IN VIVO GENERATION AND ELIMINATION OF ANGIOTENSIN IN THE RAT

1. Combined high performance liquid chromatography (HPLC) and radio-immunoassays were used to study the in vivo kinetics of the renin-angiotensin system in the rat. The HPLC-verified plasma concentrations of angiotensin I (AI) were 1.0 nmol/L (0.52-1.6) in anaesthetized normal and 4.2 nmol/L (2.5-7.0) in salt-depleted rats. The plasma concentrations of angiotensin II (AII) were 0.07 nmol/L (0.04-0.13) in anaesthetized normal and 1.0 (0.60-1.6) nmol/L in salt-depleted rats. 2. The fate of injected AI and AII passing through the vascular bed of the lungs was determined. Two-thirds of the injected AI was converted to AII and one-third was unchanged after a single passage through the lungs. Only trace amounts of angiotensin III (AIII), the only other metabolite, were demonstrated. 3. This verifies that the majority of AI is metabolized through AII. Injected AII disappeared from the circulation with formation of only trace amounts of AIII, the half-life being about 10 s. This corresponds to a calculated in vivo generation rate of AII of about 12 nmol/L per h in normal rats. It is in agreement with the AI generation rate (plasma renin activity) measured as 9.5 nmol/L per h in vitro.     

CORRELATION OF PLASMA ANGIOTENSIN II CONCENTRATION AND PLASMA RENIN ACTIVITY DURING ACUTE HYPOXIA IN DOGS

The effects of isocapnic hypoxia on plasma renin activity (PRA) and angiotensin II (AII) concentration were studied in anaesthetized, artificially ventilated dogs. Regression analysis of plasma AII concentration vs PRA, both measured by radioimmunoassay (RIA), was used as an index of converting enzyme activity in vivo. PaO2 decreased from 82 to 30 mmHg but regression analysis did not reveal any inhibition of AII production within the limits of detection of this method (less than 20% inhibition). We conclude that systemic converting enzyme activity, assessed by in vivo measurement and correlation of PRA and AII, is not inhibited by severe hypoxia.     

ANGIOTENSIN II BLUNTS, WHILE AN ANGIOTENSIN-CONVERTING ENZYME INHIBITOR AUGMENTS, REFLEX SYMPATHETIC INHIBITION IN HUMANS*

1. The role of angiotensin (Ang)II in and the effects of angiotensin-converting enzyme (ACE) inhibitors on the regulation of sympathetic neural activity were examined in humans. 2. We measured baseline values of muscle sympathetic nerve activity (MSNA) and its reflex inhibition in 28 patients with essential hypertension with elevated plasma renin activity (PRA; > 1.0 ng/mL per h = 0.28 ng/L per s) before and after either acute or chronic oral administration of an ACE inhibitor or placebo and in 20 normotensive subjects before and after infusion of either AngII (5 ng/kg per min = 4.8 pmol/kg per min) or vehicle (5% dextrose). Muscle sympathetic nerve activity was recorded from the tibial nerve and its reflex inhibition was evaluated during pressor responses to bolus injection of phenylephrine (2 micrograms/kg, i.v.). 3. Blood pressure was significantly decreased (P < 0.01) after the acute oral administration of captopril (25 mg), accompanied by a slight increase in MSNA in patients with essential hypertension compared with control patients who received placebo administration. Reflex changes in MSNA were significantly augmented after oral administration of captopril (-4.1 +/- 0.5 vs -6.2 +/- 0.6%/mmHg, respectively; P < 0.01), with a significant reduction of plasma AngII, while they were not affected by placebo administration. 4. In contrast, acute AngII infusion was accompanied by decreases in both PRA and MSNA in normotensive subjects. Reflex changes in MSNA were significantly reduced after AngII infusion (-11.0 +/- 0.8 vs -7.4 +/- 1.0%/mmHg, respectively; P < 0.01) but not after vehicle alone. 5. Chronic ACE inhibition by 12 week oral imidapril administration (5-10 mg/day) significantly (P < 0.05) decreased baseline values of MSNA, which were accompanied by a significant (P < 0.05) increase in the reflex inhibition of MSNA, while plasma concentrations of noradrenaline were unaffected. 6. These results indicate that AngII blunts reflex inhibition of sympathetic neural activity and that inhibition of the renin-angiotensin system by an ACE inhibitor augments reflex regulation of sympathetic neural activity and reduces baseline values in patients with essential hypertension.     

PLASMA RENIN RESPONSE TO ACUTE BLOCKADE OF ANGIOTENSIN II IN THE ANAESTHETIZED RAT

SUMMARY 1. Anaesthetized rats were infused intravenously for 1 h with a specific antagonist of angiotensin II, 1-Sar-8-Ala-angiotensin II (P-113), at a rate of 5 μ/kg per min, or with saline.
2. Blood samples were taken, before and after infusion, for measurement of plasma renin activity (PRA) and plasma renin concentration (PRC).
3. Saline infusion did not affect PRA or PRC.
4. Infusion of P-113 produced steep and highly significant increases in PRA (652%) and PRC (724%), despite a slight rise in mean arterial pressure.
5. Within 30 min of terminating P-113 infusion, PRA fell to 300%, and PRC to 278% of pre-infusion levels, and subsequently continued to fall.
6. It is suggested that the hypersecretion of renin produced by P-113 is due to blockade of the inhibitory control normally exerted by endogenous angiotensin II on renin release.     

DYNAMIC CHANGES IN PLASMA INACTIVE RENIN LEVELS IN BARTTER''S SYNDROME AFTER ADMINISTRATION OF CAPTOPRIL AND ANGIOTENSIN II

Changes in plasma active and inactive renin concentration (ARC and IRC) after captopril administration and angiotensin II (AII) infusion were studied in six patients with Bartter's syndrome. A single oral dose of captopril (8-25 mg) lowered the blood pressure and increased both ARC and IRC. AII infusion elevated blood pressure, suppressed ARC and increased IRC. In this syndrome of high renin levels, infused AII appeared to increase inactive renin secretion by reducing its conversion to active renin. On the other hand, an acute fall in AII levels and/or renal perfusion pressure by captopril increased both active and inactive renin. This indicates that the increase in the secretion of inactive renin, stimulated by captopril, might exceed any increase in its conversion to active renin in patients with Bartter's syndrome, in whom the production of renin is accelerated, and conversion of inactive renin to active renin probably already operates near its maximum.     

CHANGES IN CARDIAC ANGIOTENSIN CONVERTING ENZYME AFTER MYOCARDIAL INFARCTION AND HYPERTROPHY IN RATS

1. Cardiac angiotensin-converting enzyme (ACE) is localized in high concentration in cardiac valves, coronary vessels, right and left atrium and right and left ventricle. 2. Cardiac ACE is functionally active in converting angiotension I to angiotensin II. 3. The level of cardiac ACE measured by radioinhibitor binding or by quantitative in vitro autoradiography was greatly increased after experimental myocardial infarction in the rat. The increase was greatest in the fibrous scar tissue of the free left ventricular wall infarct, but there were also significant increases in the ACE concentration in the four chambers of the heart. 4. Treatment with enalapril for 4 weeks following coronary ligation inhibited cardiac ACE, including the high levels found in the scar in the left ventricular free wall. 5. There was a close relationship between the systolic blood pressure and left ventricular mass in several models of experimental hypertension, despite varying degrees of activation of the renin-angiotensin system. However no relationship between the degree of left ventricular hypertrophy and changes in cardiac ACE could be determined. 6. Inhibition of cardiac ACE may contribute to the beneficial effect of ACE inhibitors in cardiac hypertrophy and remodelling, and may play a part in the cardioprotective role of ACE inhibitor.     

CIRCULATING ANGIOTENSIN II LEVELS UNDER REPEATED ADMINISTRATION OF LISINOPRIL IN NORMAL SUBJECTS

1. To examine the effect of chronic administration of angiotensin I-converting enzyme (ACE) inhibitor on circulating angiotensin II (AII) concentration, 20 mg of lisinopril was administered once daily for 7 consecutive days to eight healthy volunteers. 2. Plasma ACE activity was inhibited to less than approximately 30% of the pretreatment level during the repeated administration. 3. Mean arterial pressure (MAP) was slightly but significantly reduced during the administration period. Plasma AII concentration measured by an established method using high performance liquid chromatography combined with a radioimmunoassay, however, was maintained at approximately the pretreatment level when it was measured at 24 h intervals after each administration of lisinopril. 4. With the gradual recovery of ACE activity following discontinuation of administration, the plasma AII concentration correlated with AI concentration (r = 0.46), and also with the product of AI and ACE activity (AI x ACE; r = 0.80), corresponding to the formula obtained from the kinetics of ACE activity. No correlation was observed between MAP and AII levels throughout the study period. 5. We conclude that in normal subjects repeatedly administered with ACE inhibitor, the AII level in the circulation is still determined by an elevated level of AI and any remaining ACE activity, thus maintaining AII at pretreatment levels. We confirmed that it is not necessary to achieve a decrease in plasma AII concentration through the chronic administration of ACE inhibitor in order to effectively lower blood pressure.     

PLASMA RENIN ACTIVITY OF RATS AND RABBITS IMMUNIZED AGAINST ANGIOTENSIN II

SUMMARY 1. Plasma renin activity (PRA) was studied in rats and rabbits actively immunized against angiotensin II (A II).
2. The mean PRA values (in ng angiotensin I/ml per h) were 3.5 (s.e.m. = 0.4) in ten control mock-immunized rats, and 3.3 (s.e.m. = 0.5) in seven rats having demonstrable sustained immunity to A II.
3. In six unanaesthetized rabbits, the appearance of antibody directed against A II was associated with a 263% increase in PRA during the first week, whereas the PRA of five control mock-immunized rabbits, tested at the same stage, exhibited no such change.
4. The PRA returned to control values by 3 months in the rabbits immunized against A II, although the antibody titres remained high.
5. The finding of significantly elevated PRA early in the course of production of antibody against A II is consistent with the properties of a negative feedback system, the renin hypersecretion being provoked by partial blockade of the normal inhibitory effect of A II on renin release. However, in chronically immune animals, the PRA is normal.     

FAILURE OF SUPPRESSION OF INTRARENAL ANGIOTENSIN II IN THE CONTRALATERAL KIDNEY OF ONE-CLIP TWO-KIDNEY HYPERTENSIVE RATS

1. Two-kidney one-clip hypertension was produced in rats by application of a 0-20 mm clip to the left renal artery. 2. After three weeks, the animals had mildly elevated PRA, kidney renin was elevated in the clipped kidney and markedly suppressed in the contralateral kidney when compared to values in sham-operated rats. 3. Intrarenal All was elevated in the clipped kidney either when compared to the contralateral kidney or to values in the controls. However, in the contralateral kidney, All was not different from controls. 4. Changes in intrarenal AH do not, therefore, always parallel those of renal renin. Failure of suppression of All in the contralateral kidney is likely to be of pathogenetic importance in development of hypertension in this experimental model.     

INTRARENAL ACTION OF ANGIOTENSIN II IN RESTORING RENAL ARTERY PRESSURE AFTER ACUTE RENAL ARTERY STENOSIS

1. The renal artery of conscious dogs was acutely narrowed over 30 s to reduce renal artery pressure distal to the stenosis to 40 mmHg and the stenosis was maintained for 1 h. The distal renal artery pressure was rapidly restored to a plateau slightly below pre-stenosis values within 10–15 min. Rises in systemic blood pressure and plasma renin activity were small and transient. 2. This restoration was an active process, mediated by the intrarenal effects of angiotensin II (AH), since it was greatly diminished or abolished when the renal artery was narrowed in the presence of angiotensin I-converting enzyme inhibitor or angiotensin receptor antagonist (l-Sar-8-Ue All). However, it was not diminished by ‘total’ autonomic effector blockade. 3. This angiotensin II-mediated restoration of renal artery pressure may be of homeostatic significance for the maintenance of glomerular filtration rate.     

STIMULATION OF THE GASTRIC SODIUM MONITOR REDUCES HEPATIC ANGIOTENSIN-CONVERTING ENZYME ACTIVITY

1. The natriuresis engendered by stimulation of the gastric sodium monitor is mediated in part by a decrease in the circulating concentration of angiotensin II (AngII). This decrease is due to a decrease in synthesis rather than to an increase in metabolism. We investigated the role of changes in plasma and hepatic angiotensin-converting enzyme (ACE) activity in this decrease in AngII synthesis. 2. Male Sprague-Dawley rats were equilibrated on a low-sodium diet for 7 days. On the day of experiment, rats were anaesthetized and received either a sodium load of 1.5 mmol/kg as 3 mol/L saline or an equivalent volume of an iso-osmotic urea solution by direct gastric puncture. Blood was sampled and livers were harvested at 0 and 30 min after sodium or urea administration. Angiotensin-converting enzyme was measured in serum and tissue homogen-ates by generation of histidyl-leucine. 3. In the liver, ACE activity decreased from control after both sodium (P < 0.005) and urea (P < 0.025) administration. The decrease was greater in the group that received saline compared with rats that received urea (P < 0.05). Serum ACE decreased in response to urea (P < 0.025) but not sodium administration. 4. We conclude that stimulation of the gastric sodium monitor results in a decrease in ACE activity in the liver. This decrease in ACE activity may be contributory to the decrease in AngII synthesis.     

AUTORADIOGRAPHIC LOCALIZATION OF ANGIOTENSIN-CONVERTING ENZYME AND ANGIOTENSIN II BINDING SITES IN EARLY ATHEROMA-LIKE LESIONS IN RABBIT ARTERIES

1. Early atheroma-like lesions in rabbits are associated with increased sensitivity to serotonin. The localization and distribution of angiotensin-converting enzyme (ACE) and angiotensin II (AII) binding sites has been studied in these developing lesions by quantitative in vitro autoradiography. 2. In sham-operated control vessels, ACE was localized predominantly to intimal and adventitial sites, whereas in lesioned arteries the level of ACE detected in these regions was significantly reduced. 3. In control vessels AII receptor binding was distributed largely in the outer media, whereas in lesioned vessels AII receptor binding was dispersed throughout the media with the highest levels of binding in the outer media. 4. There was a significant amount of binding associated with ACE and with AII receptors in the extra-adventitial inflammatory tissue of lesioned arteries. 5. Apparent loss of ACE from intimal and adventitial sites may be a consequence of tissue remodelling and cellular proliferation, while the appearance of ACE in abnormal sites could play a role in AII production by the vessel wall. The role of ACE and AII in the developing atheroma-like lesions needs to be investigated further.     

VASOCONSTRICTOR RESPONSES TO COMPONENTS OF THE RENIN-ANGIOTENSIN SYSTEM IN CYCLOSPORIN-INDUCED HYPERTENSION IN THE RAT

1. Since plasma renin activity is increased in cyclosporin A (CsA)-induced hypertension in the rat, the role of the vascular renin-angiotensin system (RAS) in CsA-induced hypertension was investigated in rat mesenteric resistance vessels. 2. Female Wistar rats received CsA (10 mg/kg per day, s.c.) or vehicle for 30 days. CsA treatment increased tail-cuff systolic blood pressure (CsA treated 135 ± 3 mmHg vs control 125 ± 1 mmHg, P<0.0001). 3. Mesenteric resistance arteries (200–300 μm) were isolated and mounted in a microvessel myograph. Concentration-response curves to tetradecapeptide renin substrate (10-¹¹-10^?6 mol/L), angiotensin I (10-^l1-10^?6 mol/L) and angiotensin II (10-¹²-10^?6 mol/L) showed no differences between CsA-treated and control groups. 4. Mesenteric vascular angiotensin-converting enzyme (ACE) characteristics were determined by radioligand binding. There were no differences in the content or affinity of ACE between CsA-treated and control rats. 5. These results suggest that the mesenteric vascular RAS does not play a major role in CsA-induced hypertension in the rat.     

SERUM ANGIOTENSIN CONVERTING ENZYME ACTIVITY AND PLASMA RENIN ACTIVITY IN EXPERIMENTAL MODELS OF RATS

1. Serum angiotensin converting enzyme activity (ACEA) and plasma renin activity (PRA) were determined in rats under different experimental conditions such as: nephrotic syndrome (NS), bilateral nephrectomy (BN), renovascular hypertension (RH), dehydration (DEH), anaesthesia (AN), low sodium diet (LSD) and high sodium diet (HSD), and injection with propranolol (PRO) and isoprenaline (ISO). 2. PRA increased in LSD, AN, NS, RH, DEH and IPT groups, and decreased in HSD, BN, and PRO groups. Serum ACEA did not change in RH, HSD, IPT, DEH, AN, and PRO groups, increased in NS group, and decreased in LSD and BN groups. 3. Serum ACEA changed in the opposite direction to PRA only in the LSD group. This finding suggests that ACE may limit the full expression of the renin-angiotensin system in the LSD group, but not in the other groups.     

DISAPPEARANCE RATE OF CIRCULATING RENIN AFTER BILATERAL NEPHRECTOMY IN THE RAT

SUMMARY 1. Plasma renin concentration (PRC) was measured in eight male rats at intervals during a 6 h period after bilateral nephrectomy.
2. PRC fell rapidly, reaching 51% of the mean control level by 10 min, 29% by 30 min, and 2% by 4 h.
3. Under the conditions of the experiment, the half-life of circulating endogenous renin in the rat was about 10 min.     

EFFECT OF DELAPRIL ON THE VASCULAR ANGIOTENSIN II RELEASE IN ISOLATED HIND LEGS OF THE SPONTANEOUSLY HYPERTENSIVE RAT: EVIDENCE FOR POTENTIAL RELEVANCE OF VASCULAR ANGIOTENSIN II TO THE MAINTENANCE OF HYPERTENSION

1. In view of a recent interesting hypothesis that the vascular renin-angiotensin system (RAS) plays an important role in the maintenance of hypertension, we examined the effect of delapril (DP), a newly developed angiotensin converting enzyme inhibitor (ACEI), on angiotensin II (Ang II) release from isolated perfused hind legs of spontaneously hypertensive rats (SHR) in comparison with normotensive rats of Wistar-Kyoto strain (WKY). 2. Male SHR and WKY were given DP orally (10 mg/kg per day) for 2 weeks. Isolated hind legs of these rats were perfused with angiotensinogen-free Krebs-Ringer solution, and Ang II released into the perfusate was determined directly by extraction with Sep-Pak C18 cartridges connected to the perfusion system. 3. Delapril produced a sustained antihypertensive action in SHR but not in WKY. The spontaneous release of Ang II in SHR was 112.9 +/- 17.6 pg during the first 30 min of perfusion, which was somewhat greater than that in WKY (96.5 +/- 9.8 pg). An active metabolite of DP, delapril diacid (DPD), when added to the perfusion medium, suppressed the Ang II release in a dose-dependent manner in the two strains. Oral pretreatment of DP for 2 weeks suppressed the Ang II release by 60% in WKY and more pronouncedly by 73% in SHR. 4. These results suggest the presence of a functional RAS in vascular tissues which contributes to the maintenance of vascular tone of SHR, and that ACEI including DP exerts their antihypertensive effect through inhibition of vascular Ang II release in this animal model of human hypertension.     

BLOOD PRESSURE RESPONSE TO NOREPINEPHRINE AND ANGIOTENSIN II IN THE OFFSPRING OF PARATHYROIDECTOMIZED MOTHER RATS

1. The basal blood pressure and the drug-stimulated pressor response were studied in male offspring in the sixth generation (5d-PTx-F6) of rats parathyroidectomized on day 5 of pregnancy. 2. The systolic blood pressure in conscious 5d-PTx-F6 rats measured by a tail cuff was significantly higher (P less than 0.001) than in control rats. 3. Venous plasma renin activity was significantly lower (P less than 0.001) in the 5d-PTx-F6 rats. Plasma levels of calcium, sodium, and potassium in the 5d-PTx-F6 rats and in the controls did not differ. 4. Systolic arterial pressure response to intravenously (i.v.) administered angiotensin II (150 ng/kg) in the 5d-PTx-F6 rats was significantly greater than in the controls (P less than 0.05), when blood pressure was measured directly through a pressure transducer under anaesthesia. 5. Pressor responses to norepinephrine (5 micrograms/kg, i.v.) in the 5d-PTx-F6 rats were significantly lower (P less than 0.05) than in the controls, when blood pressure was measured directly under anaesthesia. 6. The findings suggest that the 5d-PTx-F6 rats undergo functional alterations of the renin-angiotensin and sympathetic nervous systems for cardiovascular regulations.     

PRESSOR SENSITIVITY TO ANGIOTENSIN I AND ANGIOTENSIN II DURING THE DEVELOPMENT OF EXPERIMENTAL RENAL HYPERTENSION IN THE RAT

Treatment with the potent angiotensin converting enzyme inhibitor perindopril completely prevented any rise in blood pressure in the 2-kidney, 1-clip (2K1C) model of renal hypertension in rats. Withdrawal of this inhibitor was followed by a slow rise in blood pressure. In 2K1C rats treated with perindopril, pressor responses to angiotensin I fell during the treatment period, but returned to normal after the inhibitor was stopped. Pressor responses to angiotensin II (AII) increased during treatment with perindopril; this was presumably due to increased receptor sensitivity consequent on the falls in endogenous AII levels. Responses to AII fell to control levels after the inhibitor was stopped. It is concluded that an increased pressor sensitivity to AII is not the cause of the slowly developing hypertension in the 2K1C model of hypertension, and that the slow pressor response to AII must be due to other factors.