 1-7 mas receptor were upregulated in the olmesartan-treated group compared with those of vehicle-treated DCM rats. In contrast, Olmesartan treatment effectively suppressed the myocardial protein and mRNA expressions of inflammatory markers in comparison to the vehicle-treated DCM rats. Olmesartan treatment significantly reduced fibrosis, hypertrophy and their marker molecules (OPN, CTGF, ANP and GATA-4, respectively), as well as matrix metalloproteinases compared with those of vehicle-treated DCM rats. Enhanced myocardial protein levels of phospho-p38 MAPK, phospho-JNK and phospho MAPKAPK-2 in the vehicle-treated DCM rats were prevented by olmesartan treatment. In addition, olmesartan treatment significantly lowered the protein expressions (Nitrotyrosine, p47phox and p67phox) and superoxide radical production compared with those of vehicle-treated DCM rats. Our present study might serve as a new therapeutic target of DCM in cardiovascular diseases and cardiac myosin-induced DCM via the modulation of ACE-2/ANG 1-7 mas receptor axis in rats with DCM after myosin-immunisation.     

Effect of AT(1) receptor blockade on cardiac apoptosis in angiotensin II-induced hypertension

Angiotensin II (ANG II) via AT(1) receptors induces apoptosis in cardiomyocytes in vitro. We tested the hypothesis that in vivo AT(1) receptor stimulation is accompanied by cardiac apoptosis and attempted to elucidate the molecular mechanisms involved in the death signaling pathway. Male Sprague-Dawley rats received ANG II (120 ng x kg(-1) x min(-1) sc) for 7 days with or without the AT(1) receptor antagonist losartan (10 mg x kg(-1) x day(-1) orally). Cardiac function was assessed by echocardiography. Apoptosis in the heart was detected and quantified by in situ TdT-mediated dUTP nick-end labeling (TUNEL) and radiolabeled DNA laddering. Expression of bax, bcl-2, caspase 3, and AT(1) and AT(2) receptors was examined by Western blot analysis. Activity of caspase 3 was also measured by a fluorometric immunosorbent enzyme assay. Tail cuff systolic blood pressure was elevated (P < 0.01, n = 6) in ANG II-infused rats (173 +/- 3 mmHg) versus controls (111 +/- 2 mmHg) and reduced by losartan (134 +/- 4 mmHg). Cardiac function was essentially unchanged in ANG II-infused rats. Increased internucleosomal DNA cleavage by TUNEL assay and radiolabeled DNA laddering showed results compatible with enhanced cardiomyocyte apoptosis in the hearts of ANG-II infused rats. The bax-to-bcl-2 ratio, expression of the active form of caspase 3 (17 kDa), and activity of caspase 3 in the hearts of the ANG II group increased more than twofold above controls. Protein expression of AT(1) and AT(2) receptors was significantly increased in ANG II-infused rats compared with control rats. Losartan-treated ANG II-infused rats exhibited normalized apoptosis, bax, caspase 3 activity, and AT(1) receptors. ANG II stimulation of AT(1) receptors in the heart in vivo is associated with an increased rate of apoptosis without major hemodynamic consequences. Bax and caspase 3 are involved in the apoptotic signaling pathway in this experimental paradigm.     

Predominant activation of endothelin-dependent cardiac hypertrophy by norepinephrine in rat left ventricle

Locally released endothelin (ET)-1 has been recently identified as an important mediator of cardiac hypertrophy. It is still unclear, however, which primary stimulus specifically activates ET-dependent signaling pathways. We therefore examined in adult rats (n = 51) the effects of a selective ET(A) receptor antagonist in experimental models of cardiac hypertrophy, in which myocardial growth is predominantly initiated by a single primary stimulus. Rats were exposed to mechanical overload (ascending aortic stenosis), increased levels of circulating ANG II (ANG II infusion combined with hydralazine), or adrenergic stimulation (infusion of norepinephrine in a subpressor dose) for 7 days. All experimental treatments significantly increased left ventricular weight/body weight ratios compared with untreated rats, whereas systolic left ventricular peak pressure was increased only after ascending aortic stenosis. ET(A) receptor blockade exclusively reduced norepinephrine-induced cardiac hypertrophy and atrial natriuretic peptide gene expression. Blood pressure levels and heart rates remained unaffected during ET(A) receptor blockade in all experimental groups. These data indicate that in rat left ventricle, the ET-dependent signaling pathway leading to early development of cardiac hypertrophy and fetal gene expression is primarily activated by norepinephrine.     

Angiotensin II attenuates myocardial interstitial acetylcholine release in response to vagal stimulation

Although ANG II exerts a variety of effects on the cardiovascular system, its effects on the peripheral parasympathetic neurotransmission have only been evaluated by changes in heart rate (an effect on the sinus node). To elucidate the effect of ANG II on the parasympathetic neurotransmission in the left ventricle, we measured myocardial interstitial ACh release in response to vagal stimulation (1 ms, 10 V, 20 Hz) using cardiac microdialysis in anesthetized cats. In a control group (n = 6), vagal stimulation increased the ACh level from 0.85 +/- 0.03 to 10.7 +/- 1.0 (SE) nM. Intravenous administration of ANG II at 10 microg x kg(-1) x h(-1) suppressed the stimulation-induced ACh release to 7.5 +/- 0.6 nM (P < 0.01). In a group with pretreatment of intravenous ANG II receptor subtype 1 (AT(1) receptor) blocker losartan (10 mg/kg, n = 6), ANG II was unable to inhibit the stimulation-induced ACh release (8.6 +/- 1.5 vs. 8.4 +/- 1.7 nM). In contrast, in a group with local administration of losartan (10 mM, n = 6) through the dialysis probe, ANG II inhibited the stimulation-induced ACh release (8.0 +/- 0.8 vs. 5.8 +/- 1.0 nM, P < 0.05). In conclusion, intravenous ANG II significantly inhibited the parasympathetic neurotransmission through AT(1) receptors. The failure of local losartan administration to nullify the inhibitory effect of ANG II on the stimulation-induced ACh release indicates that the site of this inhibitory action is likely at parasympathetic ganglia rather than at postganglionic vagal nerve terminals.     

缺氧对右心室最大心肌心血流量的影响

为了探讨氧对冠状血管贮备方法的影响,我们观察了缺氧对血流动力学及右心室最大心肌血汉量的变化。结果表明,急性缺氧引起的ＰａＯ２、心输出量及氧运送量降低,但右心室心肌血流量增加,右心室最大与安静血流量比值降低,生缺氧时ＰａＯ２降低,血球比积和右心室生理指数增加,氧运送量和右心室血流量正常,但最大血流量降低,小动脉增厚、外胶元增加,以上结果提示,慢性缺氧对冠状血管贮备减少可能是小动脉壁增厚、外胶元增加和     

缺氧对右心室最大心肌血流量的影响

为了探讨缺氧对冠状血管贮备力的影响,我们观察了缺氧时大鼠血流动力学及右心室最大心肌血流量的变化。结果表明,急性缺氧引起ＰａＯ２、心输出量及氧运送量降低,但右心室心肌血流量增加,右心室最大与安静血流量比值降低。慢性缺氧时ＰａＯ２降低,血球比积和右心室重量指数增加,氧运送量和右心室血流量正常,但最大血流量降低,小动脉增厚、外膜胶元增加。以上结果提示,慢性缺氧对冠状血管贮备减少可能是小动脉壁增厚、外膜胶元增加和血液粘滞性增加及右心室肥大的结果。     

Effects of a Novel Bradykinin B1 Receptor Antagonist and Angiotensin II Receptor Blockade on Experimental Myocardial Infarction in Rats

Background

The aim of the present study was to evaluate the cardiovascular effects of the novel bradykinin B1 receptor antagonist BI-113823 following myocardial infarction (MI) and to determine whether B1 receptor blockade alters the cardiovascular effects of an angiotensin II type 1 (AT1) receptor antagonist after MI in rats.

Methodology/Principal Findings

Sprague Dawley rats were subjected to permanent occlusion of the left descending coronary artery. Cardiovascular function was determined at 7 days post MI. Treatment with either B1 receptor antagonist (BI-113823) or AT1 receptor antagonist (irbesartan) alone or in combination improved post-MI cardiac function as evidenced by attenuation of elevated left ventricular end diastolic pressure (LVEDP); greater first derivative of left ventricular pressure (± dp/dt max), left ventricle ejection fraction, fractional shorting, and better wall motion; as we as reductions in post-MI up-regulation of matrix metalloproteinases 2 (MMP-2) and collagen III. In addition, the cardiac up-regulation of B1 receptor and AT1 receptor mRNA were markedly reduced in animals treated with BI 113823, although bradykinin B2 receptor and angiotensin 1 converting enzyme (ACE1) mRNA expression were not significantly affected by B1 receptor blockade.

Conclusions/Significance

The present study demonstrates that treatment with the novel B1 receptor antagonist, BI-113823 improves post-MI cardiac function and does not influence the cardiovascular effects of AT1 receptor antagonist following MI.