The renin-angiotensin system and its relation to hypertension

Dysnatremias, Mortality, and Kidney Failure in CKD: Findings From the Chronic Renal Insufficiency Cohort (CRIC) Study

Rationale & Objective

Dysnatremias have been associated with an increased risk of mortality in the chronic kidney disease (CKD) population. Our objective is to identify the prevalence of and risk factors associated with dysnatremias in a CKD population and assess the association of dysnatremias with kidney failure and mortality among patients with CKD enrolled in the Chronic Renal Insufficiency Cohort Study.

Study Design

Analysis of prospective cohort study.

Setting & Participants

Adult patients aged 21-74 years with CKD from the Chronic Renal Insufficiency Cohort study.

Predictors

Baseline and time-dependent hyponatremia and hypernatremia.

Outcomes

All-cause mortality and kidney failure.

Analytical Approach

Baseline characteristics were compared using χ2 tests for categorical variables, analysis of variance for age, and Kruskal-Wallis tests for laboratory variables. Cox proportional hazards models and competing risk models were used to evaluate the association between baseline sodium level and overall mortality.

Results

Of a total of 5,444 patients with CKD, 486 (9%) had hyponatremia and 53 (1%) had hypernatremia. Altogether, 1,508 patients died and 1,206 reached kidney failure. In adjusted Cox models, time-dependent dysnatremias were strongly associated with mortality for both hyponatremia (HR, 1.38; 95% CI, 1.16-1.64) and hypernatremia (HR, 1.54; 95% CI, 1.04-2.29). Factors associated with hyponatremia included female sex, diabetes, and hypertension. Regardless of age, time-dependent hypernatremia was associated with an increased risk of kidney failure (HR, 1.64; 95% CI, 1.06-2.53). Baseline and time-dependent hyponatremia were associated with an increased risk of kidney failure in patients younger than 65 (baseline hyponatremia HR, 1.30; 95% CI, 1.03-1.64 and time-dependent hyponatremia HR, 1.36; 95% CI, 1.09-1.70) but not among patients aged >65 years.

Limitations

Inability to establish causality and lack of generalizability to hospitalized patients.

Conclusions

Dysnatremias are prevalent among ambulatory CKD patients and are associated with mortality and kidney failure. Time-dependent dysnatremias were significantly associated with mortality in patients with CKD. Time-dependent hypernatremia was associated with progression to kidney failure. Baseline and time-dependent hyponatremia were associated with an increased risk of progression to kidney failure in those younger than 65 years.

Renin-angiotensin system impairs macrophage lipid metabolism to promote age-related macular degeneration in mouse models

Metabolic syndrome, a condition involving obesity and hypertension, increases the risk of aging-associated diseases such as age-related macular degeneration (AMD). Here, we demonstrated that high-fat diet (HFD)-fed mice accumulated oxidized low-density lipoprotein (ox-LDL) in macrophages through the renin–angiotensin system (RAS). The ox-LDL-loaded macrophages were responsible for visual impairment in HFD mice along with a disorder of the retinal pigment epithelium (RPE), which is required for photoreceptor outer segment renewal. RAS repressed ELAVL1, which reduced PPARγ, impeding ABCA1 induction to levels that are sufficient to excrete overloaded cholesterol within the macrophages. The ox-LDL-loaded macrophages expressed inflammatory cytokines and attacked the RPE. An antihypertensive drug, angiotensin II type 1 receptor (AT1R) blocker, resolved the decompensation of lipid metabolism in the macrophages and reversed the RPE condition and visual function in HFD mice. AT1R signaling could be a future therapeutic target for macrophage-associated aging diseases, such as AMD.

Nagai et al. show that mice fed high-fat diet (HFD) accumulate oxidized low-density lipoprotein in macrophages through the renin–angiotensin system, which impairs visual function. They find that angiotensin II type 1 receptor (AT1R) improves the visual function of HFD mice, suggesting AT1R signaling as a potential therapeutic target for age-related macular degeneration.

Advances in understanding the renin-angiotensin-aldosterone system (RAAS) in blood pressure control and recent pivotal trials of RAAS blockade in heart failure and diabetic nephropathy

The renin-angiotensin-aldosterone system (RAAS) plays a fundamental role in the physiology of blood pressure control and the pathophysiology of hypertension (HTN) with effects on vascular tone, sodium retention, oxidative stress, fibrosis, sympathetic tone, and inflammation. Fortunately, RAAS blocking agents have been available to treat HTN since the 1970s and newer medications are being developed. In this review, we will (1) examine new anti-hypertensive medications affecting the RAAS, (2) evaluate recent studies that help provide a better understanding of which patients may be more likely to benefit from RAAS blockade, and (3) review three recent pivotal randomized trials that involve newer RAAS blocking agents and inform clinical practice.

Association of renin and aldosterone with ethnicity and blood pressure: the Multi-Ethnic Study of Atherosclerosis

BACKGROUND

Although variations in plasma renin activity (PRA) and aldosterone have been examined in whites and blacks, the association of these hormones with blood pressure in multiethnic populations has not been described.

METHODS

We measured PRA and aldosterone in 1,021 participants in the Multi-Ethnic Study of Atherosclerosis not taking antihypertensives and examined the association between ethnicity and PRA/aldosterone and the association between PRA/aldosterone with systolic blood pressure (SBP).

RESULTS

Average age was 62 (SD = 9) years, and 49% of participants were women. Median PRA was 0.51 (interquartile range (IQR) = 0.29-0.87) ng/ml/hour, and median aldosterone was 12.6 (IQR = 9.1-17.1) ng/dl. After age and sex adjustment, compared with whites, blacks had 28% lower PRA and 17.4% lower aldosterone, and Hispanics had 20.1% higher PRA but similar aldosterone levels. After multivariable adjustment, compared with whites, only Hispanic ethnicity independently associated with higher PRA (0.18ng/ml/hour; 95% confidence interval (CI) = 0.06-0.31). Blacks had lower aldosterone (-1.7ng/dl; 95% CI = -3.2 to -0.2) compared with whites. After multivariable adjustment, PRA was associated with lower SBP in whites (-3.2mm Hg; 95% CI = -5.2 to -1.2 per standardized unit PRA), Chinese (-3.5mm Hg; 95% CI = -6.2 to -0.80 per standardized unit), and Hispanics (-2.3mm Hg; 95% CI = -4.1 to -0.6 per standardized unit) but not blacks. Aldosterone was associated with higher SBP only in Hispanics (2.5mm Hg; 95% CI = 0.4-4.5 per SD).

CONCLUSIONS

Compared with whites, blacks have lower aldosterone and Hispanics have higher PRA. Aldosterone had significant associations with higher SBP in Hispanics compared with other groups, a finding that may suggest a different mechanism of hypertension.

Plasma levels of prorenin and renin in blacks and whites: their relative abundance and associations with plasma aldosterone concentration

BACKGROUND

All renin arises from prorenin. The proportion of renin relative to prorenin could influence overall renin-angiotensin-aldosterone activity. We sought to determine whether prorenin levels were related to extracellular volume, as reflected by the levels of plasma renin activity (PRA), and to aldosterone.

METHODS

We analyzed plasma levels of prorenin, renin, and aldosterone, as well as their interactions, in 129 young blacks and whites.

RESULTS

Blacks had lower plasma renin concentration (PRC) and PRA, but had prorenin levels similar to whites (69 pg/ml in blacks vs. 62 pg/ml in whites, P = 0.41). As a result, the renin-to-total renin ratio was significantly lower in blacks (11.5% in blacks as compared to 19.8% in whites; P = 0.0001). Because prorenin also resides in tissues including the adrenal where it can bind to a specific receptor to generate angiotensin II, we examined the relationship of prorenin levels to plasma aldosterone concentrations (PAC). While a positive association between PRC and PAC was found in both blacks and whites, PAC was positively related to prorenin in whites (P = 0.04) but negatively in blacks, an observation that we hypothesize was due to reduced prorenin-to-renin conversion in blacks.

CONCLUSIONS

We observed a disproportionately high level of prorenin in blacks. These high circulating prorenin levels however do not result in greater adrenal angiotensin II and aldosterone production in healthy young blacks.

Glomerular hyperfiltration and increased proximal sodium reabsorption in subjects with type 2 diabetes or impaired fasting glucose in a population of the African region

BACKGROUND. Glomerular hyperfiltration (GHF) is a well-recognized early renal alteration in diabetic patients. As the prevalence of GHF is largely unknown in populations in the African region with respect to normal fasting glucose (NFG), impaired fasting glucose (IFG) and type 2 diabetes [diabetes mellitus (DM)], we conducted a cross-sectional study in the Seychelles islands among families including at least one member with hypertension. METHODS. The glomerular filtration rate (GFR), effective renal plasma flow (ERPF) and proximal tubular sodium reabsorption were measured using inulin, p-aminohippurate (PAH) and endogenous lithium clearance, respectively. Twenty-four-hour urine was collected on the preceding day. RESULTS. Of the 363 participants (mean age 44.7 years), 6.6% had IFG, 9.9% had DM and 63.3% had hypertension. The prevalence of GHF, defined as a GFR >140 ml/min, was 17.2%, 29.2% and 52.8% in NFG, IFG and DM, respectively (P trend <0.001). Compared to NFG, the adjusted odds ratio for GHF was 1.99 [95% confidence interval (CI) 0.73-5.44] for IFG and 5.88 (2.39-14.45) for DM. Lithium clearance and fractional excretion of lithium were lower in DM and IFG than NFG (P < 0.001). CONCLUSION. In this population of African descent, subjects with impaired fasting glucose or type 2 diabetes had a high prevalence of GHF and enhanced proximal sodium reabsorption. These findings provide further insight on the elevated incidence of nephropathy reported among African diabetic individuals.

Sodium, blood pressure, and ethnicity: what have we learned?

An enormous amount of research has yielded significant knowledge about ethnic differences in sodium homeostasis and blood pressure regulation. Consistent findings such as greater sodium-sensitivity, lower potassium excretion and high higher serum sodium levels in African Americans need further exploration to define more precise physiological mechanisms. The genetic alleles associated with sodium homeostasis in relation to blood pressure have accounted for only a small proportion of the variance in blood pressure. Several allelic variants differ in frequency among ethnic groups and heat-adapted genetic variants have a high prevalence in low latitudes and hot, wet climates which lends support to the "sodium retention" hypothesis. The blood pressure disparities between African Americans and whites may, in part, be due to different allelic frequencies of genes associated with sodium homeostasis. However, with advances in genomics, environmental factors tend to be neglected in research. Better measures of environmental stress have recently been developed by anthropologists and should be included in research designs by investigators in other disciplines. Public health efforts should encourage food producers to reduce sodium content of its products, and physicians should encourage patients to reduce consumption of high sodium packaged and fast foods.

Ethnic Differences in Renal Responses to Furosemide

Blacks have a greater tendency to retain Na than whites. The present study sought evidence for ethnic differences in parameters reflective of Na uptake by the Na,K,2Cl cotransporter in the thick ascending limb, namely, the urine concentration and urinary excretion of certain cations before and after furosemide administration (40 mg IV). Subjects were healthy (ages 18 to 36 years). During the preceding overnight period, urine volume was lower, and osmolality was higher in blacks than in whites, an ethnic difference that disappeared when water intake was restricted to infused normal saline (60 mL/h). Plasma vasopressin levels were higher in black males than in other sex/ethnic groups. Baseline urinary excretion rates of K, Ca, and Mg were significantly lower in blacks than in whites. After furosemide (0 to 1 hour), K and Ca excretion rates increased, but the proportionate ethnic difference decreased from 44% to 22% and from 22% to 10%, respectively, consistent with blacks having more basal Na,K,2Cl cotransporter activity to inhibit. During a later postfurosemide period (1 to 5 hours), urinary concentrations of Ca and Mg recovered more slowly in blacks, consistent with greater reuptake in the thick ascending limb. In summary, there were distinct ethnic differences in renal handling of Ca and Mg basally and in response to furosemide that were consistent with a more active Na,K,2Cl cotransporter in the thick ascending limb in blacks. An increase in vasopressin levels appeared to explain greater urine concentrations in black males but not black females.

Modifier Locus on Mouse Chromosome 3 for Renal Vascular Pathology in AT 1A Receptor-Deficiency

We previously showed that the phenotype of mice with targeted disruption of the gene encoding the AT 1A receptor ( Agtr1a ), the major murine AT 1 receptor isoform, is strongly influenced by recessive genetic modifiers derived from the C57BL/6 or 129 inbred strains. To further evaluate the genetic modifiers on the C57BL/6 background, we performed backcrosses between F 1 (C57BL/6×129) and C57BL/6 Agtr1a −/− mice and analyzed the progeny, focusing on the development of structural lesions in the renal vasculature. In affected animals, these lesions are characterized by medial thickening of small arteries and arterioles in the kidney that are reminiscent of vascular lesions in patients with nephrosclerosis. Among 180 consecutive progeny, 170 (94%) survived to completion of the study. On masked pathological examination at age 8 months, 86 had intermediate to severe vascular lesions whereas 84 had no detectable lesions. Based on a hypothetical model of a single recessive modifier locus arising from the C57BL/6 background, the observed proportion of affected animals among the backcross progeny was not statistically different from that predicted by χ 2 analysis (51% versus 50%; P =0.88). We next performed genomic microsatellite analysis in a subset of 121 backcross progeny using a panel of markers spanning ≈15 cM intervals across the mouse genome. By 2-point analysis, we found a region spanning 5 cM on chromosome 3, with significant linkage to the development of renal vascular lesions (LOD score: 3.3 to 3.8).

ACE inhibition actively promotes cell survival by altering gene expression

We tested the effect of ACE inhibition on the survival of bovine retinal (REC) and choroidal (CEC) endothelial cells (EC) in culture. The ACE inhibitor captopril delayed the apoptotic tube collapse of REC on Matrigel for >15 days. Captopril treatment of confluent monolayers (2-8 weeks) followed by slow starvation (2-4 weeks) increased EC viability by approximately 200%. Two-week captopril exposures were sufficient to confer maximal protection. Only vehicle-treated EC demonstrated apoptotic features such as membrane blebbing and DNA laddering. By RT-PCR, the starvation marker p202 was upregulated only in starved cells. In REC, captopril upregulated the pro-survival proteins mortalin-2, uPA, and uPAR while downregulating the anti-growth sprouty-4 and tPA. In CEC, captopril also upregulated tPA and its inhibitor PAI-1. Amiloride (uPA inhibitor) blocked the captopril-induced increase in EC survival, secondary sprouting, and invasion in Matrigel. The pro-survival effects of captopril involve the reprogramming of genes involved in cell survival and immortalization.

Racial difference in the activity of the amiloride-sensitive epithelial sodium channel

Compared with whites, blacks appear to retain additional sodium that suppresses secretion of renin and aldosterone. The epithelial sodium channel (ENaC) is an aldosterone-regulated site for sodium reabsorption. ENaC activity could be higher in blacks, contributing to sodium retention or, alternatively, lower because of reduced stimulation by aldosterone. To examine the level of ENaC activity in blacks relative to whites, blood pressure (BP) responses to amiloride (5 mg/d), an inhibitor of ENaC, were measured in 20 black and 25 white normotensive young people. After 1 week, systolic BP decreased by 3.0+/-9.2 (SD) and diastolic by 2.8+/-8.3 mm Hg in the whites, whereas systolic BP increased by 2.5+/-7.1 and diastolic by 3.8+/-8.0 mm Hg in the blacks; the racial difference in the BP response was significant for both systolic (P=0.034) and diastolic BP (P=0.010). As ENaC activity increases, renal secretion of potassium increases proportionately, and in a larger sample of subjects, the urinary potassium excretion rate was lower in the blacks (n=301) than in the whites (n=461): 3.2+/-0.1 versus 3.8+/-0.1 mmol/mmol creatinine (P=0.0001). The concentration of serum potassium was higher in the blacks (n=81) than in the whites (n=167): 4.36+/-0.05 versus 4.21+/-0.03 (P=0.012). In summary, a favorable BP response to amiloride in the whites as well as the evidence for greater retention of potassium in the blacks is consistent with blacks having less ENaC activity than whites. We suggest that increased sodium retention in blacks occurring at other nephron sites suppresses aldosterone secretion and in turn ENaC function.

The neurohormonal natural history of essential hypertension: towards primary or tertiary aldosteronism?

Use of the aldosterone-to-renin ratio has controversially suggested that approximately 10% of hypertensives have primary aldosteronism, and most of these individuals are thought to have idiopathic hyperaldosteronism. The usual renin-angiotensin system control is intact in these individuals and is similar to that in low renin and essential hypertensives, differing only in the degree of sensitivity. There is recent evidence suggesting that hyperaldosteronism relates to aldosterone synthase genetic polymorphism, and also that increased angiotensin II stimulation of the adrenal glands appears to paradoxically upregulate the receptors increasing angiotensin II sensitivity. Taken together, the possibility arises that, in susceptible hypertensives, hyperaldosteronism could be acquired. Indeed, it is well known that renin-driven renovascular hypertension is associated with the development of hyperaldosteronism. Hypothetically, within the wider hypertensive population, these findings set the scene that angiotensin II adrenal sensitivity increases over time until the secretion of aldosterone becomes "autonomous" and hence "tertiary" aldosteronism in a significant proportion of hypertensives.

Blood volume expansion among blacks: an hypothesis

Black persons are believed to have developed in a tropical environment and an expanded blood volume would have facilitated adaptation to such an environment. Throughout most of the first half of the life cycle blacks have slower heart rates than whites, and several lines of evidence suggest that heart rate is inversely related to blood volume. Review of blood volume studies in normotensive whites and normotensive blacks does not permit any conclusion concerning the blood volume hypothesis. Among patients with hypertension results of blood volume studies are inconsistent, but indirect evidence suggests that black patients with hypertension have greater plasma volume expansion than white patients with hypertension.

Baroreceptor function in the hypertensive black African

Hypertension in the black African differs in some respects from white Europeans: complications due to accelerated atherosclerosis are rare and treatment with beta-blockers alone is ineffective. It is not known if baroreceptor function is depressed in African hypertensives to the same extent as it is in whites. Therefore, we have assessed baroreceptor reflex sensitivity (BRS) by the phenylephrine method in 19 African hypertensive patients living in the Gambia, West Africa. The results were compared to predicted BRS values for white patients of the same age and blood pressure calculated from a regression equation derived from 61 hypertensive patients studied in Oxford. It was found that baroreceptor reflex sensitivity was reduced in the African hypertensives and the log mean BRS was similar to the predicted value for Europeans of the same age and level of blood pressure (0.473 +/- 0.24 msec/mmHg and 0.489 +/- 0.21 msec/mmHg respectively). The resting mean arterial pressure in the African patients varied from 117 to 194 mmHg. The results indicated that African hypertensives have a depression of baroreflex sensitivity which is similar to European hypertensive patients.

Low and high renin essential hypertension: a comparison of clinical and biochemical characteristics

Based on a renin-sodium nomogram, patients with essential hypertension can be divided into low, normal and high renin subgroups. To define more clearly the clinical and biochemical characteristics of the two extreme groups, data of 83 patients with low renin essential hypertension were compared with those from 42 patients with high renin essential hypertension. In a further analysis, data or 27 age-and sex-matched patients from these two groups were compared with data from 27 matched subjects from a normotensive control group. Compared with high renin patients, low renin patients more frequently are female, older, and have higher systolic blood pressures and pulse pressures. The latter two characteristics are age-related. The similarity of duration of hypertension in both groups and the lack of correlation between age and plasma renin activity suggest that low renin hypertension is a separate entity rather than just a later phase of the same disease. Patients with low renin hypertension had better renal function than high renin patients or normotensive control subjects. High renin patients showed signs of hemoconcentration: hemoglobin, hematocrit, and total protein were higher than in low renin patients and normotensive controls. A significant difference between the slopes of the regression lines relating renin activity and sodium excretion for the two renin subgroups indicates that renin release in low renin patients is less sensitive to alterations in sodium balance. Both groups of patients had similarly elevated aldosterone levels, suggesting an increased sensitivity of aldosterone secretion to angiotensin II in low renin hypertensives. The relationship between aldosterone and potassium excretion was similar in both groups, indicating a comparable mineralocorticoid sensitivity.

The effect of splanchnic block on renin production and renal haemodynamics in hypertensive patients

In eighteen patients with hypertension the effect of a splanchnic block was studied with respect to the plasma renin activity (PRA) in the renal vein and to the renal haemodynamics. A significant reduction of the PRA was noted during splanchnic block, both in kidneys with arterial stenosis, and in those without. The decrease in renin activity took place despite a simultaneous decrease in renal vascular resistance, which in itself should increase the secretion of renin. This supports the view that the sympathetic nervous system dominates over the baroreceptors in patients with hypertension at the pressure levels investigated and if the sodium intake is restricted.

Skin color, ethnicity, and blood pressure II: Detroit whites

Census areas in Detroit were ranked for their stress scores based on instability (e.g., crime, marital break up) and socioeconomic status. Four areas were selected for detailed study: 1) high stress, population predominantly black and 2) white, and 3) low stress, population predominantly black and 4) white. A sample was drawn from each area of persons of the predominant race, 25-60 years old, married and living with spouse, and having relatives in the Detroit Area. Nurses interviewed such persons; three blood pressure readings were taken during the first half-hour of medical history, and skin color was rated. While lighter skin color showed a negligible relation to higher blood pressure, a four-category division of European national background based on a skin color cline in Europe from Northern areas to the Mediterranean was significantly associated with a nurse-rating of skin color. The rank order of this four category variable, white ethnicity, was related linearly to both systolic and diastolic blood pressure means. Respondents with parents from Mediterranean countries had the lowest pressures and those from Northern Europe had the highest. The relationship is stronger for women than men. The relationship was independent of nine other control variables including age, overweight, smoking etc., and high and low stress areas, although of greater magnitude for the high than low stress groups. In this article, findings in whites are compared with prior results in blacks. Findings suggest that physiological and biological correlates exist that can be explored profitably by future research.

Characteristics of hypertension in the black population

The renin-angiotensin system was examined in 146 black patients with essential hypertension. Classification into three categories was made according to plasma renin activity as measured by the radioimmunoassay of Angiotensin I and the accompanying sodium excretion. Differences among patients in the three renin groups (low, normal, and high) were not observed with respect to the incidence of cerebrovascular and cardiovascular events. No other discriminating variables could be identified by multivariate discriminant analysis. Low renin patients were distinguished in having a reduced sodium excretion compared to normal and high renin patients. Although total exchangeable sodium was not measured in this group of patients, other investigators have reported a higher exchangeable sodium in low renin patients than in those with normal or high values. This possibility, together with recent evidence in experimental models of low renin hypertension, that the affinity of angiotensin for its vascular receptors may be sodium dependent suggests that the incidence of vascular events may relate more specifically to angiotensin-vascular receptor interaction than to measurements of circulating renin.

Post-hydralazine renin release in the diagnosis of renovascular hypertension

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Metabolic studies on hypertensive patients with suppressed plasma renin activity not due to hyperaldosternosm

Metabolic data obtained during sodium depletion (10-mEq Na, 90-mEq K diet plus thiazide) from 13 hypertensive patients (HT) with low plasma renin activity (PRA) were compared with data from 15 normotensive subjects and three patients with renovascular hypertension (RHT). With Na depletion plasma renin activity and urinary aldosterone excretion increased promptly in NT and RHT. In contrast, PRA in HT after 5 days of Na depletion was only one third that of the NT after 2 days of depletion, and aldosterone excretion did not change significantly. This depressed renin and aldosterone response in HT can be overcome by extending the depletion period and administering spironolactone. In HT, Na and loss of weight was significantly lower than in NT. HT Negroes retained more potassium than HT whites and four out of seven NT Negroes and none of eight NT white subjects had a positive K balance. The hypertension of patients with low PRA appears to be due to genetic or environmental factors or to both.

Renin release during acute reduction of arterial pressure in normotensive subjects and patients with renovascular hypertension

In normotensive subjects, acute reduction of mean arterial pressure to from 60 to 75 mm Hg by infusion of sodium nitroprusside caused significant increase in renin activity of renal venous plasma and also in the renal-systemic difference of renin activity. At the same time, the products of the renal-systemic difference of renin activity and renal plasma flow increased significantly, whereas renin substrate activity of plasma was unchanged, indicating that there was an increase in renin release during reduction in pressure. Renin activity of renal venous plasma, expressed in logarithms, showed a significant correlation with the degree of reduction in pressure; an increase in renin activity became significant when mean arterial pressure was reduced to below a level of 70 to 75 mm Hg. There was a striking difference in the renal response to reduction in pressure between patients with renovascular hypertension and normotensive subjects. In 10 renovascular hypertensive patients, significant increase in renin release occurred from the involved kidney at mean arterial pressures ranging from 90 to 137 mm Hg; the threshold at which renin release increased was shifted to a range much higher than that in normotensive subjects. Furthermore, the magnitude of renin release from the involved kidney was significantly greater when compared to that in normotensive subjects. In contrast, in the contralateral uninvolved kidney, no significant release of renin was detected during reduction in pressure. The renal mechanism controlling renin secretion appears to be operative at higher systemic arterial pressure levels and with enhanced responsiveness in the involved kidney of renovascular hypertensive patients; the findings are consistent with the hypothesis that the renin-angiotensin system participates in maintaining hypertension in this disease.