ALDOSTERONE DYSREGULATION WITH AGING PREDICTS RENAL-VASCULAR FUNCTION AND CARDIO-VASCULAR RISK

Abstract

Aging and abnormal aldosterone regulation are both associated with vascular disease. We hypothesized that aldosterone dysregulation influences the age-related risk of renal- and cardio-vascular disease. We conducted an analysis of 562 subjects who underwent detailed investigations under conditions of liberal and restricted dietary sodium intake (1,124 visits) in a Clinical Research Center. Aldosterone regulation was characterized by the ratio of maximal suppression-to-stimulation (supine serum aldosterone on a liberal sodium diet divided by the same measure on a restricted sodium diet). We previously demonstrated that higher levels of this Sodium-modulated Aldosterone Suppression-Stimulation Index (SASSI) indicate greater aldosterone dysregulation. Renal plasma flow (RPF) was determined via p-aminohippurate clearance to assess basal renal hemodynamics, and the renal-vascular responses to dietary sodium manipulation and angiotensin II (AngII) infusion. Cardiovascular risk was calculated using the Framingham Risk Score. In univariate linear regression, older age (β= -4.60, p<0.0001) and higher SASSI (β= -58.63, p=0.001) predicted lower RPF and a blunted RPF response to sodium loading and AngII infusion. We observed a continuous, independent, multivariate-adjusted interaction between age and SASSI, where the inverse relationship between SASSI and RPF was most apparent with older age (p<0.05). Higher SASSI and lower RPF independently predicted higher Framingham Risk Score (p<0.0001) and together displayed an additive effect. Aldosterone regulation and age may interact to mediate renal-vascular disease. Our findings suggest that the combination of aldosterone dysregulation and renal-vascular dysfunction could additively increase the risk of future cardiovascular outcomes; therefore, aldosterone dysregulation may represent a modifiable mechanism of age-related vascular disease.

INTRODUCTION

Aldosterone is a key hormonal mediator of cardiovascular and renal disease, independent of its effect on blood pressure.^{1, 2} In overtly dysregulated states such as primary aldosteronism, there is an increased incidence of adverse vascular outcomes including left ventricular hypertrophy and proteinuria compared with essential hypertensive controls.^3-5 Human clinical trials have demonstrated that mineralocorticoid blockade results in robust improvements in cardiovascular mortality,^{6, 7} and renal-vascular outcomes such as albuminuria,^{7, 8} further supporting the pathologic role of aldosterone on the cardiovascular system. A better understanding of the spectrum of aldosterone dysregulation and its interaction with other cardiovascular disease risk factors may improve efforts to prevent adverse cardiovascular outcomes and target individuals at highest risk.

Even within the normal range, higher aldosterone levels have been associated with increased rates of hypertension⁹ and cardiovascular mortality.¹⁰ However, in the majority of cases, it is not a single measurement of aldosterone that indicates whether regulation is “normal” or “abnormal”, but rather it is the “appropriateness” of the aldosterone concentration in the context of the specific environmental and physiologic milieu. High aldosterone levels may be pathologic in some circumstances, but may be adaptive and physiologic in others. For example, under conditions of high dietary sodium intake, a relative failure to suppress aldosterone has been associated with obesity, insulin resistance, and dyslipidemia.^{11, 12} Conversely, a relative inability to stimulate aldosterone in response to severely restricted sodium intake, or following infusion of exogenous angiotensin II, has also been associated with unfavorable cardiovascular risk.^12-14 Human and animal studies have previously reported that the process of aging may be associated with a progressive impairment in adrenal aldosterone stimulation, with such impairment possibly related to increased tissue exposure to angiotensin II (AngII),^{15, 16} thereby providing one potential mechanism to account for age-related vascular diseases.

We have previously reported the development of a comprehensive index to integrate aldosterone responses in an effort to characterize the spectrum of aldosterone regulation.¹⁷ The Sodium-modulated Aldosterone Suppression-Stimulation Index (SASSI) is the ratio of aldosterone measured on a liberal sodium diet (where adrenal aldosterone should be suppressed) to aldosterone measured on a restricted sodium diet (where adrenal aldosterone should be stimulated). A failure to suppress and/or stimulate appropriately results in a high SASSI, indicating dysregulated aldosterone physiology. We previously showed that the SASSI strongly predicted cardio-metabolic abnormalities and the metabolic syndrome, and provided a more comprehensive assessment than single aldosterone concentrations when evaluating individuals with only a few cardio-metabolic risk factors.¹⁷

We therefore hypothesized that aldosterone dysregulation, as represented by SASSI, would predict renal-vascular dysfunction and cardiovascular risk. Given the prior observations linking age with dysregulated aldosterone physiology, we further hypothesized that SASSI and age may interact to predict vascular risk. Herein, we present analyses to test these hypotheses on a large population of individuals (n=562) without baseline kidney or cardiac disease, who completed a strictly controlled study protocol to characterize SASSI and renal-vascular hemodynamics.

METHODS

Study Population

Cross-sectional analyses from the International Hypertensive Pathotype (HyperPATH) dataset were conducted. The HyperPATH dataset consists of a multicenter cohort of adults who underwent detailed profiling of the renin-angiotensin-aldosterone system (RAAS) under strict control of known confounders including medication use, dietary sodium intake, posture, and time of day. Participants were studied at Brigham and Women’s Hospital (Boston, MA, USA), University of Utah Medical Center (Salt Lake City, UT, USA), Hospital Broussais (Paris, France), University of Rome (Rome Italy), and Vanderbilt University (Nashville, TN, USA). For the analyses presented herein, subjects were only included if they completed both study visits (1 on liberal [LIB] and 1 on restricted [RES] sodium intake) with available data for serum aldosterone measures as well as renal plasma flow (RPF) obtained under both LIB and RES dietary sodium conditions. We excluded from our analyses those participants with inadequate sodium balance under each dietary condition (urine sodium <150mmol/24h on LIB diet or urine sodium >40mmol/24h on RES diet) and those with evidence suggestive of primary hyperaldosteronism (aldosterone-to-renin ratio>30, serum aldosterone>12ng/dL, and urinary aldosterone excretion rate>15mcg/24h on LIB diet). With these criteria, our study population included 562 subjects who completed a total of 1,124 study visits.

The detailed inclusion and exclusion criteria for the HyperPATH cohort in general have been outlined in detail^{14, 18} and are described briefly in the Online Supplement. All subjects provided informed consent, and all study procedures were approved the Institutional Review Boards of each study site.

Study Protocol

The HyperPATH protocol has been described in detail previously.^{14, 18} In brief, if applicable, antihypertensive medications were weaned during a 1-3 month washout period prior to study procedures to ensure interpretability of RAAS measurements. Participants completed study procedures under two controlled dietary sodium conditions: LIB (200mmol/day) and RES (10mmol/day). Following 5-7 days of study diet, subjects were admitted to the Clinical Research Center for overnight supine rest prior to beginning study procedures in the morning. Blood pressure was measured while supine in the morning after overnight rest using the average of five readings from a Dinamap automated device (Critikon, Tampa, FL, USA). Sodium balance was confirmed with a 24-hour urine collection and body mass index (BMI) was recorded. On the morning after admission, blood pressure and fasting baseline measures including glucose, total cholesterol, and high density lipoprotein (HDL) were obtained at the initiation of weight-based para-aminohippurate (PAH) infusion for determination of renal plasma flow (RPF).

PAH was given initially as a bolus (8 mg/kg) followed by a continuous infusion of 12mg/min, as previously described.¹³ PAH clearance was used to calculate RPF. Upon measuring baseline plasma renin activity (PRA) and serum aldosterone, AngII was infused at 3ng/kg/min for 60 minutes, after which PAH and serum aldosterone were measured again.¹³ Identical procedures were performed on both the LIB and RES diets with a one-week interval between studies. Because the LIB diet more closely approximates a typical ad lib western diet,¹⁹ basal RPF under LIB conditions was used in all analyses of RPF, consistent with prior studies.¹³ Because dietary salt loading and restriction influence circulating blood volume and thus RPF, the RPF response to dietary salt intake (ΔRPF_sodium, also regarded as the salt-sensitivity of the renal-vasculature) was calculated as the difference between RPF on the LIB diet and RPF on the RES diet (ΔRPF_sodium= [RPF on LIB diet] – [RPF on RES diet]). AngII infusion increases adrenal aldosterone secretion and also causes direct glomerular arteriolar vasoconstriction and thus reduces RPF. This change in RPF in response to AngII (ΔRPF_AngII) serves as an indicator of renal-vascular health and the local renal-vascular RAAS, where a blunted ΔRPF_AngII indicates greater local RAAS activity and poorer renal-vascular health.^{13, 20, 21} The ΔRPF_AngII was calculated as the difference between RPF before and after AngII infusion on the LIB diet (ΔRPF_AngII = the change in RPF in response to AngII infusion on LIB diet).

In order to confirm our RPF findings using a more common clinical measure of renal function, we calculated the estimated glomerular filtration rate (eGFR) using the CKD-EPI²² and MDRD formulas,²³ which incorporate age, sex, race, and serum creatinine. Because our results did not differ based on which formula was used, only the results of analyses using eGFR calculated by the CKD-EPI equation are included herein.

To characterize dynamic aldosterone regulation, we calculated the SASSI using the ratio of supine serum aldosterone on the LIB diet to supine serum aldosterone on the RES diet. Impaired suppression of aldosterone on LIB diet and/or impaired stimulation on RES diet is manifested in a high SASSI. As previously described, higher values of SASSI indicate more abnormal regulation of aldosterone and are better predictors of severity of the metabolic syndrome than are single aldosterone measures alone.¹⁷

In addition, we further confirmed our findings using two complementary indices: SAUSSI and SASSI-II.¹⁷ The ratio of 24-hour urinary aldosterone excretion rates on LIB and RES diets were used to calculate the corresponding urinary index: the Sodium-Modulated Aldosterone Urinary Suppression-to-Stimulation Index (SAUSSI). A second serum index, the SASSI-II (Sodium-Modulated Aldosterone Suppression-to-Stimulation Index with AngII infusion), was calculated using the ratio of supine serum aldosterone on the LIB diet to supine serum aldosterone after AngII infusion on the RES diet. AngII infusion while on a RES diet is expected to provide maximal adrenal aldosterone stimulation, thereby providing an extreme physiological extension of SASSI. In addition, AngII infusion induces renal-vascular vasoconstriction to reduce RPF, thereby the SASSI-II is a useful corollary to the ΔRPF_AngII. Higher values of both SAUSSI and SASSI-II are also indicative of impaired physiologic regulation of aldosterone.

Framingham risk score (FRS) was selected as a well-validated, age-dependent measure of cardiovascular risk.²⁴ Using FRS, estimated 10-year risk of coronary heart disease was calculated from a composite of age, sex, smoking status, total cholesterol, HDL, and untreated systolic blood pressure.²⁴ Smoking status was established by self-report at the initial screening visit. Average systolic blood pressure and measures of total and HDL cholesterol obtained on the LIB diet were used in the FRS calculation since the LIB diet approximates the typical ad lib western diet.¹⁹

Laboratory Assays

Full details of the relevant laboratory assays are included in the Online Supplement.

Statistical Analysis

We assessed the relationships between aldosterone regulation (SASSI), age, and renal-vascular function (RPF) using linear regression with three additive models. Univariate regression was used to analyze the relationship between SASSI and RPF. Because age is a prominent risk factor for renal-vascular disease,²⁵ multivariate Model 1 included adjustment for age. Model 2 included additional adjustments for known vascular disease risk factors, namely race, gender, BMI, and the presence of diabetes. Model 3 further adjusted for systolic blood pressure (SBP), which is known to correlate with SASSI,¹⁷ and is strongly associated with age, renal function, and vascular disease. As all subjects were studied off of blood pressure medications, there was a continuous range of SBP values within the study population. Given the possibility of a distinct effect of hypertension status on renal function or cardiovascular risk, the Model 3 analyses were repeated using the categorical variable of hypertension (yes/no) in place of SBP. However, since this did not change the results appreciably, only the results of analyses using continuous SBP in Model 3 are described below. Continuous interaction modeling adjusted for all covariates (Model 3 + interaction term) was used to assess for effect modification between SASSI and age.

In analyses of aldosterone dysregulation and cardiovascular risk (FRS), we evaluated a subset of 352 individuals from the original population of 562 since 210 individuals had missing data for total cholesterol. When FRS was the outcome, multivariate regression models included adjustment only for race, BMI, and the presence of diabetes because all other relevant covariates were components of the FRS itself. A p-value <0.05 was considered statistically significant. All statistical analyses were performed using SAS 9·3 (SAS Institute, Cary, N.C., U.S.A.).

RESULTS

Study Populations

The study population was characterized by a mean age of 45.8 ± 0.5 years and mean estimated GFR of 86.8 ± 0.8 mL/min. Other baseline characteristics are shown in Table 1. There were no appreciable differences with regard to demographic characteristics, blood pressure, SASSI, or kidney function between the total study population (n=562) and the subset used with complete data available for subsequent FRS assessments (n=352).

Table 1.

Baseline Characteristics.

Characteristics	All Subjects (n=562)
Age (yrs)	45.8 ± 0.5
Gender – #(%):
Female	225 (40.0)
Male	337 (60.0)
Race – #(%):
Caucasian	462 (82.4)
African-American	77 (13.7)
Other	22 (3.9)
Diabetic – #(%)	52 (9.3)
Hypertensive – #(%)	334 (59.4)
Current Smoker – # (%)	31 (5.5)
Total Cholesterol (mg/dL)	182.7 ± 2.0
HDL Cholesterol (mg/dL)	43.0 ± 0.7
LDL Cholesterol (mg/dL)	107.3 ± 1.6
Screening Serum Creatinine (mg/dL)	0.98 ± 0.01
eGFR (mL/min)((CKD-EPI)	86.8 ± 0.8
eGFR (mL/min) (MDRD)	79.7 ± 0.8
SASSI	0.36 ± 0.01
	LIB Diet	RES Diet
BMI (kg/m2)	27.7 ± 0.2	27.1 ± 0.2
SBP (mmHg)	133.1 ± 1.0	120.3 ± 0.8
DBP (mmHg)	79.7 ± 0.6	73.5 ± 0.5
Urine Sodium (mmol/24hr)*	231 (88)	11 (11.1)
Aldosterone (ng/dL)*	3.2 (3.3)	14.5 (12.9)
PRA (ng/mL·h)*	0.3 (0.4)	1.9 (2.4)
Basal RPF (mL/min)	518.4 ± 4.9	497.7 ± 5.1

Characteristics are shown for the total population used in the primary analyses. Parameters that varied depending on liberal sodium (LIB) or restricted sodium (RES) diets appear in the corresponding divided columns. Values are presented as mean ± SEM unless otherwise indicated.

^*Denotes median(IQR) and is presented for variables that are not normally distributed. SASSI indicates Sodium-modulated Aldosterone Suppression-Stimulation Index; eGFR, estimated Glomerular Filtration Rate; BMI, Body Mass Index; PRA, Plasma Renin Activity; and RPF, Renal Plasma Flow.

Aldosterone Dysregulation, Age, and Renal-Vascular Dysfunction

Lower RPF was predicted by higher SASSI (β= -58.63, p=0.001) and by older age (β= -4.60, p<0.0001) in univariate analyses. Estimated glomerular filtration rate, a more common clinical metric of renal clearance based on serum creatinine measures, was correlated with RPF measured under experimental conditions (r = 0.339, β= +2.16, p<0.0001), and was similarly predicted by higher SASSI (β= -9.93, p=0.0006). When aldosterone dynamics were represented by urinary measures (SAUSSI) instead of serum measures (SASSI), we confirmed that lower RPF was also predicted by higher SAUSSI (β= -61.98, p=0.009).

Because both higher SASSI¹⁷ and declining renal function²⁵ have been previously associated with older age, we closely examined the role that aging may play in this relationship. We first crudely stratified the population by median age (47 years). For those younger than or equal to the median, there was no significant relationship between SASSI and RPF (Table 2); however, for those with age greater than the median, a significant relationship between SASSI and RPF was present which persisted despite multivariate adjustments for all variables, except SBP (Table 2). Fully adjusted multivariate interaction modeling (Model 3 + interaction term) confirmed that SASSI interacted continuously and independently with age to predict RPF (p-interaction=0.027); higher SASSI in young individuals did not appreciably influence RPF, but this relationship was increasingly evident with advancing age (Figure 1).

Table 2.

Aldosterone Dysfunction as a Predictor of Renal Plasma Flow (RPF): Stratified by Median Age.

Regression Model	Age ≤ Median		Age > Median
	β	p	β	p
Unadjusted	+21.9	0.44	-62.2	0.003
Model 1	+52.4	0.06	-56.1	0.01
Model 2	+34.7	0.23	-46.9	0.02
Model 3	+49.5	0.10	-35.3	0.08

Effect estimates (β) and probability values (p) are presented for the relationship between SASSI and basal renal plasma flow in univariate and multivariate models. Model 1 is adjusted for age. Model 2 additionally adjusts for race, gender, body mass index, and the presence of diabetes. Model 3 additionally adjusts for systolic blood pressure. In those participants older than the median (47 years), there is a significant inverse association between SASSI and RPF that is significant through multivariate adjustment until systolic blood pressure is added.

Figure 1. Interaction between age and SASSI predicts renal plasma flow (RPF).

For every quartile of aldosterone dysregulation (SASSI), we observe an inverse relationship between age and renal-vascular function. In contrast, in the younger age quartiles (Q1-Q2), RPF is relatively independent of SASSI, whereas with older age (Q3-Q4), there is an increasingly apparent negative relationship between SASSI and renal-vascular function. In an adjusted, continuous, interaction model, there was a significant interaction between age and SASSI in predicting RPF (p-interaction<0.027), suggesting that aldosterone dysregulation may modify the effect of age on renal-vascular function.

We further examined the relationships between SASSI, RPF, and age by evaluating them under experimental conditions where physiologic provocations were used to dynamically characterize SASSI and RPF. Examination of these relationships under extreme physiologic manipulation was used to confirm existing associations. We first assessed the relationship between SASSI and the RPF response to dietary sodium manipulation (ΔRPF_sodium= [RPF on LIB diet] – [RPF on RES diet]), where a blunted ΔRPF_sodium indicates an abnormal renal-vascular response to sodium manipulation. Consistent with our initial findings, higher SASSI associated with a lower ΔRPF_sodium (β= -23.43, p=0.06), though this relationship did not reach statistical significance. However, when stratified by median age, we confirmed that higher SASSI predicted a blunted ΔRPF_sodium in older, but not younger, individuals (age≤ median: β= +2.94, p=0.90; age>median: β= -28.25, p=0.037). We then evaluated the relationship between SASSI-II (the ratio of supine serum aldosterone on LIB diet to supine serum aldosterone after AngII infusion on RES diet) and ΔRPF_AngII (ΔRPF_AngII = the change in RPF in response to AngII infusion on LIB diet) where AngII infusion provides a maximal stimulus to raise serum aldosterone and to reduce RPF. Consistent with our aforementioned findings, higher SASSI-II predicted a blunted ΔRPF_AngII in univariate analyses (β= +57.49, p=0.020), and this relationship was also predominant with older, but not younger, age (age≤ median: β= -3.74, p=0.93; age>median: β= +62.91, p=0.015).

Aldosterone Dysregulation, Renal-Vascular Hemodynamics, and Cardiovascular Disease Risk

Given our findings of a relationship between aldosterone regulation and renal-vascular hemodynamics modified continuously by age, we explored whether these findings could be extended to cardiovascular disease risk. Framingham Risk Score (FRS), an age-dependent measure of 10-year risk of coronary heart disease, was calculated for participants with all available data. In our current study population, which was recruited to safely tolerate antihypertensive withdrawal and an AngII infusion, median FRS was 9 for both men and women, corresponding to a median 10-year coronary heart disease risk of 5% and 1%, respectively. Despite this relatively low-risk population, lower RPF and lower calculated eGFR each predicted higher FRS (RPF: β= -0.02, p<0.0001; eGFR: β= -0.12, p<0.0001), as expected. More notably, higher FRS was also significantly and independently predicted by higher SASSI in multivariate modeling adjusted for race, BMI, and the presence of diabetes (β= +6.95, P<0.0001). Together, higher SASSI (p<0.0001) and lower RPF (p<0.0001) appeared to display an additive effect in predicting FRS in our multivariate adjusted model (Figure 2). However, neither discrete nor continuous interaction modeling revealed an interaction between renal function and SASSI in predicting FRS.

Figure 2. Aldosterone dysregulation and impaired renal-vascular hemodynamics may additively contribute to CVD risk.

Higher SASSI and lower Renal Plasma Flow (RPF) appear to display an additive effect in predicting Framingham Risk Score (FRS), such that higher SASSI in combination with lower RPF correspond to the highest FRS. Q1-Q4 indicate quartiles 1-4 of RPF and of SASSI.

DISCUSSION

Our findings suggest that impaired aldosterone regulation may be an important mediator of the age-related decline in renal-vascular function. Using a novel index, SASSI, to characterize the spectrum of aldosterone regulation under fixed environmental conditions, we observed that the degree of aldosterone dysregulation predicted impairments in renal-vascular hemodynamics. Notably, our study participants all had normal GFR, therefore indicating the detection of sub-clinical renal-vascular dysfunction in association with aldosterone dysregulation. Older age modified this relationship, suggesting that with advancing age, the presence of aldosterone dysregulation could compound the risk of renal dysfunction. We scrutinized these new relationships and interactions, and effectively confirmed them, by analyzing the outcomes of provocations that modulated SASSI and RPF in physiologically meaningful manners. Though hard outcomes could not be assessed in this cross-sectional analysis, we demonstrate for the first time a clear interaction between aldosterone regulation, aging, and renal-vascular function, and raise the possibility that aldosterone dysregulation may represent a key feature of age-related vascular disease risk.

We here extend the findings of prior studies, which have provided evidence for aldosterone as a mediator of endothelial dysfunction,^{26, 27} and renal²⁸ and cardiovascular disease.^{1, 2} In preclinical in vitro and animal studies, aldosterone has been shown to cause nephropathy and cardiomyopathy via increased oxidative stress, inflammation, and fibrosis.^28-32 In hyperaldosteronism, inappropriately elevated aldosterone levels have been linked to impaired vascular function and increased cardiovascular disease.⁴ Furthermore, clinical studies have demonstrated a mortality benefit of mineralocorticoid receptor blockade in congestive heart failure^{6, 33} and a benefit in reducing albuminuria in both diabetic^{8, 34-36} and nondiabetic nephropathy.^{28, 37} These prior human studies largely relied on single measures of aldosterone with variable degrees of control for confounders. We here report a robust relationship using an integrated aldosterone measure reflective of responses to dynamic conditions and obtained under strict conditions of medication, postural, and dietary control in a large cohort of subjects. Furthermore, existing clinical studies have largely assessed aldosterone in the context of pathologic aldosterone excess and preexisting vascular disease.

The current knowledge of mineralocorticoid receptor activation permits an intuitive connection between the inability to suppress adrenal aldosterone release appropriately in the setting of high dietary sodium intake and vascular disease. In contrast, the failure to stimulate adrenal aldosterone secretion appropriately in the setting of restricted sodium intake is less intuitive; however, prior studies have shown that blunted aldosterone secretion is also associated with adverse cardiometabolic profiles.^12-14 We speculate that progressive impairment in the adrenal aldosterone-producing apparatus prevents adequate secretion in times of restricted sodium and adequate suppression in times of plentiful sodium balance, resulting in a limited “dynamic range” of aldosterone responsiveness. This dynamic range of adrenal aldosterone responses to dietary sodium challenges is effectively represented by SASSI, and appears to worsen with older age.¹⁷ The SASSI captures not only individuals with a defect in aldosterone suppression or stimulation, but also those in whom both suppression and stimulation are impaired; therefore, despite being a “cross-sectional” assessment of adrenal function, SASSI provides a comprehensive snapshot of individual adrenal physiology.

We here demonstrate that within a population of individuals without chronic kidney disease or primary hyperaldosteronism, there is a range of subclinical renal-vascular dysfunction and aldosterone dysregulation, and that these parameters are correlated. Our demonstration of an interaction between age and aldosterone dysregulation further suggests a potential mechanism underlying age-related decline in vascular function; the relationship between aging and the impairment in renal function may be enhanced by dysregulated aldosterone physiology. In this regard, interventions to target aldosterone (such as mineralocorticoid blockade) to mitigate renal-vascular disease and nephropathy may be most effective in the subset of older individuals with an identified abnormality in aldosterone regulation.

Beyond the implications for renal-vascular disease, aldosterone levels within the normal range have been found to predict cardiovascular and all-cause mortality in a prospective study of patients presenting for coronary angiography.¹⁰ We extend this finding now to a population without cardiovascular disease, other than mild to moderate hypertension, showing that those individuals with abnormal physiologic aldosterone responses have higher Framingham Risk Scores, an estimate of 10-year coronary heart disease risk. Together, aldosterone dysregulation and impaired renal-vascular hemodynamics appear to additively compound FRS. This affirms the importance of considering aldosterone dysregulation not only as a renal-vascular, but also as a cardiovascular risk factor in its own right, and as a potential target of intervention warranting future study. It should be acknowledged that age is independently associated with aldosterone dysregulation, renal dysfunction, and cardiovascular risk, and therefore may be a central bridge linking all of these factors; however, we studied a relatively young and healthy population with a narrow age range (mean 46, interquartile range 41-54 years), suggesting that age alone is unlikely to account for the observed differences in cardiovascular risk. Furthermore, our findings were apparent despite this narrow age range, suggesting that with a larger range of ages our findings may have been more dramatic. Future intervention studies that modulate aldosterone physiology or action may disentangle the relative contributions of aging alone and aldosterone dysregulation alone on vascular outcomes.

Our study has the benefit of several strengths. First, aldosterone physiology was carefully characterized using measures obtained under standardized conditions of time of day, fixed sodium intake, withdrawal of interfering medications, and overnight supine posture. Second, SASSI incorporates the ability of aldosterone to be stimulated and suppressed, a measure shown to be more sensitive than either measure alone for the presence of cardio-metabolic risk.¹⁷ Third, in assessing renal vascular function, we determined RPF experimentally using PAH clearance instead of relying on a calculated GFR, which is a crude measure and particularly susceptible to inaccuracy at the high end of the range.³⁸ This permitted assessment of sub-clinical changes in renal-vascular function in individuals with otherwise normal GFR.

However, some limitations should be acknowledged. First, the cross-sectional design of our study provides insights into associations but cannot determine causality. The possibility of reverse causation, such that declining renal-vascular function promotes aldosterone dysregulation, must be considered. If this were the case, we would expect to see a similar relationship between RPF and PRA, an upstream stimulus for aldosterone production. However, we have previously shown that PRA responses to dietary sodium modulation do not account for the relationship between cardio-metabolic risk and SASSI.¹⁷ Second, though our findings were repeatedly affirmed using several related indices and physiological maneuvers, it should be noted that the addition of SBP in the most complete adjusted multivariate model resulted in some loss of statistical significance. This may be due to the fact that there is a close collinear relationship between aldosterone dysregulation and blood pressure, and adjusting for SBP may inappropriately eliminate the observed effect. Third, unlike the aldosterone-to-renin-ratio (ARR), the SASSI is defined under fixed and extreme dietary sodium conditions and is therefore not practical for clinical use. Instead, of focusing on aldosterone suppressibility alone as the ARR does, the SASSI incorporates experimentally-determined aldosterone responses to both suppression and stimulation, thereby providing an experimental tool for characterizing the comprehensive dynamic range of aldosterone regulation to help uncover underlying pathophysiology. Fourth, while we evaluated renal function using both RPF and estimated GFR, we lacked data on proteinuria to provide a more complete assessment of the impact of renal-vascular abnormalities on renal outcomes. Finally, though the Framingham Risk Score has been extensively validated,^{24, 39} it remains an estimate of cardiovascular disease risk, and as expected, corresponding 10-year coronary heart disease risk in our selected study population was low.

PERSPECTIVES

In summary, we found associations suggesting that aldosterone dysfunction interacts synergistically with aging to predict renal-vascular dysfunction. Furthermore, abnormal aldosterone physiology and reduced renal-vascular function in combination appear to additively increase age-dependent future cardiovascular risk. These findings support consideration of aldosterone dysregulation as a mediator of age-related vascular disease and reinforce the need for additional studies examining aldosterone physiology and the benefits of aldosterone-targeted interventions.

NOVELTY AND SIGNIFICANCE.

What is new?

We examined the role of aldosterone as a mediator of age-related vascular dysfunction using a novel index to represent the “dynamic range” or adrenal aldosterone production. Using this index, we evaluated whether dysregulated aldosteorone physiology could predict experimentally determined measures of renal blood flow and the Framingham Risk Score, and whether interactions with age were influential.

What is relevant?

Characterization of the dynamic range of aldosterone regulation can reveal important pathways in the renal-vascular and cardio-vascular decline that occurs with age.

Summary

There is a synergistic interaction between aldosterone dysregulation and aging that predicts reduced renal-vascular function, implicating control of aldosterone in the pathophysiology of age-related vascular disease. Even in a relatively young and healthy population, aldosterone dysregulation and renal vascular function appear to increase age-dependent cardiovascular risk.