Prolyl oligopeptidase is involved in release of the antifibrotic peptide Ac-SDKP

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a ubiquitous tetrapeptide hydrolyzed almost exclusively by angiotensin-converting enzyme (ACE). Chronic treatment with Ac-SDKP decreases cardiac and renal fibrosis and inflammatory cell infiltration in hypertensive rats. However, very little is known about endogenous synthesis of Ac-SDKP, except that thymosin-beta4 may be the most likely precursor. Two enzymes are potentially able to release Ac-SDKP from thymosin-beta4: prolyl oligopeptidase (POP) and endoproteinase asp-N. POP is widely present and active in several tissues and biological fluids, whereas endoproteinase asp-N appears to be lacking in mammals. Therefore, we hypothesized that POP is the main enzyme involved in synthesizing the antifibrotic peptide Ac-SDKP. We investigated in vitro and in vivo production of Ac-SDKP. Using kidney cortex homogenates, we observed that Ac-SDKP was generated in a time-dependent manner in the presence of exogenous thymosin-beta4, and this generation was significantly inhibited by several POP inhibitors (POPi), Z-prolyl-prolinal, Fmoc-prolyl-pyrrolidine-2-nitrile, and S17092. Long-term administration of S17092 in rats significantly decreased endogenous levels of Ac-SDKP in the plasma (from 1.76+/-0.2 to 1.01+/-0.1 nM), heart (from 2.31+/-0.21 to 0.83+/-0.09 pmol/mg protein), and kidneys (from 5.62+/-0.34 to 2.86+/-0.76 pmol/mg protein). As expected, ACE inhibitors significantly increased endogenous levels of Ac-SDKP in the plasma, heart, and kidney, whereas coadministration of POPi prevented this increase. We concluded that POP is the main enzyme responsible for synthesis of the antifibrotic peptide Ac-SDKP.

Antifibrotic effect of Ac-SDKP and angiotensin-converting enzyme inhibition in hypertension

OBJECTIVE

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a potent natural inhibitor of hematopoietic stem cell proliferation which is degraded mainly by angiotensin-converting enzyme (ACE). In vitro, Ac-SDKP inhibits collagen production by cardiac fibroblasts; while in vivo it blocks collagen deposition in the left ventricle (LV) of rats with hypertension or myocardial infarction (MI). In addition, it reportedly prevents and reverses macrophage infiltration in the LV of rats with MI. We tested the hypothesis that when Ac-SDKP is infused at doses that cause plasma concentrations similar to those observed after ACE inhibition, it mimics the anti-inflammatory and antifibrotic effects of ACE inhibitors (ACEi) in the heart, and, further, that these effects are independent of changes in blood pressure.

DESIGN AND METHODS

Rats were divided into five groups: (1) controls, (2) Ang II (750 microg/kg per day, s.c.), (3) Ang II + captopril (100 mg/kg per day in drinking water), (4) Ang II + Ac-SDKP (400 microg/kg per day, s.c.), and (5) Ang II + Ac-SDKP (800 microg/kg per day, s.c.). We measured LV cell proliferation, inflammatory cell infiltration, cytokine expression, hypertrophy and fibrosis.

RESULTS

Plasma Ac-SDKP was five-fold higher in rats given ACEi and four- and ten-fold higher in rats given 400 and 800 microg/kg per day Ac-SDKP, respectively. ACEi significantly decreased Ang II-induced cell proliferation (Ki-67), LV macrophage/mast cell infiltration, transforming growth factor-beta, connective tissue growth factor and collagen deposition without affecting hypertension, LV hypertrophy or myocyte cross-sectional area, and these effects were mimicked by exogenous Ac-SDKP (400 microg/kg per day) which raised plasma Ac-SDKP to levels similar to ACEi. BP was not decreased by either ACEi or Ac-SDKP.

CONCLUSIONS

We concluded that Ac-SDKP may be an important mediator of the anti-inflammatory and antifibrotic effects of ACEi in hypertension independent of its hemodynamic effects.

Role of a selective aldosterone blocker in mice with chronic heart failure

BACKGROUND

Spironolactone, a nonselective aldosterone blocker, has a cardioprotective effect; however, significant endocrine side effects limit its use. Eplerenone is a new selective aldosterone blocker. We investigated whether eplerenone attenuates cardiac remodeling and improves function in a mouse model of heart failure and whether coadministration of eplerenone and an angiotensin-converting enzyme inhibitor (ACEi) provides better cardioprotection than either agent alone.

METHODS AND RESULTS

C57BL/6J mice were subjected to myocardial infarction (MI) by ligating the left anterior descending coronary artery. Two weeks later, the mice were either left untreated or treated with (1) eplerenone, (2) ACEi, or (3) eplerenone plus ACEi for 12 weeks. Systolic blood pressure (SBP) was measured and echocardiography performed before MI and weekly thereafter. At the end of the study, interstitial collagen fraction (ICF) and myocyte cross-sectional area (MCSA) were examined histologically. We found that (1) eplerenone significantly improved ejection fraction and cardiac output and decreased left ventricular (LV) systolic area, LV weight, ICF, and MCSA independently of changes in SBP compared with untreated animals; (2) ACEi had similar beneficial effects, accompanied by a significant reduction in SBP; and (3) combined treatment offered limited additional benefit beyond monotherapy.

CONCLUSIONS

In mice with MI, eplerenone attenuates progression of heart failure comparably to ACEi, and its effect is independent of BP lowering.

Ac-SDKP reverses inflammation and fibrosis in rats with heart failure after myocardial infarction

Inflammation may play an important role in the pathogenesis of cardiac fibrosis in heart failure (HF) after myocardial infarction (MI). N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a naturally occurring antifibrotic peptide whose plasma concentration is increased 4- to 5-fold by angiotensin-converting enzyme inhibitors. We tested the hypothesis that in rats with HF after MI, Ac-SDKP acts as an anti-inflammatory cytokine, preventing and also reversing cardiac fibrosis in the noninfarcted area (reactive fibrosis), and thus affording functional improvement. We found that Ac-SDKP significantly decreased total collagen content in the prevention group from 23.7+/-0.9 to 15.0+/-0.7 microg/mg and in the reversal group from 22.6+/-2.2 to 14.4+/-1.6 (P<0.01). Interstitial collagen volume fraction and perivascular collagen were likewise significantly reduced. We also found that infiltrating macrophages were reduced from 264.7+/-8.1 to 170.2+/-9.2/mm2, P<0.001 (prevention), and from 257.5+/-9.1 to 153.1+/-8.5 mm2, P<0.001 (reversal), while transforming growth factor (TGF)-beta-positive cells were decreased from 195.6+/-8.4 to 129.6+/-5.7/mm2, P<0.01 (prevention), and from 195.6+/-8.4 to 130.7+/-10.8/mm2, P<0.01 (reversal). Ac-SDKP did not alter either blood pressure or left ventricular hypertrophy (LVH); however, it depressed systolic cardiac function in the prevention study while having no significant effect in the reversal group. We concluded that Ac-SDKP has an anti-inflammatory effect in HF that may contribute to its antifibrotic effect; however, this decrease in fibrosis without changes in LVH was not accompanied by an improvement in cardiac function.

Ac-SDKP reverses cardiac fibrosis in rats with renovascular hypertension

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a natural substrate for the N-terminal active site of angiotensin-converting enzyme (ACE). We previously reported that Ac-SDKP prevented cardiac fibrosis in rats with renovascular or aldosterone-salt hypertension. However, it is not clear whether Ac-SDKP reverses cardiac fibrosis in hypertension, nor the mechanism(s) involved. In the present study, we tested the hypothesis that Ac-SDKP reversal of hypertension-induced cardiac fibrosis involves a decrease in transforming growth factor-beta (TGF-beta) and/or connective tissue growth factor (CTGF). In 2-kidney, 1-clip (2K-1C) hypertensive rats, Ac-SDKP at 400 or 800 microg/kg per day SC was started 8 weeks after hypertension and cardiac fibrosis were established and was continued for 8 weeks. Left ventricular (LV) collagen in rats with 2K-1C plus vehicle at 8 and 16 weeks after clipping was similar but higher than in the sham group (P<0.05). Ac-SDKP at 400 and 800 microg/kg per day, which increased plasma Ac-SDKP 2- and 5-fold, respectively, reversed the increase in LV collagen in a dose-dependent manner. The mechanism by which Ac-SDKP reverses LV fibrosis does not appear to depend on ACE inhibition by Ac-SDKP, since we found that Ac-SDKP at various doses did not affect blood pressure responses to exogenous angiotensin I or bradykinin. However, Ac-SDKP reversed the increase in LV TGF-beta and CTGF compared with rats with 2K-1C plus vehicle (P<0.005). We concluded that in hypertension, Ac-SDKP reverses cardiac fibrosis, perhaps due in part to a decrease in TGF-beta and CTGF in the heart.

Long-term effect of N-acetyl-seryl-aspartyl-lysyl-proline on left ventricular collagen deposition in rats with 2-kidney, 1-clip hypertension

BACKGROUND

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a natural inhibitor of pluripotent hematopoietic stem cell proliferation. Ac-SDKP plasma concentration is increased 5-fold after angiotensin-converting enzyme inhibition. Here we studied the effect of Ac-SDKP on monocyte/macrophage infiltration, fibroblast proliferation, and collagen deposition in the rat heart in renovascular hypertension.

METHODS AND RESULTS

We investigated whether long-term Ac-SDKP administration would prevent left ventricular (LV) hypertrophy and interstitial collagen deposition in rats with 2-kidney, 1-clip (2K-1C) hypertension. Ac-SDKP (400 microgram. kg(-1). d(-1)) did not affect development of hypertension. Mean blood pressure was similar in rats with 2K-1C hypertension whether they were given vehicle or Ac-SDKP and was higher than in controls. Both LV weight and cardiomyocyte size were significantly increased in rats with 2K-1C hypertension compared with controls and were unaffected by Ac-SDKP. Proliferating cell nuclear antigen- and monocyte/macrophage-positive cells were increased in the LV of 2K-1C hypertensive rats; this increase was significantly blunted by Ac-SDKP (P<0.001). LV interstitial collagen fraction was also increased in 2K-1C hypertensive rats given vehicle (10.1+/-0.8%) compared with sham (5.3+/-0.1%, P<0.0001), and this increase was prevented by Ac-SDKP (5.4+/-0.4%, P<0.001).

CONCLUSIONS

Ac-SDKP inhibited monocyte/macrophage infiltration, cell proliferation, and collagen deposition in the LV of hypertensive rats without affecting blood pressure or cardiac hypertrophy, suggesting that it may be partly responsible for the cardioprotective effect of angiotensin-converting enzyme inhibitors.

Effects of angiotensin-converting enzyme inhibitor and angiotensin type 1 receptor antagonist in deoxycorticosterone acetate-salt hypertensive mice lacking Ren-2 gene

We previously reported that inhibition of angiotensin-converting enzyme (ACE) prevented the hypertension and left ventricular hypertrophy induced by deoxycorticosterone acetate-salt (DOCA-salt) in 129/SvEvTac mice, which have 2 renin genes (Ren-1 and Ren-2). In the present study, we induced hypertension by uninephrectomy and DOCA-salt in mice having only the Ren-1 gene (C57BL/6J) and investigated the effect of an ACE inhibitor (ramipril, 4 mg. kg(-)(1). d(-)(1)) and an angiotensin type 1 (AT(1)) receptor antagonist (L-158809, 4 mg. kg(-)(1). d(-)(1)) on the development of hypertension, cardiac hypertrophy, and renal injury. After 4 weeks of treatment, systolic blood pressure in DOCA-salt mice was significantly increased (128+/-2 mm Hg) compared with controls (109+/-2 mm Hg) (P:<0.001), while plasma renin concentration was decreased by 97% (P:<0.001). DOCA-salt also induced left ventricular and renal hypertrophy and renal damage as manifested by proteinuria. Collagen content in the left ventricle and kidney was significantly higher in DOCA-salt mice (P:<0.001). Urinary albumin (P:<0.05) and proliferating cell nucleic antigen-positive cells in the tubules and interstitium of the renal cortex (P:<0.001) were significantly increased in the DOCA-salt group. Neither the ACE inhibitor nor the AT(1) antagonist had any antihypertensive effect; however, they partially prevented cardiac hypertrophy and completely inhibited left ventricular collagen deposition. In the kidney, both the ACE inhibitor and AT(1) antagonist partially reduced the increase in collagen but had no effect on hypertrophy. They also significantly prevented the effect of DOCA-salt on urinary albumin and proliferating cell nucleic antigen expression in the kidney. Despite the lack of an antihypertensive effect, both ACE inhibitor and AT(1) antagonist prevented cardiac remodeling and renal damage. Our results indicate that ACE inhibitors and AT(1) antagonists exert beneficial effects on the heart and kidney in DOCA-salt hypertensive mice independently of their effects on blood pressure.

Effect of N-acetyl-seryl-aspartyl-lysyl-proline on DNA and collagen synthesis in rat cardiac fibroblasts

N:-Acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a natural inhibitor of pluripotent hematopoietic stem cell entry into the S phase of the cell cycle and is normally present in human plasma. Ac-SDKP is exclusively hydrolyzed by ACE, and its plasma concentration is increased 5-fold after ACE inhibition in humans. We examined the effect of 0.05 to 100 nmol/L Ac-SDKP on 24-hour (3)H-thymidine incorporation (DNA synthesis) by cardiac fibroblasts both in the absence and presence of 5% FCS. Captopril (1 micromol/L) was added in all cases to prevent the degradation of Ac-SDKP. Treatment of cardiac fibroblasts with 5% FCS increased thymidine incorporation from a control value of 12 469+/-594 to 24 598+/-1051 cpm (P:<0.001). Cotreatment with 1 nmol/L Ac-SDKP reduced stimulation to control levels (10 373+/-200 cpm, P:<0.001). We measured hydroxyproline content and incorporation of (3)H-proline into collagenous fibroblast proteins and found that Ac-SDKP blocked endothelin-1 (10(-8) mol/L)-induced collagen synthesis in a biphasic and dose-dependent manner, causing inhibition at low doses, whereas high doses had little or no effect. It also blunted the activity of p44/p42 mitogen-activated protein kinase in a biphasic and dose-dependent manner in serum-stimulated fibroblasts, suggesting that the inhibitory effect of DNA and collagen synthesis may depend in part on blocking mitogen-activated protein kinase activity. Participation of p44/p42 in collagen synthesis was confirmed, because a specific inhibitor for p44/p42 activation (PD 98059, 25 micromol/L) was able to block endothelin-1-induced collagen synthesis, similar to the effect of Ac-SDKP. The fact that Ac-SDKP inhibits DNA and collagen synthesis in cardiac fibroblasts suggests that it may be an important endogenous regulator of fibroblast proliferation and collagen synthesis in the heart. Ac-SDKP may participate in the cardioprotective effect of ACE inhibitors by limiting fibroblast proliferation (and hence collagen production), and therefore it would reduce fibrosis in patients with hypertension.

Antifibrotic effects of N-acetyl-seryl-aspartyl-Lysyl-proline on the heart and kidney in aldosterone-salt hypertensive rats

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) inhibits not only hematopoietic cell proliferation but also fibroblast proliferation and collagen synthesis in vitro. Ac-SDKP also prevents collagen deposition and cell proliferation in the left ventricle (LV) in rats with renovascular hypertension (renin dependent). However, it is not clear whether Ac-SDKP has similar effects in a model of renin-independent hypertension (aldosterone-salt). Using a hypertensive rat model of cardiac and renal fibrosis created by chronic elevation of circulating aldosterone (ALDO) levels, we examined the effect of Ac-SDKP on blood pressure, cardiac and renal fibrosis and hypertrophy, and proliferating cell nuclear antigen (PCNA) expression in the LV and left kidney. Uninephrectomized rats were divided into 4 groups: (1) controls that received tap water, (2) rats that received ALDO (0.75 microgram/h SC) and 1% NaCl/0.2% KCl in drinking water (ALDO-salt), (3) rats that received ALDO-salt plus Ac-SDKP 400 microgram. kg(-1). day(-1) SC, and (4) rats that received ALDO-salt plus Ac-SDKP 800 microgram. kg(-1). d(-1) SC. After 6 weeks of treatment, the ALDO-salt group was found to have significantly increased blood pressure with decreased body weight and plasma renin concentration (P<0.05), LV and renal hypertrophy as well as renal injury, significantly increased collagen content in both ventricles and kidney as well as increased collagen volume fraction in the LV (P<0.0001), and significantly increased interstitial and perivascular PCNA-positive cells in the LV and kidney (P<0.0001). Ac-SDKP at 800 microgram. kg(-1). d(-1) markedly prevented cardiac and renal fibrosis (P<0.005) without affecting blood pressure or organ hypertrophy. It also suppressed PCNA expression in the LV and kidney in a dose-dependent manner. We concluded that Ac-SDKP prevents increased collagen deposition and cell proliferation in the heart and kidney in ALDO-salt hypertensive rats. Because ACE inhibitors increase plasma and tissue Ac-SDKP and decrease cardiac and renal fibrosis, we speculate that Ac-SDKP may participate in the antifibrotic effect of ACE inhibitors.

Upregulated Expression of Rat Heart Intercellular Adhesion Molecule-1 in Angiotensin II- but Not Phenylephrine- Induced Hypertension

-Intercellular adhesion molecule-1 (ICAM-1), part of an immunoglobulin-like superfamily of adhesion molecules, is involved in several cardiovascular diseases. We investigated whether in vivo angiotensin II (Ang II) increases ICAM-1 in rats. Sprague-Dawley rats were infused with vehicle or Ang II (750 µg. kg(-1). d(-1) SC) for 7 days. The contribution of Ang II receptors to ICAM-1 expression was investigated with a nonpeptide Ang II type 1 (AT(1)) receptor antagonist losartan (30 mg. kg(-1). d(-1) in drinking water). Systolic blood pressure was elevated in Ang II-treated animals compared with sham-treated controls, and losartan blocked this increase. Tumor necrosis factor (TNF)-alpha (5 µg/kg IP bolus), a prototype inducer of ICAM-1, was administered as a positive control for ICAM-1 expression. After treatment, hearts were frozen in liquid nitrogen; homogenates were subjected to SDS-PAGE and immunoblotted with an anti-rat ICAM-1 monoclonal antibody. We detected a predominantly high-molecular-weight band in homogenates from non-TNF-alpha-treated rats, which was enhanced by 80+/-5% in TNF-alpha-treated rats. This band measured approximately 200 kDa, which is the molecular weight of ICAM-1 in its native dimer form. The same band was detected in homogenates from sham and Ang II-treated rats, with the latter showing a 150+/-10% increase in ICAM-1 versus sham controls. Immunoprecipitation of rat heart homogenates with anti-rat ICAM-1 antibody resulted in a dominant band of the same molecular weight as samples not treated with antibody. Losartan prevented enhanced expression of ICAM-1 in the presence of Ang II but had no effect on basal ICAM-1 expression. Phenylephrine, an alpha-agonist (3 mg. kg(-1). d(-1) ), was infused for 1 week but had no effect on ICAM-1 expression, even though systolic blood pressure was elevated to the same level as in rats treated with Ang II. Thus, heart ICAM-1 expression is enhanced via AT(1) receptor activation independent of hypertension. Ang II-induced ICAM-1 expression was time and dose dependent, with maximal expression occurring within 5 to 7 days at 100 to 750 µg/kg Ang II. Immunohistochemical staining demonstrated markedly increased ICAM-1 levels in the perivascular area in Ang II-infused rats. Monocyte/macrophage accumulation was significantly greater in Ang II-treated rat hearts than in sham-treated hearts (10+/-1; P:<0.001; n=5). Thus during inflammation, overexpression of ICAM-1 may contribute to cardiovascular damage in diseases characterized by increased activity of the renin-angiotensin system.

Effect of Chronic Blockade of the Kallikrein-Kinin System on the Development of Hypertension in Rats

-The kallikrein-kininogen-kinin system is an important vasodilator and vasodepressor component of the cardiovascular system. Acting mainly through B(2) receptors, kinins may counterbalance the pressor effect of angiotensin II, salt, and mineralocorticoids plus salt. Using rats lacking the bradykinin precursors low- and high-molecular-weight kininogen or a B(2) kinin receptor antagonist (icatibant), we investigated whether absence or blockade of the kallikrein-kinin system alters blood pressure (BP) in rats given (1) chronic infusion of Ang II, (2) a normal or high salt diet, or (3) chronic administration of deoxycorticosterone acetate (DOCA) plus salt. We confirmed the genotype and phenotype of Brown Norway Katholiek rats (BNK) and found that they had a G-to-A point mutation on the kininogen gene compared with Brown Norway (BN) and Sprague-Dawley (SD) rats, very low levels of high-molecular-weight kininogen (17+/-3 ng/mL) compared with BN and SD (1814+/-253 and 2397+/-302 ng/mL, respectively; P:<0.01), and plasma low-molecular-weight kininogen concentrations below detectable limits compared with 1773+/-74 and 1781+/-140 ng/mL for BN and SD, respectively. Basal BP was the same in BNK and BN. Chronic infusion of icatibant did not alter BP in BN or Wistar rats. At doses that blocked the acute effect of bradykinin, icatibant did not potentiate the pressor effect of a chronic subpressor or pressor dose of angiotensin II in male and female Wistar rats nor that of a high salt diet (2%) plus unilateral nephrectomy in male Wistar rats. Moreover, blockade of the kallikrein-kininogen-kinin system in either BN rats given a very high dose of icatibant or kinin-deficient rats (BNK) did not potentiate the pressor effect of angiotensin II (nonpressor dose) or a high salt (3% NaCl) diet given for 2 weeks. Established DOCA-salt hypertension was not exaggerated in rats treated with icatibant but was partially attenuated by ramipril (1.5 mg. kg(-)(1). d(-)(1) for 7 days; P:<0.002). This antihypertensive effect was abolished by icatibant (P:<0.002, ramipril versus ramipril plus icatibant). These results suggest that endogenous kinins do not participate in the maintenance of normal blood pressure or antagonize the development of hypertension induced by chronic infusion of angiotensin II, a high salt diet, or DOCA-salt. However, kinins appear to play an important role in the antihypertensive effect of angiotensin-converting enzyme inhibitors in DOCA-salt hypertension.

Effect of Ac-SDKP on Cardiac and Renal Collagen Deposition in Aldosterone-Salt Hypertension

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Ac-SDKP is a natural substrate for the N-terminal active site of ACE. We have data showing that Ac-SDKP inhibits collagen synthesis both in vitro and in vivo . In the present study, we tested whether Ac-SDKP prevents cardiac and renal fibrosis in a renin-independent model of hypertension (aldosterone-salt) characterized by renal and cardiac fibrosis. Uninephrectomized adult SD rats were divided into 3 groups: control (saline) and aldosterone-salt (18 μg/d) alone or combined with Ac-SDKP 800 μg/kg/d.Saline, aldosterone or Ac-SDKP was given s.c. via osmotic pump for 6 weeks. We found that Ac-SDKP decreased collagen content (hydroxyproline assay) in the LV, right ventricle (RV) and kidney, and LV collagen fraction, with no significant effect on BP and LV hypertrophy (Table). Our results suggest that Ac-SDKP prevents cardiac and renal fibrosis in renin-independent hypertension. This indicates that Ac-SDKP could participate in the antifibrotic effect of ACEi, since ACEi inhibit fibrosis in this model of hypertension and markedly increase plasma Ac-SDKP.

Effects of Ac-Sdkp, a Natural Inhibitor of Hematopoietic Stem Cell Proliferation, on Cardiac Collagen Deposition in Rats with Myocardial Infarction

P56

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a potent natural inhibitor of hematopoietic stem cell proliferation. Recently, it has been shown to inhibit collagen production by cardiac fibroblasts in vitro and collagen content in the left ventricle (LV) of 2K1C hypertensive rats in vivo . We hypothesized that Ac-SDKP may decrease myocardial collagen deposition in rats with myocardial infarction (MI), thereby reducing interstitial fibrosis and improving cardiac function. Lewis inbred rats were subjected to MI by ligating the left anterior descending coronary artery. Either vehicle or Ac-SDKP (400 mg/kg/day, i.p. by osmotic mini-pump) was given 2 months after MI and continued for another 2 months. Systolic blood pressure (SBP) was measured once a month by tail cuff. LV diastolic dimension (LVDd), mass, ejection fraction (EF) and cardiac output (CO) were measured monthly by echocardiography. At the end of the experiment, mean blood pressure (MBP), LV end-diastolic pressure (LVEDP) and dP/dt were determined. Collagen content in the non-infarcted area of the LV was measured by hydroxyproline assay. We found that Ac-SDKP had no effect on most hemodynamic and cardiac functional parameters. However, it significantly decreased collagen content from 26.1 ±2.2 (vehicle) to 14.7 ±4.8 mg/mg dry weight (Ac-SDKP); P < 0.001, associated with an increase in negative dP/dt from 5451.8 ±1240.9 (vehicle) to 7030.3 ±763.2 mm Hg/sec (Ac-SDKP); P < 0.01. These results suggest that Ac-SDKP could be a potent tool to slow down the process of myocardial interstitial fibrosis and improve post-MI diastolic function.

Echocardiographic assessment of cardiac function in conscious and anesthetized mice

Using a high-frequency linear transducer (15L8), we studied 1) the feasibility of performing echocardiography in nonanesthetized mice compared with mice given pentobarbital sodium (Pento) or a mixture of ketamine and xylazine and 2) the feasibility of echocardiographic evaluation of left ventricular (LV) hypertrophy, dilatation, and function in mice with two-kidney, one-clip hypertension or myocardial infarction (MI). Heart rate (HR) in awake mice was 658 +/- 9 beats/min; Pento and ketamine plus xylazine reduced HR to 377 +/- 11 and 293 +/- 19 beats/min, respectively, associated with a significant decrease in shortening fraction (SF), ejection fraction (EF), and cardiac output (CO) and an increase in LV end-diastolic (LVEDD) and end-systolic dimensions (LVESD). Mice with 4 wk of two-kidney, one-clip hypertension had increased LV mass (15.62 +/- 0. 62 vs. 22.17 +/- 1.79 mg) without altered LV dimensions, SF, EF, or CO. Mice studied 4 wk post-MI exhibited obvious LV dilatation and systolic dysfunction, as evidenced by increased LVEDD and LVESD and decreased SF, EF, and CO. Our findings clearly show the adverse impact of anesthesia on basal cardiac function and the difficulty in interpreting data obtained from anesthetized mice. We believe this is the first study to demonstrate the feasibility of using echocardiography to assess cardiovascular function in the nonanesthetized mouse.

Reactive oxygen species: role in the relaxation induced by bradykinin or arachidonic acid via EDHF in isolated porcine coronary arteries

Although endothelium-derived hyperpolarizing factor (EDHF) is thought to be a cytochrome P-450 product (arachidonic acid metabolite) in some tissues, in porcine coronary arteries (PCAs) its nature remains unclear. Because phospholipase A2 and C are involved in the synthesis and/or release of EDHF in the PCA, the arachidonic acid (AA) pathway may be involved. In the presence of the cyclooxygenase inhibitor indomethacin (10(-5) M) and the NOS inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME; 10(-4) M), both bradykinin (BK; 10(-9)-10(-6) M) and AA (10(-7)-10(-4) M) induced dose-dependent relaxation of PGF2alpha-contracted PCA rings, which was blocked by a high extracellular concentration of KCl (30 mM) or pretreatment with ouabain, a Na+/K+-adenosine triphosphatase (ATPase) inhibitor (5 x 10(-7) M). Eicosatetraynoic acid (ETYA; 20 microM), which inhibits all AA pathways, slightly affected the response to BK and AA; however, lipoxygenase or cytochrome P-450 inhibitors had no effect, suggesting that relaxation is independent of these enzymatic pathways. Because endothelial cells can generate reactive oxygen species (ROS) via metabolism of AA and independent of cyclooxygenase activity, we also studied (a) whether ROS can relax the PCA, as well as the mechanism(s) involved, and (b) the role of ROS in BK- and AA-induced relaxation. Xanthine (X; 100 microM) plus xanthine oxidase (XO; 0.02 U/ml) induced time-dependent relaxation of PGF2alpha-contracted PCA rings in the presence of indomethacin and L-NAME. Dilatation was not affected by superoxide dismutase (SOD; 500 U/ml) but was abolished by catalase (300 U/ml), suggesting that hydrogen peroxide (H2O2) is involved. When rings were contracted by depolarizing them with 30 mM KCl, X/XO failed to elicit relaxation. Ouabain abolished the response to X/XO, suggesting that X/XO may induce relaxation by hyperpolarizing vascular smooth muscle cells via stimulation of the Na+/K+-ATPase pump. We therefore questioned whether ROS might be involved in BK- and AA-induced relaxation. Because catalase combined with SOD had little or no effect, we concluded that in the PCA, the relaxation induced by BK via EDHF involves some mechanism independent of NO, AA metabolism, or ROS.

Effect of ACE inhibitor on DOCA-salt- and aortic coarctation-induced hypertension in mice: do kinin B2 receptors play a role?

Kinins have been shown to play an important role in the cardioprotective effect of ACE inhibitors (ACEi) during heart failure and ischemia-reperfusion. However, it is controversial as to whether kinins oppose the hypertensinogenic effect of deoxycorticosterone acetate plus salt (DOCA-salt) or aortic coarctation and whether they mediate both chronic antihypertensive and cardiac antihypertrophic effects of ACEi in hypertension. Using normal 129/SvEvTac mice and mice lacking the bradykinin B2 receptor gene (B2-KO), we investigated whether (1) the hypertensinogenic effect of DOCA-salt or aortic coarctation is enhanced in B2-KO mice and (2) the chronic antihypertensive and antihypertrophic effects of an ACEi (ramipril, 4 mg. kg-1. d-1) are mediated by B2 receptors in aortic coarctation (6 weeks)- and DOCA-salt (4 weeks)-induced hypertension. Before surgery, there was no difference between 129/SvEvTac and B2-KO mice in terms of blood pressure and heart weight, suggesting that kinins are not essential to maintaining normal blood pressure. DOCA-salt (volume expansion) or aortic coarctation (renin-dependent) induced similar hypertension and left ventricular hypertrophy (LVH) in 129/SvEvTac and B2-KO mice, suggesting that kinins do not play an essential role in the development of DOCA-salt- or aortic coarctation-induced hypertension. We found that B2 receptors mediate only the early (1 week) but not the late phase (4 weeks) of the chronic hypotensive effect of ACEi in DOCA-salt hypertension. On the other hand, chronic ACE inhibition prevented the development of hypertension and LVH in both 129/SvEvTac and B2-KO mice given DOCA-salt or subjected to aortic coarctation, suggesting that kinins do not participate in the chronic antihypertensive and antihypertrophic effects of ACEi in these 2 models of hypertension. Thus, in mice, kinins acting via B2 receptors do not participate in (1) maintenance of normal basal blood pressure, (2) establishment and maintenance of hypertension induced by DOCA-salt or aortic coarctation, and (3) chronic antihypertensive and cardiac antihypertrophic effects of ACEi in DOCA-salt and aortic coarctation hypertension.

Role of nNOS in blood pressure regulation in eNOS null mutant mice

The role of neural nitric oxide synthase (nNOS) in regulating blood pressure (BP) remains uncertain. Recently it was reported that in mice lacking functional endothelial NOS (eNOS) genes (-/-), acute administration of a nonselective NOS inhibitor, Nw-nitro-L-arginine, decreased mean BP, suggesting that NO released by non-eNOS isoforms increases BP. Because the inducible NOS isoform is not constitutively expressed and when induced causes hypotension, we hypothesize that it is NO produced by nNOS that increases BP in the absence of eNOS activity. To test this hypothesis, we studied the acute effect of selective and nonselective nNOS inhibitors on BP and cerebellar NOS activity in eNOS (-/-), wild-type (+/+), and heterozygous (+/-) mice as well as in +/+ mice with renovascular hypertension. Because it is not known whether the decrease in BP caused by acute NOS inhibition in -/- mice can occur chronically, we also studied the effect of chronic NOS inhibition on both BP and cerebellar NOS activity. eNOS (-/-) mice had higher BP than +/+ or +/-mice, and acute administration of the selective nNOS inhibitor 7-nitroindazole (7-NI) decreased their mean BP from 137+/-13 to 124+/-12 mm Hg (P<0.01). In +/+, +/-, or renovascular hypertensive +/+ mice, 7-NI caused a small but insignificant rise from 105+/-5 to 110+/-6 mm Hg, from 115+/-9 to 119+/-13 mm Hg, and from 146+/-6 to 150+/-6 mm Hg, respectively. Fifteen minutes after administration of 7-NI, cerebellar NOS activity decreased by 70%; however, this inhibitory effect was brief, since 2 hours after 7-NI administration NOS returned toward control values. Chronic oral or intraperitoneal administration of 7-NI did not inhibit cerebellar NOS activity, whereas the nonselective NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME) decreased this activity by 50%. Therefore, we studied the effect of chronic L-NAME administration (4 weeks) on BP. In -/- mice, chronic L-NAME administration decreased BP from 135+/-4 to 120+/-3 mm Hg (P<0.05), whereas in +/+ and +/-mice, as expected, it increased BP from 109+/-2 to 125+/-3 mm Hg (P<0.001) and from 107+/-6 to 119+/-5 mm Hg (P<0.02), respectively. After L-NAME administration was stopped, BP returned to baseline. These results suggest that in eNOS -/- mice, NO derived from nNOS increases BP both acutely and chronically.

Autacoids mediate coronary vasoconstriction induced by nitric oxide synthesis inhibition

Inhibition of nitric oxide (NO) synthesis results in coronary vasoconstriction. Using a Langendorff rat heart preparation, we tested the hypothesis that this vasoconstriction is caused by the unopposed effect of the autacoids prostaglandin H2 (PGH2) or thromboxane A2 (TxA2) or both through a mechanism that involves oxygen free radicals. The vasoconstriction induced by NO synthesis inhibition was studied with two different NO synthase inhibitors, N(omega)-nitro-L-arginine methyl ester (L-NAME) and N(omega)-monomethyl-L-arginine (L-NMMA). We found that the decrease in coronary flow (CF) induced by L-NAME (from 19.3 +/- 0.9 to 13.2 +/- 0.9 ml/min; p < 0.001) and L-NMMA (from 20.1 +/- 0.4 to 15.0 +/- 0.3 ml/min; p < 0.001) was completely blocked by the cyclooxygenase inhibitor indomethacin. A different cyclooxygenase inhibitor (ibuprofen), a PGH2/TxA2-receptor antagonist (SQ29548), and a TxA2 synthase inhibitor (CGS 13080) also completely abolished the vasoconstrictor effect of L-NAME, suggesting that this vasoconstriction is mediated by TxA2. Two different scavengers of superoxide radical anions (O2-), the enzyme superoxide dismutase (SOD) and a cell-permeable SOD mimic, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol), also blocked the vasoconstriction induced by NO synthesis inhibition. In contrast, catalase, which inactivates hydrogen peroxide (H2O2), failed to do so, indicating that O2- is needed for the vasoconstrictor effect of L-NAME, whereas H2O2 is not. To determine whether O2- acts on the conversion of PGH2 to TxA2 or at the receptor or postreceptor level, we studied whether the vasoconstriction induced by exogenous PGH2 or the TxA2 receptor agonist U46619 is blocked by scavengers of O2-. CF decreased by 50% with PGH2 (from 21 +/- 2.1 to 10.6 +/- 5.8 ml/min; p < 0.01), and this decrease was abolished by SOD and Tempol but not catalase. However, SOD had no effect on the vasoconstriction induced by U46619, which decreased CF by 45% (from 17.3 +/- 2.5 to 9.5 +/- 1.8 ml/min; p < 0.01). In addition, PGH2 increased the release of TxB2 (the stable metabolite of TxA2) in the coronary effluent (from 5.1 +/- 1.2 to 136.1 +/- 11.8 pg/ml/min). The release of TxB2 was significantly lower in hearts treated with SOD (76.8 +/- 14.2 pg/ml/min) and CGS (65.7 +/- 13.9 pg/ml/min). We conclude that the coronary vasoconstriction induced by inhibition of NO synthesis is the result of the unopposed effect of the autacoid TxA2 through activation of its receptor, and that O2- is necessary for conversion of PGH2 to TxA2.

Role of kinins and nitric oxide in the antihypertrophic effect of ramipril

We examined the effect of non-antihypertensive doses of the angiotensin-converting enzyme inhibitor ramipril, kinins, and/or nitric oxide on left ventricular hypertrophy in rats with aortic coarctation. We investigated the effect of either HOE 140, a specific B2 receptor antagonist, or NG-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, on the antihypertrophic effect of ramipril at non-antihypertensive doses (10 micrograms/kg per day) failed to alter left ventricular hypertrophy significantly, although a small decrease was obtained. Given at a dose of 1 mg/kg per day for 6 weeks, ramipril prevented increased blood pressure and left ventricular hypertrophy after aortic coarctation. Neither of these effects was blocked by simultaneous administration of HOE 140 (500 micrograms/kg per day). In rats with aortic coarctation treated with L-NAME, blood pressure increased further but left ventricular weight did not. Ramipril (1 mg/kg per day) significantly reduced left ventricular hypertrophy, although blood pressure was still higher than in rats given water alone. The slope of the correlation between left ventricular weight and blood pressure in rats that received L-NAME was significantly lower than in rats that did not (0.52 versus 1.29; P = .008). This suggests that for each 1 mm Hg that the blood pressure increased, the increase in left ventricular weight was less in the L-NAME groups. Thus, only antihypertensive doses of ramipril possessed antihypertrophic activity. Kinins did not participate in the chronic antihypertensive and antihypertrophic effects of ramipril. In hypertension induced or aggravated by chronic nitric oxide synthase, L-NAME partially impaired development of left ventricular hypertrophy for reasons that are unclear.

Role of B1 and B2 receptors and of nitric oxide in bradykinin-induced relaxation and contraction of isolated rat duodenum

Bradykinin (BK) and its analogues induce a typical biphasic response (relaxation followed by contraction) in the isolated rat duodenum. We studied the role of B1 and B2 BK receptors and nitric oxide (NO) in relaxation and contraction of the isolated rat duodenum. Both effects are concentration-dependent: BK has shown an EC50 (contraction) of 3.8 +/- 1.9 x 10(-7) M and an IC50 (relaxation) of 3.0 +/- 0.7 x 10(-9). Similar results were obtained with the selective B2 receptor agonists [Hyp3,Tyr(Me)8]-BK and [Phe8 psi (CH2-NH)Arg9]-BK, showing an EC50 of 9.6 +/- 1.9 x 10(-7) M and 5.6 +/- 2.9 x 10(-7) M and an IC50 of 3.5 +/- 0.6 x 10(-10) M and 6.8 +/- 1.7 x 10(-10) M, respectively. Furthermore, the effects induced by these three agonists were not altered when tissues were treated with 42.1 microM Mergetpa, a carboxypeptidase N inhibitor. While the relaxant and contractile effects elicited by BK were significantly inhibited in the presence of Hoe 140 (0.7 microM), a selective B2 receptor antagonist, those induced by the selective B1 receptor agonist desArg9-BK were not. Furthermore, [Leu8]-desArg9-BK (2.6 microM), which is both a pure and selective B1 receptor antagonist, acted as an agonist on the rat duodenum, inducing a biphasic relaxant and contractile effect. These relaxant and contractile effects were not altered by drugs that inhibit or stimulate NO production, such as L-NAME (200 microM), a combination of L-NAME (200 microM) and indomethacin (2.5 microM), L-arginine (1 mM), or superoxide dismutase (20 U/ml). However, the contractile effect was significantly reduced when tissues were preincubated with methylene blue (100 microM), which inhibits activation of guanylate cyclase. We conclude that 1) BK and its analogues selectively activate a B2 receptor, producing a biphasic effect (relaxation and contraction); 2) DesArg9-BK may either acts via a different receptor which might be another B1 receptor subtype or a typical B1 receptor where [Leu8]-desArg9-BK acts as a partial agonist; and 3) neither NO nor the prostaglandin pathway mediates BK-induced relaxation in the isolated rat duodenum.

Receptors for bradykinin and related kinins: a critical analysis

Kinins exert a variety of biological actions and have been implicated in the pathogenesis of inflammation, pain, asthma, and other diseases. Kinins act through specific receptors that are widespread and belong to two major categories, B1 and B2. B2 has been cloned and shown to be of the rhodopsin type, consisting of seven hydrophobic membrane domains connected by extracellular and intracellular loops. Recent pharmacological findings from various laboratories suggest the existence of new receptor types, which have been named B3, B4, and B5. These findings are analysed critically, especially with respect to the criteria that have been used for affirming the existence of new receptor entities. The analysis is restricted to data obtained in isolated organs, almost exclusively smooth muscle preparations. Criteria for receptor characterization and classification are the order of potency of agonists and the apparent affinities of antagonists. The analysis reveals that receptors for bradykinin and related kinins are of two types, B1 and B2. B1 mediates the rapid acute response (smooth muscle contraction or relaxation) as well as some effects occurring more slowly (e.g., collagen synthesis). B1 receptor functions have been shown to be modulated by interleukins. B2 receptors are responsible for most of the kinins' biological effects, including arterial vasodilatation, plasma extravasation, venoconstriction, activation of sensory fibers (e.g., fibers for pain), and stimulation of the release of prostaglandins, endothelium-dependent relaxing factor (from endothelia), noradrenaline (from nerve terminals and adrenals), and other endogenous agents. The pharmacological characteristics of the receptor sites (B2) mediating this array of biological effects show differences between species, and two B2 receptor subtypes are proposed, namely B2A (rabbit, dog, and possibly man) and B2B (guinea pig, hamster, rat). B2A and B2B receptor subtypes have been characterized by using fairly selective agonists and competitive antagonists (e.g., D-Arg[Hyp3, D-Phe7,Leu8]BK). Noncompetitive antagonists (non-equilibrium), such as HOE 140, do not discriminate between B2A and B2B subtypes. Species differences cannot account for the multiplicity of receptors that have been proposed for rat vas deferens, pre- and post-junctional sites, and rat uterus, guinea pig ileum, and rat blood pressure. The existence of hypothetical new receptor sites was based on data obtained with partial agonists and have not been substantiated by data obtained with potent pure antagonists. The B3 receptor, proposed to explain the unusual behaviour of the guinea pig tracheal response to kinins, has to be carefully reconsidered after the finding that HOE 140 acts as a pure antagonist on this tissue and shows a fairly high affinity for the tracheal site.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of a novel binding site for 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]bradykinin on epithelial membranes of guinea pig ileum

We have recently shown that (a) [125I-Tyr8]bradykinin (BK) recognized bradykinin binding sites in guinea pig epithelium membranes with a Kd value of 1.6 nM and a Bmax of 156 fmol/mg protein, and (b) B2 agonists and some B2 antagonists, such as D-Arg-[Hyp3,D-Phe7,Leu8]BK, inhibited this specific binding with a Ki value of 32 nM. In the present study, we have radioiodinated the B2 antagonist Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK and have performed a full characterization of the binding properties of this tracer in the same membrane preparation. Equilibrium experiments performed in the absence or presence of an excess of BK (10(-5) M) showed that 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK specifically labelled two different sites. One of these is the same as the site labelled by [125I-Tyr8]BK, and this indicates that 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK interacts specifically with kinin B2 receptors. Equilibrium experiment performed in the presence of an excess of BK (10(-5) M) indicated that specific binding of 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK to the second site is also saturable and Scatchard analysis showed that the site is of high affinity with a Kd of 16.8 nM and a Bmax of 2.08 pmol/mg protein. Surprisingly, unlabelled B2 agonists such as bradykinin, [Tyr8]BK, [Leu8]BK, [Hyp3,Tyr8(OMe)]BK, D-Arg-[Hyp3]BK and kallidin were found to be inactive on this second site. A series of B2 receptor antagonists, Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK, D-Arg-[Hyp3,D-Phe7,Leu8]BK, D-Arg-[Hyp3,Leu5,8,D-Phe7]BK, D-Arg-[Hyp3,Gly6,D-Phe7,Leu8]BK and D-Arg-[Hyp3,Thi5,8,D-Phe7]BK inhibited 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK binding with Ki values of 25.0, 20.9, 15.8, 64.6 and 6606.9 nM respectively. On the other hand, [Thi5,8,D-Phe7]BK did not interfere with 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK but was found to be a potent inhibitor of [125I-Tyr8]BK binding (Ki = 53.7 nM). As expected, B1 receptor agonists, antagonists and peptides non-related to BK such as substance P, neurokinin A, neurokinin B, angiotensin II, bombesin, vasopressin and the calcitonin gene related peptide were unable to compete with 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK. The results show that 125I-Tyr-D-Arg-[Hyp3,D-Phe7,Leu8]BK is interacting with two distinct binding sites in the guinea pig epithelium: one is the well known bradykinin B2 receptor and the other is a new, non-characterized binding site that interacts exclusively with some bradykinin receptor antagonists.

Pharmacological characterization of a new highly potent B2 receptor antagonist (HOE 140: D-Arg-[Hyp3,Thi5,D-Tic7,Qic8]bradykinin)

HOE 140 (D-Arg-[Hyp3,Thi5,D-Tic7,Oic8]bradykinin), a new B2 antagonist, was compared to R-493 (D-Arg[Hyp3-D-Phe7,Leu8]bradykinin) with respect to inhibition of the responses of seven isolated smooth muscle preparations to bradykinin. R-493 was found to exert: (a) high antagonistic activity on the rabbit jugular vein (pA2 of 8.86), (b) moderate activity on the rabbit aorta, guinea-pig ileum, hamster urinary bladder and human urinary bladder (pA2 of 5.76, 6.77, 7.16 and 7.15, respectively) and (c) a stimulatory effect on the guinea-pig trachea. On the other hand, HOE 140 showed identical apparent affinities (8.36-9.12) on all preparations except the rabbit aorta where it was inactive and the guinea-pig trachea where the compound was an antagonist (pA2: 7.42) without agonistic effect. HOE 140 is specific and selective for B2 receptors since it was inactive against angiotensin II, substance P, neurokinin A, desArg9-bradykinin, noradrenaline or acetylcholine in the various preparations. R-493 inhibited the contractile effects of bradykinin competitively, while HOE 140 was not competitive even at low concentrations (7.7 x 10(-9) M). These results demonstrate that HOE 140 is a potent B2 antagonist with high affinity, specific for kinin receptors and selective for the B2 receptor type, but is non-competitive. HOE 140 is the first bradykinin receptor antagonist that acts as such on the guinea-pig trachea without showing any agonistic activity.

125I-Tyr,D-Arg[Hyp3,D-Phe7,Leu8]BK, a radiolabelled B2 antagonist specifically interacts with two distinct binding sites on epithelial membranes of guinea pig ileum

Kinins are endogenously formed peptides that have diverse biological actions, including effects on the gastrointestinal tract. In the search of selective ligands, we studied the binding properties of a selective B2 radioiodinated antagonist (Tyr,D-Arg[Hyp3,D-Phe7,Leu8]BK) on epithelial membranes of guinea pig ileum. Equilibrium binding experiments showed that 125I-Tyr,D-Arg[Hyp3,D-Phe7,Leu8]BK specifically labels two different sites. One of these sites is the conventional B2 receptor. The new tracer recognized this site with a Kd of 34.7 nM and revealed a Bmax of 156 fmol/mg protein. In equilibrium binding experiments 125I-Tyr,D-Arg[Hyp3,D-Phe7,Leu8]BK also recognized a second specific site. Scatchard analysis showed that this second site was of high affinity (Kd of 16.8 nM) and very abundant (Bmax of 2.08 pmol/mg protein). Surprisingly, the natural B2 agonists bradykinin and kallidin were unable to inhibit the specific binding of 125I-Tyr,D-Arg[Hyp3,D-Phe7,Leu8]BK to the second site. A series of B2 antagonists failed to inhibit the specific binding of the new radiolabelled peptide. As expected, non related peptides such as angiotensin II, neurokinin A and B, substance P, vasopressin, calcitonin gene related peptide and bombesin were also inactive. These results show that the new tracer is interacting with two distinct binding sites in epithelial membranes of guinea pig ileum. One is the well known bradykinin B2 receptor and the other is a new, non characterized binding site that interacts exclusively with bradykinin receptor antagonists.