Is the renin system necessary?

Renin-Angiotensin-Aldosterone System: From History to Practice of a Secular Topic

Renin-angiotensin-aldosterone system (RAAS) inhibitors are standard care in patients with hypertension, heart failure or chronic kidney disease (CKD). Although we have studied the RAAS for decades, there are still circumstances that remain unclear. In this review, we describe the evolution of the RAAS and pose the question of whether this survival trait is still necessary to humankind in the present age. We elucidate the benefits on cardiovascular health and kidney disease of RAAS inhibition and present promising novel medications. Furthermore, we address why more studies are needed to establish a new standard of care away from generally prescribing ACEi or ARB toward an improved approach to combine drugs tailored to the needs of individual patients.

Genomic Approaches to Hypertension

Genomic insights and analyses of Mendelian hypertension (HTN) syndromes and Genome-Wide Association study (GWAS) on essential hypertension have contributed to the depth of understanding of the genetics origins of hypertension. Mendelian syndromes are important for the field, since such knowledge leads to specific insights about disease pathogenesis and the potential for precision medicine. The clinical impact of findings of on essential hypertension is continuously evolving, and the insights accrued will refine efforts to combat the societal impact of hypertension. Comprehensive identification of all genomic variants of hypertension, along with their individual associated mechanisms, is paving the way forward in the era of personalized medicine. The overriding challenge for care providers is to reduce health inequities through improved compliance and, perhaps, new paradigms for implementation science that incorporate genomic medicine.

Genetics of hypertension: from experimental animals to humans

Essential hypertension affects 20 to 30% of the population worldwide and contributes significantly to cardiovascular mortality and morbidity. Heridability of blood pressure is around 15 to 40% but there are also substantial environmental factors affecting blood pressure variability. It is assumed that blood pressure is under the control of a large number of genes each of which has only relatively mild effects. It has therefore been difficult to discover the genes that contribute to blood pressure variation using traditional approaches including candidate gene studies and linkage studies. Animal models of hypertension, particularly in the rat, have led to the discovery of quantitative trait loci harbouring one or several hypertension related genes, but translation of these findings into human essential hypertension remains challenging. Recent development of genotyping technology made large scale genome-wide association studies possible. This approach and the study of monogenic forms of hypertension has led to the discovery of novel and robust candidate genes for human essential hypertension, many of which require functional analysis in experimental models.

Hypertension and genome-wide association studies: combining high fidelity phenotyping and hypercontrols

Among the common complex diseases, hypertension has been particularly unlucky in the recent surge of positive results from genome-wide association studies. We summarize the evidence that would support continuing the effort in the hunt for a genetic basis for hypertension. The problems facing the genetic studies for hypertension are not unique, but phenotypic characterization, heterogeneity and high prevalence make it a special case requiring a more individualized approach. We argue that, even in the presence of a strong environmental component to hypertension risk, the common disease/common variant model is relevant for hypertension and discuss the issues involved in designing a genome-wide association study for hypertension. It is likely that the individual odds ratios for disease variants will be less than 1.3 and, although individually these effect sizes are minor, the combination of even a few such common polymorphisms can have substantial population attributable risks. The identification of hypertension gene variants should provide new insight into the disease susceptibility, progression and severity. This will lead to the identification of potential targets for lifestyle and pharmacological interventions, with the ultimate goal of improving prevention, diagnosis and treatment.

Renal effects of angiotensin converting enzyme inhibitors in hypertension

This review focuses on the renal effects of the angiotensin converting enzyme inhibitors, captopril and enalapril. Emphasis is placed on the renal response to these drugs in patients with primary essential hypertension, and in hypertension accompanying renal parenchymal disease. Specifically reviewed are the renal function and hemodynamic, salt and water, body fluid composition, and urinary protein excretion responses. The interruption of the renin-angiotensin-aldosterone axis has the potential to produce a variety of favorable renal responses, including reduction of renal vascular resistance, enhancement of renal blood flow, enhancement of glomerular filtration rate, acute natriuresis, sustained diuresis, and a decrease in urinary protein excretion. Data in support of these potential renal perturbations are presented and discussed. The results suggest that the angiotensin converting enzyme inhibitors are important therapeutic agents in the treatment of hypertensive disease, in that they may modify pathophysiologic renal abnormalities encountered in this disease state.

Diuretic treatment in decompensated cirrhosis and congestive heart failure: effect of posture

The diuretic effect of the supine position was evaluated in six patients with cirrhosis and ascites and six with congestive cardiac failure. After fasting overnight in bed the patients received bumetanide 1 mg intravenously and were then immediately randomly assigned to either bed rest in the supine position or normal daily activity in the upright position for the next six hours. Two days later the procedure was repeated, the patients being assigned to the other posture. The diuretic response was similar in patients with heart failure and cirrhosis, and was significantly greater in the supine than in the upright position: mean 1133 v 626 ml/6 h (p less than 0.01). The natriuresis was similarly larger during recumbency: mean sodium 96 v 45 mmol(mEq)/6h (p less than 0.01), and the excreted potassium in six hours was similar in both postures. The glomerular filtration rate was 100 and 66 ml/min (p less than 0.01) and heart rate 76 and 83 beats/min (p less than 0.01) in the supine and upright positions respectively. Plasma concentrations of noradrenaline, renin, and aldosterone were all raised even when the patient adopted the supine position, and a further significant rise was observed during the upright position. The results suggest that the attenuated response to intravenous bumetanide in the upright position and during normal daily activity may be due to the activation of several homeostatic mechanisms that may reduce the excretion of water and salt.

Developmental aspects of the renal responses to hemorrhage during converting-enzyme inhibition in fetal lambs

The role of the renin-angiotensin system in modulating the renal hemodynamic and functional responses to reductions of fetoplacental blood volume (8.8-35.5%) was studied in two groups of fetal lambs (less than 120 days and greater than 130 days gestation; term 145 days) during infusion of either captopril (experimental fetuses) or dextrose 5% in water (control fetuses). At high hemorrhage levels (level III), renal blood flow decreased and renal vascular resistance increased significantly in both groups of fetuses (less than 120 days and greater than 130 days), either treated or not treated with captopril. However, at low hemorrhage levels (levels I and II), and contrary to what was observed in young fetuses (less than 120 days), near-term fetuses (greater than 130 days) receiving captopril showed neither significant decreases in renal blood flow nor increases in renal vascular resistance, whereas untreated fetuses of the same gestational ages demonstrated significant decreases in renal blood flow and increases in renal vascular resistance. It was found in both less than 120 day and greater than 130 day fetuses that hemorrhage is associated with a decrease in urinary flow rate and free water clearance accompanied by an increase in urine osmolality and sodium reabsorption. It was shown that captopril does not modify this response. The present study also demonstrated that the blood pressure response to hemorrhage was characterized by a similar decrease in less than 120 day fetuses, whether treated or untreated with captopril. On the other hand, blood pressure did not change in control fetuses greater than 130 days, but decreased slightly in captopril-treated fetuses during hemorrhage. Taken together, the present results tend to suggest that the renin-angiotensin system may be an important modulator of the renal hemodynamic response to low level hemorrhage as fetuses approach term, and may be more important in controlling blood pressure in near-term than in young fetuses.

The clinical application of converting enzyme inhibitors

Chronic blockade of the renin angiotensin system became possible when orally active inhibitors of angiotensin converting enzyme, the enzyme which catalyzes the transformation of angiotensin I into angiotensin II, were synthetized. Two compounds, captopril and enalapril, have been investigated in clinical studies. The decrease of the pressor response to exogenous angiotensin I and of the circulating levels of angiotensin II following administration of these inhibitors has been demonstrated to be directly related to the degree of suppression of plasma angiotensin converting enzyme activity. These inhibitors have been shown to normalize blood pressure alone in some hypertensive patients whereas in many others, satisfactory blood pressure control can be achieved only after the addition of a diuretic. Captopril and enalapril also markedly improve cardiac function of patients with chronic congestive heart failure. Chronic blockade of the renin angiotensin system has therefore provided an interesting new approach to the treatment of clinical hypertension and heart failure.

Plasma catecholamines and cardiovascular responses during converting enzyme inhibition in normotensive and hypertensive man

The hemodynamic and plasma catecholamine responses to isometric exercise and head up tilt were investigated in normotensive and hypertensive subjects during normal and low sodium intake and before and during the administration of the converting enzyme inhibitors teprotide or captopril. Although teprotide and captopril decreased significantly the mean arterial pressure during both sodium intakes the normal pattern of hemodynamic response to hand grip and head up tilt was preserved. Moreover changes of plasma catecholamines during hand grip and head up tilt were not affected either by teprotide or captopril. When following the administration of teprotide or captopril fainting occurred either in the seated position or during head up tilt it was associated not only with a decrease in arterial pressure but also with a concurrent reduction in cardiac output and/or hypovolemia. It is concluded that converting enzyme inhibition does not impair significantly the cardiovascular responses which are mediated via the sympathetic nervous system either in normotensive or in hypertensive subjects.

Sodium intake alters the effects of norepinephrine on the renin system and the kidney

To examine the interactions between sodium intake and the sympathetic nervous system and their influences on the blood pressure control system we studied eight normotensive men after high (800 mEq/d) and low (10 mEq/d) sodium intake. We measured blood pressure, arterial, venous and urinary norepinephrine (NE), glomerular filtration rate (GFR), renal blood flow (RBF), plasma renin activity (PRA) and aldosterone (PA), and the fractional excretion of sodium (FENa) and potassium (FEK) before and during incremental infusion of norepinephrine. High salt intake influenced the sensitivity to NE as well as subsequent pressor responses. The NE-induced decrease in RBF and GFR was not different on high and low sodium intakes. A significant decrease in FENa (p less than 0.05) with NE infusion could only be seen during high sodium intake. A significant increase in PRA (p less than 0.01) and PA (p less than 0.05) was induced by NE only during the low sodium period. These observations reveal previously unrecognized qualitative and quantitative interactions between sodium homeostasis and norepinephrine which are capable of influencing blood pressure in man.

Effect of captopril on cardiovascular, sympathetic and vasopressin responses to hemorrhage

Spontaneous blood pressure recovery following hemorrhage in anesthetized rats is almost completely blocked by administration of captopril. This occurs despite comparable or enhanced increases in plasma levels of catecholamines and vasopressin. These findings suggest that angiotensin II, the formation of which is blocked by captopril, is essential to acute blood pressure recovery following hemorrhage and that changes in catecholamines or vasopressin are insufficient to sustain pressure recovery. The data also indicate that angiotensin II formation is not essential in the hemorrhagic stimulation of vasopressin or catecholamines.