Direct radioimmunoassay of human renin: comparison with renin activity in plasma and amniotic fluid

Mineralocorticoid hypertension

Hypertension affects about 10 - 25% of the population and is an important risk factor for cardiovascular and renal disease. The renin-angiotensin system is frequently implicated in the pathophysiology of hypertension, be it primary or secondary. The prevalence of primary aldosteronism increases with the severity of hypertension, from 2% in patients with grade 1 hypertension to 20% among resistant hypertensives. Mineralcorticoid hypertension includes a spectrum of disorders ranging from renin-producing pathologies (renin-secreting tumors, malignant hypertension, coarctation of aorta), aldosterone-producing pathologies (primary aldosteronism - Conns syndrome, familial hyperaldosteronism 1, 2, and 3), non-aldosterone mineralocorticoid producing pathologies (apparent mineralocorticoid excess syndrome, Liddle syndrome, deoxycorticosterone-secreting tumors, ectopic adrenocorticotropic hormones (ACTH) syndrome, congenitalvadrenal hyperplasia), and drugs with mineraocorticoid activity (locorice, carbenoxole therapy) to glucocorticoid receptor resistance syndromes. Clinical presentation includes hypertension with varying severity, hypokalemia, and alkalosis. Ratio of plasma aldosterone concentraion to plasma renin activity remains the best screening tool. Bilateral adrenal venous sampling is the best diagnostic test coupled with a CT scan. Treatment is either surgical (adrenelectomy) for unilateral adrenal disease versus medical therapy for idiopathic, ambiguous, or bilateral disease. Medical therapy focuses on blood pressure control and correction of hypokalemia using a combination of anti-hypertensives (calcium channel blockers, angiotensin converting enzyme inhibitors, or angiotensin receptor blockers) and potassium-raising therapies (mineralcorticoid receptor antagonist or potassium sparing diuretics). Direct aldosterone synthetase antagonists represent a promising future therapy.

Adrenergic regulation of renin release and effects on angiotensin and aldosterone

This survey discloses the main mechanisms regulating renin release from the kidneys. Stimulation or inhibition of renin at least during a normal sodium intake seems to depend mostly on the sympathetic nervous system and be mediated through beta 1-adrenoceptors. The suppression of renin release is maintained during long-term treatment with both selective (beta 1) and non-selective (beta 1 + beta 2)-adrenoceptor blocking drugs. The role of alpha-adrenoceptors on renin release is less clear, both stimulating and suppressive effects having been described after treatment with alpha 1-adrenoceptor blocking therapy (i.e. prazosin). In certain conditions, i.e. when renal vascular resistance is increased or renal perfusion pressure augmented, renal prostaglandins (PG) especially PGE2, may play an important part in renin release. Angiotensin II (A II) and aldosterone generally follow the shifts in renin release. Thus, a decrease in both A II and plasma aldosterone is seen during long-term treatment with beta-adrenoceptor-blockade and may contribute to the blood-pressure lowering effect of these drugs.

Malignant Hypertension With Intestinal Ischemia Secondary to Juxtaglomerular Cell Tumor

Malignant hypertension is a well-defined condition associated with high blood pressure and acute target-organ damage. Although 95% of cases are secondary to essential hypertension, its etiological profile is broad. Juxtaglomerular cell tumor is a rare condition, with only approximately 65 cases reported to date. We describe a patient with malignant hypertension with acute renal failure and intestinal ischemia secondary to a juxtaglomerular cell tumor. We believe this is the first case of juxtaglomerular cell tumor causing malignant hypertension. The diagnostic approach and treatment are discussed.

Measurement of plasma active renin by solid phase radioimmunoassay using monoclonal antibodies

A direct radioimmunoassay (RIA) for plasma active renin concentration (ARC) was evaluated by using plasma samples obtained from hospitalized normal volunteers and hypertensive patients. The direct renin RIA was performed by using a pair of anti-renin monoclonal antibodies and a sandwich method. It is suggested that an agitator should be used during the incubation, because the magnetic solid phase was precipitated and could not be suspended well in plasmas. Further, the thawed reagents should not be used for the assay. A highly significant correlation (r = 0.96, p less than 0.01) was found between ARC and enzymatic activity of renin (PRA) in plasma samples obtained from hypertensive patients. The mean values of ARC were 22.8 +/- 3.6 pg/ml in normal subjects, 22.5 +/- 5.3 in patients with EH having medium levels of PRA, 113.7 +/- 11.7 in patients with renovascular hypertension, and undetectable in patients with primary aldosteronism. The results indicated good and reliable performance of the direct renin RIA, which is clinically useful to investigate the renin-angiotensin system.

Renin inhibition: immunological procedures and renin inhibitor peptides

Renin inhibitors represent an alternative to angiotensin-converting enzyme inhibitors (ACEI) for the treatment of hypertension. They inhibit the renin-angiotensin system at its first and rate limiting step, the renin-angiotensinogen reaction. Passive administration of angiotensinogen or renin antibodies lowers blood pressure in primates to the same extent as ACEI. Chronic active immunization against renin decreases blood pressure markedly in normotensive marmosets. Renin can be inhibited by peptides derived from its prosegment. The design of compounds based on pepstatin and on angiotensinogen sequence has led to very potent and specific human renin inhibitors. Such inhibitors are active by the IV route in primates but still lack of good oral activity.

Pharmacological investigations of a new renin inhibitor in normal sodium-unrestricted volunteers

1. CGP 38 560 A, a low-molecular-weight, non-peptidic renin inhibitor, was well tolerated upon intravenous and oral administration to recumbent healthy volunteers on an unrestricted-sodium diet. 2. After intravenous infusion over 30 min at a rate of 100 ml h-1, doses of 50, 125 and 250 micrograms kg-1 appear to induce a long-lasting inhibition of plasma renin activity. Plasma angiotensin II was decreased in a dose-dependent manner during the infusion and thereafter reverted to the initial level. A concomitant dose-related increase in active plasma renin was observed. Blood pressure was unaffected. The plasma levels of CGP 38 560 reached during infusion were at least 2000-fold higher than the theoretical inhibitory concentration based on in vitro results. 3. After oral administration in doses of 50, 100 and 200 mg CGP 38 560 A, inhibition of plasma renin activity was observed, but plasma active renin was unchanged. Blood pressure also remained unaffected. 4. CGP 38 560 was rapidly cleared from plasma with a half-life of 7.6 min for the first phase and 63 min for the second phase. Plasma levels were 100-fold lower after oral administration than after infusion, indicating a low degree of absorption (less than 1% oral bioavailability).

Modulation of renin synthesis by lipoxygenase products in cultured human mesangial cells

Modulation of renin synthesis by lipoxygenase products has been studied in cultured human mesangial cells under basal conditions and in the presence of prostaglandin (PG) E2. Total renin and cyclic AMP productions were stimulated in a dose-dependent manner (0.1-10 microM) by PGE2. The stimulatory effect of PGE2 on renin production was inhibited by 12-hydroxyeicosatetraenoic acid (12-HETE) between 0.1 and 100 nM. Extracellular and intracellular renin were affected similarly. Neither basal and PGE2-dependent cyclic AMP nor basal cyclic GMP productions were modified. 15-Hydroxyeicosatetraenoic acid (15-HPETE), 12-hydroperoxyeicosatetraenoic acid (12-HPETE) and 15-hydroperoxyeicosatetraenoic acid (15-HPETE) had the same effects as 12-HETE. Intracellular calcium concentration was not modified in the presence of 12-HETE. Since oleyl-2-acetylglycerol (OAG), an analog of diacylglycerol, also inhibited PGE2-stimulated renin production, it is hypothesized that the effect of the lipoxygenase products is mediated via protein kinase C stimulation.

Monoclonal antibodies to human renin: properties and applications

A series of 11 different monoclonal antibodies generated against human kidney renin have been characterised. Their binding affinity, inhibition of renin activity, epitope distribution, crossreactivity with related enzymes and finally in vivo pharmacological effects were analysed. All antibodies were found to be specific for primate renin recognising 6 independent antigenic structures on the renin molecule. They expressed different effects on renin activity namely (1) no inhibition, (2) only partial, or (3) complete inhibition. Partially inhibiting antibodies demonstrated specific degrees of inhibition (30, 60 or 80%). One antibody, R-36-16, demonstrated an IC 50 of 1.3 X 10(-11) M/L and, when injected into marmosets, induced complete inhibition of plasma renin activity and reduction of blood pressure. Using a selected pair of antibodies a radioimmunoassay has been established providing a fast and highly reproducible determination of human and marmoset immunoreactive renin, detecting both active and inactive renin down to concentrations of 10 pg/ml (1.25 X 10(-17) moles of renin per 50 microliter sample).

Isolation of renin-producing human cells by transfection with three simian virus 40 mutants

A human juxtaglomerular cell (JGC) tumor was used for the immortalization of renin-secreting cells. The transfection of primary JGC with three different simian virus 40 (SV40) mutants resulted in the continuous production of renin-secreting cells. The most efficient renin-producing cells (producing about 400 pg of renin per 24 hr per ml of culture medium) were those transfected with the PAS SV40 mutant. The renin production was stable and the cell cultures have been maintained for greater than 1 year. Two types of cells were cultured together and could not be separated: round and birefringent cells, which exhibited features of mast cells, and elongated cells containing myofilaments and secretory granules. Immunocytochemical staining showed the presence of renin in this latter cell type. The renin produced by the transfected cells was not stored within the cells but was released rapidly into the medium. More than 95% of the renin produced was prorenin, which, after activation, had characteristics similar to those of pure human standard renin as to its enzymatic, immunologic, and biochemical properties, except that it was less glycosylated. These stable JGC tumoral cell lines provide a unique system for studying human renin biosynthesis and its regulation in vitro.

Electrophoretic characterization of active renin from human kidney and inactive renin from a human chorionic cell culture

Enzymatically inactive human renin from chorionic cells in culture is significantly distinct in polyacrylamide gel electrophoresis (pH 8.17, 0 degree C) from active human kidney renin. The inactive renin is larger and more basic than the active renin; their molecular weights derived from gel electrophoretic retardation coefficients relate as 47.5/35.3 kDa, their valences (net protons/molecule) as 2.14/1.85. In gel electrofocusing conducted in a mixture of simple buffers, both inactive and active renins exhibit 2 components at the steady-state. The molecular size and basicity of inactive renin are consistent with the hypothesis that it may be a precursor (prorenin), although the possibility that it is an inhibitor complex cannot be ruled out.

Renin storage and cell differentiation in juxtaglomerular cell tumors: an immunohistochemical and ultrastructural study of three cases

Three renin-secreting juxtaglomerular cell tumors were studied by ultrastructural and immunocytochemical methods. Both active and inactive renins were identified in tumor extracts. By immunofluorescence and the peroxidase-antiperoxidase (PAP) method with antirenin antiserum, immunolabeling was intracytoplasmic and irregularly distributed throughout the tumor tissue. Electron microscopic examination revealed various types of secretory granules, including atypical giant crystalloid protogranules in one case, and the postembedding PAP procedure showed labeling of all types of granules. Acid phosphatase staining was observed within secretory granules and autophagic vacuoles. The process of renin storage and release is discussed. The presence in one case of a neural component and a distal tubular structure supports the view of a hamartomatous lesion.

New monoclonal antibodies directed against human renin. Powerful tools for the investigation of the renin system

Monoclonal antibodies directed against human renin were obtained by the fusing of myeloma cells with spleen cells from Balb/c or high-responder Biozzi mice injected with pure tumoral or highly purified renal renin. These procedures resulted in the production of seven stable monoclonal antibodies to human renin. Antibodies in the hybridoma culture medium were screened by binding to pure iodinated renin or insolubilized renin in a solid phase assay. The concentration of purified antibodies that provided a 50% binding to iodinated renin varied from 1 X 10(-10) to 1 X 10(-7) M. Two monoclonal antibodies were found to be potent inhibitors of renin enzymatic activity in vitro, behaving as noncompetitive inhibitors (Ki, 1 to 4 X 10(-10) M). They were specific for primate renin. Three monoclonal antibodies provided suitable immunoadsorbants for renin purification. One of these immunoadsorbants was used for large-scale purification of the renal enzyme, resulting in an 825-fold renin enrichment in a single step. Two antibodies were able to distinguish between active and inactive renin and enabled concomitant separation and purification of the two enzyme forms in various biological fluids. Monoclonal antibodies also stained human and monkey renal renin when indirect immunofluorescence and peroxidase-antiperoxidase techniques were used. A highly sensitive radioimmunometric assay of renin was constructed with two monoclonal antibodies. The sensitivity of this improved assay should permit the detection of renin in normal human plasma. Monoclonal antibodies have been shown to be superior to polyclonal antibodies in the following areas: the separation of active from inactive renin, the purification of renin from biological fluids, and the setting up of a direct assay of plasma renin.

Diagnosis and treatment of renin-secreting tumors. Report of three cases

During the past 10 years, we have found renin-secreting renal juxtaglomerular cell tumors in three hypertensive patients (two women, one man, aged 22, 69, and 21 years, respectively). The major chemical and biological findings revealed the association of severe hypertension with hypokalemia and increased plasma renin activity and plasma aldosterone. The diagnosis of such tumors is difficult, and two of the three patients were followed up for four and five years respectively before undergoing surgery. The pharmacological blockade of the renin system by various agents (beta-blockers, angiotensin II antagonists, and captopril) and its effects on blood pressure and plasma renin activity proved to be unreliable. Renal venous catheterization for renin measurements failed to provide adequate localization of the tumor. Direct radioimmunoassay, however, showed the total plasma renin to be markedly elevated. In addition, renal arteriography showed an avascular area corresponding to the renin-secreting tumor in each of the three patients. All three patients were cured of hypertension and hypokalemia by excision of the tumor.

Immunocytochemical localisation of renin in nephroblastoma

An antibody to pure human renin and an immunoperoxidase technique were used to stain immunoreactive renin in nephroblastomas. In an unselected series of surgically removed nephroblastomas immunoreactive renin was found in 10 of 19 tumours (53%). Tissue obtained at necropsy from 12 cases of disseminated nephroblastoma was also studied, and immunoreactive renin in metastatic tumours was found in three cases. Patients' blood pressures and serum potassium concentrations showed no evidence of excess renin activity. It is therefore suggested that if nephroblastomas contain renin as commonly as this study suggests, then any hormone secreted may be biologically inactive.

Renin biosynthesis by human tumoral juxtaglomerular cells. Evidences for a renin precursor

Renin biosynthesis was studied in a juxtaglomerular cell tumor. The tumoral tissue had a high renin content (180 Goldblatt Units/g of tissue), was heavily stained by immunofluorescence using human renin antiserum, and exhibited numerous characteristic secretory granules by electron microscopy. In one series of experiments, renin biosynthesis was studied in tissue slices, by following the incorporation of radiolabeled amino acids into specific immunoprecipitable renin. Time course studies showed that renin was first synthesized in a high molecular weight form, 55,000 mol wt, i.e., 10,000 mol wt higher than that of active renin, and was then converted into a 44,000-mol wt form. In a second series of experiments renin tumoral cells were cultured. Small, round, birefringent cells obtained after collagenase digestion produced renin in both primary culture and subculture media. After 5 d most of the renin found in the culture medium was inactive, but could be activated by trypsin treatment. The tumoral tissue exhibited a strong renin immunofluorescence and numerous secretory granules were observed by electron microscopy. In contrast, the renin-producing cells isolated from this tumor and grown in culture showed little renin immunofluorescence and no secretory granule could be observed. The renin-producing cells in primary culture and subculture were pulsed with radiolabeled amino acids, and immunoprecipitable radiolabeled renin was found in the culture media, thus demonstrating the actual biosynthesis of the enzyme. This renin was not stored inside cultured cells but was rapidly released into the medium and had a molecular weight of 55,000. No conversion of this inactive high molecular weight renin into the active, 44,000 mol wt form of renin was observed. We postulate the existence of two pathways for the processing, packaging, and secretion of renin in the tumoral cells: in juxtaglomerular cells of tumoral tissue renin is synthesized as a preprorenin and rapidly converted into prorenin (55,000 mol wt), which is in turn packaged in secretory granules where it is processed into active renin (44,000 mol wt) and finally secreted; in the cultured tumoral cells renin is still biosynthesized as a preprorenin molecule and then converted into prorenin, but is neither stored as granules nor processed into active renin. In this case the renin is released in an inactive form.

Characterization of inactive renin ("prorenin") from renin-secreting tumors of nonrenal origin. Similarity to inactive renin from kidney and normal plasma

Inactive renin comprises well over half the total renin in normal human plasma. There is a direct relationship between active and inactive renin levels in normal and hypertensive populations, but the proportion of inactive renin varies inversely with the active renin level; as much as 98% of plasma renin is inactive in patients with low renin, whereas the proportion is consistently lower (usually 20-60%) in high-renin states. Two hypertensive patients with proven renin-secreting carcinomas of non-renal origin (pancreas and ovary) had high plasma active renin (119 and 138 ng/h per ml) and the highest inactive renin levels we have ever observed (5,200 and 14,300 ng/h per ml; normal range 3-50). The proportion of inactive renin (98-99%) far exceeded that found in other patients with high active renin levels. A third hypertensive patient with a probable renin-secreting ovarian carcinoma exhibited a similar pattern. Inactive renins isolated from plasma and tumors of these patients were biochemically similar to semipurified inactive renins from normal plasma or cadaver kidney. All were bound by Cibacron Blue-agarose, were not retained by pepstatin-Sepharose, and had greater apparent molecular weights (Mr) than the corresponding active forms. Plasma and tumor inactive renins from the three patients were similar in size (Mr 52,000-54,000), whereas normal plasma inactive renin had a slightly larger Mr than that from kidney (56,000 vs. 50,000). Inactive renin from each source was activated irreversibly by trypsin and reversibly by dialysis to pH 3.3 at 4 degrees C; the reversal process followed the kinetics of a first-order reaction in each instance. The trypsin-activated inactive renins were all identical to semipurified active renal renin in terms of pH optimum (pH 5.5-6.0) and kinetics with homologous angiotensinogen (Michaelis constants, 0.8-1.3 microM) and inhibition by pepstatin or by serial dilutions of renin-specific antibody. These results indicate that a markedly elevated plasma inactive renin level distinguishes patients with ectopic renin production from other high-renin hypertensive states. The co-production of inactive and active renin by extrarenal neoplasms provides strong presumptive evidence that inactive renin is a biosynthetic precursor of active renin. The unusually high proportion of inactive renin in plasma and tumor extracts from such patients is consistent with ineffective precursor processing by neoplastic tissue, suggesting that if activation of "prorenin" is involved in the normal regulation of active renin levels it more likely occurs in the tissue of origin (e.g., kidney) than in the circulation.

Inhibition of renin in the primate Callithrix jacchus (common marmoset)

The effects of an antiserum against human kidney renin and of H-142, a peptide inhibitor of human renin, were studied in the primate Callithrix jacchus (common marmoset). In severely volume-depleted (low-salt diet and repetitive injections of furosemide), conscious marmosets, the antiserum reduced blood pressure to the same extent as the converting enzyme inhibitor teprotide (-31 +/- 7 SEM mmHg and -30 +/- 5 mmHg). In mildly volume-depleted (normal salt diet and single injection of furosemide), conscious marmosets, H-142 and teprotide induced a similar fall in blood pressure (-14 +/- 4 mmHg and -15 +/- 2 mmHg). When H-142 was infused after the injection of teprotide, it had no further effect on blood pressure. Conversely, teprotide was ineffective when injected during the infusion of H-142. These results show that in marmosets blood pressure can be lowered by specific inhibitors of human renin. A converting-enzyme inhibitor has similar effects. Hence the renin-angiotensin system appears to contribute significantly to the maintenance of blood pressure in conscious, volume-depleted marmosets.

In vitro biosynthesis of human renin: evidence for a high molecular weight form

Primary cultures of human chorionic cells have been developed. They secrete an inactive form of renin which can be activated by trypsin treatment. In biosynthesis experiments forming labelled renin the inactive renin exhibited a molecular weight (54 KD) higher than active kidney renin (44 KD). The same high molecular weight renin was obtained during biosynthesis experiments performed on tissue slices of human infarcted kidney. Therefore it is postulated that 54 KD biosynthetized renin might be the precursor of active renin.

Quantitation of inactive renin in human and dog plasma: techniques for activation

For human samples quantitation of inactive renin can be carried out by incubation with trypsin under defined conditions, followed by RIA of the activated renin. For dog samples we were unable to obtain evidence for the presence of inactive renin in the plasma by using trypsin, acid or cold to activate. Increases in angiotensin generation did occur with trypsin and acid but they both changed renin substrate such that the rate of angiotensin generation by exogenous renin was increased at pH 7.4, but not at pH 5.7; also following trypsin or acid treatment angiotensin I was cleaved from renin substrate by a plasma acid protease that normally does not cleave renin substrate in plasma. Therefore, for dog samples, it is important to demonstrate that an increase in the rate of angiotensin generation is indeed due to activation of inactive renin and not to changes in pH optimum of renin with angiotensinogen or to the effect of another enzyme.

Direct radioimmunoassays of renin and renin substrate during converting-enzyme inhibition

In 10 severely hypertensive patients, on a low sodium diet, converting enzyme inhibition increased plasma renin activity and decreased plasma renin substrate. The use of direct radioimmunoassays for both the enzyme and its substrate showed that the number of immunoreactive renin molecules increased from 11.3 +/- 4.9 to 31.7 +/- 25.3 pmol 1(-1) whereas the number of immunoreactive renin substrate molecules decreased from 1.04 +/- 0.35 to 0.74 +/- 0.16 mumol 1(-1). The direct radioimmunoassay for angiotensinogen gave higher values than the direct enzymatic assay, and during converting enzyme inhibition, the difference between both methods increased in proportion to the rise in circulating renin. It is concluded that the difference between the renin substrate radioimmunoassay, which measures angiotensinogen and des-angio I-angiotensinogen, and the renin substrate enzymatic assay which only measures "active" substrate, is an index of the increased consumption of renin substrate, in a situation where the fall in angiotensin II enhances renin release and decreases renin substrate release.

Monoclonal antibodies binding renal renin

Somatic-cell fusion of normal antibody-producing spleen cells with cells from a plasmacytoma culture results in a culture of hybrid cells from which a monoclonal line may be selected. These lines are immortal and may be amplified as tumors in syngeneic animals to produce large quantities of antibodies characterized by molecular homogeneity. We report the application of this technique to the production of antibodies binding canine renin. Balb/c mice were immunized with pure canine renal renin and their spleen cells fused with the NS-1 myeloma line. In two separate fusions, nine clones of cells were isolated that bound canine renal renin but did not cross-react with a number of protein antigens tested. One of these antibodies cross-reacted with renins of several different species, including human renin. Binding inhibition studies carried out with one of these monoclonal antibodies demonstrated a dissociation constant for renin of 10(-7) M. These monoclonal antibodies have great potential in answering significant questions concerning the structure, biosynthesis, tissue localization, and physiologic actions of renin.