Cardiovascular disease and genetics of the renin-angiotensin system

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

A1166C genetic variation of the angiotensin II type I receptor gene and susceptibility to coronary heart disease: collaborative of 53 studies with 20,435 cases and 23,674 controls

OBJECTIVE

Angiotensin II induces vasoconstriction and vascular smooth muscle growth via stimulation of the angiotensin II type I receptor (AGTR1). Some studies have reported an association between a genetic variant (A1166C) in the 3' un-translated region of AGTR1 and increased risk of coronary heart disease (CHD), but other have yielded apparently conflicting results.

METHODS

Literature-based meta-analyses were performed on 48 papers including 53 studies published before June 2008 in relation to the A1166C polymorphism (NCBI, dbSNP: rs5186) of the AGTR1, involving a total of 20,435 CHD cases and 23,674 controls. We also explored potential sources of heterogeneity and conducted appropriate stratified analyses.

RESULTS

In a combined analysis, the per-allele odds ratio (OR) for CHD of the A1166C polymorphism was 1.11 (95% confidence interval: 1.03-1.19), but there is an indication of publication bias and heterogeneity among the 53 studies. Sample size and study quality were significant sources of heterogeneity among studies of the A1166C polymorphism with possibly overestimates in studies of smaller sample-size and poor-quality. When the analyses were restricted to 11 larger studies (≥500 cases), and to 8 high-quality studies (quality score: ≥11 points), the summary per-allele odds ratios were 0.992 (95% confidence interval, 0.944-1.042) and 0.990 (95% confidence interval, 0.915-1.072), respectively.

CONCLUSIONS

An overall weak association between the A1166C polymorphism and CHD is observed but this is likely to be due to publication bias and heterogeneity between studies. There were no significant associations among the larger sample-size and high-quality studies which are less prone to selective publication and have greater power to detect a true association.

Essential arterial hypertension and polymorphism of angiotensinogen M235T gene

Several candidate genes, chosen from the renin- angiotensin system, were examined for their association with essential hypertension. The genes of the renin- angiotensin system (RAS) are good candidates for such an approach because this system is well known to be involved in the control of blood pressure. One of these candidate genes is the gene encoding for angiotensinogen (the most important gene of the RAS associated with essential hypertension in the most population, is the gene for angiotensin-converting enzyme- ACE). One DNA polymorphism within exon 2- with threonine instead of methionine at position 235 (M235T) was found to be significantly associated with hypertension. The objective of this study is the analysis of M235T polymorphism in angiotensinogen gene in Romanian patients with essential hypertension as well as controls. We examined 38 patients with essential hypertension and 21 normotensive patients. In order to identify the M235T angioteninogen variant, we used the following methods: DNA extraction, PCR amplification and enzymatic digestion of the PCR product using Tth 111I restriction endonuclease enzyme. In the study groups, the M235T variant (Met?Thr in aminoacid position 235) was found more frequently in hypertensive patients (81,57%), than in control subjects (66,66%). We identified 52,63% M235T heterozygotes in the hypertensive group compared with 47,61% in the control group, and 28,94% T235T homozygotes in the hypertensive group compared with 19,04% in the control group. The results of our study suggest an association of the M235T polymorphism in the gene encoding angiotensinogen with essential hypertension.

Regression of left ventricular mass with captopril and metoprolol, and the effects on glucose and lipid metabolism

OBJECTIVE

Angiotensin II and insulin have been suggested to promote the development of hypertensive left ventricular (LV) hypertrophy. We compared the effects of captopril and metoprolol on the regression of LV mass and the relation to insulin sensitivity.

DESIGN

51 previously untreated non-diabetic hypertensive patients (mean age 51 +/- 8 years, body mass index, BMI 25.9 +/- 3.2 kg/m2, office blood pressure, 158/102 mmHg) were randomized to captopril or metoprolol; a low-dose diuretic and/or a calcium cannel antagonist were added, if needed.

INTERVENTIONS

LV mass index (LVMI; by echocardiography) and 24-h ambulatory blood pressure were examined at baseline, 6 and 12 months. At baseline and 12 months, insulin sensitivity index (MI) was calculated by a hyperinsulinemic-euglycemic insulin clamp technique.

RESULTS

Blood pressures were reduced similarly in both groups. LVMI (115 +/- 21 g/m2 at baseline) was reduced in both groups (p < 0.01), but more with captopril than with metoprolol (e.g. -16 vs -7 g/m2, i.e. -13 vs -6%, at 12 months, p < 0.01). MI decreased by 6% with captopril (p = 0.05) and by 23% with metoprolol (p < 0.01), with no difference between the groups. Changes in LVMI were not related to changes in MI in the two groups, or when all patients were analyzed together. High-density lipoprotein (HDL)-cholesterol decreased (p < 0.05) by both drugs, with small effects on low-density lipoprotein (LDL), and triglycerides increased by 30% with metoprolol (p < 0.01).

CONCLUSION

Blockade of the renin-angiotensin-aldosterone system has a role beyond that of blood pressure reduction in the regression of LV mass. There was no relationship between regression of LV mass and improvement in insulin sensitivity. We could not confirm a beneficial effect of ACE inhibition on insulin sensitivity. Thus, our results do not support the importance of insulin in the control of LV geometry.

Gene-gene interaction in the RAS system in the predisposition to myocardial infarction in elder population of St. Petersburg (Russia)

The aim of this study was to estimate the frequencies of some DNA polymorphisms of two genes of the renin-angiotensin system (RAS), M235T angiotensinogen gene and insertion-deletion polymorphism in angiotensin-converting enzyme gene, in older (>55 years old) myocardial infarction survival and control groups. For this purpose 198 myocardial infarction (MI) patients and 152 randomly selected healthy persons have been analyzed. We have not found any differences in allele and genotype distribution in the above-mentioned genes for either group. However, statistical research showed a significant increase of double homozygotes IITT in the group of MI patients as compared with those in the control group. In this respect we suggested that gene-gene interaction in the RAS system may be considered to be a predisposing factor for MI development.

The renin-angiotensin system in essential hypertension: associations with cardiovascular risk

The importance of the renin-angiotensin system (RAS) in blood pressure regulation is well established. High RAS activity has also been implicated in connection with elevated cardiovascular risk in patients with essential hypertension. Data from epidemiological studies have related high plasma renin levels in essential hypertensive patients to cardiovascular complications. However, whether renin itself is a risk factor of cardiovascular events or just acts as a marker for other risk factors still remains to be elucidated. Several possible mechanisms that could be responsible for the association between elevated RAS activity and cardiovascular risk are reviewed. The concept of high RAS activity being a cardiovascular risk factor is strongly supported by results from large clinical studies showing the beneficial effects of angiotensin-converting enzyme inhibitors and angiotensin II receptor blockers in congestive heart failure and hypertension. Knowing more about the exact mechanisms of the association between high RAS activity and cardiovascular complications would enable us to profile the treatment of high blood pressure more specifically to improve outcome in individuals or groups of patients.

The renin-angiotensin system and the heart: beyond 2000

Renin-angiotensin is both a circulatory and a local tissue system. However, in most circumstances, renin present in the heart and blood vessels is taken up from the plasma, and the kidney is the prime source of this renin. Tissues can then modulate and control the production of angiotensin II. In various organs, angiotensin II has local actions. In the heart, working through the AT1 receptor, it increases contractility and may cause cardiocyte hypertrophy. Indirectly, the heart is also very much affected by the vascular actions of angiotensin II. However, the net result on the heart is the product of an important interaction between a large number of factors. There is little doubt that an inappropriately high plasma (and tissue) level of renin, related to sodium balance, is associated with increased left ventricular hypertrophy and cardiovascular complications. The genetic approach will lead to an understanding of genomic risk factors in hypertension and may identify control systems--at present unknown--which will lead to a new approach to the prevention of cardiovascular death. It is, however, unlikely that such a factor will clearly predict an individual's prognosis, but it will, rather, identify groups at risk. Constellations of genomic interactions with multiple environment factors reduce this power. Modification of the response to angiotensin II using present treatment, or interrupting the response at more distal sites, may enable us to achieve the beneficial effects on cardiac hypertrophy without the detrimental effects.