Angiotensin-II Biochemistry and Physiology: Update on Angiotensin-II Receptor Blockers

Foetal kidney maldevelopment in maternal use of angiotensin II type I receptor antagonists

We report renal lesions observed in a foetus exposed throughout pregnancy to angiotensin II type I (AT 1) receptor antagonists. The mother suffered from essential hypertension and was treated with Cozaar (losartan 50 mg). Autopsy examination of the foetus revealed severe renal lesions, including tubular dysgenesis, hypertrophy of the endothelial and medial cells lining the arterial and arteriolar walls, hyperplasia of the juxtaglomerular apparatus and poorly developed vasa recta. Similar lesions have already been observed in foetuses of women treated with angiotensin-converting enzyme antagonists and also in foetuses and neonates of animals undergoing experimental blockade of the renin-angiotensin system. The purpose of this report is to describe structural lesions observed in the kidneys, and, particularly, vascular lesions. Our results suggest that the use of AT 1 receptor antagonists during pregnancy may have a severe deleterious effect on kidney development in the foetus.

Effects of Blocking the Renin-Angiotensin System on Expression and Translocation of Protein Kinase C Isoforms in the Kidney of Diabetic Rats

BACKGROUND

High glucose and angiotensin II (Ang II) can activate protein kinase C (PKC) in diabetes mellitus. However, it is not clear which isoform of PKC is activated by glucose or Ang II. Our study focused on the effects of angiotensin blockade, using the angiotensin-converting enzyme inhibitor fosinopril, the Ang II receptor blocker irbesartan and their combination, on the expression and translocation of PKC isoforms alpha and betaII in the renal cortex and medulla in diabetes.

METHODS

Hyperglycemia was induced with streptozotocin and diabetic rats were randomized to 4 groups: diabetic control, irbesartan group (40 mg/kg daily), fosinopril group (40 mg/kg daily) and combination group (irbesartan plus fosinopril, 20 mg/kg daily, respectively); age-matched normal rats served as normal control. After 4 weeks, expression and translocation of PKC-alpha and -betaII in the renal cortex and medulla were assessed by immunohistochemistry and Western immunoblotting.

RESULTS

The expression of PKC-alpha in the membrane and cytosol fractions from the renal cortex was significantly higher in diabetic rats (276.83 +/- 32.44% in membrane, 149.04 +/- 23.42% in cytosol) than that in normal ones. The expression of PKC-betaII in the renal cortex of diabetic rats decreased significantly in the membrane (50.00 +/- 11.68%, p < 0.05) and remained unchanged in the cytosol (94.51 +/- 11.69%, p > 0.05) compared with normal controls. Treatment with irbesartan, fosinopril and their combination partially corrected the abnormalities mentioned above. For the expression of PKC-alpha and -betaII in the medulla, no difference was detected among the 5 groups.

CONCLUSION

The renin-angiotensin system was implicated in the pathogenesis of diabetic nephropathy by regulating the activation of PKC isoforms.

Influence of angiotensin II type 1-receptor antagonist CV11974 on infarct size and adjacent regional function after ischemia-reperfusion in dogs

The presence of nonischemic regional dysfunction at the adjacent region of the ischemic myocardium was demonstrated in clinical studies. Recent studies demonstrated an angiotensin II type 1 (AT1)-receptor antagonist reduced myocardial ischemia-reperfusion injury. We investigated the role of the adjacent region after reperfusion by studying the effects of AT1-receptor antagonist on myocardial function and infarct size. We investigated 12 open-chest anesthetized dogs undergoing 90 min of left anterior descending coronary artery occlusion followed by 4 h of reperfusion. Six dogs injected with an AT1-receptor antagonist (CV11974) immediately after reperfusion were compared with 6 control dogs. Percent systolic shortening (%SS) was measured by two sets of the pair sonomicrometer crystals implanted to adjacent and remote nonischemic myocardium. After 4 h of reperfusion, infarct size was measured. There were no significant differences of the %SS at baseline between two regions. In both groups, %SS at adjacent region after reperfusion was significantly decreased as compared with remote region. There were no significant differences between the two groups. Infarct size, as a percentage of the area at risk, was smaller in the AT, group than in control group (25.49+/-7.53% vs 68.58+/-26.88% P<0.01). AT1-receptor antagonist reduces infarct size. This effect is not related to the change of regional myocardial function at adjacent region after reperfusion.

Update in pharmacologic treatment of hypertension

Initial pharmacologic therapy for hypertension is low-dose thiazide diuretics, beta-blockers, and ACE inhibitors. Increasing data have confirmed that ACE inhibitors have specific benefit in patients with diabetes, atherosclerosis, left ventricular dysfunction, and renal insufficiency. CCBs are alternative agents for ISH in the elderly and appear to decrease stroke with perhaps less protection against progression of renal insufficiency and proteinuria, CAD mortality and new onset heart failure versus other initial agents, especially ACE inhibitors. ARBs are well tolerated and effective blood pressure lowering agents but have not been confirmed as effective as ACE inhibitors for reducing renal progression, clinical events, or mortality from heart failure. Effective pharmacologic antihypertensive therapy may avoid disabling and undetected cerebrovascular disease, cognitive dysfunction, and disturbing symptoms of elevated blood pressure. Vasopeptidase inhibitor, such as omapatrilat, and endothelin-1 antagonist, such as bosentan, may become future agents approved for the reduction of morbidity and mortality with hypertension. The ALLHAT trial continues to examine the potential benefits and harms of amlodipine versus chlorthalidone and lisinopril in a diverse high-risk population. Based on ALLHAT data, however, doxazosin is no longer an acceptable initial pharmacological agent. Intensive pharmacologic treatment with blood pressure lowering to less than 130/85 mm Hg is recommended with diabetes, renal insufficiency, and heart failure with additional goal of less than 125/75 mm Hg with renal failure and proteinuria greater than 1 g/24 h, based on multiple outcome studies.

Angiotensin II receptor blockers: review of the binding characteristics

Several angiotensin II receptor blockers (ARBs), including candesartan cilexetil, irbesartan, losartan, telmisartan, and valsartan, are currently approved by the US Food and Drug Administration (FDA) for the treatment of patients with hypertension. These agents share a common mechanism of action-antagonism of the angiotensin type 1 (AT1) receptor-and as a result, they block a number of angiotensin II effects that are relevant to the pathophysiology of cardiovascular disease, including vasoconstriction, renal sodium reabsorption, aldosterone and vasopressin secretion, sympathetic activation, and vascular and cardiac hyperplasia and hypertrophy. Unlike the angiotensin converting enzyme (ACE) inhibitors, these new drugs block the effects of angiotensin II regardless of whether it is produced systemically in the circulation or locally via ACE- or non-ACE-dependent pathways in tissues. ARBs also block the angiotensin II-induced feedback regulation of renin release, resulting in an increase in angiotensin II levels. With the AT1 receptor blocked, angiotensin II is available to activate the angiotensin type 2 (AT2) receptor, which mediates several potentially beneficial effects in the cardiovascular system, including vasodilation, antiproliferation, and apoptosis. Thus, ARBs provide a highly selective approach for regulating the effects of angiotensin II.

Candesartan cilexetil in combination with low-dose hydrochlorothiazide is effective in severe hypertension

This randomized, double-blind, placebo-controlled multicenter study was designed to evaluate the efficacy, tolerability, and safety of candesartan cilexetil in a diverse population of patients with severe systemic hypertension (diastolic blood pressure > or =110 mm Hg). After a placebo run-in period, patients were given hydrochlorothiazide (HCTZ) 12.5 mg once daily for 1 week. A total of 217 patients with sitting diastolic blood pressure >95 mm Hg while receiving HCTZ were then randomized in a 2:1 ratio to receive candesartan cilexetil or placebo once daily for 4 weeks. Candesartan cilexetil was started at 8 mg and titrated to 16 mg if needed for blood pressure control; background HCTZ was continued in both groups. Candesartan cilexetil was significantly more effective than placebo in lowering trough diastolic and systolic blood pressure. The mean change in trough sitting diastolic blood pressure from the end of the HCTZ run-in period to the week 4 endpoint (primary study endpoint) was -9.1 mm Hg with candesartan cilexetil and -3.1 mm Hg with placebo (p = 0.0001). A higher percentage of patients treated with candesartan cilexetil than placebo responded to treatment (53% vs 29%) and achieved diastolic blood pressure <90 mm Hg (32% vs 15%). Subgroup analyses indicated that candesartan cilexetil was especially effective in patients with higher trough diastolic blood pressure at randomization, and it was significantly more effective than placebo in both black and nonblack patients alike as well as in women and men. Candesartan cilexetil was safe and well tolerated, with an adverse-event profile comparable to placebo. These results demonstrate that candesartan cilexetil added to background HCTZ therapy is effective and well tolerated in lowering blood pressure in patients with severe systemic hypertension.

Combination of a calcium antagonist, a lipid-peroxidation inhibitor, and an angiotensin AT1-receptor antagonist provides additive myocardial infarct size-limiting effect in pigs

The calcium antagonist felodipine, the lipid-peroxidation inhibitor H290/51, and the angiotensin II type 1 (AT1)-receptor antagonist candesartan all exert beneficial effects on myocardial ischemia/reperfusion injury. This study was undertaken to test the hypothesis that a combination of these drugs with different pharmacologic properties could exert additive cardioprotective effects. Anesthetized pigs were subjected to 45 min of left anterior descending coronary artery occlusion followed by 240 min of reperfusion. Five groups of pigs were randomly given either 0.65 microM (7 nmol/kg) felodipine, 1.0 microM (3.1 microg/kg) H 290/51, 4.2 microM (20 microg/kg) candesartan, a cocktail of these three drugs, or vehicle (n = 6 for each) for 30 min starting at 5 min before reperfusion by coronary venous retroinfusion, which delivers drugs specifically to the ischemic regions. Systolic segment shortening (%SS) was measured by sonomicrometer. The myocardial area at risk and the final infarct size were determined by Evans blue and 2,3,5-triphenyl tetrazolium chloride staining. The hemodynamics did not change significantly during the study. In the vehicle group, the recovery of coronary flow was not maintained during reperfusion, and it was significantly lower after 240 min of reperfusion than during the preischemic period (p < 0.05). The coronary flow in the drug-treated groups was approximately the same by the end of the reperfusion period as that before the induction of ischemia. In the ischemic myocardium, %SS slightly recovered during reperfusion in the four drug-treated groups, but not in the vehicle group. The infarct size, expressed as a percentage of the myocardial area at risk, was smaller in all four drug-treated groups compared with the vehicle group. The infarct size in the cocktail group was significantly smaller than that in the groups given felodipine, H290/51, or candesartan alone. These results demonstrate that a combination of a calcium antagonist, a lipid-peroxidation inhibitor, and an angiotensin AT1-receptor antagonist has an additive effect on infarct limitation, indicating that combined therapy with agents having different pharmacologic modes of action may provide better cardioprotection than any of the drugs alone. The findings also support the view that reperfusion injury is possibly mediated by a combination of mechanisms.

Effects of the insurmountable angiotensin AT1 receptor antagonist candesartan and the surmountable antagonist losartan on ischemia/reperfusion injury in rat hearts

Two angiotensin AT1 receptor antagonists with different receptor binding characteristics, candesartan (insurmountable antagonism) and losartan (surmountable antagonism), were compared as regards their effects on angiotensin II-induced vasoconstriction and on myocardial ischemia/reperfusion injury. In isolated rat hearts perfused under constant flow, it was found that at equipotent concentrations candesartan (10 nM) and losartan (3 microM) almost completely inhibited the angiotensin II-induced increase in coronary perfusion pressure. However, if a washout period was introduced before the angiotensin II challenge, the effect of losartan quickly vanished, while that of candesartan remained. In hearts subjected to 25 min of global ischemia and 45 min of reperfusion, pre-treatment with candesartan (10 nM) or losartan (3 microM) immediately prior to ischemia improved the recovery of left ventricular developed pressure as compared to the effect of vehicle (69 +/- 3.2 and 64 +/- 2.3 vs. 44 +/- 6.2%, respectively; mean +/- S.E.M, P < 0.05). When ischemia was initiated following 30 min of washout after drug administration, the recovery of left ventricular developed pressure was higher in the candesartan group (73 +/- 3.2%, P < 0.05), but not in the losartan group (63 +/- 2.8%), than in the vehicle group (58 +/- 4.8%). The cumulative creatine kinase release during the first 30 min of reperfusion in the washout experiments was lower in the candesartan group (28.5 +/- 2.30 U, P < 0.05), but not in the losartan (40.8 +/- 6.73 U) group, than in the vehicle group (48.1 +/- 4.35 U). No significant difference between groups in left ventricular end-diastolic pressure and coronary perfusion pressure was found. The present results demonstrate that angiotensin AT1 receptor antagonists at equipotent concentrations could differ in their cardioprotective effects in hearts subjected to ischemia/reperfusion. It is suggested that the insurmountable AT1 receptor characteristics of candesartan could provide more persistent cardioprotection than the surmountable receptor characteristics of losartan.

The angiotensin II AT1-receptor antagonist candesartan improves functional recovery and reduces the no-reflow area in reperfused ischemic rat hearts

It is not yet clear if cardiac angiotensin II is involved in the pathophysiology of myocardial ischemia/ reperfusion injury. The aim of this study was to investigate the effect of the angiotensin II AT1-receptor antagonist candesartan on myocardial functional recovery in isolated rat hearts subjected to ischemia and reperfusion. Three groups of hearts perfused in the Langendorff mode with Krebs-Henseleit buffer under constant pressure received either vehicle (n = 7), candesartan, 1 nM (n = 6), or 100 nM (n = 7) at the start of 30 min of global ischemia. The recovery of the double product was significantly higher in the candesartan, 100 nM, group (75+/-9.2%) than in the vehicle group (40+/-5.1%; p < 0.05). At the end of 30 min of reperfusion, left ventricular end diastolic pressure was lower in rats given candesartan, 100 nM, than in rats given vehicle (10+/-4.3 vs. 38+/-4.8 mm Hg; p < 0.05). After ischemia and reperfusion, there was a large no-reflow area in the vehicle group (28+/-3.1% of the left ventricle), which was reduced by candesartan, 100 nM (12+/-1.3%; p < 0.05). In rats given candesartan, 1 nM, there was a trend toward a higher recovery of the double product (73+/-13.4%), a lower left ventricular end-diastolic pressure (29+/-6.6 mm Hg), and a smaller no-reflow area (19+/-3.5% of the left ventricle) compared with the rats receiving vehicle. These trends did, however, not reach statistical significance. Our results demonstrate that candesartan reduces myocardial ischemia/reperfusion injury, thus indicating that endogenous cardiac angiotensin II is involved in the tissue injury after myocardial ischemia and reperfusion.

Angiotensin-converting enzyme inhibitors and angiotensin II receptor antagonists in the treatment of heart failure caused by left ventricular systolic dysfunction

Activation of the renin-angiotensin-aldosterone system (RAAS) in left ventricular systolic dysfunction is a critically important determinant in the pathophysiologic processes that lead to progression of heart failure and sudden death. Angiotensin II, acting at the specific angiotensin receptor (AT1-R), activates a series of intracellular signaling sequences which are ultimately expressed within the cardiovascular system as vasoconstriction and associated vascular hypertrophy and remodeling. Angiotensin converting enzyme (ACE) inhibition leads to increases in the vasodilatory peptides bradykinin and substance P and at least an initial reduction in angiotensin II concentrations. AT1-R blocking drugs prevent access of angiotensin II to the AT1-R and thus prevent cellular activation. ACE inhibitors have clearly been demonstrated through a large number of clinical trials to increase survival in congestive heart failure, primarily by reducing the rate of progression of left ventricular dilatation and decompensation. However, this beneficial effect diminishes over time. Preliminary short-term clinical studies evaluating the efficacy of AT1-R blocking drugs in the treatment of heart failure have suggested that they elicit similar hemodynamic and neuroendocrine effects as do the ACE inhibitors. The combination ACE inhibitors and AT1-R blocking drugs offer the theoretical advantage of increasing bradykinin while blocking the actions of angiotensin II, and thus possibly show a synergistic effect. Again, preliminary studies have yielded encouraging results that are difficult to interpret because neither ACE inhibitor nor the AT1-R blocking drug doses were titrated to tolerance. Pharmacological manipulation of the RAAS has led to better understanding of its role in heart failure and improved clinical outcomes.

Angiotensin II type 1 receptor blockade with candesartan protects the porcine myocardium from reperfusion-induced injury

Angiotensin-converting enzyme (ACE) inhibitors reduce myocardial ischemia/reperfusion injury. It is unclear whether reduced formation of angiotensin II or attenuated degradation of bradykinin is responsible for the beneficial effects. We investigated the role of endogenous angiotensin II in ischemia/reperfusion injury by studying the effects of the angiotensin II type 1 receptor antagonist candesartan on myocardial function, infarct size, and perfusion after ischemia/reperfusion. Anesthetized pigs were subjected to 45 min of regional ischemia and 240 min of reperfusion. Starting 5 min before reperfusion, four groups of pigs (n = 6 in each) received coronary venous retroinfusion of candesartan (0.2, 2, or 20 microg/kg) or vehicle for 30 min. Myocardial regional blood flow was measured with radioactive microspheres in two separate groups (n = 6 in each) given 20 microg/kg candesartan or vehicle. Retroinfusion of 20 microg/kg of candesartan improved recovery of left ventricular systolic segment shortening measured by sonomicrometry in the ischemic area compared with 0.2 microg/kg of candesartan and vehicle. Infarct size, as a percentage of the area at risk, was smaller in the 2 and 20 microg/kg groups than in the vehicle group (39.1 +/- 11.6% and 34.8 +/- 10.2% vs. 78.3 +/- 8.9%, p < 0.01). There was no difference between candesartan and vehicle in their effects on regional myocardial blood flow. Angiotensin II type 1 receptor blockade supports myocardial functional recovery and reduces infarct size. This effect is not related to improved regional myocardial blood flow during reperfusion.