Renal secretion of inactive renin

Human plasma contains a non-dialyzable factor which is not enzymatically active in its native form but shows renin-like activity after exposure to low pH (inactive renin). In 3 out of 13 patients with renovascular hypertension the renal to peripheral vein ratio for inactive renin on the affected side was above 1.40, indicating renal release of this form of renin. In 4 cases a high renal to peripheral vein ratio for active renin was associated with a ratio for inactive renin below 0.80, indicating renal activation of circulating inactive renin. The results suggest a renal mechanism for modulating the degree of activation of renin. They have some practical implications for the pre-operative evaluation of patients with renovascular hypertension.

Body fluid volumes and the response of renin and aldosterone to short- and long-term thiazide therapy of essential hypertension

Plasma volume (PV), extracellular fluid volume (ECV) serum electrolytes, renin and aldosterone were measured before and after 1 week and 4 months of hydrochlorothiazide (HCT) treatment, 50 mg twice daily, in nine male patients with uncomplicated essential hypertension. All studies were carried out under strictly standardized conditions in a metabolic ward. After 1 week of HCT treatment, significant reductions were found in PV and ECV, but after 4 months only ECV was significantly reduced. During HCT therapy, renin and aldosterone were permanently elevated whereas serum sodium and potassium were lowered. After 1 week, renin was inversely correlated with PV and ECV and directly correlated with heart rate. After 4 months, renin was inversely correlated with serum sodium. These results indicate a permanent decrease in ECV during long-term HCT therapy. It is further suggested that the mechanisms responsible for the renin response during short- and long-term HCT treatment are different, changes in body fluid volumes and increased neural activity being responsible for the initial rise in renin, and serum sodium being the predominant factor during chronic treatment.

Intrarenal angiotensin II: interstitial and cellular levels and site of production

BACKGROUND

Both local production and angiotensin II subtype 1 (AT1) receptor-mediated uptake from the circulation contribute to the high levels of angiotensin (Ang) II in the kidney. It is largely unknown where Ang II is produced in the kidney and how much of it originates from the circulation.

METHODS

The concentrations of endogenous and 125I-labeled Ang I and II were measured in renal tissue and in blood from pigs receiving systemic infusions of 125I-Ang I. Pigs were either untreated or treated with the angiotensin converting enzyme (ACE) inhibitor captopril or the AT1 receptor antagonist eprosartan.

RESULTS

125I-Ang I was undetectable in renal tissue but the steady-state concentrations of 125I-Ang II in cortical and medullary tissue were four and two times the concentration in arterial blood plasma, respectively. The tissue concentrations of endogenous Ang II were 100 and 60 times higher than in arterial plasma. Eprosartan reduced 125I-Ang II accumulation by 90%, but did not lower tissue Ang II. Captopril did not alter either 125I-Ang II accumulation or tissue Ang II.

CONCLUSIONS

The bulk of Ang II in the kidney is cell-associated. The high tissue/blood concentration ratio of endogenous Ang II may depend on the same mechanism as demonstrated for 125I-Ang II, that is, AT1 receptor-mediated binding to cells and endocytosis. If so, the results indicate that most renal AT1 receptors are exposed to locally generated Ang II rather than Ang II from the circulation. We propose the existence of a low-Ang II vascular system-related interstitial compartment that is separate from tubular fluid, where, according to micropuncture studies, Ang II levels might be high.

Resistance to antihypertensive medication as predictor of renal artery stenosis: comparison of two drug regimens

BACKGROUND

Renal artery stenosis is among the most common curable causes of hypertension. The definitive diagnosis is made by renal angiography, an invasive and costly procedure. The prevalence of renal artery stenosis is less than 1% in non-selected hypertensive patients but is higher when hypertension is resistant to drugs.

OBJECTIVE

To study the usefulness of standardised two-drug regimens for identifying drug-resistant hypertension as a predictor of renal artery stenosis.

DESIGN AND SETTING

Prospective cohort study carried out in 26 hospitals in The Netherlands.

PATIENTS

Patients had been referred for analysis of possible secondary hypertension or because hypertension was difficult to treat. Patients < or =40 years of age were assigned to either amlodipine 10 mg or enalapril 20 mg, and patients >40 years to either amlodipine 10 mg combined with atenolol 50 mg or to enalapril 20 mg combined with hydrochlorothiazide 25 mg. Renal angiography was performed: (1) if hypertension was drug-resistant, ie if diastolic pressure remained > or =95 mm Hg at three visits 1-3 weeks apart or an extra drug was required, and/or (2) if serum creatinine rose by > or =20 micromol/L (> or =0.23 mg/dL) during ACE inhibitor treatment.

RESULTS

Of the 1106 patients with complete follow-up, 1022 had been assigned to either the amlodipine- or enalapril-based regimens, 772 by randomisation. Drug-resistant hypertension, as defined above, was identified in 41% of the patients, and 20% of these had renal artery stenosis. Renal function impairment was observed in 8% of the patients on ACE inhibitor, and this was associated with a 46% prevalence of renal artery stenosis. In the randomised patients, the prevalence of renal artery stenosis did not differ between the amlodipine- and enalapril-based regimens.

CONCLUSIONS

In the diagnostic work-up for renovascular hypertension the use of standardised medication regimens of maximally two drugs, to identify patients with drug-resistant hypertension, is a rational first step to increase the a priori chance of renal artery stenosis. Amlodipine- or enalapril-based regimens are equally effective for this purpose.

Prorenin accumulation and activation in human endothelial cells: importance of mannose 6-phosphate receptors

ACE inhibitors improve endothelial dysfunction, possibly by blocking endothelial angiotensin production. Prorenin, through its binding and activation by endothelial mannose 6-phosphate (M6P) receptors, may contribute to this production. Here, we investigated this possibility as well as prorenin activation kinetics, the nature of the prorenin-activating enzyme, and M6P receptor-independent prorenin binding. Human umbilical vein endothelial cells (HUVECs) were incubated with wild-type prorenin, K/A-2 prorenin (in which Lys42 is mutated to Ala, thereby preventing cleavage by known proteases), M6P-free prorenin, and nonglycosylated prorenin, with or without M6P, protease inhibitors, or angiotensinogen. HUVECs bound only M6P-containing prorenin (K(d) 0.9+/-0.1 nmol/L, maximum number of binding sites [B(max)] 1010+/-50 receptors/cell). At 37 degrees C, because of M6P receptor recycling, the amount of prorenin internalized via M6P receptors was >25 times B(max). Inside the cells, wild-type and K/A-2 prorenin were proteolytically activated to renin. Renin was subsequently degraded. Protease inhibitors interfered with the latter but not with prorenin activation, thereby indicating that the activating enzyme is different from any of the known prorenin-activating enzymes. Incubation with angiotensinogen did not lead to endothelial angiotensin generation, inasmuch as HUVECs were unable to internalize angiotensinogen. Most likely, therefore, in the absence of angiotensinogen synthesis or endocytosis, M6P receptor-mediated prorenin internalization by endothelial cells represents prorenin clearance.

Angiotensin converting enzyme is the main contributor to angiotensin I-II conversion in the interstitium of the isolated perfused rat heart

OBJECTIVES

Recent studies in homogenized hearts suggest that chymase rather than angiotensin converting enzyme (ACE) is responsible for cardiac angiotensin I to angiotensin II conversion. We investigated in intact rat hearts whether (i) enzymes other than ACE contribute to angiotensin I to angiotensin II conversion and (ii) the localization (endothelial/extra-endothelial) of converting enzymes.

DESIGN AND METHODS

We used a modified version of the rat Langendorff heart, allowing separate collection of coronary effluent and interstitial fluid. Hearts were perfused with angiotensin I (arterial concentration 5-10 pmol/ml) under control conditions, in the presence of captopril (1 micromol/l) or after endothelium removal with 0.2% triton X-100. Endothelium removal was verified as the absence of a coronary vasodilator response to 10 nmol bradykinin. Angiotensin I and angiotensin II were measured in coronary effluent and interstitial fluid with sensitive radioimmunoassays.

RESULTS

In control hearts, 45% of arterial angiotensin I was metabolized during coronary passage, partly through conversion to angiotensin II. At steady-state, the angiotensin I concentration in interstitial fluid was three to four-fold lower than in coronary effluent, while the angiotensin II concentrations in both fluids were similar. Captopril and endothelium removal did not affect coronary angiotensin I extraction, but increased the interstitial fluid levels of angiotensin I two- and three-fold, respectively, thereby demonstrating that metabolism (by ACE) as well as the physical presence of the endothelium normally prevent arterial angiotensin I from reaching similar levels in coronary effluent and interstitial fluid. Captopril, but not endothelium removal, greatly reduced the angiotensin II levels in coronary effluent and interstitial fluid. With the ACE inhibitor, the angiotensin II/I ratios in coronary effluent and interstitial fluid were 83 and 93% lower, while after endothelium removal, the ratios were 33 and 71% lower.

CONCLUSIONS

In the intact rat heart, ACE is the main contributor to angiotensin I to angiotensin II conversion, both in the coronary vascular bed and the interstitium. Cardiac ACE is not limited to the coronary vascular endothelium.

High-affinity prorenin binding to cardiac man-6-P/IGF-II receptors precedes proteolytic activation to renin

Mannose-6-phosphate (man-6-P)/insulin-like growth factor-II (man-6-P/IgF-II) receptors are involved in the activation of recombinant human prorenin by cardiomyocytes. To investigate the kinetics of this process, the nature of activation, the existence of other prorenin receptors, and binding of native prorenin, neonatal rat cardiomyocytes were incubated with recombinant, renal, or amniotic fluid prorenin with or without man-6-P. Intact and activated prorenin were measured in cell lysates with prosegment- and renin-specific antibodies, respectively. The dissociation constant (K(d)) and maximum number of binding sites (B(max)) for prorenin binding to man-6-P/IGF-II receptors were 0.6 +/- 0.1 nM and 3,840 +/- 510 receptors/myocyte, respectively. The capacity for prorenin internalization was greater than 10 times B(max). Levels of internalized intact prorenin decreased rapidly (half-life = 5 +/- 3 min) indicating proteolytic prosegment removal. Prorenin subdivision into man-6-P-free and man-6-P-containing fractions revealed that only the latter was bound. Cells also bound and activated renal but not amniotic fluid prorenin. We concluded that cardiomyocytes display high-affinity binding of renal but not extrarenal prorenin exclusively via man-6-P/IGF-II receptors. Binding precedes internalization and proteolytic activation to renin thereby supporting the concept of cardiac angiotensin formation by renal prorenin.

Subcellular localization of angiotensin II in kidney and adrenal

OBJECTIVES

To investigate whether tissue angiotensin II generation occurs intra- or extracellularly, we studied the subcellular localization of angiotensin II in kidney and adrenal, two organs with high endogenous angiotensin II concentrations.

DESIGN AND METHODS

Tissues were obtained, following a 1 h infusion of 125I-angiotensin I or 125I-angiotensin II to simultaneously determine the localization of plasma-derived angiotensin II, from five control pigs and four pigs that had been pretreated with the AT1 receptor antagonist eprosartan. Subcellular organelles, prepared by differential centrifugation from homogenized tissue, were characterized using organelle-specific markers.

RESULTS

125I-angiotensin II and angiotensin II were present in all organelles, with identical distribution profiles. In mitochondria-enriched fractions the relative specific activities [RSAs = (concentration per mg protein in fraction)/(concentration per mg protein in homogenate)] of the two peptides were similar to those in homogenate, whereas in cytosol-enriched fractions their RSAs were five- to 10-fold lower (P< 0.05 versus homogenate). In microsome- as well as in lysosome-enriched fractions the RSAs of 125I-angiotensin II and angiotensin II were two- to four-fold higher than in homogenate (P < 0.05), and their RSAs were also higher in renal nuclei-enriched fractions (P< 0.05). Eprosartan increased plasma angiotensin II to a larger degree than tissue angiotensin II and greatly reduced tissue 125I-angiotensin II. This led to similar decreases in the tissue/plasma concentration ratios of 125I-angiotensin II and angiotensin II. The subcellular distribution of both angiotensin II peptides was not affected by eprosartan.

CONCLUSIONS

Local angiotensin II synthesis in adrenal and kidney occurs predominantly extracellularly, and is followed by rapid AT1 receptor-mediated endocytosis, thereby leading to high intracellular angiotensin II levels.

Cardiomyocytes bind and activate native human prorenin : role of soluble mannose 6-phosphate receptors

Cardiomyocytes bind, internalize, and activate recombinant human prorenin through mannose 6-phosphate/insulin-like growth factor II (M6P/IGFII) receptors. To investigate whether this also applies to native human prorenin, neonatal rat myocytes were incubated for 4 hours at 37 degrees C with various prorenin-containing human body fluids. Uptake and activation by M6P/IGFII receptors were observed for plasma prorenin from subjects with renal artery stenosis and/or hypertension and for follicular fluid prorenin. The total amount of cellular renin and prorenin (expressed as percentage of the levels of renin and prorenin in the medium) after 4 hours of incubation was 4 to 10 times lower than after incubation with recombinant human prorenin. Although plasma contains alkaline phosphatases capable of inactivating the M6P label as well as soluble M6P/IGFII receptors that block prorenin binding in a competitive manner and proteins (eg, insulin, IGFII) that increase the number of cell-surface M6P/IGFII receptors, these factors were not responsible for the modest uptake of native human prorenin. Uptake did not occur during incubation of myocytes with plasma prorenin from anephric subjects or with amniotic fluid prorenin, and this was not due to the presence of excessively high levels of M6P/IGFII receptors and/or phosphatase activity in these fluids. In conclusion, myocytes are capable of binding, internalizing, and activating native human prorenin of renal and ovarian origin through M6P/IGFII receptors. Differences in prorenin glycosylation and/or phosphorylation as well as the concentration of soluble M6P/IGFII receptors and growth factors affecting cell-surface M6P/IGFII receptor density determine the amount of prorenin entering the heart and thus cardiac angiotensin II production.

Angiotensin-converting enzyme inhibition and angiotensin II type 1 receptor blockade prevent cardiac remodeling in pigs after myocardial infarction: role of tissue angiotensin II

BACKGROUND

The mechanisms behind the beneficial effects of renin-angiotensin system blockade after myocardial infarction (MI) are not fully elucidated but may include interference with tissue angiotensin II (Ang II).

METHODS AND RESULTS

Forty-nine pigs underwent coronary artery ligation or sham operation and were studied up to 6 weeks. To determine coronary angiotensin I (Ang I) to Ang II conversion and to distinguish plasma-derived Ang II from locally synthesized Ang II, (125)I-labeled and endogenous Ang I and II were measured in plasma and in infarcted and noninfarcted left ventricle (LV) during (125)I-Ang I infusion. Ang II type 1 (AT(1)) receptor-mediated uptake of circulating (125)I-Ang II was increased at 1 and 3 weeks in noninfarcted LV, and this uptake was the main cause of the transient elevation in Ang II levels in the noninfarcted LV at 1 week. Ang II levels and AT(1) receptor-mediated uptake of circulating Ang II were reduced in the infarct area at all time points. Coronary Ang I to Ang II conversion was unaffected by MI. Captopril and the AT(1) receptor antagonist eprosartan attenuated postinfarct remodeling, although both drugs increased cardiac Ang II production. Captopril blocked coronary conversion by >80% and normalized Ang II uptake in the noninfarcted LV. Eprosartan did not affect coronary conversion and blocked cardiac Ang II uptake by >90%.

CONCLUSIONS

Both circulating and locally generated Ang II contribute to remodeling after MI. The rise in tissue Ang II production during angiotensin-converting enzyme inhibition and AT(1) receptor blockade suggests that the antihypertrophic effects of these drugs result not only from diminished AT(1) receptor stimulation but also from increased stimulation of growth-inhibitory Ang II type 2 receptors.

The effect of balloon angioplasty on hypertension in atherosclerotic renal-artery stenosis. Dutch Renal Artery Stenosis Intervention Cooperative Study Group

BACKGROUND

Patients with hypertension and renal-artery stenosis are often treated with percutaneous transluminal renal angioplasty. However, the long-term effects of this procedure on blood pressure are not well understood.

METHODS

We randomly assigned 106 patients with hypertension who had atherosclerotic renal-artery stenosis (defined as a decrease in luminal diameter of 50 percent or more) and a serum creatinine concentration of 2.3 mg per deciliter (200 micromol per liter) or less to undergo percutaneous transluminal renal angioplasty or to receive drug therapy. To be included, patients also had to have a diastolic blood pressure of 95 mm Hg or higher despite treatment with two antihypertensive drugs or an increase of at least 0.2 mg per deciliter (20 micromol per liter) in the serum creatinine concentration during treatment with an angiotensin-converting-enzyme inhibitor. Blood pressure, doses of antihypertensive drugs, and renal function were assessed at 3 and 12 months, and patency of the renal artery was assessed at 12 months.

RESULTS

At base line, the mean (+/-SD) systolic and diastolic blood pressures were 179+/-25 and 104+/-10 mm Hg, respectively, in the angioplasty group and 180+/-23 and 103+/-8 mm Hg, respectively, in the drug-therapy group. At three months, the blood pressures were similar in the two groups (169+/-28 and 99+/-12 mm Hg, respectively, in the 56 patients in the angioplasty group and 176+/-31 and 101+/-14 mm Hg, respectively, in the 50 patients in the drug-therapy group; P=0.25 for the comparison of systolic pressure and P=0.36 for the comparison of diastolic pressure between the two groups); at the time, patients in the angioplasty group were taking 2.1+/-1.3 defined daily doses of medication and those in the drug-therapy group were taking 3.2+/-1.5 daily doses (P<0.001). In the drug-therapy group, 22 patients underwent balloon angioplasty after three months because of persistent hypertension despite treatment with three or more drugs or because of a deterioration in renal function. According to intention-to-treat analysis, at 12 months, there were no significant differences between the angioplasty and drug-therapy groups in systolic and diastolic blood pressures, daily drug doses, or renal function.

CONCLUSIONS

In the treatment of patients with hypertension and renal-artery stenosis, angioplasty has little advantage over antihypertensive-drug therapy.

Functional importance of angiotensin-converting enzyme-dependent in situ angiotensin II generation in the human forearm

To assess the importance for vasoconstriction of in situ angiotensin (Ang) II generation, as opposed to Ang II delivery via the circulation, we determined forearm vasoconstriction in response to Ang I (0.1 to 10 ng. kg(-1). min(-1)) and Ang II (0.1 to 5 ng. kg(-1). min(-1)) in 14 normotensive male volunteers (age 18 to 67 years). Changes in forearm blood flow (FBF) were registered with venous occlusion plethysmography. Arterial and venous blood samples were collected under steady-state conditions to quantify forearm fractional Ang I-to-II conversion. Ang I and II exerted the same maximal effect (mean+/-SEM 71+/-4% and 75+/-4% decrease in FBF, respectively), with similar potencies (mean EC(50) [range] 5.6 [0.30 to 12.0] nmol/L for Ang I and 3.6 [0.37 to 7.1] nmol/L for Ang II). Forearm fractional Ang I-to-II conversion was 36% (range 18% to 57%). The angiotensin-converting enzyme (ACE) inhibitor enalaprilat (80 ng. kg(-1). min(-1)) inhibited the contractile effects of Ang I and reduced fractional conversion to 1% (0.1% to 8%), thereby excluding a role for Ang I-to-II converting enzymes other than ACE (eg, chymase). The Ang II type 1 receptor antagonist losartan (3 mg. kg(-1). min(-1)) inhibited the vasoconstrictor effects of Ang II. In conclusion, the similar potencies of Ang I and II in the forearm, combined with the fact that only one third of arterially delivered Ang I is converted to Ang II, suggest that in situ-generated Ang II is more important for vasoconstriction than circulating Ang II. Local Ang II generation in the forearm depends on ACE exclusively and results in vasoconstriction via Ang II type 1 receptors.

Cardiac interstitial fluid levels of angiotensin I and II in the pig

OBJECTIVE

To study whether cardiac interstitial fluid levels of angiotensin I and II (Ang I and II) can be monitored in vivo, using the microdialysis technique, and to assess the contribution of plasma-derived angiotensins to the interstitial fluid levels of these peptides.

DESIGN AND METHODS

Microdialysis probes were placed in the left ventricular (LV) myocardium of eight anaesthetized pigs, three of which were untreated and five treated with the angiotensin II type 1 (AT1) receptor antagonist L-158,809 (10 mg intracoronary). All pigs were given a 1 h intracoronary infusion of 125I-Ang II. Aortic and coronary venous blood samples were taken under steady-state conditions, and interstitial dialysate was collected during the entire infusion period. Immediately after stopping the infusion, LV tissue pieces were obtained at various time points.

RESULTS

L-158,809 did not affect the levels of endogenous Ang I and II or the levels of plasma 125I-Ang II. Aortic Ang I and II levels (22 and 16 fmol/ml; geometric mean of eight pigs) were comparable to coronary venous Ang I and II levels, whereas the coronary venous 125I-Ang II levels (6650 c.p.m./ml) were approximately 30 times higher than those in the aorta. Tissue Ang I and II levels were 5 and 17 fmol/g, respectively. In untreated animals, the 125I-Ang II levels per g LV tissue were similar to the levels per ml coronary venous plasma, and the ex vivo half-life of tissue 1251-Ang II was > 30 min. In treated animals, tissue 125I-Ang II was < 5% of coronary venous 125I-Ang II and became undetectable within 15 min. 125I-Ang II, Ang I and Ang II levels in the interstitial fluid were close to or below the detection limit (200 c.p.m., 60 fmol and 20 fmol per ml, respectively) in all animals.

CONCLUSIONS

Plasma and myocardial interstitial fluid angiotensin levels are of the same order of magnitude. Plasma Ang II does not contribute to the interstitial fluid level of Ang II, most likely because of its rapid metabolism in the vascular wall. Binding to AT1 receptors protects Ang II against metabolism.

Angiotensin I to angiotensin II conversion in the human forearm and leg. Effect of the angiotensin converting enzyme gene insertion/deletion polymorphism

OBJECTIVE

The angiotensin-converting enzyme (ACE) gene I/D polymorphism accounts for part of the variation in ACE concentration; subjects with one or two D alleles have approximately 25 and 50% higher ACE levels, respectively, than subjects with two I alleles. Data from studies on the pressor effects of angiotensin (Ang) I in DD compared with II subjects are inconsistent, because enhanced conversion in DD subjects may have been masked by a decreased responsiveness to Ang II. Here we quantify ACE genotype-related Ang I to Ang II conversion in the human forearm and leg using non-pressor 125I-Ang I infusions.

DESIGN AND METHODS

Infusions were given to 12 women and 17 men (age 24-67 years) who were undergoing renal vein sampling followed by renal angiography for diagnostic purposes. 125I-Ang I was infused for 20 min into the right antecubital vein, and blood samples for the measurement of 125I-labelled and endogenous Ang I and Ang II were taken from the aorta, the left antecubital vein and a femoral vein under steady-state conditions. Genotype frequencies were determined by polymerase chain reaction.

RESULTS

Fractional conversion (i.e. the percentage of arterially delivered 125I-Ang I that is converted to 125I-Ang II) in the forearm (38+/-4, 30+/-3 and 31+/-6% in 8 II, 16 ID and 5 DD subjects, respectively; mean +/- SEM) and leg (52+/-4, 48+/-3 and 42+/-5%) was similar in all three groups. In addition, no genotype-related differences in plasma Ang II/I ratio (a measure of ACE activity) were observed at the three sampling sites.

CONCLUSIONS

Regional Ang I to Ang II conversion does not parallel the previously described D allele-related differences in ACE concentration, suggesting that effects other than enhanced conversion may underlie the reported associations between the D allele and various cardiovascular diseases.

Inter-observer variability in the angiographic assessment of renal artery stenosis. DRASTIC study group. Dutch Renal Artery Stenosis Intervention Cooperative

OBJECTIVE

To assess inter-observer agreement in the interpretation of renal angiograms.

DESIGN

Comparison of the assessment of renal angiograms by three experienced radiologists, who evaluated the number of renal arteries and the presence, location, aspect and severity of a renal artery stenosis.

SETTING

General hospital and university hospital serving urban and rural populations.

PATIENTS

Patients with difficult-to-treat hypertension referred for diagnostic work-up; 312 angiograms with the intra-arterial digital subtraction technique were obtained from 289 consecutive patients.

MAIN OUTCOME MEASURES

Inter-observer agreement was tested for the following parameters: number of arteries per kidney, presence of stenosis, location of stenosis (truncal, ostial), aspect of stenosis (concentric, eccentric, post-stenotic dilatation), severity of stenosis (reduction of lumen diameter in categories of 30%, 40%, etc. to 100%), and overall quality of the angiographic images. Kappa (kappa) values and weighted kappa between the three pairs of radiologists were used as estimates of inter-observer agreement

RESULTS

Agreement about the number of renal arteries was reasonable (kappa = 0.50-0.72), as was agreement about the presence of stenosis (kappa = 0.68-0.86). Agreement about stenosis location and aspect was poor (kappa = 0.26-0.47 and kappa = 0.15-0.26, respectively). There was general agreement about the severity of stenosis (weighted kappa = 0.65-0.70), but it was not possible to distinguish between 50 and 60% stenosis or between 60 and 70% stenosis (kappa < 0.40). No correlation was found between agreement on severity of stenosis and the quality of the images.

CONCLUSIONS

It is not realistic to make statements about what degree of renal artery stenosis is clinically significant, as long as the intra-arterial angiogram with digital subtraction remains the gold standard. It is likewise risky to rely too strongly on stenosis morphology as visualized by renal angiography in choosing between balloon angioplasty and stent deployment.

Vasoconstriction by in situ formed angiotensin II: role of ACE and chymase

OBJECTIVE

To assess the importance, for vasoconstriction, of in situ angiotensin (Ang) II generation, as opposed to ang II delivery to AT receptors via the organ bath fluid.

METHODS

Ang I and II concentration-response curves in human and porcine coronary arteries (HCAs, PCAs) were constructed in relation to estimates of the clearances of Ang I and II (ClAngI, ClAngII) from the organ bath and the release of newly formed Ang II (RAngII) into the bath fluid. HCAs were from 25 heart valve donors (age 5-54 years), and PCAs from 14 pigs (age 3 months).

RESULTS

Ang I- and II-evoked constrictions were inhibited by the AT1 receptor antagonist, irbesartan, and were not influenced by the AT2 receptor antagonist, PD123319. In HCAs Ang II was only three times more potent than Ang I, wheres, in the experiments with Ang I, comparison of ClAngI with ClAngII and RAngII indicated that most of the arterially produced Ang II did not reach the bath fluid. Also in PCAs Ang I and II showed similar potency. In HCAs both the ACE inhibitor, captopril, and the chymase inhibitor, chymostatin, inhibited Ang I-evoked vasoconstriction, while only chymostatin had a significant effect on ClAngI. In PCAs Ang I-evoked vasoconstriction was almost completely ACE-dependent.

CONCLUSIONS

This study points towards the functional importance of in situ ACE- and chymase-dependent Ang II generation, as opposed to Ang II delivery via the circulation. It also indicates that functionally relevant changes in local Ang I-II conversion are not necessarily reflected by detectable changes in circulating Ang II.

Increase in serum prorenin precedes onset of microalbuminuria in patients with insulin-dependent diabetes mellitus

AIMS/HYPOTHESIS

The renin-angiotensin system is possibly involved in the pathogenesis of diabetic nephropathy. The most striking change in renin-angiotensin system components in blood of patients with diabetic nephropathy is an increased prorenin concentration. We investigated prospectively serum concentrations of renin-angiotensin system components and the time course of prorenin increase in normoalbuminuric diabetic patients developing microalbuminuria.

METHODS

Patients (n = 199) with Type I (insulin-dependent) diabetes mellitus and normoalbuminuria at baseline were prospectively followed for 10 years. The prorenin concentrations and other variables possibly associated with the occurrence of microalbuminuria, were investigated by Cox-regression analysis.

RESULTS

Of the patients 29 developed microalbuminuria. Glycated haemoglobin values were higher at baseline in these patients. Serum prorenin was similar at baseline but rose in the 29 patients before the development of microalbuminuria and was stable in patients with stable albumin excretion. Renin, angiotensinogen and angiotensin converting enzyme serum concentrations were stable in both groups. Prorenin and glycated haemoglobin were independent prognostic factors for the development of microalbuminuria. A prognostic index, based on these variables, was constructed to estimate the relative risk of developing microalbuminuria.

CONCLUSIONS/INTERPRETATION

Increase in serum prorenin precedes onset of microalbuminuria in normotensive patients with insulin-dependent diabetes mellitus. High concentrations of prorenin in combination with high values of glycated haemoglobin can be used as a predictor of development of microalbuminuria.

Plasma renin and prorenin and renin gene variation in patients with insulin-dependent diabetes mellitus and nephropathy

BACKGROUND

The most striking abnormality in the renin angiotensin system in diabetic nephropathy (DN) is increased plasma prorenin. Renin is thought to be low or normal in DN. In spite of altered (pro)renin regulation the renin gene has not been studied for contribution to the development of DN.

METHODS

We studied plasma renin, prorenin, and four polymorphic markers of the renin gene in 199 patients with IDDM and DN, and in 192 normoalbuminuric IDDM controls matched for age, sex, and duration of diabetes. Plasma renin and total renin were measured by immunoradiometric assays. Genotyping was PCR-based.

RESULTS

Plasma renin was increased in patients with nephropathy (median (range), 26.3 (5.2-243.3) vs 18.3 (4.2-373.5) microU/ml in the normoalbuminuric group, P<0.0001). Prorenin levels were elevated out of proportion to renin levels in nephropathic patients (789 (88-5481) vs 302 (36-2226) microU/ml, P<0.0001). Proliferative retinopathy had an additive effect on plasma prorenin, but not on renin. DN was associated with a BglI RFLP in the first intron of the renin gene (bb-genotype: n=106 vs 82 in DN and normoalbuminuric patients respectively, P=0.037), but not with three other polymorphisms in the renin gene. A trend for association of higher prorenin levels with the DN-associated allele of this renin polymorphism was observed in a subgroup of patients with DN (bb vs Bb+BB, P=0.07).

CONCLUSIONS

The results indicate that in DN there is an increase in both renin and prorenin levels. A renin gene polymorphism may contribute weakly to DN. Although speculative, one of the renin gene alleles could lead to increased renin gene expression, leading to higher renin and prorenin levels. These may play a role in the pathogenesis of DN.

Cultured neonatal rat cardiac myocytes and fibroblasts do not synthesize renin or angiotensinogen: evidence for stretch-induced cardiomyocyte hypertrophy independent of angiotensin II

OBJECTIVE

The hypertrophic response of cardiomyocytes exposed to mechanical stretch is assumed to depend on the release of angiotensin (Ang) II from these cells. Here we studied the synthesis of renin-angiotensin system (RAS) components by cardiac cells under basal conditions and after stretch.

METHODS

Myocytes and fibroblasts were isolated by enzymatic dissociation from hearts of 1-3-day-old Wistar rat strain pups, grown for 1 day in serum-supplemented medium and then cultured in a chemically defined, serum-free medium. Medium and cell lysate were collected 5 days later or after exposure of the cells to cyclic stretch for 24 h. Prorenin, renin and angiotensinogen were measured by enzyme-kinetic assay; Ang I and Ang II were measured by radioimmunoassay after SepPak extraction and HPLC separation.

RESULTS

Prorenin, but none of the other RAS components, could be detected in the medium of both cell types. However, its levels were low and the Ang I-generating activity corresponding with these low prorenin levels could not be inhibited by the specific rat renin inhibitor CH-732, suggesting that it was most likely due to bovine and/or horse prorenin sequestered from the serum-containing medium to which the cells had been exposed prior to the serum-free period. When incubated with Ang I, both myocytes and fibroblasts generated Ang II in a captopril-inhibitable manner. Myocyte and fibroblast cell lysates did not contain prorenin, renin, angiotensinogen, Ang I or Ang II in detectable quantities. Stretch increased myocyte protein synthesis by 20%, but was not accompanied by Ang II release into the medium.

CONCLUSION

Cardiac myocytes and fibroblasts do not synthesize renin, prorenin or angiotensinogen in concentrations that are detectable or, it not detectable, high enough to result in Ang II concentrations of physiological relevance. These cells do synthesize ACE, thereby allowing the synthesis of Ang II at cardiac tissue sites when renin and angiotensinogen are provided via the circulation. Ang II is not a prerequisite to observe a hypertrophic response of cardiomyocytes following stretch.

Improved immunoradiometric assay for plasma renin

BACKGROUND

Our renin IRMA overestimated renin in plasmas with high prorenin-to-renin ratios. We suspected that the overestimation of renin was caused less by cross-reactivity of the renin-specific antibody with prorenin than by a conformational change of prorenin into an enzymatically active form during the assay.

METHODS

Because the inactive form of prorenin converts slowly into an active form at low temperature, we raised the assay temperature from 22 degrees C to 37 degrees C, simultaneously shortening the incubation time from 24 to 6 h. The former IRMA was performed in <1 working day with these modifications.

RESULTS

The comeasurement of prorenin as renin was eliminated. Reagents were stable at 37 degrees C, and the new and old IRMAs were comparable in terms of precision and accuracy. The functional lower limit of the assay (4 mU/L) was below the lower reference limit (9 mU/L). The modified IRMA agreed closely with the activities measured with an enzyme-kinetic assay. Results were not influenced by the plasma concentration of angiotensinogen. At normal angiotensinogen concentrations, the IRMA closely correlated with the classical enzyme-kinetic assay of plasma renin activity.

CONCLUSION

The modified IRMA, performed at 37 degrees C, avoids interference by prorenin while retaining the desirable analytical characteristics of the older IRMA and requiring less time.

Uptake and proteolytic activation of prorenin by cultured human endothelial cells

OBJECTIVE

To investigate the mechanisms of vascular uptake of prorenin and renin and to explore the possibility of vascular activation of prorenin.

DESIGN AND METHODS

Human umbilical vein endothelial cells (HUVECs) cultured in a chemically defined medium were incubated with recombinant human prorenin or renin in the presence or absence of putative inhibitors of renin internalization. Cell surface-bound and internalized prorenin or renin were separated by the acid-wash method and were quantified by enzyme-kinetic assays. The activation of prorenin was also monitored by a direct immunoradiometric assay (IRMA) with use of a monoclonal antibody directed against the -p24-Arg to -1p-Arg C-terminal propeptide sequence of prorenin.

RESULTS

Prorenin and renin were internalized at 37 degrees C in a dose-dependent manner; with 1000 microU prorenin/ml medium, the quantity of cell-associated prorenin after 3 h of incubation was 9.3 +/- 1.0 microU/4 x 10(5) cells, and with 75,000 microU/ml medium it was 670 +/- 75 microU/4 x 10(5) cells (mean +/- SD; n = 5). Results for renin were similar. Prorenin that had been treated with endoglycosidase H to remove N-linked oligosaccharides was not internalized. Addition of mannose 6-phosphate (M-6-P) to the medium caused a dose-dependent inhibition of renin and prorenin internalization. Fifty per cent inhibition was observed at 70 micromol/M-6-P, whereas mannose 1-phosphate, glucose 6-phosphate and alpha-methylmannoside at this concentration had no effect Ammonium chloride (50 mmol/l) and monensin (10 micromol/l) also inhibited internalization. Prorenin was activated by HUVECs, and cell-activated prorenin was only found in the internalized fraction, whereas the surface-bound prorenin remained inactive. Thus, it appears that the activation of prorenin took place at the time of its internalization or thereafter. The results of the prorenin IRMA indicated that activation was associated with proteolytic cleavage of the propeptide.

CONCLUSIONS

Our findings provide evidence for M-6-P receptor-dependent endocytosis of (pro)renin and proteolytic prorenin activation by vascular endothelial cells.

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.

Human renal and systemic hemodynamic, natriuretic, and neurohumoral responses to different doses of L-NAME

Experimental evidence indicates that the renal circulation is more sensitive to the effects of nitric oxide (NO) synthesis inhibition than other vascular beds. To explore whether in men the NO-mediated vasodilator tone is greater in the renal than in the systemic circulation, the effects of three different intravenous infusions of NG-nitro-L-arginine methyl ester (L-NAME; 1, 5, and 25 microg. kg-1. min-1 for 30 min) or placebo on mean arterial pressure (MAP), systemic vascular resistance (SVR), renal blood flow (RBF), renal vascular resistance (RVR), glomerular filtration rate (GFR), and fractional sodium and lithium excretion (FENa and FELi) were studied in 12 healthy subjects, each receiving randomly two of the four treatments on two different occasions. MAP was measured continuously by means of the Finapres device, and stroke volume was calculated by a model flow method. GFR and RBF were estimated from the clearances of radiolabeled thalamate and hippuran. Systemic and renal hemodynamics were followed for 2 h after start of infusions. During placebo, renal and systemic hemodynamics and FENa and FELi remained stable. With the low and intermediate L-NAME doses, maximal increments in SVR and RVR were similar: 20.4 +/- 19.6 and 23.5 +/- 16.0%, respectively, with the low dose and 31.4 +/- 26.7 and 31.2 +/- 14.4%, respectively, with the intermediate dose (means +/- SD). With the high L-NAME dose, the increment in RVR was greater than the increment in SVR. Despite a decrease in RBF, FENa and FELi did not change with the low L-NAME dose, but they decreased by 31.2 +/- 11.0 and 20.2 +/- 6.3%, respectively, with the intermediate dose and by 70.8 +/- 8.1 and 31.5 +/- 15.9% with the high L-NAME dose, respectively. It is concluded that in men the renal circulation is not more sensitive to the effects of NO synthesis inhibition than the systemic circulation and that the threshold for NO synthesis inhibition to produce antinatriuresis is higher than the threshold level to cause renal vasoconstriction.

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.