Angiotensin I-to-II conversion in the human renal vascular bed

OBJECTIVE

During previous studies in humans and pigs, using infusions of 125I-angiotensin into the right antecubital vein or the left cardiac ventricle, we were unable to demonstrate conversion of arterial angiotensin I in the renal vascular bed. The arterial 125I-angiotensin I levels in these studies may have been too low to result in detectable renal venous 125I-angiotensin II levels, especially in view of the extensive degradation of angiotensins in the kidney. To overcome this problem, we now infused 125I-angiotensin I directly into the renal artery.

DESIGN AND METHODS

Five subjects (three women, two men) with essential hypertension (n = 4) or unilateral renal artery stenosis (n = 1), not treated with an ACE inhibitor, were given a 10-min infusion of 125I-angiotensin I (3.6+/-0.4 x 10(6) cpm/min, mean +/- SEM) into the left (n = 4) or right (n = 1) renal artery. Blood samples for the measurement of endogenous and radiolabelled angiotensin I and II were taken under steady-state conditions from the aorta and the renal vein of the 125I-angiotensin I-perfused kidney.

RESULTS

At steady-state, the levels of 125I-angiotensin I in renal venous blood were 5-6 fold lower, and those of 125I-angiotensin II were 4-5 fold higher than in renal arterial blood. On the basis of these levels, angiotensin I extraction in the renal vascular bed was calculated to be 80+/-3%, of which 9+/-1% was due to angiotensin I-to-II conversion. The renal venous levels of endogenous angiotensin I were 50% higher than its arterial levels, whereas the levels of endogenous angiotensin II were 50% lower in renal venous blood than in arterial blood. Taking into consideration the regional metabolism of arterially delivered angiotensins, and the generation of angiotensin I in circulating blood by plasma renin activity, it could be calculated that renal venous angiotensin I is largely derived from renal tissue sites, and that renal venous angiotensin II has no other sources than arterially delivered angiotensin I and II and angiotensin I generated by plasma renin activity in the renal vascular bed.

CONCLUSIONS

Less than 10% of arterially delivered angiotensin I is converted to angiotensin II in the renal vascular bed. Conversion of angiotensin I generated at renal tissue sites does not contribute to the level of angiotensin II in the renal vein, although it is the main source of angiotensin II in renal tissue. Thus, the intrarenal formation of angiotensin II is highly compartmentalised.

Probing epitopes on human prorenin during its proteolytic and non-proteolytic activation

The conformational changes of prorenin (PR) that are associated with its reversible non-proteolytic activation and irreversible proteolytic activation were monitored with immunoradiometric assays, using antibodies against epitopes belonging to the propeptide or the renin part of PR. Binding of PR to the renin inhibitor remikiren or protonation of PR resulted in the slowly progressive and simultaneous expression (t1/2 congruent with3.5-5.0 h at 4 degreesC) of epitopes of the N-terminal and C-terminal halves of the propeptide and an epitope that is manifest on renin but not on native non-activated PR. During reversible PR activation-inactivation, expression and disappearance of these epitopes coincided with the appearance and disappearance of enzyme activity. Cleavage of the propeptide from the renin part of PR by plasmin, as demonstrated by the failure of remikiren to unmask the N-terminal and C-terminal propeptide epitopes, was, with some time lag, followed by the simultaneous expression (t1/2 congruent with60 min at 4 degreesC) of the renin-specific epitope and enzymatic activity. Based on these findings we propose a model for the non-proteolytic activation of PR that involves the formation of an intermediary form of activated PR with the following properties: (1) the covalently bound propeptide has moved out of the active-site cleft, so that binding sites are exposed to active site ligands, (2) the propeptide is still not in the 'relaxed' conformation that is characteristic for fully, non-proteolytically, activated PR, and (3) the N-terminal part of the renin polypeptide chain has not yet attained the proper location that is required for enzymatic activity.

AT1 receptor A/C1166 polymorphism contributes to cardiac hypertrophy in subjects with hypertrophic cardiomyopathy

The development of left ventricular hypertrophy (LVH) in subjects with hypertrophic cardiomyopathy (HCM) is variable, suggesting a role for modifying factors such as angiotensin II. We investigated whether the angiotensin II type 1 receptor (AT1-R) A/C1166 polymorphism, the angiotensin-converting enzyme (ACE) insertion/deletion (I/D) polymorphism, and/or plasma renin influence LVH in HCM. Left ventricular mass index (LVMI) and interventricular septal thickness were determined by 2-dimensional echocardiography in 104 genetically independent subjects with HCM. Extent of hypertrophy was quantified by a point score (Wigle score). Plasma prorenin, renin, and ACE were measured by immunoradiometric or fluorometric assays, and ACE and AT1-R genotyping were performed by polymerase chain reactions. The ACE D allele did not affect any of the measured parameters except plasma ACE (P<0.04). LVMI was higher (P<0.05) in patients carrying the AT1-R C allele (190+/-8.3 g/m2) than in AA homozygotes (168+/-7.2 g/m2), and similar patterns were observed for interventricular septal thickness (23.0+/-0.7 versus 21. 6+/-0.7 mm) and Wigle score (7.0+/-0.3 versus 6.3+/-0.3). Plasma renin was higher (P=0.05) in carriers of the C allele than in AA homozygotes. Multivariate regression analysis, however, revealed no independent role for renin in the prediction of LVMI. Plasma prorenin and ACE were not affected by the AT1-R A/C1166 polymorphism, nor did the ACE and AT1-R polymorphisms interact with regard to any of the measured parameters. We conclude that the AT1-R C1166 allele modulates the phenotypic expression of hypertrophy in HCM, independently of plasma renin and the ACE I/D polymorphism.

A clinical prediction rule for renal artery stenosis

BACKGROUND

Renal artery stenosis is a rare cause of hypertension. The gold standard for diagnosing renal artery stenosis, renal angiography, is invasive and costly.

OBJECTIVE

To develop a prediction rule for renal artery stenosis from clinical characteristics that can be used to select patients for renal angiography.

DESIGN

Logistic regression analysis of data from a prospective cohort of patients suspected of having renal artery stenosis. A prediction rule was derived from the regression model for use in clinical practice.

SETTING

26 hypertension clinics in The Netherlands.

PATIENTS

477 hypertensive patients who underwent renal angiography because they had drug-resistant hypertension or an increase in serum creatinine concentration during therapy with angiotensin-converting enzyme inhibitors.

RESULTS

Age, sex, atherosclerotic vascular disease, recent onset of hypertension, smoking history, body mass index, presence of an abdominal bruit, serum creatinine concentration, and serum cholesterol level were selected as predictors. The regression model was reliable (goodness-of-fit test, P > 0.2) and discriminated well between patients with stenosis and those with essential hypertension (area under the receiver-operating characteristic curve, 0.84). The diagnostic accuracy of the regression model was similar to that of renal scintigraphy, which had a sensitivity of 72% and a specificity of 90%.

CONCLUSIONS

In the diagnostic workup of patients suspected of having renal artery stenosis, the clinical prediction rule can be considered as an alternative to renal scintigraphy. It can help to select patients for renal angiography in an efficient manner by reducing the number of angiographic procedures without the risk for missing many renal artery stenoses.

Angiotensin production by the heart: a quantitative study in pigs with the use of radiolabeled angiotensin infusions

BACKGROUND

Beneficial effects of ACE inhibitors on the heart may be mediated by decreased cardiac angiotensin II (Ang II) production.

METHODS AND RESULTS

To determine whether cardiac Ang I and Ang II are produced in situ or derived from the circulation, we infused 125I-labeled Ang I or II into pigs (25 to 30 kg) and measured 125I-Ang I and II as well as endogenous Ang I and II in cardiac tissue and blood plasma. In untreated pigs, the tissue Ang II concentration (per gram wet weight) in different parts of the heart was 5 times the concentration (per milliliter) in plasma, and the tissue Ang I concentration was 75% of the plasma Ang I concentration. Tissue 125I-Ang II during 125I-Ang II infusion was 75% of 125I-Ang II in arterial plasma, whereas tissue 125I-Ang I during 125I-Ang I infusion was <4% of 125I-Ang I in arterial plasma. After treatment with the ACE inhibitor captopril (25 mg twice daily), Ang II fell in plasma but not in tissue, and Ang I and renin rose both in plasma and tissue, whereas angiotensinogen did not change in plasma and fell in tissue. Tissue 125I-Ang II derived by conversion from arterially delivered 125I-Ang I fell from 23% to <2% of 125I-Ang I in arterial plasma.

CONCLUSIONS

Most of the cardiac Ang II appears to be produced at tissue sites by conversion of in situ-synthesized rather than blood-derived Ang I. Our study also indicates that under certain experimental conditions, the heart can maintain its Ang II production, whereas the production of circulating Ang II is effectively suppressed.

Determinants of interindividual variation of renin and prorenin concentrations: evidence for a sexual dimorphism of (pro)renin levels in humans

BACKGROUND

Plasma renin concentrations are an important factor in cardiovascular risk profiling.

OBJECTIVE

To investigate the effects of sex, medication, and anthropometric factors that may contribute to the interindividual variation in the plasma concentrations of renin and its precursor prorenin.

DESIGN AND METHODS

Prorenin and renin levels in 327 men and 383 women, aged 52-69 years, who participated in a 1994 reexamination of a previous population survey in Bavaria, were measured by immunoradiometric assay.

RESULTS

Prorenin and renin levels in men were significantly higher than those in women, those in women without estrogen replacement therapy were significantly higher than those in women with estrogen replacement therapy, and those in diabetics were significantly higher than those in nondiabetics. Prorenin level was correlated negatively to blood pressure and positively to age and the use of diuretics; it was normal in subjects using angiotensin converting enzyme inhibitors and beta-adrenergic antagonists (beta-blockers). Renin level was correlated negatively to atrial natriuretic peptide level and the use of beta-blockers, and it was elevated above normal levels in subjects using angiotensin converting enzyme inhibitors and diuretics as well as in subjects who had previously suffered myocardial infarction. After exclusion of data for women being administered estrogen replacement therapy, multivariate analysis revealed that sex (P<0.001), age (P<0.02), blood pressure (P<0.002), diabetes (P<0.05), and the use of angiotensin converting enzyme inhibitors (P<0.002), beta-blockers (P<0.001), and diuretics (P<0.05) were independent determinants of plasma prorenin. Plasma renin was independently related to atrial natriuretic peptide level (P<0.01) and the use of angiotensin converting enzyme inhibitors (P<0.001), beta-blockers (P<0.001), and diuretics (P<0.05).

CONCLUSIONS

These data demonstrate that there is a sexual dimorphism of prorenin levels in humans, suggesting that sex hormones affect the regulation of the renin gene. Data confirm previous reports of elevated prorenin levels in diabetics and older subjects, as well as of lower than normal prorenin levels in subjects with hypertension in smaller populations. Our findings may help to clarify the potential (patho)physiologic functions of prorenin and to identify the factors that influence the constitutive secretion and intracellular processing of this prohormone.

Localization and production of angiotensin II in the isolated perfused rat heart

We used a modification of the isolated perfused rat heart, in which coronary effluent and interstitial transudate were separately collected, to investigate the localization and production of angiotensin II (Ang II) in the heart. During combined renin (0.7 to 1.5 pmol Ang I/mL per minute) and angiotensinogen (6 to 12 pmol/mL) perfusion (4 to 8 mL/min) for 60 minutes (n=3), the steady-state levels of Ang II in interstitial transudate in two consecutive 10-minute periods were 4.3+/-1.5 and 3.6+/-1.5 fmol/mL compared with 1.1+/-0.4 and 1.1+/-0.6 fmol/mL in coronary effluent (mean+/-half range). During perfusion with Ang II (n=5), steady-state Ang II in interstitial transudate was 32+/-19% of arterial Ang II compared with 65+/-16% in coronary effluent (mean+/-SD, P<.02). During perfusion with Ang I (n=5), Ang II in interstitial transudate was 5.1+/-0.6% of arterial Ang I compared with 2.2+/-0.3% in coronary effluent (P<.05). The tissue concentration of Ang II in the combined renin/angiotensinogen perfusions (per gram) was as high as the concentration in interstitial transudate (per milliliter). Addition of losartan (10(-6) mol/L) to the renin/angiotensinogen perfusion (n=3) had no significant effect on the tissue level of Ang II, whereas losartan in the perfusions with Ang I (n=5) or Ang II (n=5) decreased tissue Ang II to undetectably low levels. The results indicate that the heart is capable of producing Ang II and that this can lead to higher levels in tissue than in blood plasma. Cardiac Ang II does not appear to be restricted to the extracellular fluid. This is in part due to AT1-receptor-mediated cellular uptake of extracellular Ang II, but our results also raise the possibility of intracellular Ang II production.

Angiotensin-converting enzyme inhibition does not correct early defects in renal and vascular permeability in diabetes mellitus

1. In diabetes mellitus a selective increase in the excretion of albumin generally precedes the occurrence of demonstrable loss of glomerular size-selectivity. However, even in this (microalbuminuric) phase of diabetic nephropathy a defect in glomerular barrier function can be demonstrated during infusion of atrial natriuretic peptide. 2. The aim of this study was to investigate whether angiotensin-converting enzyme inhibition could prevent the proteinuric response to atrial natriuretic peptide in these patients. We performed infusions of atrial natriuretic peptide (0.01 microgram min-1 kg-1) in 10 patients with insulin-dependent diabetes mellitus and microalbuminuria (urinary albumin excretion 90 +/- 44 mg/day), both before and after 1 month of treatment with enalapril (20 mg once daily). 3. Despite a 40% reduction in proteinuria, angiotensin-converting enzyme inhibition did not prevent the atrial natriuretic peptide-induced increase in protein excretion. Both before and during angiotensin-converting enzyme inhibition, atrial natriuretic peptide infusion resulted in a significant increase in the fractional excretion of large dextran molecules, which is compatible with an increase in flow through large unrestrictive 'shunt' pores. Atrial natriuretic peptide infusion also induced an increase in the transcapillary escape rate of albumin and angiotensin-converting enzyme inhibition also failed to prevent this effect of atrial natriuretic peptide on peripheral capillary permeability. 4. We conclude that angiotensin-converting enzyme inhibition during 1 month does not correct the capillary barrier function defect in patients with diabetes mellitus and microalbuminuria that is unmasked by atrial natriuretic peptide infusion.

Continuous arterio-venous haemodiafiltration: hydraulic and diffusive permeability index of an AN-69 capillary haemofilter

The dependence of uraemic solute clearance on the hydraulic and diffusive permeability index of an AN-69 capillary haemofilter is investigated during the treatment of patients with continuous arterio-venous haemodiafiltration (CAVHD). A mathematical model is presented to calculate solute clearance and the hydraulic and diffusive permeability index parameters from clinical data and to predict the blood flow rate entering the extra-corporeal circuit from the manufacturer's specifications and blood viscosity. By measuring the flow rates, the patient's mean arterio-venous pressure difference and uraemic solute clearance under different clinical and operational conditions, mathematical model equations are evaluated. During the average survival time of an AN-69 capillary haemofilter of about five days, it is found that both the hydraulic permeability index and the diffusive permeability index decline over treatment time, independent of the haemofilter resistance to blood flow. The measured haemofilter resistance to blood flow is three times higher than the haemofilter resistance predicted from the manufacturer's specifications and blood viscosity. Predicting the blood flow rate entering the extra-corporeal circuit from the arterial haematocrit, plasma protein concentration and temperature and the manufacturer's specifications is not reliable.

Mannose 6-phosphate receptor-mediated internalization and activation of prorenin by cardiac cells

The binding and internalization of recombinant human renin and prorenin (2500 microU/mL) and the activation of prorenin were studied in neonatal rat cardiac myocytes and fibroblasts cultured in a chemically defined medium. Surface-bound and internalized enzymes were distinguished by the addition of mannose 6-phosphate to the medium, by incubating the cells both at 37 degrees C and 4 degrees C, and by the acid-wash method. Mannose 6-phosphate inhibited the binding of renin and prorenin to the myocyte cell surface in a dose-dependent manner. At 37 degrees C, after incubation at 4 degrees C for 2 hours, 60% to 70% of cell surface-bound renin or prorenin was internalized within 5 minutes. Intracellular prorenin was activated, but extracellular prorenin was not. The half-time of activation at 37 degrees C was 25 minutes. Ammonium chloride and monensin, which interfere with the normal trafficking and recycling of internalized receptors and ligands, inhibited the activation of prorenin. Results obtained with cardiac fibroblasts were comparable to those in the myocytes. This study is the first to show experimental evidence for the internalization and activation of prorenin in extrarenal cells by a mannose 6-phosphate receptor-dependent process. Our findings may have physiological significance in light of recent experimental data indicating that angiotensin I and II are produced at cardiac and other extrarenal tissue sites by the action of renal renin and that intracellular angiotensin II can elicit important physiological responses.

Angiotensin II-mediated growth and antigrowth effects in cultured neonatal rat cardiac myocytes and fibroblasts

Angiotensin II (Ang II) stimulates cardiovascular growth and remodeling via AT1 receptors. Recent experiments have shown that Ang II may also exert antiproliferative effects via AT2 receptors. We studied the effects of Ang II on protein and DNA content and synthesis rate in unstimulated and endothelin-1 (ET-1)-stimulated neonatal rat cardiomyocytes and fibroblasts, isolated from 1-3-day-old Wistar strain pups. Total protein and total DNA, as well as [3H]leucine and [3H]thymidine incorporation were measured following incubation with either vehicle, Ang II, ET-1 or Ang II+ET-1, both in the presence or absence of the AT1 receptor blocker losartan or the AT2 receptor blocker PD123319. In myocytes, ET-1 increased total protein (+38% relative to control) as well as [3H]leucine (+66%) and [3H]thymidine (+77%) incorporation. Ang II did not affect any of these parameters, nor did it influence the ET-1-induced responses. However, in the presence of PD123319 Ang II stimulated [3H]leucine (+24%) and [3H]thymidine (+30%) incorporation. In fibroblasts, ET-1 and Ang II did not significantly affect total DNA and [3H]thymidine incorporation. Ang II tended to increase total protein in these cells, an effect which was significant only in the presence of PD123319 (+17%). Ang II stimulated [3H]leucine incorporation (+24%) in fibroblasts. This effect was absent with losartan and enhanced in the presence of PD123319. These data demonstrate that AT1 receptor-mediated proliferative effects of Ang II in neonatal cardiac cells may become apparent only when its AT2 receptor-mediated antigrowth effects are blocked. The net growth effect of Ang II therefore depends on the cellular AT1/AT2 receptor ratio. Ang II does not appear to interfere with ET-1-induced effects.

Angiotensin II type 1 (AT1) receptor-mediated accumulation of angiotensin II in tissues and its intracellular half-life in vivo

Angiotensin II (Ang II) is internalized by various cell types via receptor-mediated endocytosis. Little is known about the kinetics of this process in the whole animal and about the half-life of intact Ang II after its internalization. We measured the levels of 125I-Ang II and 125I-Ang I that were reached in various tissues and blood plasma during infusions of these peptides into the left cardiac ventricle of pigs. Steady-state concentrations of 125I-Ang II in skeletal muscle, heart, kidney, and adrenal were 8% to 41%, 64% to 150%, 340% to 550%, and 680% to 2100%, respectively, of the 125I-Ang II concentration in arterial blood plasma (ranges of six experiments). The tissue concentrations of 125I-Ang I were less than 5% of the arterial plasma concentrations. 125I-Ang II accumulation seen in heart, kidney, and adrenal was almost completely blocked by a specific Ang II type 1 (AT1) receptor antagonist. Steady-state concentrations of 125I-Ang II were reached within 30 to 60 minutes in the tissues and within 5 minutes in blood plasma. The in vivo half-life of intact 125I-Ang II in heart, kidney, and adrenal was approximately 15 minutes, compared with 0.5 minute in the circulation. Thus, Ang II, but not Ang I, from the circulation is accumulated by some tissues, and this is mediated by AT1 receptors. The time course of this process and the long half-life of the accumulated Ang II support the contention that this Ang II has been internalized after its binding to the AT1 receptor, so that it is protected from rapid degradation by endothelial peptidases. The results of this study are in agreement with growing evidence of an important physiological role for internalized Ang II.

Prorenin, renin, angiotensinogen, and angiotensin-converting enzyme in normal and failing human hearts. Evidence for renin binding

BACKGROUND

A local renin-angiotensin system in the heart is often invoked to explain the beneficial effects of ACE inhibitors in heart failure. The heart, however, produces little or no renin under normal conditions.

METHODS AND RESULTS

We compared the cardiac tissue levels of renin-angiotensin system components in 10 potential heart donors who died of noncardiac disorders and 10 subjects with dilated cardiomyopathy (DCM) who underwent cardiac transplantation. Cardiac levels of renin and prorenin in DCM patients were higher than in the donors. The cardiac and plasma levels of renin in DCM were positively correlated, and extrapolation of the regression line to normal plasma levels yielded a tissue level close to that measured in the donor hearts. The cardiac tissue-to-plasma concentration (T/P) ratios for renin and prorenin were threefold the ratio for albumin, which indicates that the tissue levels were too high to be accounted for by admixture with blood and diffusion into the interstitial fluid. Cell membranes from porcine cardiac tissue bound porcine renin with high affinity. The T/P ratio for ACE, which is membrane bound, was fivefold the ratio for albumin. Cardiac angiotensinogen was lower in DCM patients than in the donors, and its T/P ratio was half that for albumin, which is compatible with substrate consumption by cardiac renin.

CONCLUSIONS

These data in patients with heart failure support the concept of local angiotensin production in the heart by renin that is taken up from the circulation. Membrane binding may be part of the uptake process.

The place of renal scintigraphy in the diagnosis of renal artery stenosis. Fifteen years of clinical experience

BACKGROUND

Renal scintigraphy with radiolabeled pentetic acid (diethylenetriamine pentaacetic acid [DTPA]) or, more recently, mertiatide (mercaptoacetyltriglycine [MAG3]), with or without captopril challenge, is widely recommended as a diagnostic test for renal artery stenosis.

OBJECTIVES

To address (1) whether the diagnostic accuracy has been improved by the use of captopril and the introduction of mertiatide and (2) whether a renal scan that shows abnormalities is a useful criterion to select patients for renal arteriography.

PATIENTS AND METHODS

A standard diagnostic protocol, using both scintigraphy and arteriography, was followed in 505 consecutive high-risk hypertensive patients who were evaluated for renovascular hypertension at the University Hospital Dijkzigt, Rotterdam, the Netherlands, from 1978 to 1992.

RESULTS

Renal artery stenosis (> or = 50%) was present in 263 patients. When the single-kidney fractional uptake was used as a diagnostic criterion, a specificity of 0.90 was obtained at a cutoff value of 35% for the worst kidney in scintigraphy using pentetic acid without captopril challenge (n = 225) and at a cutoff value of 37% after captopril challenge (n = 280). This was associated with sensitivity levels of 0.65 and 0.68, respectively. The difference between the uptake of pentetic acid with and without captopril challenge in the 85 patients who were studied under both circumstances was no more accurate as a predictor of renal artery stenosis. In the 93 patients who were studied with mertiatide as well as with pentetic acid, both after captopril challenge, the diagnostic accuracy was no better with mertiatide than with pentetic acid; mertiatide failed to offer any advantage not only when the single-kidney fractional uptake was used as a criterion, but also with the use of other scintigraphic parameters (eg, time to peak, time to pyelum, overall shape of renographic curve, and kidney size).

CONCLUSIONS

The diagnostic accuracy of renal scintigraphy has not been improved by the introduction of mertiatide or by the use of captopril. The usefulness of scintigraphy as a diagnostic test for the presence of renal artery stenosis remains questionable. The physician will always confront either a substantial number of arteriograms that do not show abnormalities when renal scintigraphy is omitted as a screening step or a substantial number of missed diagnoses when a renal scan that shows abnormalities is used as a prerequisite for arteriography.

Renin-angiotensin system components in the interstitial fluid of the isolated perfused rat heart. Local production of angiotensin I

We used a modification of the isolated perfused rat heart, in which coronary effluent and interstitial transudate were separately collected, to investigate the uptake and clearance of exogenous renin, angiotensinogen, and angiotensin I (Ang I) as well as the cardiac production of Ang I. The levels of these compounds in interstitial transudate were considered to be representative of the levels in the cardiac interstitial fluid. During perfusion with renin or angiotensinogen, the steady-state levels (mean +/- SD) in interstitial transudate were 64 +/- 34% (P < .05 for difference from the arterial level, n = 8) and 108 +/- 42% (n = 6) of the arterial level, respectively; the levels in coronary effluent were not significantly different from those in interstitial transudate. Ang I was not detectable in interstitial transudate during perfusion with Tyrode's buffer or angiotensinogen. It was very low in interstitial transudate during perfusion with renin and rose to much higher levels during combined renin and angiotensinogen perfusion. The total production rate of Ang I present in interstitial fluid could be largely explained by the renin-angiotensinogen reaction in the fluid phase of the interstitial compartment. In contrast, the total production rate of Ang I present in coronary effluent and the net ejection rate of Ang I via coronary effluent were, respectively, 4.6 +/- 2.2 and 2.8 +/- 1.3 (P < .01 and P < .05 for difference from 1.0, n = 6) times higher than could be explained by Ang I formation in the fluid phase of the intravascular compartment. Ang I from the interstitial fluid contributed little to the Ang I in the intravascular fluid and vice versa. These data reveal two tissue sites of Ang I production, ie, the interstitial fluid and a site closer to the blood compartment, possibly vascular surface-bound renin. There was no evidence that the release of locally produced Ang I into coronary effluent and interstitial transudate occurred independently of blood-derived renin or angiotensinogen.

Plasma semicarbazide-sensitive amine oxidase is elevated in patients with congestive heart failure

OBJECTIVE

Semicarbazide-sensitive amine oxidase (SSAO) is present in various mammalian tissues, especially in vascular smooth muscle cells, but also in plasma. The enzyme has been suggested to play a role in vascular endothelial damage through conversion of amines into cytotoxic aldehydes, ammonia and hydrogen peroxide. Endothelial dysfunction is present in diabetes mellitus (DM) and congestive heart failure (CHF). Elevated plasma SSAO activities have been reported in patients with DM, but no data on patients with CHF are as yet available.

METHODS AND RESULTS

Plasma SSAO was measured in 271 patients with CHF and compared to values in 77 controls. SSAO was found to be elevated in patients with CHF compared to controls (589 +/- 252 vs. 455 +/- 114 mU/l; P < 0.0001). Plasma SSAO was higher in NYHA class III/IV than in class III (662 +/- 288 vs. 555 +/- 226 mU/l; P = 0.004) and also higher in patients with concomitant DM than in those without (706 +/- 248 vs. 557 +/- 245 mU/l; P < 0.0001). Plasma SSAO correlated with plasma atrial natriuretic peptide (r = 0.42; P < 0.0001), with plasma norepinephrine (r = 0.27; P < 0.0001) and with left ventricular ejection fraction (r = -0.13; P = 0.0162). Multiple regression analysis showed atrial natriuretic peptide, norepinephrine, DM and cardiothoracic ratio to be the main determinants of plasma SSAO.

CONCLUSION

The finding of elevated plasma SSAO in CHF, increasing with severity of the disease and with the concomitant presence of DM, supports the suggestion that SSAO may be involved in the pathogenesis of vascular endothelial damage. Plasma SSAO may be a useful parameter in assessing severity of CHF and in prognostic evaluation. Pharmacologic manipulation of SSAO activity might be an interesting new concept for prevention of vascular endothelial damage in various vascular disease entities.

Renal artery stenosis towards the year 2000

FUNCTIONAL DIAGNOSTIC TESTS FOR SELECTING PATIENTS FOR RENAL ARTERIOGRAPHY: The functional diagnostic tests now available to select patients for renal arteriography, such as captopril-renal scintigraphy and the captopril-peripheral vein renin test, have a high degree of diagnostic accuracy, 70% sensitivity at 90% specificity, but they are still not good enough to assess the large population of hypertensives for renal artery stenosis. The use of the angiotensin converting enzyme (ACE) inhibitor challenge and the introduction of a new pharmacon, technetium-labelled mercapto-triglycine, have not sufficiently increased the value of scintigraphy and further improvement in the near future seems unlikely. THE USE OF CLINICAL CRITERIA TO IDENTIFY HIGH-RISK PATIENTS: A more realistic approach to the diagnosis of renal artery stenosis is to seek simple and sensible clinical criteria to identify high-risk patients. The blood pressure response to 2 months of treatment with a standardized two-drug regimen is a suitable criterion. Interim analysis of an ongoing prospective multicentre study in the Netherlands indicates that the prevalence of renal artery stenosis is as high as 25% in patients who are resistant to the combination of 10 mg amlodipine + 50 mg atenolol, compared with 15% in patients resistant to the combination of 20 mg enalapril + 25 mg hydrochlorothiazide. When drug resistance is used as the selection criterion for arteriography, the total number of cases detected with intra-arterial digital subtraction angiography does not seem to differ according to which of these drug regimens is used. FUTURE POSSIBILITIES: The current enthusiasm for percutaneous transluminal renal angioplasty (PTRA) and stenting may be premature. With further improvement of non-invasive techniques to visualize the renal arteries, such as colour Doppler ultrasound with microparticle contrast enhancement or magnetic resonance angiography, it should become more attractive to follow patients with renal artery stenosis while they are being treated with drugs rather than automatically resorting to PTRA.

Is there an internal cardiac renin-angiotensin system?

The beneficial effects of angiotensin converting enzyme (ACE) inhibitors in heart failure appear to be independent, at least in part, of their effect on blood pressure. The existence of a local cardiac renin angiotensin system is often suggested as an explanation. It has been known for some time that a substantial proportion of arterially delivered angiotensin I is converted to angiotensin II by ACE of the coronary vascular endothelium. The levels of angiotensin II in cardiac tissue are several times the levels of angiotensin II in circulating blood. Recent evidence suggests that most of the angiotensin II in the heart is not derived from angiotensin I in the circulation, and that most of the angiotensin I in cardiac tissue is generated in the tissue itself. On the other hand, renin mRNA levels are very low or undetectable in the normal heart. In addition, studies on the effects of bilateral nephrectomy on the cardiac tissue levels of renin, angiotensin I, and angiotensin II in pigs have indicated that cardiac renin originates from the kidney and that cardiac generation of angiotensin I and angiotensin II depends on renin from the kidney. Intracardiac synthesis of renin may occur under pathological conditions and during fetal development. The fact that angiotensins are generated by the heart raises the possibility of local mechanisms to regulate the concentrations of these peptides at certain tissue sites. For example, preliminary evidence suggests that binding of renin to cardiac membranes is a mechanism by which renin is taken up by the heart. A specific renin binding protein has been identified in cardiac tissue. Cardiac ACE levels may also influence local angiotensin II formation and are, in part, determined by the so called insertion/deletion ACE gene polymorphism. More detailed knowledge on the site of angiotensin generation and on its regulation will improve our understanding of the role of the renin-angiotensin system in cardiac function, hypertrophy, and postinfarction remodelling.

Comparison of N-terminal pro-atrial natriuretic peptide and atrial natriuretic peptide in human plasma as measured with commercially available radioimmunoassay kits

Atrial natriuretic peptide (ANP) has become an important parameter for assessing the condition of patients with cardiac disease. Recently, attention has also focused on N-terminal pro-atrial natriuretic peptide (NtproANP) in this context. NtproANP circulates in plasma in higher concentration, is more stable ex vivo, and may be a better parameter for cardiac function over time. We have evaluated a new commercially available radiommunoassay kit for NtproANP and compared results and method with those of ANP measurements. The NtproANP kit was found to be reliable and easy to use (no plasma extraction step is necessary), with good reproducibility (coefficients of variation 7-15%). Normal values in 15 healthy laboratory workers, 25 healthy elderly subjects and 25 patients with heart failure were 207 +/- 70, 368 +/- 134 and 1206 +/- 860 pmol/l, respectively, 8.3, 11.8 and 13.0 times higher, respectively, than corresponding ANP concentrations. NtproANP correlated well with ANP (r 0.64-0.78). We conclude that plasma NtproANP measurement may be a good alternative to plasma ANP measurement: technically, it is easier to perform, and NtproANP is more stable in plasma. Whether NtproANP is a better diagnostic and prognostic parameter than ANP remains to be further established.

Cyclosporin A impairs the nocturnal blood pressure fall in renal transplant recipients

In renal transplant recipients, hypertension and a diminished nocturnal blood pressure fall are frequently found. To investigate whether this diminished nocturnal blood pressure fall is related to the use of cyclosporin A or to other factors, such as the use of glucocorticoids, we measured 24-hour ambulatory blood pressure in 18 renal transplant recipients both before and 16 weeks after conversion from cyclosporin A to azathioprine. Renal blood flow and glomerular filtration rate were estimated from 131I-hippurate and 125I-iothalamate clearances, respectively, and plasma concentrations of renin, atrial natriuretic peptide, norepinephrine, prostaglandin E2, and thromboxane B2 were determined. During cyclosporin A treatment, mean 24-hour blood pressure was 117 +/- 3 mm Hg, and the nocturnal fall in blood pressure was 4 +/- 9 mm Hg. A nondipping diurnal blood pressure pattern was present in 13 patients. After conversion to azathioprine, mean 24-hour blood pressure decreased to 109 +/- 3 mm Hg (P < .001), the nocturnal fall increased to 9 +/- 6 mm Hg, and the number of patients with a nondipping diurnal blood pressure pattern decreased to 9. The nocturnal fall in heart rate (17 +/- 10 beats per minute) during cyclosporin A did not change after conversion. Body weight and plasma concentrations of norepinephrine and renin did not change. Plasma concentrations of prostaglandin E2 and thromboxane B2 decreased after conversion, as did plasma atrial natriuretic peptide. Renal blood flow and glomerular filtration rate increased after conversion. In conclusion, cyclosporin A appears to be involved in the disturbance of the circadian blood pressure rhythm in renal transplant recipients. Although the precise mechanism is unclear. the elevated plasma atrial natriuretic peptide and slightly suppressed plasma renin concentrations suggest that intravascular volume expansion may contribute to the observed hemodynamic alterations.

Clinical validation of renin monoclonal antibody-based sandwich assays of renin and prorenin, and use of renin inhibitor to enhance prorenin immunoreactivity

Newly developed IRMAs to measure the plasma concentrations of renin and prorenin were validated for clinical use and compared with a classical enzyme kinetic assay. The IRMAs involve two monoclonal antibodies, one that reacts equally well with renin and prorenin and one that recognizes renin well but prorenin only minimally. Prorenin reactivity with the second antibody was enhanced by adding the renin inhibitor, Remikiren, to plasma. The complex of prorenin with this active-site ligand undergoes a conformational change, whereby prorenin is converted into a form that cannot be differentiated from renin by the IRMA. The linear working range of the assay was 4.0-3000 mU/L. The concentration of prorenin was calculated by subtracting the assay result obtained without Remikiren (i.e., renin) from the result obtained with Remikiren (i.e., renin plus prorenin). No more than 2% of prorenin present in plasma was detected as renin. The interassay CVs for renin quantification were 18%, 13%, and 8% at low, medium, and high concentrations, respectively. The interassay CV for calculated prorenin was 8% at both low and high concentrations. The IRMA results were highly correlated with those of an enzyme kinetic assay in healthy subjects; in patients with such conditions as primary hyperaldosteronism, renovascular hypertension, and low-, medium-, and high-renin essential hypertension; and in women undergoing gonadotropin stimulation.

Clinical validation of renin monoclonal antibody-based sandwich assays of renin and prorenin, and use of renin inhibitor to enhance prorenin immunoreactivity

Newly developed IRMAs to measure the plasma concentrations of renin and prorenin were validated for clinical use and compared with a classical enzyme kinetic assay. The IRMAs involve two monoclonal antibodies, one that reacts equally well with renin and prorenin and one that recognizes renin well but prorenin only minimally. Prorenin reactivity with the second antibody was enhanced by adding the renin inhibitor, Remikiren, to plasma. The complex of prorenin with this active-site ligand undergoes a conformational change, whereby prorenin is converted into a form that cannot be differentiated from renin by the IRMA. The linear working range of the assay was 4.0-3000 mU/L. The concentration of prorenin was calculated by subtracting the assay result obtained without Remikiren (i.e., renin) from the result obtained with Remikiren (i.e., renin plus prorenin). No more than 2% of prorenin present in plasma was detected as renin. The interassay CVs for renin quantification were 18%, 13%, and 8% at low, medium, and high concentrations, respectively. The interassay CV for calculated prorenin was 8% at both low and high concentrations. The IRMA results were highly correlated with those of an enzyme kinetic assay in healthy subjects; in patients with such conditions as primary hyperaldosteronism, renovascular hypertension, and low-, medium-, and high-renin essential hypertension; and in women undergoing gonadotropin stimulation.