Efficacy of intrarenal ACE-inhibition estimated from the renal response to angiotensin I and II in humans

The Genetic Determinants of Renal Impairment Following Cardiac Surgery

Cardiac surgery is frequently performed, and acute renal dysfunction is a common adverse event following this procedure. Cardiac surgery-related renal injury independently predicts longer hospital stays and greater rates of morbidity and mortality. Although much work has been completed toward better understanding of this phenomenon, the state of knowledge concerning surgery-related renal injury remains limited. Currently, there is no effective paradigm to identify patients who are at risk for this condition; the specific mechanisms of renal injury during surgery are incompletely understood; and few therapies exist to prevent or treat this phenomenon. To better understand this common clinical problem, recent research has focused on the importance of genetic variability within the physiological and patho-physiological systems that underlie renal dysfunction following cardiac surgery. Emphasizing the importance of using genetics to elucidate molecular mechanisms of this disease, this article reviews the current literature on genetic polymorphisms and post cardiac surgery-related renal dysfunction.

Rationale for Combining Blockers of the Renin-Angiotensin System

Blockade of the renin-angiotensin system (RAS) with angiotensin I-converting enzyme (ACE) inhibitors and AT1-receptor (AT1R) blockers has become one of the most successful therapeutic approaches in medicine. The question is no longer whether RAS inhibition helps, but rather how we can optimize inhibition to achieve optimal cardiovascular and renal protection. Indeed, numerous data have shown that the RAS is not blocked fully over 24 hours with current doses of RAS blockers because they trigger a counter-regulatory renin release that can offset pharmacologic inhibition of the RAS. This absence of full blockade may have clinical implications. Combination therapy with ACE inhibitors and AT1R antagonists thus has been proposed to inhibit the biological effects of the reactive renin release triggered by single-site RAS inhibition. By using this approach, numerous experimental and clinical studies have suggested that this combination therapy has additive or synergistic effects on blood pressure and on the prevention of cardiovascular and renal lesions. Although similar intensity of RAS blockade can be achieved by either combination therapy or by using high doses of an AT1-receptor antagonist given alone, the ACE inhibitor present in the combination interferes with the bradykinin-nitric oxide pathway and the N-acetyl-Ser-Asp-Lys-Pro metabolism, which both may have additional biological effects.

Angiotensin II–nitric oxide interaction in the kidney

PURPOSE OF REVIEW

The balance of angiotensin II and nitric oxide determines the sensitivity of the tubuloglomerular feedback mechanism, renal vascular resistance and filtration rate. Angiotensin II induces nitric oxide release, but the role of angiotensin II receptors here is not fully understood. Further, the angiotensin II-nitric oxide interaction can be modulated by reactive oxygen species. This review focuses on the angiotensin II-nitric oxide interaction and their modulation by reactive oxygen species in the control of renal blood flow.

RECENT FINDINGS

Ideas about the role of angiotensin II type 1 and angiotensin II type 2 receptors are extended by the observation of angiotensin II type 1-mediated nitric oxide release with direct effects on vascular tone, tubuloglomerular feedback and sympathetic neurotransmission. Angiotensin receptors elicit disparate effects on intrarenal circulation. Angiotensin II-nitric oxide interactions are modulated by reactive oxygen species, as shown by angiotensin II type 1-mediated activation of superoxide and depression of antioxidant enzymes leading to reduced nitric oxide concentration - mechanisms that may be also important in angiotensin II-dependent hypertension.

SUMMARY

Recent studies show that angiotensin II stimulates the nitric oxide system via angiotensin II type 1 and angiotensin II type 2 receptors, whereas receptors exert different effects on renal and medullary flow. The interaction via angiotensin II type 1 is modulated by reactive oxygen species.

ANGIOTENSIN II-NITRIC OXIDE INTERACTION IN GLOMERULAR ARTERIOLES

1. Resistance changes of the afferent and efferent arterioles determine blood flow and filtration rate in the kidney. The tone of both vessels results from the influence of nerves and humoral and paracrine factors, through a balance of constrictor and dilator systems. Angiotensin (Ang) II and nitric oxide (NO) are important factors determining vascular tone. 2. In the present review, we show that, in addition to the basal production of NO, a specific and significant AngII-induced release of NO occurs in glomerular arterioles. Data from investigations of arteriolar contraction, as well as from fluorescence measurements of NO, in the presence of selective angiotensin AT(1) and AT(2) receptor antagonists indicate an AT(1) receptor-stimulated release of NO in afferent arterioles. 3. The AngII-induced liberation of NO could prevent glomerular arterioles from a marked constriction, particularly in situations of high AngII levels in the kidney.

Nitrendipine and amlodipine mimic the acute effects of enalapril on renal haemodynamics and reduce glomerular hyperfiltration in patients with chronic kidney disease

Antihypertensive drugs may have an important effect on glomerular haemodynamics. In chronic nephropathy patients, we compared the effect on glomerular haemodynamics of two second-generation dihydropyridinic agents, nitrendipine and amlodipine, with a first generation dihydropyridinic agent and an ACE-inhibitor, enalapril. In all, 32 patients (pts), divided into four groups, received the different drugs: ENA (enalapril, eight pts), NIF (nifedipine, eight pts), NIT (nitrendipine, eight pts) AML (amlodipine, eight pts). The study assessed the effect on glomerular haemodynamics of a single administration of the test drug in baseline conditions and in glomerular hyperfiltration experimentally induced by amino-acid infusion. The glomerular filtration rate (GFR, measured by inulin clearance), effective renal plasma flow (ERPF, measured by p-aminohippurate clearance), renal vascular resistances (RVR) and filtration fraction (FF) were assessed. Administration of AML and NIT test dose reduced FF, as did ENA, but not NIF, in both baseline (AML: P=0.005; NIT: P=0.02; ENA: P=0.007) and glomerular hyperfiltration conditions (AML: P=0.0003; NIT: P=0.03; ENA: P=0.00006). In baseline conditions, only ENA resulted in a significant drop in the GFR (P=0.008), while NIF, NIT and AML induced a significant increase (P=0.003, 0.03, 0.0001, respectively). However, in hyperfiltration conditions, NIT (0.08) and AML (0.00003) caused a decrease in the GFR, as did ENA (0.0003) but not NIF. In all the experimental conditions, a RVR reduction and an ERPF increase were observed. Single dose of NIT and AML were effective in attenuating the effect of amino-acid infusion on glomerular filtration, similar to ENA; this effect of NIT and AML on the glomerular filtration rate is not observed under basal conditions.

Salt intake and non-ACE pathways for intrarenal angiotensin II generation in man

Angiotensin-converting enzyme (ACE) plays a crucial role in the generation of angiotensin II (Ang II) via conversion from angiotensin I (Ang I). There has been substantial recent interest in non-ACE pathways of Ang II generation in the heart, large arteries, and the kidney. In the case of the human kidney, studied when in balance on a low-salt diet, the renal haemodynamic response to Ang II antagonists substantially exceeds the renal response to ACE inhibitors (ACE-I), suggesting that about 30-40% of Ang II-generation occurs via non-ACE pathways. In this study, we examined the relative contribution of non-ACE pathways, by comparing the response to candesartan and to captopril at the top of the dose-response in normal humans when in balance on a low-salt, as well as a high-salt, diet. As anticipated on a low-salt diet, the increase in renal plasma flow (RPF) in response to candesartan (165+/-14 mL/min/1.73 m2) significantly exceeded the response to captopril (118+/-12 mL/min/1.73 m2; p<0.01). In subjects studied on a high-salt diet, the response to candesartan (97+/-20 mL/min/1.73 m2) also significantly exceeded the response to captopril on the same diet(30+/-15 mL/min/1.73 m2; p<0.01). This remarkable response to candesartan in subjects on a high-salt diet,when compared with the response to captopril,suggests that non-ACE-dependent Ang II generation was influenced less than the classical renal pathway with an increase in salt intake, so that the percentage of Ang II generated via the non-ACE pathway rose to the 60-70% range.

Effect of ACE inhibition on pressor, renal vascular, and adrenal responses to infusion of angiotensin I in normal subjects eating a low-salt diet

To examine the influence of angiotensin-converting enzyme (ACE) on pressor, renal vascular, and adrenal responses during angiotensin I (Ang I) infusion, we studied 10 normotensive, healthy men. Each was in balance with a 10-mEq sodium, 100-mEq potassium intake and was studied before and during ACE inhibition with enalapril. Ang I (3, 10, and 30 ng/kg/min) was infused in each subject. Then ACE inhibition was instituted with enalapril for 3 days, which induced the anticipated fall in blood pressure, plasma Ang II, and aldosterone concentration, and rise in renal plasma flow. During ACE inhibition only the 30-ng/kg/min Ang I dose raised plasma Ang II levels. There was a spectrum, however, in the end-organ response to Ang I during ACE inhibition. Responses of plasma aldosterone concentration and blood pressure were in excellent accord with the reduction in Ang II formation. On the other hand, responses of the renal blood supply were substantially less inhibited than anticipated. Under the conditions of this study, ACE inhibition led to nonuniform changes in the response to exogenous Ang I, suggesting intrarenal conversion of Ang I to Ang II.

Role of kinins and angiotensin II in the vasodilating action of angiotensin converting enzyme inhibition in rat renal vessels

OBJECTIVE

To assess directly the vasodilating effects of angiotensin converting enzyme (ACE) inhibition in different renal vessels and to determine the role of kinins and angiotensin II (ANGII) therein.

METHODS

Lumen diameters of different vessels and glomerular blood flows were measured in cortical and juxtamedullary glomeruli by in-vivo microscopy in the split hydronephrotic kidney of anesthetized female Wistar rats.

RESULTS

Injection of the ACE inhibitor quinapril at a dose of 0.9 mg/kg intravenously, which blocks conversion of locally applied angiotensin I (1 mumol/l), increased glomerular blood flows by 39 +/- 6 and 18 +/- 4% in cortical and juxtamedullary glomeruli, respectively, due to vasodilatation in all renal vessels. The most pronounced vasodilatation was observed in interlobular arteries (19 +/- 2%) and in cortical afferent arterioles (16 +/- 3%). Pretreatment of the hydronephrotic kidney by local application of 40 nmol/l Hoe140, a bradykinin B2 receptor antagonist, or 3 mumol/l valsartan, an ANGII type 1 receptor antagonist, attenuated the vasodilatation in response to quinapril. ANGII receptor blockade affected only weakly, whereas bradykinin receptor blockade blunted markedly, the quinapril-induced vasodilatation, suggesting that kinins play an important role in our experimental model. Administration of valsartan, which abrogated the renal vasoconstriction induced by 10 nmol/l ANGII completely, caused vasodilation of magnitude similar to that caused by administration of quinapril. Yet, the vasodilatation induced by the combination of valsartan and quinapril was significantly larger than that induced by administration of quinapril alone in interlobular arteries, afferent arterioles, and cortical efferent arterioles.

CONCLUSIONS

Our results indicate that kinins and ANGII can contribute to the renal vasodilatation in response to ACE inhibitors, but ACE inhibitors appear to have only minor effects on ANGII levels in those renal vessels, which are the well-known sites of renin expression.