Effect of angiotensin-converting enzyme inhibitors on glomerular eicosanoid production in normotensive and spontaneously hypertensive rats

Higher levels of antioxidant defenses in enalapril-treated versus non-enalapril-treated hemodialysis patients

We previously reported chronic treatment with angiotensin-converting enzyme inhibitors (ACEis) increases antioxidant defenses in mice. In the present study, however, we examined various antioxidant defenses in chronic hemodialysis (HD) patients either treated with enalapril (10 mg/d) for at least 6 months (+ACEi; n = 11) or untreated (-ACEi; n = 11). The relationship between antioxidant status and HD was investigated by determining oxidative stress markers and antioxidant defenses in a group of chronic HD patients (n = 33) and a group of age-matched controls (n = 29). The effect of a single HD session on those parameters was also evaluated. Before an HD session (pre-HD), HD patients had significantly lower levels of red blood cell (RBC) glutathione (GSH), selenium-dependent glutathione peroxidase activity (RBC-Se-GPx), plasma ubiquinol-10, and alpha-tocopherol than controls. In a randomly selected group of patients (n = 19), a single HD session caused an additional decrease in RBC-GSH and plasma ubiquinol-10 levels. Plasma thiobarbituric acid reactive substance (TBARS) levels were significantly greater in pre-HD patients than controls. Post-HD plasma TBARS levels were similar to control values. The cohort of +ACEi HD patients had greater pre-HD RBC-GSH content, RBC-Se-GPx activity, and plasma beta-carotene concentrations than -ACEi patients (RBC-GSH: +ACEi, 3.1 +/- 0.9 micromol/mL packed RBCs [PRBCs]; -ACEi, 1.2 +/- 0.3 micromol/mL PRBCs [P < 0.05 v +ACEi]; RBC-Se-GPx: +ACEi, 5.8 +/- 0.7 U/mL PRBCs; -ACEi, 4.3 +/- 0.2 U/mL PRBCs [P < 0.05 v +ACEi]; plasma beta-carotene: +ACEi, 0.54 +/- 0.16 micromol/L plasma; -ACEi, 0.19 +/- 0.05 micromol/L plasma [P < 0.05 v +ACEi]). Results show profound alterations in the circulating antioxidant systems of chronic HD patients and that additional oxidative stress occurs during the HD procedure. In addition, in +ACEi HD patients, the levels of several antioxidant defenses are greater than in those in -ACEi HD patients.

Prostacyclin release by rat cardiac fibroblasts: inhibition of collagen expression

Cardiac fibroblasts, as the source of extracellular matrix for the left ventricle, subserve important functions to cardiac remodeling and fibrotic development following myocardial infarction or with pressure-overload cardiac hypertrophy. The fibroblast may be the target cell for angiotensin-converting enzyme inhibitors (ACEI) that are cardioprotective and reverse collagen deposition and remodeling but whose mechanisms of action remain controversial. Because we previously documented phenotypic differences between cardiac fibroblasts from the spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) left ventricle, the present study evaluated whether phenotypic differences also exist in the release of endogenous arachidonic acid metabolites or in the activation of phospholipase D, and the importance of observed differences to the formation of collagen and the mechanism of action of ACEI. The experimental design compared endogenous sources of arachidonic acid with exogenous prelabeling of cells. Angiotensin II stimulated greater arachidonic acid release than bradykinin, and WKY cells were more responsive than SHR. The major prostanoid formed by cardiac fibroblasts was prostaglandin I2 (PGI2), with more prostacyclin production by WKY cells than SHR cells both under nonstimulated conditions and in response to angiotensin II or bradykinin. Beraprost, a PGI2 analogue, was shown to decrease growth rate and DNA synthesis of fibroblasts and to inhibit mRNA expression for collagen types I and III, with SHR cells being less responsive to beraprost than WKY cells. These results potentially implicate eicosanoid metabolism, particularly PGI2, in collagen formation, fibrotic development, and cardiac remodeling, and they imply that the SHR genetic hypertension model may be predisposed to excess cardiac fibrosis.

ACE inhibitor effects on platelet function in stages I-II hypertension

Angiotensin II enhances platelet aggregation through activation of the G protein-linked pathway present in platelets. Studies of several angiotensin-converting enzyme (ACE) inhibitors have demonstrated marked differences on platelets. Therefore this prospective, randomized, double-blind, crossover study compared the ex vivo effects of equivalent antihypertensive doses of captopril, enalapril, and fosinopril on platelet aggregation and thromboxane B2 (TxB2) formation in subjects with stage I-II essential hypertension. Nineteen male subjects with a baseline mean seated blood pressure of 141 +/- 3/100 +/- 1 mm Hg were enrolled. The decline in mean arterial pressure after 4 weeks of stable dosing was 10 +/- 1, 12 +/- 1, and 11 +/- 1 mm Hg for captopril, enalapril, and fosinopril, respectively (p = NS). There was no significant change in adenosine diphosphate (ADP)-, epinephrine-, or thrombin-stimulated platelet aggregation from baseline or between ACE inhibitors. Compared with baseline, fosinopril decreased TxB2 concentrations 27.5-67.6% with all stimuli after 1 and 5 min. Captopril also decreased TxB2 formation, but this effect was stimulus and time dependent. Enalapril consistently increased TxB2 concentrations, independent of stimuli or time. We conclude that different ACE inhibitors have distinct effects on platelet TxB2 formation without significant effects on platelet aggregation. Fosinopril may be a direct antagonist ofTxA2 synthase, suggesting benefit in syndromes of platelet activation or vascular occlusion.

Stress-induced regulation of the renal peripheral benzodiazepine receptor: possible role of the renin-angiotensin system

The etiology of the decrease in renal peripheral benzodiazepine receptor (PBR) binding caused by stress was studied in rats. Prior investigations suggest that the response of the renal PBR to stress occurs independently of the hypothalamo-pituitary-adrenal (HPA) axis and sympathetic nervous system. The present experiments tested the hypothesis that the renin-angiotensin system is involved in regulating the PBR. Eighty min of brief, intermittent tailshocks caused increases in plasma renin activity and decreases in renal PBR binding. The stress-induced decrease in renal PBR binding was reversed by pretreatment with captopril. Acute administration of angiotensin II (ANG II) alone caused reductions in PBR binding in kidney, heart, and cerebral cortex. These data suggest that ANG II may be an endogenous factor responsible for regulating the PBR in several tissues during stress.

Dose-dependent effects of angiotensin converting enzyme (ACE) inhibitors on glomerular prostanoid production by normotensive rats

1. This study was designed to investigate whether the angiotensin converting enzyme (ACE) inhibitors, captopril, enalapril and fosinopril have a dose-dependent effect on the production of prostaglandin E2 (PGE2), prostaglandin I2 (prostacyclin, PGI2) and thromboxane A2 (TxA2) by glomeruli isolated from normotensive Wistar-Kyoto rats. 2. Measurements of glomerular prostanoid production were made under basal conditions and in the presence of excess exogenous arachidonic acid. 3. All three ACE inhibitors demonstrated dose-dependent effects upon glomerular prostanoid production which varied with the individual ACE inhibitor. 4. Enalapril induced a dose-dependent increase in the ratio of (PGE2 + PGI2)/TxA2, from 2.17 +/- 0.20 to 5.35 +/- 0.84 and to 10.0 +/- 1.16 with the low and high doses of enalapril respectively. In contrast, the high dose of captopril tended to reduce the ratio when compared to the low dose. 5. The results obtained in this study suggest that although all three ACE inhibitors appear to induce prostacyclin synthetase and/or modulate phospholipase A2 (PLA2) activity, these effects differ with the ACE inhibitor studied and the dose employed. 6. This study has demonstrated dose-dependent effects of three ACE inhibitors on glomerular prostanoid production which may be significant in modulating glomerular haemodynamics and growth characteristics of glomerular cells.

Oxidant stress and glomerular prostanoid production: influence of angiotensin converting enzyme inhibition

1. The effect of H2O2 (4.7 x 10(-9) -4.7 x 10(-3) M) on prostanoid production by isolated glomeruli from normotensive (WKY) and, spontaneously hypertensive rats (SHR) has been studied. 2. Oxidant stress significantly increased synthesis of prostaglandin E2 (PGE2), I2 (PGI2) and thromboxane A2 (TxA2) by glomeruli from both strains whereas the ratio (PGE2 + PGI2)/TxA2 increased in only SHR. 3. Pre-incubation of glomeruli with the angiotensin converting enzyme inhibitors captopril or lisinopril, had virtually no effect on H2O2-induced synthesis of individual prostanoids nor on the ratio (PGE2 + PGI2)/TxA2 by glomeruli from either WKY or SHR. 4. The findings suggest that H2O2-induced changes in glomerular function may be mediated, in part, by PGs but fail to support the suggestion that the ability of ACEI to protect glomeruli from H2O2-induced damage is determined by PGs.

Glomerular injury induced by hydrogen peroxide: modifying influence of ACE inhibitors

The sensitivity of isolated glomeruli from normotensive (Wistar-Kyoto, WKY) and spontaneously hypertensive (SHR) strains to oxidant stress was studied by determining the incidence of pyknosis, karyohexis and karyolysis after incubation with different concentrations of hydrogen peroxide (H2O2) (4.7 x 10(-9) - 10(-3) M). Even though the proportion of glomeruli containing nuclei that demonstrated these features increased progressively with increasing concentrations of H2O2, the number of severely damaged glomeruli was relatively small even at concentrations of 4.7 x 10(-3) M. Examination of the surface epithelial cells of glomeruli using scanning electron microscopy revealed no evidence of disturbance of the macroscopic or podocyte structure or, of increased blebbing after H2O2-treatment. These data suggest damage to nuclei is an early result of ROS stress on glomeruli. Preincubation of WKY glomeruli with captopril or lisinopril resulted in a significant drop in the proportion of WKY glomeruli demonstrating structural damage after oxidant stress. In contrast, preincubation of SHR glomeruli with lisinopril had no effect on oxidant-induced changes in the morphology of SHR glomeruli, whereas captopril effected a significant increase in the proportion of glomeruli demonstrating damage at all concentration of H2O2.