ROLE OF THE RENIN-ANGIOTENSIN SYSTEM IN HYPERTENSION PRODUCED BY UNILATERAL RENAL ARTERY CONSTRICTION IN THE RAT

1. Conscious rats which had undergone unilateral renal artery constriction were infused for 1 h with a specific antagonist of angiotensin II, 1-Sar-8-Ala-angiotensin II (P-113). 2. There was a highly significant correlation between the change in blood pressure induced by P-113 and the pre-infusion plasma renin concentration (PRC), regardless of initial blood pressure or the duration of stenosis. However, the blood pressure fall was not significantly greater in nineteen hypertensive rats than in eleven which remained normotensive. P-113 did not abolish the hypertension. 3. The extent to which angiotensin II supports blood pressure in rats with renal artery constriction is directly related to the PRC.     

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.     

REMODELLING OF THE VASCULAR SYSTEM IN RESPONSE TO HYPERTENSION AND DRUG THERAPY

1. Arterial remodelling is an important mechanism in the pathophysiology of hypertension and its complications, being involved in the decrease of vascular reserve, the autoregulation of cerebral blood flow and the development of atherosclerosis. There is now evidence that, in addition to several other growth factors, vasoactive peptides such as angiotensin II may act as vascular smooth muscle growth-promoting substances. Based on these data, the effects of perindopril, a potent and long-lasting angiotensin-converting enzyme (ACE) inhibitor, on structural and mechanical properties of the arterial wall have been studied in animal models of hypertension. Perindopril completely reversed the aortic medial hypertrophy and arterial stiffening observed in renovascular hypertensive rats and in spontaneously hypertensive rats. The effect of perindopril was consistent with the potent inhibition of vascular ACE, and emphasized the potential role of angiotensin II as a vascular growth modulator. Whether the time constant of remodelling is similar or not in the heart and large vessels remains an important question that requires further investigation.     

THE RENIN-ANGIOTENSIN SYSTEM AND HYPERTENSIVE LEFT VENTRICULAR HYPERTROPHY IN THE RAT

The relationship between left ventricle-to-body weight ratio (LV/BW), mean arterial pressure (MAP, the average of 12 h of intra-arterial monitoring in conscious animals) and plasma renin activity (PRA) was assessed at 12 weeks after unilateral clipping or sham operation in twenty-four one-kidney rats and twenty-six two-kidney rats. The degree of hypertension and left ventricular hypertrophy were similar in one-kidney and two-kidney groups. There was a close linear relationship between LV/BW and MAP which was similar in one-kidney and two-kidney rats. PRA was significantly elevated only in two-kidney rats and was not related to the degree of hypertrophy after accounting for the effect of PRA on MAP. These data suggest that activation of the renin-angiotensin system does not contribute directly to myocardial hypertrophy in rats with renal-clip hypertension.     

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.     

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.     

HYPERTENSION ALTERS SLOPE AND RANGE BUT NOT SENSITIVITY TO VASOCONSTRICTOR AND VASODILATOR AGENTS IN THE RABBIT HINDQUARTER

The vascular reactivity (slope and range) and location parameter (ED50) of dose-response curves in the hindquarter vascular bed in conscious rabbits previously subjected to renal cellophane wrapping or sham operation were examined. The animals were instrumented with pulsed Doppler flow transducers and chronic indwelling aortic catheters for intra-arterial drug infusion. The rabbits were ganglion-blocked with mecamylamine before constructing full dose-response curves to intra-arterial infusions of methoxamine, noradrenaline, angiotensin II, acetylcholine, adenosine and serotonin. Curves relating dose to conductance were fitted to the experiments involving constrictors and curves relating dose to vascular resistance fitted to those involving dilator drugs. With both classes of drugs the reactivity was significantly higher in the hypertensive animals than in sham-operated rabbits. There was no difference in sensitivity (ED50 value) between the hypertensive and control animals for any agonist tested. The increased reactivity, but not sensitivity, could be entirely accounted for by the vascular amplifier action of medial hypertrophy in hypertension.     

ATTENUATION OF PRESSOR RESPONSES TO SYMPATHETIC STIMULI IN THE RAT BY ENALAPRIL

Intravenous administration to pithed Wistar rats of the angiotensin converting enzyme inhibitor enalapril (0.1-1.0 mg/kg) lowered the diastolic blood pressure and reduced pressor responses occurring during electrical stimulation (1-30 Hz) of the spinal sympathetic outflow. These doses of enalapril given intravenously also attenuated pressor responses to intravenous injection of the muscarinic ganglion stimulant McNeil-A-343 (50, 100, 150 micrograms/kg) and noradrenaline (0.1-5.0 micrograms/kg). Enalapril (1.0 mg/kg, i.v.) reduced pressor responses to the nicotinic ganglion stimulant 1,1-dimethyl-4-phenyl-piperazinium (300 micrograms/kg, i.v.). These results confirmed that the actions of enalapril resemble those of captopril in the pithed rat, by causing reductions in both blood pressure and pressor responses to sympathetic stimuli.     

CYCLOSPORIN HYPERTENSION IN THE WISTAR RAT: ROLE OF UNINEPHRECTOMY

1. The effects of cyclosporin A (CyA) solution (1–50 mg/kg per day), CyA powder (5–20 mg/kg per day) and vehicle, and the effects of renal mass reduction on CyA induced hypertension, were examined in conscious Wistar rats. 2. CyA solution consistently increased blood pressure at 5–20 mg/kg per day and the powder at 10–20 mg/kg per day. The vehicle had no consistent effect. 3. Both CyA solution and powder inhibited gain in bodyweight. There was no consistent effect on food intake, water intake or urine volume. Plasma sodium (Na⁺), and Na⁺ and potassium (K⁺) excretion were reduced by CyA 20 mg/ kg per day. 4. Renal function was impaired by CyA powder at 20 mg/kg per day and plasma renin activity (PRA) was increased by all doses of CyA powder. 5. Thus, uninephrectomy did not potentiate CyA solution (5 mg/kg per day) induced increases in blood pressure and the effects of CyA do not appear to be a consequence of nephrotoxicity in Wistar rats.     

FUNCTIONS OF THE RENIN-ANGIOTENSIN SYSTEM DURING DEVELOPMENT

1. From studies in chronically catheterized fetal sheep and other species, it can be shown that the renin-angiotensin system (RAS) is active during intra-uterine life. Levels of angiotensin II (AII) in fetal sheep are similar to maternal. 2. The fetal RAS plays a role in maintenance of arterial pressure. The extent to which it does so depends on the level of activity of the system. 3. The distribution of renin within the fetal rat kidney is much more widespread than in the adult. The fetal kidney, like other vascular beds has high levels of the AT₂ angiotensin receptor subtype. With maturation the proportion of the AT₁ receptor subtype increases. 4. Blockade of the fetal RAS with angiotensin converting enzyme (ACE) inhibitors or with the non-peptide AII antagonist (losartan) caused a fall in fetal glomerular filtration rate (GFR) and a rise in renal blood flow (RBF). AII reverses the fall in GFR even though RBF decreases. 5. The fraction of the filtered sodium load reabsorbed by the proximal tubule was not affected when the fetal RAS was blocked by captopril or losartan. High doses of infused AII had no effect on renal reabsorption of sodium, in the short term, but in the long term depressed fractional proximal reabsorption. 6. Only in high doses does AII stimulate the secretion of aldosterone from the fetal adrenal. 7. Since the fetal RAS is responsible for maintenance of GFR and physiological levels of AII do not stimulate either proximal tubular sodium reabsorption nor aldosterone secretion, it is proposed that during intra-uterine life the fetal RAS maintains the renal excretion of sodium and water into the amniotic cavity, thus ensuring an adequate volume of amniotic fluid for normal growth and development.     

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.     

RESPONSE OF PLASMA RENIN-ANGIOTENSIN SYSTEM TO A SINGLE CAPTOPRIL ADMINISTRATION IN PATIENTS RECEIVING LONG-TERM TREATMENT WITH CAPTOPRIL

1. The responses of angiotensin II (AII), AIII, aldosterone and plasma renin activity (PRA) to a single dose of captopril were investigated in hypertensive patients receiving long-term (more than 1 year) captopril therapy (CT patients) and compared with those of non-treated hypertensive patients (NT patients). 2. Baseline levels of AII and aldosterone were significantly lower in CT patients than in NT patients. AIII tended to be lower and PRA was slightly higher in CT than in NT patients, but these differences were not significant. 3. A single administration of captopril (50 mg orally) significantly decreased plasma levels of AII, AIII and aldosterone as well as blood pressure in both CT and NT patients. 4. These results demonstrate that chronically repeated administration of captopril to hypertensive patients effectively reduces the daily blood pressure and concomitantly the plasma AII level to acceptable levels in patients with no experience of ACE inhibition.     

CAPTOPRIL ANTAGONIZES THE HYPOTENSIVE ACTION OF ATRIAL NATRIURETIC PEPTIDE IN THE ANAESTHETIZED RAT

Atrial natriuretic peptide (8-33; ANP) caused a prolonged hypotensive response following intravenous injection in anaesthetized rats. This response was abolished by captopril treatment and restored by concomitant angiotensin II infusion. These results suggest that ANP exerts its hypotensive action in the anaesthetized rat by the antagonism of the vasoconstrictor action of endogenous angiotensin II.     

HYPERTENSION CORRECTED AND ALDOSTERONE RESPONSIVENESS TO RENIN-ANGIOTENSIN RESTORED BY LONG-TERM DEXAMETHASONE IN GLUCOCORTICOID-SUPPRESSIBLE HYPERALDOSTERONISM

Two males with glucocorticoid-suppressible hyperaldosteronism had hyperaldosteronism, hypertension and hypokalaemia corrected by continuous administration of physiological doses of dexamethasone for more than a year. During long-term dexamethasone treatment: (a) Plasma renin activity increased from subnormal to high normal levels, with normal posture-mediated increases; (b) Plasma aldosterone became responsive to angiotensin infusion, a new observation; (c) A fall in plasma aldosterone between 0800 h (recumbent) and 1000 h (upright) was replaced by a rise; (d) Plasma aldosterone became suppressible with salt loading. These findings are consistent with a shift to more normal control of aldosterone by renin-angiotensin, once abnormal responsiveness to ACTH has been nullified.     

EFFECT OF TEMOCAPRIL ON HAEMODYNAMIC AND HUMORAL RESPONSES TO EXERCISE IN PATIENTS WITH MILD ESSENTIAL HYPERTENSION

1. This study was carried out to evaluate the effect of temocapril on haemodynamic and humoral responses to exercise in nine patients with mild essential hypertension (WHO stages I and II). 2. After a 4-week placebo period, temocapril was administered at a dose of 1.0 mg once daily for 2–4 weeks. Graded submaximal bicycle ergometer exercise was performed before and after temocapril treatment, and the changes in arterial blood pressure, heart rate, cardiac output (CO), and systemic vascular resistance (SVR) were evaluated. In addition, the plasma norepinephrine (NE) level was determined both at rest and peak exercise before and after temocapril treatment. 3. Both the systolic and diastolic blood pressure were reduced at rest and during exercise by temocapril treatment. No significant change in the resting heart rate and CO was observed, and the exercise-induced increase of these parameters was also not affected by temocapril. In contrast, the resting SVR was significantly decreased by temocapril, although the exercise SVR was similar during both temocapril and placebo treatment. 4. Although there was no significant change in the plasma NE level with temocapril treatment, the exercise-induced increase of plasma NE was significantly suppressed by temocapril. 5. These results indicate that temocapril reduces the blood pressure without causing any significant changes in the heart rate and CO at rest, and that it does not produce any changes in the haemodynamic response to exercise.     

THE EFFECT OF NALOXONE ON PRESSOR RESPONSES TO ANGIOTENSIN II IN ANAESTHETIZED GREYHOUNDS

In greyhounds anaesthetized with morphine and chloralose, vertebral artery infusions of angiotensin II resulted in a similar pressor response but smaller tachycardia than in greyhounds anaesthetized with chloralose alone. The pressor responses to intravenous infusions of angiotensin II were significantly larger in the morphine premedicated greyhounds. In greyhounds anaesthetized with morphine and chloralose, pressor responses to vertebral artery angiotensin II were potentiated following the administration of naloxone via either a vertebral artery or intravenously although the effect was more consistent with the former route. No such effect was seen in greyhounds anaesthetized with chloralose alone. Naloxone had no effect on the pressor responses to intravenous angiotensin II or carotid artery occlusion. The heart rate responses to vertebral artery angiotensin II in greyhounds anaesthetized with morphine and chloralose were potentiated by naloxone such that they were not significantly different from the responses obtained in greyhounds anaesthetized with chloralose alone before naloxone. As was observed with the pressor responses, the potentiation was more apparent with the vertebral artery route of administration of naloxone. There was no potentiation of the heart rate responses in the chloralose group of greyhounds. It is suggested that morphine premedication may repress the vagal withdrawal mechanism while potentiating the sympathetic vasomotor mechanism mediating the central cardiovascular actions of angiotensin II.     

MECHANISMS UNDERLYING THE DECREASE IN CIRCULATING ANGIOTENSIN II CONCENTRATION AFTER SODIUM LOADING

1. Acute sodium loading causes a rapid decrease in the circulating concentration of angiotensin II (AngII), which is apparent from 5 min after sodium administration. This could result from an increase in AngII catabolism and/or a decrease in AngII synthesis/secretion. However, the major determinant of AngII synthesis is thought to be a change in plasma renin activity, which occurs over a longer time frame (15 min). 2. To investigate the mechanisms underlying the rapid decrease in plasma AngII engendered by sodium administration, we performed metabolic clearance studies in male New Zealand white rabbits before and after a hypertonic sodium load of 1.5 mmol/kg as 0.513 mol/L saline i.v. bolus. 3. The metabolic clearance rate of AngII increased significantly from 42.2 ± 9.0 mL/min per kg before sodium to 110.8±33.7 mL/min per kg after sodium administration (P<0.05). The calculated or theoretical secretion rate decreased from 1470.7±404.2 to 573.5 ± 139.5 fmol/min per kg (P<0.025) in response to sodium. 4. We conclude that an increase in AngII metabolism and a decrease in synthesis/secretion contribute to the reduction in circulating AngII, which occurs in the first 60–90 min after sodium loading.     

INHIBITION OF RENIN-ANGIOTENSIN INDUCED DRINKING IN THE CAT BY ENZYME INHIBITORS AND BY ANALOGUE ANTAGONISTS OF ANGIOTENSIN II

SUMMARY 1. Intracerebroventricular (icv) administration of the renin inhibitor pepstatin inhibited the dipsogenic response to icv renin but did not affect drinking induced by icv angiotensin I and II.
2. Intracerebroventricular administration of an angiotensin converting enzyme inhibitor, SQ 20881, reduced the dipsogenic effect of icv renin and angiotensin I but did not affect angiotensin II induced drinking.
3. Intracerebroventricular administration of SQ 20881 reduced the dipsogenic effect of intravenous angiotensin I but did not reduce drinking elicited by intravenous angiotensin II.
4. Intracerebroventricular administration of Sar¹-Ala⁸-angiotensin II, a competitive antagonist of angiotensin II, inhibited drinking elicited by icv renin, angiotensin I and angiotensin II. The dipsogenic response to intravenous infusion of both angiotensins I and II was reduced after central administration of this antagonist.
5. Four analogue antagonists of angiotensin II given icv reversibly inhibited the dipsogenic response to icv angiotensin II. These analogues themselves possessed weak dipsogenic effects.
6. Renin-angiotensin induced drinking in the cat is mediated by the action of angiotensin II on receptors in the central nervous system.