Effects of chronic administration of angiotensin converting enzyme (ACE) inhibitors on blood pressure and tissue ACE activity in the SHR

Patterns of Renal Function in Hypertension Due to Unilateral Renal Artery Occlusion

We performed renal function studies in dogs with chronic renovascular hypertension produced by complete occlusion of a renal artery. In addition, we evaluated in anesthetized dogs the acute effects of a novel angiotensin converting enzyme inhibitor, CGS 16,617, on renal function and plasma neurohormones (epinephrine, norepinephrine and vasopressin) 4 weeks after initiation of 2 kidney, 1 clip hypertension. CGS 16,617 effectively decreased blood pressure in renal hypertensive animals. This response was associated with suppression of angiotensin II indicating effective converting enzyme inhibition. In the non-clipped kidney, acute administration of CGS 16,617 increased effective renal plasma flow but not glomerular filtration rate and urinary sodium excretion. In the clipped kidney, CGS 16,617 caused no change in any parameter of renal function. Plasma norepinephrine, epinephrine and vasopressin were unaffected by administration of CGS 16,617. These studies showed that chronic occlusion of a renal artery does not result in renal infarction because of a compensatory increase in the amount of blood provided through capsular collateral vessels. The collateral circulation which has developed in the clipped kidney explains the lack of a converting enzyme inhibitor effect.

Effects of chronic oral treatment with imidapril and TCV-116 on the responsiveness to angiotensin II in ventrolateral medulla of SHR

OBJECTIVE

To examine whether chronic oral treatment with an angiotensin-converting enzyme inhibitor imidapril and an angiotensin II type 1 receptor antagonist TCV-116 would alter the response to angiotensin II in the rostral ventrolateral medulla.

METHODS

Twelve-week-old spontaneously hypertensive rats (SHR) were treated with imidapril (20 mg/kg per day, n = 7), TCV-116 (5 mg/kg per day, n = 8) or vehicle (n = 8) for 4 weeks. Wistar- Kyoto rats (WKY) (n = 8) served as normotensive controls. At 16 weeks of age, angiotensin II (100 pmol) was microinjected into the rostral ventrolateral medulla of anaesthetized rats.

RESULTS

Blood pressure decreased significantly in the rats treated with either imidapril or TCV-116. Pressor responses to angiotensin II microinjected into the rostral ventrolateral medulla were comparable in the untreated SHR, the imidapril-treated SHR and WKY (12 +/- 2, 15 +/- 4 and 10 +/- 1 mmHg, respectively), but were abolished in SHR treated with TCV-116 (0 +/- 2 mmHg, P< 0.01). Angiotensin-converting enzyme activity in the brain stem was significantly lower in SHR treated with imidapril (0.70 +/- 0.06 nmol/mg per h), but significantly higher in SHR treated with TCV-116 (1.62 +/- 0.04 nmol/mg per h) than in the untreated SHR (1.37 +/- 0.05 nmol/mg per h).

CONCLUSIONS

Chronic oral treatment with imidapril and TCV-116 may have divergent influences on the renin-angiotensin system within the brain stem. TCV-116, but not imidapril, abolishes the pressor effect of angiotensin II in the rostral ventrolateral medulla.

Antiatherogenic effect of angiotensin converting enzyme inhibitor (benazepril) and angiotensin II receptor antagonist (valsartan) in the cholesterol-fed rabbits

The purpose of this study was to determine whether an angiotensin converting enzyme (ACE) inhibitor, benazepril, and an angiotensin receptor antagonist, valsartan, would decrease atherosclerotic severity in cholesterol-fed rabbits. Male rabbits were fed either: (a) normal rabbit chow; (b) 2% cholesterol diet; (c) 2% cholesterol diet supplemented by benazepril (3 mg/kg per day, subcutaneous injection); or (d) 2% cholesterol diet supplemented by valsartan (1 mg/kg per day, subcutaneous injection). After 12 weeks, the arteries were harvested for histomorphometry and immunohistochemistry. We observed that decreases in serum triglyceride (TG) and total cholesterol (TC) and ACE activity with benazepril-treatment were more than 60, 30, and 84%, respectively, in comparison with the cholesterol group; with valsartan-treatment, TG levels were 53% lower than in the cholesterol group, however, there was no significant difference in TC and ACE activity. The percentage of aortic surface atherosclerotic area, intimal thickness and the ratio of aortic intimal area to medial area were about 40% lower in the benazepril-treated group in comparison with those of the cholesterol group; the difference was more than 60% in the thoracic aorta. The valsartan-treated group had 23% less atherosclerotic area, less effective than benazepril treatment. The percent of PCNA-positive cells and the number of intimal proliferative cells/mm2 were significantly less in the benazepril-treated group compared with the cholesterol group (by 55 and 63%); these parameters were 35 and 17% lower, respectively, with valsartan. The ratio of proliferating macrophages to smooth muscle cells (SMCs) was 3:1 in the cholesterol group, 1:1 in the benazepril and 2:1 in the valsartan-treated group. These results indicate that benazepril could reduce atherosclerotic progression by decreasing macrophage proliferation and accumulation in the arterial wall. The mechanisms for reducing atherosclerotic progression by benazepril and valsartan may be related to reduction of TG and blockade of the angiotensin II action.

Synergistic effects of combined converting enzyme inhibition and angiotensin II antagonism on blood pressure in conscious telemetered spontaneously hypertensive rats

OBJECTIVE

To investigate the chronic effects of combined administration of an angiotensin II receptor antagonist (valsartan) and an angiotensin converting enzyme inhibitor (benazeprilat) on blood pressure and heart rate in conscious telemetered spontaneously hypertensive rats.

METHODS

Blood pressure and heart rate were monitored (by radiotelemetry) during 2-week infusions of 0.5-10 mg/kg valsartan per day and 0.5-10 mg/kg benazeprilat per day, alone or in combination, into conscious spontaneously hypertensive rats. Also, responses of blood pressure in conscious spontaneously hypertensive rats to exogenous angiotensin I and II were determined.

RESULTS

Synergistic antihypertensive effects were observed when valsartan and benazeprilat were coadministered at submaximal monotherapy doses in the range 0.5-1.5 mg/kg per day. For all combination groups, the area over the curve (mmHg x days) for lowering of blood pressure was significantly greater (synergy) than that predicted from the sum of the monotherapy responses. Combination therapy abrogated pressor responses to angiotensin I more effectively than did comparable doses of the monotherapies.

CONCLUSIONS

These results demonstrate that combination therapy aimed at interrupting operation of the renin-angiotensin system simultaneously at multiple sites can prevent the partial escape which occurs during chronic angiotensin converting enzyme inhibitor monotherapy. Furthermore, multiple-site intervention results in a more efficacious antihypertensive response than that achieved with high doses of the individual monotherapies.

Antihypertensive treatment and the responsiveness to glutamate in ventrolateral medulla

We have recently reported that the cardiovascular responses to excitatory amino acids are augmented in the rostral ventrolateral medulla of spontaneously hypertensive rats (SHR). In the present study, we investigated whether the responsiveness to excitatory amino acids would be normalized by antihypertensive treatment. Thus we treated 4-week-old SHR and age-matched Wistar-Kyoto (WKY) rats with either enalapril (25 mg/kg per day in drinking water) or vehicle for 8 weeks. At 12 weeks of age, systolic blood pressure in the untreated SHR (248+/-9 mm Hg) was significantly (P<.01) higher than that in the enalapril-treated SHR (140+/-4 mm Hg), untreated WKY rats (148+/-4 mm Hg), and enalapril-treated WKY rats (117+/-1 mm Hg). The pressor responses to L-glutamate (2 nmol) microinjected into the rostral ventrolateral medulla were similar in enalapril-treated and untreated SHR (40+/-5 and 47+/-3 mm Hg, respectively, NS), and these responses were significantly greater than that seen in the untreated WKY rats (24+/-2 mm Hg, P<.01). On the other hand, the pressor response to either N-methyl-D-aspartate, an ionotropic glutamate receptor agonist, or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, a metabotropic glutamate receptor agonist, in the enalapril-treated SHR was slightly but significantly smaller than that in the untreated SHR but was still markedly greater than those in untreated and enalapril-treated WKY rats. These results suggest that the augmented responsiveness to excitatory amino acids in the rostral ventrolateral medulla of SHR may be at least partly genetically determined and cannot be normalized by the treatment with enalapril.

Neuroendocrine and cardiovascular effects of serotonin: selective role of brain angiotensin on vasopressin

Central serotonin (5-HT) and angiotensin (ANG II) stimulate arginine vasopressin (AVP), oxytocin (OT), and adrenocorticotropin (ACTH) secretion and increase blood pressure. Studies were conducted in conscious rats to determine whether neuroendocrine activation by 5-HT requires a brain angiotensinergic intermediate pathway. In the first study, ANG II formation was inhibited by the angiotensin-converting enzyme inhibitor enalapril before injection of the 5-HT releaser/uptake inhibitor d-fenfluramine. Fenfluramine (2 mg/kg ip) stimulated AVP, OT, corticosterone, and prolactin (PRL) secretion (P<0.01). Enalapril (60 mg/l in drinking water for 4 days and 10 mg/kg ip 2 h before the rats were killed) inhibited only the AVP response (P<0.01) to d-fenfluramine. In the second study, the effect of intracerebroventricular injection of the 5-HT2A/2C antagonist LY-53857 (10 microgram), or the ANG II AT1 antagonist DuP-753 (10 microgram), on intracerebroventricular 5-HT (10 microgram)-stimulated AVP, OT, ACTH, PRL, renin secretion, mean arterial pressure (MAP) and heart rate (HR) was tested. LY-53857 inhibited the AVP, OT, and ACTH responses to 5-HT (P<0.01), whereas DuP-753 inhibited only the AVP response (P<0.01). Intraventricular injection of 5-HT increased MAP and decreased HR. The MAP response was not affected by LY-53857 or DuP-753, and at no time did MAP decline below starting levels. The decreased HR was inhibited by LY-53857 but not by DuP-753. These results demonstrate that 5-HT-induced AVP secretion is mediated selectively via brain angiotensinergic mechanisms by way of the AT1 receptor.

Age and hypertension differently affect coronary contractions to endothelin-1, serotonin, and angiotensins

BACKGROUND

Endothelium-derived substances and the renin-angiotensin system are important regulators of vascular tone. This study was designed to evaluate the effects of age and hypertension on vascular function of rat coronary arteries.

METHODS AND RESULTS

Rings of the left anterior descending coronary artery were isolated from Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) at 12 (younger) and 72 (older) weeks of age and suspended in myographs (37 degrees C, 95% O2/5% CO2) for isometric tension recording. Systolic blood pressure was higher in SHR than in WKY rats (P < .05) but was unaffected by age in both strains. Active wall tension to KCl 100 mmol/L (mN/mm) was decreased in younger (0.28 +/- 0.03, n = 9) and older SHR (0.49 +/- 0.06, n = 13) compared with age-matched WKY rats (0.87 +/- 0.05, n = 9 and 1.51 +/- 0.11, n = 11, respectively, P < .05). In both strains, active wall tension to endothelin-1 and serotonin increased with age (n = 6 to 10, P < .05) but was decreased in younger and older SHR compared with WKY rats (P < .05). Active wall tension induced by angiotensin I 10(-7) mol/L was increased in older SHR (0.19 +/- 0.04, n = 7) compared with younger SHR (0.04 +/- 0.01, n = 9) but was similar in younger and older WKY rats (0.10 +/- 0.02 versus 0.15 +/- 0.03, n = 6 to 9) and younger SHR. In younger WKY rats and SHR, pretreatment of coronary arteries with benazeprilat 10(-5) mol/L (n = 5 for each) almost completely abolished the contractions to angiotensin I 10(-7) mol/L. Active wall tension to angiotensin II 10(-7) mol/L was comparable in all four groups, but compared with the contraction to KCl 100 mmol/L, the response was already increased in younger SHR (29 +/- 3%, n = 9) compared with the younger WKY rats (14 +/- 3%, n = 9, P < .05), but it was unaffected by age in both strains. In vitro treatment of younger WKY rat and SHR coronary arteries with the nonpeptide angiotensin II (AT1) receptor antagonist valsartan 10(-5) mol/L (n = 3 for each) fully suppressed contractions to angiotensin II 10(-7) mol/L. In contrast, endothelium-independent relaxations to the nitrovasodilator sodium nitroprusside, endothelium-dependent relaxations to acetylcholine, and endothelium-dependent contractions to N omega-nitro-L-arginine methyl ester were comparable in all four groups of rats.

CONCLUSIONS

In summary, in rat coronary arteries, contractile responses to endothelin-1, serotonin, and KCl increase with age but are decreased by hypertension. In contrast, the L-arginine/nitric oxide pathway remains unaffected. The contractions to angiotensin I markedly increased with increasing duration of hypertension in the SHR only. Despite overall reduced contractile responses of SHR coronary arteries, contractions to angiotensin II were maintained. Hence, aging and hypertension affect contractile responses of rat coronary arteries to vasoconstrictor agonists differently.

Hemodynamic effects of benazepril, an angiotensin-converting enzyme inhibitor, as studied in conscious normotensive dogs

Hemodynamic effects and inhibitory effects on the pressor response to exogenous angiotensin I of benazepril (CGS 14824A), a new angiotensin-converting enzyme (ACE) inhibitor, were examined in conscious chronically instrumented normotensive dogs in comparison with those of captorpil. Oral administration of benazepril (1-10 mg/kg) and captopril (3 and 10 mg/kg) reduced the blood pressure and inhibited the pressor response to angiotensin I dose-dependently. The blood-pressure-lowering effect of benazepril was as potent as that of captopril. The onset of effects of benazepril was slower and the duration longer than that of captopril. There was no close correlation between the attenuation of pressor response to exogenous angiotensin I and the blood-pressure-lowering effect of these two agents. These results indicate that benazepril is a potent ACE inhibitor with a slow onset and a long duration. The slow onset of action may be explained by the necessity of prior conversion of this compound to an active metabolite. A mechanism or mechanisms other than that responsible for the inhibition of pressor response to exogenous angiotensin I must be taken into consideration to explain the blood-pressure-lowering effects of benazepril and captopril.

Comparisons in vitro, ex vivo, and in vivo of the actions of seven structurally diverse inhibitors of angiotensin converting enzyme (ACE)

1. Seven drugs (captopril, zofenopril, enalapril, ramipril, lisinopril, fosinopril, and SQ 29,852) were compared in vitro in homogenates of aorta, brain, heart, lung, and kidney and in sera of spontaneously hypertensive rats (SHR) both with respect to potencies of their active moieties as inhibitors of angiotensin-converting enzyme (ACE), and, where applicable, rates of hydrolysis of their prodrug ester functions. 2. In ex vivo dose-response and time-course studies, the inhibitory effects of the seven drugs on tissue ACEs and their relative distributions to SHR tissues were compared following oral administration. 3. The relative potencies of the inhibitory moieties of the drugs (in parentheses) and the normalized 'equiactive' oral doses employed for time-course studies were: SQ 29,852 (1.0), 100 mg kg-1; captopril (3.5), 30 mg kg-1; enalapril (12), 20 mg kg-1; fosinopril (13), 25 mg kg-1; zofenopril (20), 10 mg kg-1; lisinopril (24), 10 mg kg-1; and ramipril (51), 5 mg kg-1. 4. Following oral administration of the drugs to SHR, the degree and duration of ACE inhibition in aorta and lung correlated with the antihypertensive actions, with ramipril, lisinopril, and zofenopril producing effects of the greatest magnitude and duration. 5. Ramipril and enalapril did not inhibit brain ACE ex vivo; captopril and zofenopril had modest but short-lasting effects; and fosinopril, lisinopril, and SQ 29,852 had long-lasting inhibitory actions, which, with the latter two, were delayed in onset. 6. All of the drugs produced significant inhibition of kidney ACE, with ramipril and fosinopril having somewhat weaker effects, perhaps due to biliary routes of excretion. 7. Captopril, fosinopril, and particularly zofenopril inhibited cardiac ACE ex vivo with degrees and durations that were marked compared with those of the other drugs; preliminary studies with isolated hearts suggest a possible relationship between inhibition of cardiac ACE and preservation of cardiac function subsequent to ischaemia.