Influence of angiotensinase ihhibitors on the enzymatic activity of renin

Poor comparability of plasma renin activity measurement in determining patient samples: the status quo and recommendations for harmonization

OBJECTIVES

This study aims to investigate and update the consistency and comparability of plasma renin activity (PRA) assays in measuring clinical samples. The contributions of recalibration, blank subtraction, and incubation strategies to interchangeability were also explored.

METHODS

Five different laboratories were evaluated using forty-six individual plasma samples, including four liquid chromatography-tandem mass spectrometry (LC‒MS/MS) assays and one chemiluminescence immunoassay (CLIA). Spearman correlation coefficient (R), Passing-Bablok regression, and Bland‒Altman plot analyses were used to evaluate the consistency among assays. Consistency before and after recalibration, blank subtraction, and incubation strategy unification was compared.

RESULTS

A good correlation was observed among all assays (R>0.93). None of the samples measured by all assays showed coefficient variation (CV) <10 %, and 37 % of samples showed overall CVs >20 %. The 95 % confidence intervals (CIs) for slopes did not contain 1 for most assay pairs. Large relative biases (-85.1-104.2 %) were found, and 76 % (52-93 %) of samples had unacceptable biases. Recalibration reduced the calibration bias. Ignoring blank subtraction improved the comparability across all assays while unifying incubation did not.

CONCLUSIONS

The interchangeability of PRA measurement was unsatisfying. Harmonization on calibrator and ignoring blank were recommended. Unifying incubation strategy was unnecessary.

Hypertension: The role of biochemistry in the diagnosis and management

Hypertension is defined as a persistently elevated blood pressure ≥140/90mmHg. It is an important treatable risk factor for cardiovascular disease, with a high prevalence in the general population. The most common cause, essential hypertension, is a widespread disease - however, secondary hypertension is under investigated and under diagnosed. Collectively, hypertension is referred to as a "silent killer" - frequently it displays no overt symptomatology. It is a leading risk factor for death and disability globally, with >40% of persons aged over 25 having hypertension. A vast spectrum of conditions result in hypertension spanning essential through resistant, to patients with an overt endocrine cause. A significant number of patients with hypertension have multiple cardiovascular risk factors at the time of presentation. Both routine and specialised biochemical investigations are paramount for the evaluation of these patients and their subsequent management. Biochemical testing serves to identify those hypertensive individuals who are at higher risk on the basis of evidence of dysglycaemia, dyslipidaemia, renal impairment, or target organ damage and to exclude identifiable causes of hypertension. The main target of biochemical testing is the identification of patients with a specific and treatable aetiology of hypertension. Information gleaned from biochemical investigation is used to risk stratify patients and tailor the type and intensity of subsequent management and treatment. We review the approach to the biochemical investigation of patients presenting with hypertension and propose a diagnostic algorithm for work-up.

Insulin-mediated growth in aortic smooth muscle and the vascular renin-angiotensin system

Insulin has been shown to directly affect blood vessel tone and to promote vascular hypertrophy, but the mechanism of these actions remains uncertain. Because angiotensin I (Ang I)-converting enzyme inhibitors have been shown to improve insulin action and to impede the progression of vascular hypertrophy in hypertensive animal models, it is possible that the vascular properties of insulin may be mediated through the tissue renin-angiotensin system (RAS). To evaluate this relationship, we first investigated the effect of insulin on components of the RAS using cultured rat vascular smooth muscle cells (VSMCs). Insulin treatment (1000 microU/mL) markedly increased angiotensinogen mRNA expression and angiotensinogen production. We next investigated the role of the RAS in insulin-mediated cell proliferation, using [3H]thymidine uptake. Studies were done both with insulin alone and in the presence of captopril (1x10(-7) to 10(-5) mol/L) and losartan (1x10(-9) to 10(-7) mol/L). [3H]Thymidine uptake was increased significantly by 1000 microU/mL insulin, and this stimulation was reduced by 1x10(-6) mol/L captopril (-38.8%, P<0.05) and by 1x10(-8) mol/L losartan (-37. 5%, P<0.05). Further studies showed that the degree of insulin-mediated [3H]thymidine uptake in VSMCs could be duplicated by 4x10(-10) mol/L Ang II. Losartan reduced the effects of both Ang II and insulin on [3H]thymidine uptake by about 40% to 45% of baseline (P<0.05). Captopril reduced insulin-mediated [3H]thymidine uptake but did not affect Ang II-mediated [3H]thymidine uptake. In summary, insulin induced significant stimulation of angiotensinogen expression and production and stimulated growth similar to that seen with Ang II in cultured rat VSMCs. Inhibition of Ang II production or its binding to the Ang II type 1 (AT1) receptor inhibited insulin-mediated growth in a fashion similar to that seen with inhibition of Ang II-mediated growth. Thus, insulin can modulate the vascular RAS, and the effect of insulin on vascular growth may be via direct effects on angiotensinogen expression and translation operative through both the AT1 receptor and the conversion of Ang I to Ang II.

Inactivation of renin substrate by soybean trypsin inhibitors: implications for measurement of circulating inactive renin

Semipurified soybean trypsin inhibitor added to rat and human plasma leads to a concentration dependent decrease in the rate of angiotensin I generation. This inhibition is due to binding of renin substrate to the inhibitor. Renin substrate present in nephrectomized rat plasma was more susceptible to binding than substrate of the normal rat suggesting structural differences in the substrate generated following nephrectomy. Because trypsin inhibition is necessary for measurement of active and inactive renin, we examined several alternate trypsin inhibitors. The Bowman-Birk inhibitor from soybean had similar actions as purified soybean trypsin inhibitor while trypsin inhibitors from lima bean and chicken did not depress renin substrate, but did have variable effects on the measured levels of active and total plasma renin. Surprisingly, crude soybean trypsin inhibitor did not suppress renin substrate and actually increased angiotensin I generation during PRA and PRC measurements. Since the crude preparation did not suppress renin substrate, changes in the specificity of the inhibitor may occur during its purification. The augmentation of PRA and PRC may be related to angiotensinase inhibitory actions.

Disparity between blood pressure and PRA inhibition after administration of a renin inhibitor to anesthetized dogs: methodological considerations

A dissociation between changes in blood pressure (BP) and plasma renin activity (PRA) has been noted after administration of renin inhibitors. In the present study, the renin inhibitor PD 132002 was given to salt-deplete, anesthetized dogs. PRA was measured at pH 6.0 by a conventional angiotensin I (ANG I) RIA method (PRA-C) and by an ANG I antibody-trapping RIA method (PRA-AT) performed at pH 7.4. PD 132002 at 0.01, 0.1, 1, and 10 mg/kg IV, reduced BP by 3 +/- 2, 9 +/- 2, 24 +/- 4, and 39 +/- 4 mm Hg, respectively, (baseline of 136 +/- 8 mm Hg, N = 5), when infused IV over 30 minutes with a 30 minute recovery between doses. The BP response at 10 mg/kg equaled that of saralasin (20 micrograms/kg/min IV). PRA-AT (baseline of 20 +/- 6 ng ANG l/ml/hr, N = 4) was inhibited by 0%, 28% +/- 12%, 75% +/- 10%, and 97% +/- 1% at 0.01, 0.1, 1, and 10 mg/kg, respectively. Plasma concentrations of immunoreactive ANG II were also reduced dose-dependently and paralleled changes in BP. In contrast, PRA-C (baseline of 13 +/- 4 ng ANG l/ml/hr, N = 4) was inhibited by 82% +/- 8% at 0.01 mg/kg and by > 98% at higher doses. After a single dose of PD 132002 at 10 mg/kg infused over 30 minutes, BP recovery paralleled changes in immunoreactive ANG II and PRA-AT, yet PRA-C inhibition showed no recovery over the same time course. Our data support the conclusion that BP relates better to PRA-AT than PRA-C. Thus the dissociation sometimes observed in studies with renin inhibitors between changes in BP and PRA may be attributed to the assay used to determine PRA.

Chimeric rats from the fusion of genetically hypertensive and normotensive rat embryos

Structural changes in the cardiovascular musculature of the SHR during the development of hypertension appears to involve both prehypertensive hyperplastic cellular growth and hypertension induced cellular hypertrophy. The genetic factors determining these changes are not fully known, but may involve altered growth control. The role of genetic determination on the development of hypertension in the SHR was investigated by producing chimeric rats composed of a mixture of genetically homozygous SHR and normotensive cells. Preimplantation 8-cell embryos isolated from SHR and the normotensive NBR (NIH Black Wistar) rat strains were aggregated in vitro and cultured to the blastocyst stage before implantation into surrogate mothers. Chimeric rats born to the surrogate females were raised to 36-40 wks of age and the development of hypertension monitored by tail cuff pressure (BP). BP in the chimeras varied from 115 to 205 mm Hg (146 +/- 25). Heart weights were positively correlated with BP, r = 0.76, p less than 0.05, while only a marginal and non-significant correlation of aortic weight was found (r = 0.53). The renin-angiotensin system was normal in the chimeras. This model may prove useful in determining the extent of genetically mediated cellular events in the development of hypertrophy and hypertension in the SHR.

Production of angiotensinogen by cultured rat aortic smooth muscle cells

This study was conducted to further investigate angiotensinogen synthesis in rat aortic smooth muscle cells (SMC) grown in culture. tissue cultures maintained in defined medium neither grew nor synthesized angiotensinogen. However, in the presence of 5% homologous serum both cell proliferation and angiotensinogen synthesis became apparent. Substitution of normal control serum with that of bilaterally nephrectomized rats or animals given dexamethasone (10mg/kg, ip) led to a further significant increase in angiotensinogen production. In contrast, serum from adrenalectomized rats suppressed angiotensinogen synthesis below the rate observed with normal serum. A positive linear correlation (r = 0.96, p less than 0.01) was evident between the serum angiotensinogen level and the rate of de novo synthesis of this protein. No correlations were found between cell proliferation and either angiotensinogen synthesis or serum angiotensinogen levels. Dexamethasone added to serum did not stimulate the rate of angiotensinogen synthesis and appeared to inhibit cell proliferation. Stimulation or suppression of angiotensinogen synthesis was not accompanied by a statistically significant change in angiotensinogen specific mRNA. The data indicate a complex regulation of angiotensinogen in vascular smooth muscle cells in culture.

Effects of central and peripheral dopamine antagonism on aldosterone secretion: evidence for adrenal mechanism

Both domperidone (DOMP) and metoclopramide (MCP) are D2 receptor antagonists, MCP being a central and peripheral dopamine antagonist, whereas DOMP is exclusively a peripheral antagonist. MCP, but not DOMP, has been shown to stimulate aldosterone production. To elucidate whether aldosterone stimulation by dopamine antagonism is centrally mediated, we injected DOMP (28 micrograms/kg body wt) via a cannula into the third ventricle in Sprague-Dawley rats. Plasma aldosterone and renin concentration were measured before and 15 min after the injection. Centrally administered DOMP resulted in an increment in plasma aldosterone (23.8 +/- 7.4 ng/dl) that was not significantly greater than that induced by vehicle alone (15.8 +/- 4.5 ng/dl). This increase was inhibited by pretreatment with dexamethasone (100 micrograms three times daily) and attenuated by captopril (1 mg/kg ip) but not by L-beta-3,4-dihydroxyphenylalanine (30 mg/kg), thus reflecting a stress effect. Similarly, central administration of MCP (21 micrograms/kg) resulted in a significant rise in plasma aldosterone. This increase, however, was eliminated by pretreatment with dexamethasone and attenuated by captopril. Peripherally administered DOMP (280 micrograms/kg) had no effect on plasma aldosterone. The effect of DOMP and MCP on aldosterone secretion by freshly obtained adrenal capsules was also tested. Angiotensin II and MCP, but not DOMP, induced a dose-dependent increase in aldosterone secretion, with a maximal increment (15.7 +/- 5.8 ng.mg capsular protein-1.10 min-1; 50% increase) with MCP at 10(-7) M (P less than 0.01 compared with controls). Dopamine completely inhibited this MCP-induced rise in aldosterone release.(ABSTRACT TRUNCATED AT 250 WORDS)

Independent change of plasma and tissue renin in response to anesthetics

It was the purpose of this study to determine whether acute increases of endogenous plasma renin induced by administration of anesthetics cause simultaneous increases of the enzyme in aortic tissue of the rat. Administration of CO2 did not alter plasma or tissue renin. Ether, Innovar and Nembutal increased active plasma renin and had variable effects on the inactive form of the enzyme. Only Innovar, a combination of droperidol and fentanyl, increased aortic renin. The active component of Innovar was shown to be droperidol, which also increased aortic renin in 24 hour nephrectomized animals. The increase of aortic renin was, therefore, independent of changes of circulating active or inactive renin. The increase of tissue renin following droperidol was rapid, suggesting activation of an inactive tissue renin.

Effects of volume change on circulating immunoreactive atrial natriuretic factor in rats

The mammalian atrial hormone atrial natriuretic factor (ANF) has been shown to have potent natriuretic and diuretic actions as well as vasodilator effects when released into the circulation. To investigate how the levels of the circulating form of this peptide change with alteration of intravascular fluid volume, we measured immunoreactive ANF in the plasma of Wistar rats after acute saline load, acute furosemide treatment, and chronic water restriction. Circulating levels of immunoreactive ANF increased significantly (p less than 0.001) 1 minute after acute saline load and returned to normal levels within 5 minutes. Volume contraction induced by furosemide treatment of chronic water restriction significantly reduced the circulating immunoreactive ANF. These data indicate that acute volume expansion causes an immediate release of immunoreactive ANF into the general circulation and acute volume contraction results in a decline of circulating levels of immunoreactive ANF, which is maintained during chronic volume contraction. These results suggest that the atria detect alterations in intravascular fluid volume and respond by changing the levels of ANF acutely as well as chronically and thereby participate in the regulation of body fluids and, perhaps, of blood pressure.

Cyclosporin A-induced hyperreninemic hypoaldosteronism. A model of adrenal resistance to angiotensin II

We studied the effects of cyclosporin A on the renin-aldosterone axis in Sprague-Dawley rats. Two weeks of intragastric administration of cyclosporin A (5 mg/kg/day or or 20 mg/kg/day) resulted in large increases in plasma renin concentration (23 +/- 5, 70 +/- 12, and 79 +/- 11 ng/ml/hr in control rats and rats receiving 5 mg and 20 mg of cyclosporin A, respectively), with no parallel increments in plasma aldosterone. In vitro angiotensin II (ANG II)-stimulated aldosterone secretion by zona glomerulosa cells obtained from cyclosporin A-treated rats was also reduced (4.8 +/- 0.5, 1.5 +/- 0.2, and 0.2 +/- 0.2 ng/10(5) cells in control rats and rats receiving 5 mg and 20 mg of cyclosporin A, respectively). In contrast, in vitro aldosterone response to graded increments of potassium (3.7-10.7 mmol/L) or adrenocorticotropic hormone (ACTH) (10(-11)-10(-8) M) was preserved in cyclosporin A-treated rats. When added in vitro to zona glomerulosa cells from untreated rats, cyclosporin A also attenuated ANG II-stimulated aldosterone secretion, but did not affect potassium or ACTH-mediated aldosterone production. Thus, cyclosporin A-induced hyperreninemic hypoaldosteronism in the rat depends on opposing renal and adrenal effects, with a direct or feedback stimulation of renin secretion and a specific blockade of ANG II-mediated aldosterone production.

Tryptic generation of an angiotensin binding substance

Following trypsin treatment of rat or human plasma, the level of angiotensin I, generated by renin, can be significantly underestimated by radioimmunoassay due to tryptic generation of an angiotensin binding substance. The precursor of the binding substance (void volume-AcA 44 gel) was converted by trypsin to 45K. Analogous to PRC methodology, known concentrations of angiotensin I were added to control and trypsin treated human plasma after the renin incubation step to determine the influence of the binding substance on the measured levels of generated angiotensin I. Using this technique, renin levels in trypsin exposed plasma were approximately two fold higher than when measured by single point conventional assay. If plasma levels of the binding precursor change in response to renin stimulation or suppression, its activation during the trypsin treatment step of the renin assay may explain the relative lack of change of inactive renin observed following numerous in vivo maneuvers.

Renin substrate and the renin-angiotensin system in hog tissues

The individual components of the renin-angiotensin system has been identified in numerous tissues. In this study we have examined whether a functional renin-angiotensin system is operative in several hog tissues including brain, aorta, and liver. The contribution of tissue renin substrate to the rate of local angiotensin generation was also assessed. Electrophoretic differences in plasma and tissue renin substrates, indicating structural differences, were employed as an index of independence of the tissue system from that of the peripheral circulation. Our results indicate that all tissues studied had the potential to locally generate angiotensin and that renin substrate limited to rate of the renin reaction in these tissues. Electrophoretic parameters, polyacrylamide gel electrophoresis, and isoelectric focusing suggest that the tissue renin systems are of local origin. The potential magnitude of local angiotensin production is such that tissue renin-angiotensin systems may significantly contribute to the control and regulation of blood pressure and other regulatory mechanisms influenced by angiotensin.

The influence of acute stress on active and inactive renin in the rat

The purpose of this study was to determine the influence acute stress on the plasma levels of active and inactive renin in the rat. Active and inactive plasma renins were measured in calm unanesthetized normal Wistar rats (indwelling catheter) and compared to values obtained after handling, anesthesia and decapitation. All maneuvers resulted in an increase of active renin. Only anesthesia significantly reduced inactive renin. It is concluded that acute stress can significantly change the ratio of active and inactive renins in rat plasma. These changes must be taken into consideration when the influence of "physiological or pharmacological" alterations of these enzymes are under investigation.

Medroxalol combined with hydrochlorothiazide in the treatment of hypertension

The antihypertensive effect and safety of hydrochlorothiazide administration as a single drug and together with medroxalol were determined in 20 patients with primary hypertension. Following two biweekly intervals on placebo and hydrochlorothiazide, medroxalol was started at 100 mg three times a day and titrated against blood pressure response up to a maximum of 300 mg three times a day. In nine patients the effect of the single and the combined drug therapy on blood pressure during isometric handgrip exercise, on plasma renin activity, and on plasma catecholamines and their deaminated metabolites was investigated. The administration of hydrochlorothiazide was associated with a significant decrease in blood pressure, but heart rate did not change. The addition of medroxalol produced a substantial decrease in blood pressure and heart rate in both the recumbent and upright positions (P less than 0.001). Due to careful titration of medroxalol, orthostatic hypotension was observed only in one patient. Neither hydrochlorothiazide alone nor the combined drug regimen prevented or diminished the rise in blood pressure with exercise. Although plasma renin activity decreased during the combined drug therapy, there was no correlation between the initial levels or the change in plasma renin activity and the extent of decrease in blood pressure. The concentration of plasma epinephrine increased during the combined drug period, whereas catecholamine metabolites increased significantly during both periods of the study. It is concluded that medroxalol combined with hydrochlorothiazide constituted a potent and safe antihypertensive therapy for the duration of the present study.

An increase in high-molecular weight renin substrate associated with estrogenic hypertension

We have previously reported that estrogens have the potential to induce new forms of renin substrate in addition to elevating the major circulating form of this protein. One of these estrogen-induced forms had a molecular weight in excess of 150,000. In this study we have compared the plasma concentration of the high-molecular-weight renin substrate in normotensive women receiving estrogen therapy and women with estrogenic hypertension. A statistically significant elevation of this protein was associated with estrogenic hypertension and normotensive pregnant women at term. This form of renin substrate differed from the major form with respect to electrophoretic mobility, isoelectric point, and immunologic cross-reactivity. In addition, kinetic analysis indicated that this high-molecular-weight substrate has a significantly higher affinity for the enzyme renin than the major circulating form (Km = 1800 +/- 290 versus 3520 +/- 260 ng angiotensin I equivalents/ml). These results suggest that in addition to renin substrate concentration, substrate composition may play an important role in blood pressure regulation.

In vitro generation of inactive renin in hypertensive human plasma

When plasma from normal and hypertensive human subjects is incubated at pH 7.4 in the absence of angiotensinase inhibitors, a significant decline of active plasma renin occurs. The fall of active renin was more pronounced in the plasma from normotensive controls and was accompanied by a fall of total renin without change of inactive renin. In hypertensive subjects total renin levels (trypsin treatment) did not change. The fall of active renin in plasma from hypertensive subjects was due therefore to its in vitro conversion to an inactive form of renin which could be reactivated by trypsin. If reversible inactivation of active plasma renin occurs in vivo, inactivated active renin may contribute to a portion of the plasma pool of inactive renin. This form of inactive renin which may be activable in vivo would not therefore represent a renin precursor or "prorenin" of renal origin.

Clonidine, a centrally acting sympathetic inhibitor, as monotherapy for mild to moderate hypertension

Sixteen patients with uncomplicated essential hypertension were treated with 0.2 mg of clonidine three times daily as the sole antihypertensive drug. Blood pressure decreased from 167 +/- 4/105 +/- 2 to 139 +/- 3/89 +/- 2 mm Hg (mean +/- standard error of the mean) after 1 week (p less than 0.001) and remained at 140 +/- 3/90 +/- 2 mm Hg after 3 months of therapy. There were no significant changes in cardiac output, blood volume, renal blood flow or glomerular filtration rate during clonidine therapy. Clonidine significantly decreased plasma catecholamines and there was a linear correlation between the change in blood pressure and decreases in plasma catecholamine concentration (r = 0.61, p less than 0.001). There was also a significant correlation between the decreases in heart rate and blood pressure (r = 0.78, p less than 0.001). It is concluded that clonidine can be used effectively and safely as the sole agent in the treatment of mild to moderate hypertension.

Role of prolactin and the renin-angiotensin system in mediating dopaminergic control of aldosterone secretion in the rat

Blockade of dopaminergic pathways increases aldosterone levels by mechanisms that are not well delineated. Since both prolactin (PRL) and plasma renin activity (PRA) also increase after administration of dopaminergic antagonists, the aldosterone increments may be secondary to these changes. To address these questions, the relationship between plasma aldosterone (PA) and PRL responses to 2 different dopamine receptor antagonists, haloperidol and metoclopramide (MCP) was examined in rats. The PA response to MCP was compared before and after blockade of the renin-angiotensin system with saralasin and after pre-administration of L-dopa. MCP administration produced significant and parallel increments in PA and PRL whereas haloperidol increased PRL without any change in PA or PRA. L-dopa pre-treatment suppressed the early PA response to MCP. Hypophysectomy prior to MCP administration eliminated the PRL response but did not significantly alter the PA response to MCP. Our findings suggest that dopamine has an inhibitory action on the adrenal gland production of aldosterone acting independently of changes in PRL and the renin-angiotensin system.

Dopaminergic modulation of renin release

The effect of metoclopramide, a procainamide derivative with dopamine antagonistic properties, and L-dopa on plasma renin activity (PRA) was studied in adult rats. Following an intravenous bolus of metoclopramide (200 microgram/kg) to the American Wistar rat there was a significant (p less than 0.05) elevation in PRA at 10 min and a maximum response at 30 min. There was a significant depression (p less than 0.05) in PRA at 15 through 40 min following an intravenous bolus of L-dopa (30 mg/kg). Pre-administration of L-dopa delayed and blunted the PRA response to metoclopramide. However, the PRA response to this dopamine antagonist was not altered by beta blockade with propranolol and alpha blockade with phentolamine. Administration of metoclopramide resulted in considerably greater (p less than 0.01) PRA responses in spontaneously hypertensive rats than in Wistar Kyoto (WKY) normotensive controls. Administration of L-dopa resulted in similar suppression of PRA in the two groups. These results indicate that there is a dopaminergic inhibitory control mechanism for renin secretion. Dopaminergic control of renin release appears to be altered in the spontaneously hypertensive rat.

Effect of sulfhydryl reagents on the enzymatic activity of human renin: implications for renin assay

Sulfhydryl (SH) reagents are sometimes used in renin assays, yet their effects on the enzymatic activity of human renin are not clearly understood. We have employed radioimmunoassay of angiotensin I (AI) to assess the effects of dithiothreitol (DTT) and of dimerecaptopropanol (DMP) on the formation of AI at pH 5.5 and at pH 7.4. When ethylenediamine tetra-acetate, phenylmethylsulfonyl fluoride, and 8-hydroxyquinoline were used as angiotensinase inhibitors, both DTT and DMP decreased the rate of AI formation in each of two human plasma pools at both pH values. In contrast, in a system of semi-purified human kidney renin and hog renin substrate, DTT enhanced the formation of AI, increasing Vmax without changing Km at both pH values. The reaction of human kidney renin with synthetic tetradecapeptide renin substrate was stimulated by DTT at pH 7.4 but inhibited by DTT at pH 5.5. In all three systems, SH reagents altered the ratio of reaction velocity at pH 5.5 to reaction velocity at pH 7.4. We conclude that SH reagents affect the assay of human renin in complex ways which depend upon the pH of incubation and upon the subcomplex ways which depend upon the pH in incubation and upon the substrate utilized. Although the mechanism of these effects is not known, such effects probably contribute to the lack of agreement among many of the procedures for renin assay.

Studies on angiotensinogen of plasma and cerebrospinal fluid in normal and hypertensive human subjects

The presence of a renin-angiotensin system in the central nervous system (CNS) has been demonstrated by several investigators, but little is known regarding the origin of its components. In this study we have compared the immunological and physical-chemical nature of angiotensinogen in plasma and cerebrospinal fluid (CSF) of human subjects and explored whether differences are present in CSF angiotensinogen concentrations of normal and hypertensive subjects. No significant differences in the nature of plasma and CSF angiotensinogen was observed with respect to molecular weight (65-70,000) electrophoretic mobility (RFalb = 0.67 plus or minus 0.003) or angiotensin I (AI) generated (pI = 6.6). Following isoelectric focusing, the plasma angiotensinogen was shown to consist of a single component with an isoionic point of 4.40 plus or minus 0.04. CSF angiotensinogen, on the other hand, resolved into three components (pI = 4.76 plus or minus 0.02; 5.16 plus or minus 0.04; 5.76 plus or minus 0.04). Although no correlations were observed between angiotensinogen levels in the CSF or plasma with blood pressure (BP), a statistically significant difference in angiotensinogen concentration of both plasma and CSF was observed between normotensive and hypertensive subjects. The differences in the chemical and immunological nature of human plasma and CSF angiotensinogens suggest that the angiotensinogen of CSF is not of peripheral origin.

Angiotensin II, plasma renin and sodium depletion as determinants of blood pressure response to saralasin in essential hypertension

To evaluate the role of the renin-angiotensin system and sodium depletion in the hypotensive response to 1-sarcosine-8-alanine-angiotensin II (saralasin), 15 male patients with essential hypertension were studied on a diet containing 120 mEq of sodium and 100 mEq of potassium per day. After a 5-day control period, all subjects had a mild pressor response to the saralasin infusion (p less than 0.01). After 5 days of the diuretic metolazone (5 mg/day), eight of the 15 patients had a vasodepressor response; these responders had a significantly greater increase in plasma renin activity and angiotensin II concentrations than did the non-responders. Sodium deficit differed markedly (p less than 0.001) between the two groups (361 +/- 121 mEq (SD) vs 52 +/- 26 mEq sodium, respectively). The addition of spironolactone (400 mg/day) for 5 days resulted in saralasin responsiveness in all but two patients, both of whom had small sodium deficits. Thus, variability in the natriuretic response to diuretics may affect saralasin testing and limit its clinical utility.

pH-independent inhibition of plasma angiotensin I degradation: implications for renin assay

Valid enzymatic assay of plasma renin (EC 3.4.99.19) requires complete inhibition of enzymes that destroy angiotensin I. Historically, the degree of such inhibition has varied with assay pH and with the combination of protease inactivators employed. To circumvent this problem, we studied the effects of several protease inactivators on both the formation and degradation of angiotensin I by human plasma at pH 5.5 and at pH 7.4. While several reagent combinations effectively suppressed the rate of angiotensin I disappearance at one of the two pH values, three combinations of phenylmethylsulfonyl fluoride with chelating agents accomplished this at both pH values. Of these, however, only the combination of 5 mmol/l ethylenediamine tetraacetate, 1.5 mmol/l 8-hydroxyquinoline, and 7.5 mmol/l phenylmethylsulfonyl fluoride also maximized the rate of angiotensin I formation at both pH 5.5 and 7.4. Thus, the use of the latter reagent combination would facilitate comparison of renin assays performed at different pH values and would permit standardization of the protease inactivators employed in such assays.

Studies on renin activation in normal human plasma

The phenomenon of plasma renin activattion by acid dialysis and preincubation with trypsin was studied in normal human plasma. Activation of plasma renin by exposure to pH 3.3 was shown to require at least one dialysis step and could be inhibited by the presence of Trasylol, indicating the involvement of a protease in acid activation. Amniotic fluid exposed to pH 1.5 to destroy renin and renin substrate was also found to contain an enzyme capable of activating plasma renin. The Michaelis-Menten constant Km and the molecular weight of activated "renin" were found to be similar to those of normal plasma renin. Inactive renins or renin-like enzymes were partially purified from plasma by affinity chromatography on concanavalin A, precipitation with (NH4)2SO4 and isoelectric focusing. Trypsin and acid exposure gave similar results with regard to the activation of this zymogen, suggesting that trypsin and acid dialysis may increase plasma renin activity by the same mechanism.

Multiple forms of human plasma renin substrate

The objective of this investigation was to determine whether heterogeneity of plasma renin substrate could be observed in states of steroid excess and various forms of hypertensive disease. In states of stimulated renin substrate production by estrogens or glucocorticoids, multiple forms of renin substrate were apparent when stimulation was excessive. Stimulation of substrate production caused by uremia associated with hypertension showed similar results. None, or only trace quantities of the additional forms of renin substrate were evident in subjects with normal or suppressed levels of plasma renin substrate. The additional forms of renin substrate could be distinguished from the normal form on the basis of cross-reactivity with a specific antiserum to the normal form, electrophoretic mobility, and kinetic rate constants. Differences in rate constants of the various forms of plasma renin substrate may account for the altered rate of the renin reaction associated with several states of hypertension. In plasma of patients with renovascular hypertension, significant quantities of a protein which cross-reacted with the antiserum but could not generate angiotensin I were observed.

Fluorimetric assay of renin

A simple fluorimetric assay was set up to test renin within 2 h. N-acetyltetradecapeptide was synthesized and used as substrate. It was demonstrated that N-acetyl-angiotensin I and Leu-Val-Tyr-Ser were the two peptides obtained after hydrolysis by renin. Fluorescamine reaction reacted with the free NH2 of the tetrapeptide generated to induce a fluorimetric reaction detected at 395--405 nm. The Michaelis constant of the reaction was 1.87 . 10(-5) M. With this method as little as one milliGoldblatt Unit (mG.U.) of hog renin could be detected and the generation of tetrapeptide was linear with respect to the renin concentration up to 20 mG.U. The fluorimetric assay was applied to the detection of renin during its purification and to the characterization of renin inhibitors.