Abstract
Mild cases of autonomous aldosterone secretion may go unrecognized using current diagnostic criteria for primary aldosteronism (PA).
To investigate whether the inability to stimulate renin serves as a biomarker for unrecognized autonomous aldosterone secretion and mineralocorticoid receptor (MR) activation.
Six hundred sixty-three normotensive and mildly hypertensive participants, who were confirmed to not have PA using current guideline criteria and were on no antihypertensive medications.
Participants had their maximally stimulated plasma renin activity (PRA) measured while standing upright after sodium restriction. Tertiles of maximally stimulated PRA were hypothesized to reflect the degree of MR activation: lowest PRA tertile = “Inappropriate/Excess MR Activity;” middle PRA tertile = “Intermediate MR Activity;”; and highest PRA tertile = “Physiologic MR Activity.” All participants underwent detailed biochemical and vascular characterizations under conditions of liberalized sodium intake, and associations with stimulated PRA phenotypes were performed.
Participants with lower stimulated PRA had greater autonomous aldosterone secretion [higher aldosterone-to-renin ratio (P = 0.002), higher urine aldosterone excretion rate (P = 0.003), higher systolic blood pressure (P = 0.004), and lower renal plasma flow (P = 0.04)] and a nonsignificant trend toward lower serum potassium and higher urine potassium excretion, which became significant after stratification by hypertension status.
In participants without clinical PA, the inability to stimulate renin was associated with greater autonomous aldosterone secretion, impaired vascular function, and suggestive trends in potassium handling that indicate an extensive spectrum of unrecognized MR activation.
Primary aldosteronism (PA) is regarded as a form of autonomous aldosterone secretion that is independent of renin and angiotensin II and results in a syndrome of inappropriate and excessive mineralocorticoid receptor (MR) activation. The current screening and diagnostic recommendations for detecting PA (1) focus on diagnosing the more severe and overt instances of renin-independent aldosteronism, usually attributable to an aldosterone-producing adenoma or bilateral adrenal hyperplasia. However, milder forms of autonomous aldosterone secretion have been demonstrated to exist below the diagnostic thresholds of current PA guidelines, which associate with clinically relevant cardiovascular risk (2–7). This clinically unrecognized spectrum of renin-independent aldosteronism may represent a missed opportunity to reduce cardiovascular risk (3, 4, 6).
The discovery of aldosterone-producing cell clusters (APCCs) provides a potential pathologic basis for milder forms of renin-independent aldosteronism. APCCs are nonneoplastic, abnormal foci of CYP11B2 (i.e., aldosterone synthase) staining that have been observed in over 50% of morphologically normal adrenal glands (8). Further, APCCs have been shown to harbor somatic mutations known to cause autonomous aldosterone secretion and are therefore regarded as either precursor lesions to neoplastic PA and/or the basis of a prevalent syndrome of milder renin-independent aldosteronism (9, 10).
We previously demonstrated that renin is a sensitive biomarker to characterize the severity of renin-independent aldosteronism (11). Suppression of renin in PA occurs because inappropriate and excessive activation of the MR directly inhibits renin release (12) and also indirectly suppresses renin by intravascular volume expansion (13). We have shown that more severe forms of PA are characterized by a greater suppression of renin and a greater inability to stimulate renin when sodium restricted (11). In contrast, milder forms of PA are characterized by less-suppressed renin and a greater ability to stimulate renin when sodium restricted (11).
In this regard, we hypothesized that the inability to adequately and physiologically stimulate renin could be used to identify a milder and unrecognized syndrome of MR activation caused by renin-independent aldosteronism that may exist in participants that do not meet the current guideline criteria for PA (1). The ability to phenotype cases of unrecognized renin-independent aldosteronism may serve to identify individuals who are at risk for developing MR-mediated cardiovascular disease and would therefore benefit from early and targeted interventions, such as MR antagonist therapy. Herein, we present cross-sectional analyses to characterize the degree of MR activity in a large cohort of normotensive and stage I hypertensive participants that do not meet the current criteria for PA as a function of their maximally stimulated renin phenotype.
Materials and Methods
Study population and study protocol
Participants for the current cross-sectional analyses were from the Hypertensive Pathotype (HyperPATH) cohort, which has been described in detail previously (14, 15). Briefly, normotensive and stage I hypertensive subjects were recruited to participate in the HyperPATH study protocol. Normotensives were defined by blood pressure (BP) <140/90 mm Hg plus no first-degree relatives diagnosed with hypertension prior to 60 years of age. Stage I hypertensives were defined by a seated diastolic BP (DBP) ≥100 mm Hg off antihypertensive medications, ≥90 mm Hg on at least one antihypertensive medications, or treatment with at least two antihypertensive medications. All participants underwent detailed profiling of the renin-angiotensin-aldosterone system (RAAS) in a clinical research center after strict control of known confounders of RAAS activity, including antihypertensive medication use, posture, time of day, and dietary intake of sodium, potassium, and calcium (see the following paragraphs for details). Notably, women on hormone replacement therapy were excluded from study entry. All subjects provided informed consent, and the institutional review board approved all study procedures.
Antihypertensive medications, if applicable, were discontinued for a 1- to 3-month washout period prior to study entry to ensure accurate interpretation of RAAS measurements (14, 15). Participants were studied after restricted dietary sodium intake (<10 mmol/d) and also after liberal dietary sodium intake (>200 mmol/d). Both diets contained fixed amounts of potassium (100 mmol/d) and calcium (20 mmol/d). After 5 to 7 days of restricted sodium intake, participants were studied in an ambulatory clinical research center, where their plasma renin activity (PRA) was measured after 60 minutes of upright posture to measure their maximally stimulated PRA. After five to seven days on the liberal sodium intake diet, participants were admitted to the clinical research center for overnight supine rest before beginning study procedures the following morning. In the morning, supine BP was measured using the mean of five readings from a Dinamap automated device (Critikon, Tampa, FL), and PRA and serum aldosterone concentration were measured. Sodium balance was confirmed, and urine aldosterone excretion was measured via 24-hour urine collection. Renal plasma flow (RPF) was measured via para-aminohippurate (PAH) clearance. PAH was given initially as an 8 mg/kg bolus followed by a continuous infusion of 12 mg/min for 60 minutes as previously described (16).
For the current analyses described in this paper, HyperPATH participants were included only if they: (1) were compliant with the study diet as defined by urine sodium <40 mmol/24 h during the restricted sodium intake phase and >150 mmol/24 h during the liberal sodium intake phase; (2) had available data for PRA and serum aldosterone concentration; and (3) did not meet criteria for PA. We excluded participants with incidental biochemical evidence suggestive of undiagnosed PA based on currently accepted criteria (1) defined as: urine aldosterone excretion rate >12 μg/24 hour with urine sodium excretion >200 mmol/24 hour or urine aldosterone excretion rate >10 μg/24 hour with urine sodium excretion >200 mmol/24 hour and an aldosterone-to-renin ratio (ARR) >30 ng/dL per ng/mL/h. The resultant study population was 663 participants for the current analysis. A random subset of this population (n = 440) had RPF measured. Some of the data presented herein have been published previously from the HyperPATH cohort; however, the current analyses are original and have not been reported previously.
Degree of MR activity and autonomous aldosterone secretion based on stimulated renin phenotype
We used the maximally stimulated PRA values derived from the sodium-restricted upright posture protocol to develop hypothesized MR activation phenotypes (Fig. 1). We divided participants into unbiased tertiles of stimulated PRA and hypothesized that the lowest stimulated PRA category (0.10 to 4.79 ng/mL/h) represented the most abnormal physiology and corresponded to a phenotype of “Inappropriate/Excess MR Activity.” In contrast, the highest stimulated PRA category (9.71 to 47.10 ng/mL/h) was hypothesized to represent normal “Physiologic MR Activity.” Lastly, those in the middle category of stimulated PRA (4.80 to 9.70 ng/mL/h) were considered to have “Intermediate MR Activity” (Fig. 1).
Visual description of study design and hypothesis. Phenotypes of renin stimulation were determined while participants were sodium restricted and in an upright posture to obtain maximally stimulated PRA measurements. We then categorized individuals into tertiles of stimulated PRA corresponding to hypothesized degrees of MR activation: Highest (“Physiologic MR Activity”), Middle (“Intermediate MR Activity”), and Lowest (“Inappropriate/Excess MR Activity”). Our hypothesis was tested under conditions of liberalized dietary sodium intake by assessing autonomous aldosterone secretion, vascular function, and potassium regulation.
Laboratory assays
Serum aldosterone and PRA were measured in duplicate and averaged. The PRA assay (Diasorin, Inc., Stillwater, MN) had a dynamic range of 0.1 to 50 ng/mL/h; interassay variation is 5.6% to 7.6%. The serum aldosterone assay (Siemens, Los Angeles, CA) had a dynamic range of 2.5 to 120 ng/dL; interassay variation is 3.8% to 15.7%. Serum creatinine was measured upon screening prior to the study diet interventions and was used to calculate the estimated glomerular filtration rate (eGFR) using the CKD-EPI formula (17).
Statistical analysis
Our analytic approach was to assess whether hypothesized stimulated renin and MR activation phenotypes corresponded to the expected degrees of autonomous aldosterone secretion, vascular function, and MR activity (Fig. 1). Participants were categorized based on their sodium-restricted PRA phenotype as described previously. The aldosterone, vascular, and MR activation phenotypes were determined under liberalized sodium intake conditions, which approximate the typical Western diet (18).
Baseline characteristics of the study population across stimulated PRA categories were compared using analysis of variance testing for normally distributed continuous variables, Kruskal-Wallis testing for nonnormally distributed continuous variables, and χ2 testing for categorical variables. Categorical variables are reported as percentages, normally distributed continuous outcomes are reported as mean ± standard deviation, and nonnormally distributed continuous outcomes are reported as median (25th to 75th percentile interquartile range).
Autonomous aldosterone secretion
RAAS phenotypes (PRA, serum aldosterone, ARR, and urine aldosterone excretion rate) were compared across categories of stimulated PRA using multivariate linear regression analyses to assess for differences in autonomous aldosterone secretion. This allowed for testing of our hypothesis that participants with lower stimulated PRA and higher MR activity would have greater autonomous aldosterone secretion.
Vascular phenotypes
BP and RPF were compared across categories of stimulated PRA using multivariate linear regression analyses to assess for differences in vascular function. We also measured the salt sensitivity of BP and RPF (calculated as the values on the liberal sodium diet minus the values on the restricted sodium diet), as these have been shown to associate with aldosterone excess in prior studies (19–21). These measures allowed for testing of our hypothesis that participants with lower stimulated PRA and greater MR activity would have the most impaired vascular function.
MR activity (potassium regulation)
Serum potassium concentration and urine potassium excretion were compared across categories of stimulated PRA using multivariate linear regression analyses. These measures allowed for testing of our hypothesis that participants with lower stimulated PRA had correspondingly greater MR activity as demonstrated by lower serum potassium concentration and higher urine potassium excretion.
Age (22–24), sex (23, 25, 26), race (27, 28), and diabetes (29, 30) are all potential confounders of the associations with RAAS, BP, and RPF. Additionally, recent studies have shown that visceral adiposity increases RAAS activity via extra-adrenal aldosterone synthesis by adipocytes (31–34). Accordingly, age, sex, race, diabetes status, and body mass index (BMI), along with the 24-hour urine sodium excretion, were included in all of our regression models. Additionally, stratified analyses based on hypertension status were performed for RPF and potassium regulation. A P value < 0.05 was considered statistically significant. All statistical analyses were performed using SAS 9.4 (SAS Institute, Cary, NC).
Results
Study population
Baseline characteristics of the 663 study participants are shown in Table 1 (baseline characteristics are stratified by hypertensive status in Supplemental Table 1). Participants with lower stimulated PRA and hypothesized to have inappropriate/excessive MR activity were older, were proportionally more women and hypertensives, and had higher BMI. There were no differences in serum creatinine across categories; however, eGFR was lower among participants with lower stimulated PRA. There was a nonsignificant trend suggesting that the lowest stimulated PRA category had the largest proportion of African Americans when compared with the middle and highest stimulated PRA categories (21.7% vs 13.4% vs 12.2%; P = 0.06).
Characteristics of the Study Population by Stimulated Renin Phenotypes and Corresponding Hypothesized MR Activation States
| Characteristic | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile (n = 220) | “Intermediate” MR Activity Middle Stimulated PRA Tertile (n = 223) | “Physiologic” MR Activity Highest Stimulated PRA Tertile (n = 220) | P Value |
|---|---|---|---|---|
| Age (y) | 50.4 (9.2) | 44.5 (10.6) | 41.3 (12.6) | <0.0001 |
| Female | 110 (50%) | 92 (41.3%) | 75 (34.1%) | 0.003 |
| Race | 0.06 | |||
| Caucasian | 160 (73.7%) | 174 (83.2%) | 179 (84.0%) | |
| African American | 47 (21.7%) | 28 (13.4%) | 26 (12.2%) | |
| Other (Hispanic/Asian) | 10 (4.6%) | 7 (3.4%) | 8 (3.8%) | |
| Hypertension status | 167 (75.9%) | 136 (61.0%) | 113 (51.6%) | <0.0001 |
| Type 2 diabetes | 15 (9.4%) | 12 (7.8%) | 8 (5.0%) | 0.32 |
| BMI (kg/m2) | 28.2 (4.1) | 27.5 (4.6) | 26.7 (4.6) | 0.001 |
| Serum creatinine (mg/dL) | 0.97 (0.25) | 0.96 (0.20) | 0.98 (0.23) | 0.70 |
| eGFR (mL/min/1.73 m2)a | 84.6 (19.1) | 88.4 (17.2) | 91.7 (18.7) | 0.001 |
| Characteristic | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile (n = 220) | “Intermediate” MR Activity Middle Stimulated PRA Tertile (n = 223) | “Physiologic” MR Activity Highest Stimulated PRA Tertile (n = 220) | P Value |
|---|---|---|---|---|
| Age (y) | 50.4 (9.2) | 44.5 (10.6) | 41.3 (12.6) | <0.0001 |
| Female | 110 (50%) | 92 (41.3%) | 75 (34.1%) | 0.003 |
| Race | 0.06 | |||
| Caucasian | 160 (73.7%) | 174 (83.2%) | 179 (84.0%) | |
| African American | 47 (21.7%) | 28 (13.4%) | 26 (12.2%) | |
| Other (Hispanic/Asian) | 10 (4.6%) | 7 (3.4%) | 8 (3.8%) | |
| Hypertension status | 167 (75.9%) | 136 (61.0%) | 113 (51.6%) | <0.0001 |
| Type 2 diabetes | 15 (9.4%) | 12 (7.8%) | 8 (5.0%) | 0.32 |
| BMI (kg/m2) | 28.2 (4.1) | 27.5 (4.6) | 26.7 (4.6) | 0.001 |
| Serum creatinine (mg/dL) | 0.97 (0.25) | 0.96 (0.20) | 0.98 (0.23) | 0.70 |
| eGFR (mL/min/1.73 m2)a | 84.6 (19.1) | 88.4 (17.2) | 91.7 (18.7) | 0.001 |
Values are presented as mean ± standard deviation for continuous normally distributed variables or percentages for categorical data. P values were calculated via analysis of variance testing for continuous variables and χ2 testing for categorical variables.
eGFR calculated using the CKD-EPI equation.
Characteristics of the Study Population by Stimulated Renin Phenotypes and Corresponding Hypothesized MR Activation States
| Characteristic | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile (n = 220) | “Intermediate” MR Activity Middle Stimulated PRA Tertile (n = 223) | “Physiologic” MR Activity Highest Stimulated PRA Tertile (n = 220) | P Value |
|---|---|---|---|---|
| Age (y) | 50.4 (9.2) | 44.5 (10.6) | 41.3 (12.6) | <0.0001 |
| Female | 110 (50%) | 92 (41.3%) | 75 (34.1%) | 0.003 |
| Race | 0.06 | |||
| Caucasian | 160 (73.7%) | 174 (83.2%) | 179 (84.0%) | |
| African American | 47 (21.7%) | 28 (13.4%) | 26 (12.2%) | |
| Other (Hispanic/Asian) | 10 (4.6%) | 7 (3.4%) | 8 (3.8%) | |
| Hypertension status | 167 (75.9%) | 136 (61.0%) | 113 (51.6%) | <0.0001 |
| Type 2 diabetes | 15 (9.4%) | 12 (7.8%) | 8 (5.0%) | 0.32 |
| BMI (kg/m2) | 28.2 (4.1) | 27.5 (4.6) | 26.7 (4.6) | 0.001 |
| Serum creatinine (mg/dL) | 0.97 (0.25) | 0.96 (0.20) | 0.98 (0.23) | 0.70 |
| eGFR (mL/min/1.73 m2)a | 84.6 (19.1) | 88.4 (17.2) | 91.7 (18.7) | 0.001 |
| Characteristic | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile (n = 220) | “Intermediate” MR Activity Middle Stimulated PRA Tertile (n = 223) | “Physiologic” MR Activity Highest Stimulated PRA Tertile (n = 220) | P Value |
|---|---|---|---|---|
| Age (y) | 50.4 (9.2) | 44.5 (10.6) | 41.3 (12.6) | <0.0001 |
| Female | 110 (50%) | 92 (41.3%) | 75 (34.1%) | 0.003 |
| Race | 0.06 | |||
| Caucasian | 160 (73.7%) | 174 (83.2%) | 179 (84.0%) | |
| African American | 47 (21.7%) | 28 (13.4%) | 26 (12.2%) | |
| Other (Hispanic/Asian) | 10 (4.6%) | 7 (3.4%) | 8 (3.8%) | |
| Hypertension status | 167 (75.9%) | 136 (61.0%) | 113 (51.6%) | <0.0001 |
| Type 2 diabetes | 15 (9.4%) | 12 (7.8%) | 8 (5.0%) | 0.32 |
| BMI (kg/m2) | 28.2 (4.1) | 27.5 (4.6) | 26.7 (4.6) | 0.001 |
| Serum creatinine (mg/dL) | 0.97 (0.25) | 0.96 (0.20) | 0.98 (0.23) | 0.70 |
| eGFR (mL/min/1.73 m2)a | 84.6 (19.1) | 88.4 (17.2) | 91.7 (18.7) | 0.001 |
Values are presented as mean ± standard deviation for continuous normally distributed variables or percentages for categorical data. P values were calculated via analysis of variance testing for continuous variables and χ2 testing for categorical variables.
eGFR calculated using the CKD-EPI equation.
Stimulated PRA phenotypes and autonomous aldosterone secretion
Participants with lower stimulated PRA demonstrated evidence of more severe renin-independent aldosteronism. Participants with a greater inability to stimulate renin had higher ARR and higher urine aldosterone excretion when sodium loaded (Fig. 2). The inability to stimulate renin when sodium restricted corresponded with greater renin suppression when sodium loaded [PRA for lowest, middle, and highest stimulated categories: 0.20 (0.10 to 0.41) vs 0.32 (0.20 to 0.68) vs 0.40 (0.20 to 0.70) ng/mL/h; P trend < 0.0001]. These findings for ARR, urine aldosterone excretion, and PRA remained significant even after adjusting for hypertensive status in our multivariate linear regression models.
Autonomous aldosterone secretion by stimulated renin phenotypes. Participants with lower stimulated PRA had higher ARR (left) and urine aldosterone excretion rate (right). Figures present median values with 25th to 75th percentile interquartile ranges. P values reflect multivariate P trends based on linear regression models controlling for age, sex, race, BMI, diabetes mellitus, and 24-hour urine sodium. UAER, 24-hour urine aldosterone excretion rate.
Stimulated PRA phenotypes and vascular disease
Participants with lower stimulated PRA had higher systolic BP and DBP, trends that remained significant after multivariable adjustment (Table 2). There were parallel trends toward higher salt sensitivity of systolic BP and salt sensitivity of DBP (Table 2). Supplemental Table 2 shows trends in BP and BP salt sensitivity across stimulated PRA phenotypes after stratification by hypertensive status. Participants with lower stimulated PRA had higher BP among both normotensives and hypertensives, although the magnitude of difference was greater among hypertensives; similar and significant trends were seen in regard to BP salt sensitivity. Participants with lower stimulated PRA also had lower RPF and a trend suggesting lower salt sensitivity of RPF (Table 3). However, when these relationships were re-examined after stratifying the population by hypertension status, we observed that the statistical significance of the relationship between stimulated PRA phenotypes and RPF was driven by participants with hypertension (Table 3). Hypertensives had lower RPF when compared with normotensives and also a significant trend suggesting lower RPF in participants with lower stimulated PRA. An interaction analysis verified effect modification by hypertension status in the relationship between stimulated PRA and RPF (P = 0.01).
BP Characteristics by Stimulated Renin Phenotypes and Corresponding Hypothesized MR Activation States
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| SBP | 141 (23) | 132 (21) | 129 (21) | <0.0001 | 0.004 |
| DBP | 84 (13) | 79 (13) | 77 (13) | <0.0001 | 0.008 |
| SBP salt sensitivity | 15 (14) | 11 (13) | 9 (13) | <0.0001 | 0.09 |
| DBP salt sensitivity | 8 (9) | 6 (8) | 5 (8) | 0.0001 | 0.04 |
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| SBP | 141 (23) | 132 (21) | 129 (21) | <0.0001 | 0.004 |
| DBP | 84 (13) | 79 (13) | 77 (13) | <0.0001 | 0.008 |
| SBP salt sensitivity | 15 (14) | 11 (13) | 9 (13) | <0.0001 | 0.09 |
| DBP salt sensitivity | 8 (9) | 6 (8) | 5 (8) | 0.0001 | 0.04 |
Values are presented as mean ± standard deviation. P trend values were calculated using linear regression analyses in both univariate and multivariate models.
Abbreviation: SBP, systolic BP.
Adjusted for age, sex, race, diabetes mellitus, BMI, and 24-hour urine sodium.
BP Characteristics by Stimulated Renin Phenotypes and Corresponding Hypothesized MR Activation States
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| SBP | 141 (23) | 132 (21) | 129 (21) | <0.0001 | 0.004 |
| DBP | 84 (13) | 79 (13) | 77 (13) | <0.0001 | 0.008 |
| SBP salt sensitivity | 15 (14) | 11 (13) | 9 (13) | <0.0001 | 0.09 |
| DBP salt sensitivity | 8 (9) | 6 (8) | 5 (8) | 0.0001 | 0.04 |
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| SBP | 141 (23) | 132 (21) | 129 (21) | <0.0001 | 0.004 |
| DBP | 84 (13) | 79 (13) | 77 (13) | <0.0001 | 0.008 |
| SBP salt sensitivity | 15 (14) | 11 (13) | 9 (13) | <0.0001 | 0.09 |
| DBP salt sensitivity | 8 (9) | 6 (8) | 5 (8) | 0.0001 | 0.04 |
Values are presented as mean ± standard deviation. P trend values were calculated using linear regression analyses in both univariate and multivariate models.
Abbreviation: SBP, systolic BP.
Adjusted for age, sex, race, diabetes mellitus, BMI, and 24-hour urine sodium.
Renal Vascular Characteristics by Stimulated Renin Phenotypes and Corresponding Hypothesized MR Activation States
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| All participantsb | |||||
| RPF (mL/min/1.73 m2) | 475 (118) | 532 (125) | 548 (117) | <0.0001 | 0.04 |
| RPF salt sensitivity (mL/min/1.73 m2) | 4.1 (59.6) | 20.8 (51.1) | 25.7 (83.1) | 0.01 | 0.09 |
| Normotensives onlyc | |||||
| RPF (mL/min/1.73 m2) | 539 (126) | 585 (129) | 591 (127) | 0.02 | 0.39 |
| RPF salt sensitivity (mL/min/1.73 m2) | 8.9 (56.0) | 26.2 (69.0) | 20.0 (78.8) | 0.11 | 0.84 |
| Hypertensives onlyc | |||||
| RPF (mL/min/1.73 m2) | 453 (96) | 481 (112) | 498 (99) | 0.005 | 0.008 |
| RPF salt sensitivity (mL/min/1.73 m2) | 8.1 (63.2) | 13.0 (45.0) | 25.6 (80.9) | 0.04 | 0.02 |
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| All participantsb | |||||
| RPF (mL/min/1.73 m2) | 475 (118) | 532 (125) | 548 (117) | <0.0001 | 0.04 |
| RPF salt sensitivity (mL/min/1.73 m2) | 4.1 (59.6) | 20.8 (51.1) | 25.7 (83.1) | 0.01 | 0.09 |
| Normotensives onlyc | |||||
| RPF (mL/min/1.73 m2) | 539 (126) | 585 (129) | 591 (127) | 0.02 | 0.39 |
| RPF salt sensitivity (mL/min/1.73 m2) | 8.9 (56.0) | 26.2 (69.0) | 20.0 (78.8) | 0.11 | 0.84 |
| Hypertensives onlyc | |||||
| RPF (mL/min/1.73 m2) | 453 (96) | 481 (112) | 498 (99) | 0.005 | 0.008 |
| RPF salt sensitivity (mL/min/1.73 m2) | 8.1 (63.2) | 13.0 (45.0) | 25.6 (80.9) | 0.04 | 0.02 |
Values are presented as mean ± standard deviation. P trend values were calculated using linear regression analyses in both univariate and multivariate models.
Adjusted for age, sex, race, diabetes mellitus, BMI, and 24-hour urine sodium.
A random subset of 440 participants from our study population had RPF measured, with 190 of these participants being normotensive and 250 of these participants being hypertensive.
Interaction analysis verified effect modification by hypertension status in the relationship between stimulated PRA and RPF (P = 0.01).
Renal Vascular Characteristics by Stimulated Renin Phenotypes and Corresponding Hypothesized MR Activation States
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| All participantsb | |||||
| RPF (mL/min/1.73 m2) | 475 (118) | 532 (125) | 548 (117) | <0.0001 | 0.04 |
| RPF salt sensitivity (mL/min/1.73 m2) | 4.1 (59.6) | 20.8 (51.1) | 25.7 (83.1) | 0.01 | 0.09 |
| Normotensives onlyc | |||||
| RPF (mL/min/1.73 m2) | 539 (126) | 585 (129) | 591 (127) | 0.02 | 0.39 |
| RPF salt sensitivity (mL/min/1.73 m2) | 8.9 (56.0) | 26.2 (69.0) | 20.0 (78.8) | 0.11 | 0.84 |
| Hypertensives onlyc | |||||
| RPF (mL/min/1.73 m2) | 453 (96) | 481 (112) | 498 (99) | 0.005 | 0.008 |
| RPF salt sensitivity (mL/min/1.73 m2) | 8.1 (63.2) | 13.0 (45.0) | 25.6 (80.9) | 0.04 | 0.02 |
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| All participantsb | |||||
| RPF (mL/min/1.73 m2) | 475 (118) | 532 (125) | 548 (117) | <0.0001 | 0.04 |
| RPF salt sensitivity (mL/min/1.73 m2) | 4.1 (59.6) | 20.8 (51.1) | 25.7 (83.1) | 0.01 | 0.09 |
| Normotensives onlyc | |||||
| RPF (mL/min/1.73 m2) | 539 (126) | 585 (129) | 591 (127) | 0.02 | 0.39 |
| RPF salt sensitivity (mL/min/1.73 m2) | 8.9 (56.0) | 26.2 (69.0) | 20.0 (78.8) | 0.11 | 0.84 |
| Hypertensives onlyc | |||||
| RPF (mL/min/1.73 m2) | 453 (96) | 481 (112) | 498 (99) | 0.005 | 0.008 |
| RPF salt sensitivity (mL/min/1.73 m2) | 8.1 (63.2) | 13.0 (45.0) | 25.6 (80.9) | 0.04 | 0.02 |
Values are presented as mean ± standard deviation. P trend values were calculated using linear regression analyses in both univariate and multivariate models.
Adjusted for age, sex, race, diabetes mellitus, BMI, and 24-hour urine sodium.
A random subset of 440 participants from our study population had RPF measured, with 190 of these participants being normotensive and 250 of these participants being hypertensive.
Interaction analysis verified effect modification by hypertension status in the relationship between stimulated PRA and RPF (P = 0.01).
Stimulated PRA phenotypes and MR activity (potassium regulation)
Participants with lower stimulated PRA had lower serum potassium; however, this trend was not statistically significant (P trend = 0.28) (Table 4). In contrast, participants with lower stimulated PRA had higher urine potassium excretion (Table 4). These relationships were re-examined after stratifying the population by hypertension status. Normotensive participants showed no difference in serum potassium concentrations across stimulated PRA categories; however, normotensives with lower stimulated PRA had higher urine potassium excretion (Table 4). In contrast, among hypertensive participants, lower stimulated PRA associated with lower serum potassium concentrations; however, there was no difference in urine potassium excretion across stimulated PRA categories (Table 4). Interaction analyses verified effect modification by hypertension status in the relationship between stimulated PRA and serum potassium concentration (P = 0.02) as well as between stimulated PRA and urine potassium excretion (P = 0.04).
Serum Potassium and 24-Hour Urine Potassium Excretion by Stimulated Renin Phenotypes and Corresponding Hypothesized MR Activation States
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| All participants | |||||
| Serum potassium (mmol/L) | 4.11 (0.38) | 4.16 (0.31) | 4.17 (0.32) | 0.07 | 0.28 |
| Urine potassium excretion (mmol/24 h) | 76.4 (25.7) | 75.1 (24.9) | 72.9 (23.6) | 0.14 | 0.008 |
| Normotensives onlyb | |||||
| Serum potassium (mmol/L) | 4.18 (0.43) | 4.17 (0.34) | 4.11 (0.30) | 0.26 | 0.28 |
| Urine potassium excretion (mmol/24 h) | 81.1 (24.9) | 75.3 (24.3) | 72.1 (25.9) | 0.02 | 0.01 |
| Hypertensives onlyb | |||||
| Serum potassium (mmol/L) | 4.06 (0.32) | 4.14 (0.30) | 4.23 (0.34) | 0.0001 | 0.007 |
| Urine potassium excretion (mmol/24 h) | 74.4 (26.4) | 74.1 (24.5) | 73.7 (22.7) | 0.82 | 0.11 |
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| All participants | |||||
| Serum potassium (mmol/L) | 4.11 (0.38) | 4.16 (0.31) | 4.17 (0.32) | 0.07 | 0.28 |
| Urine potassium excretion (mmol/24 h) | 76.4 (25.7) | 75.1 (24.9) | 72.9 (23.6) | 0.14 | 0.008 |
| Normotensives onlyb | |||||
| Serum potassium (mmol/L) | 4.18 (0.43) | 4.17 (0.34) | 4.11 (0.30) | 0.26 | 0.28 |
| Urine potassium excretion (mmol/24 h) | 81.1 (24.9) | 75.3 (24.3) | 72.1 (25.9) | 0.02 | 0.01 |
| Hypertensives onlyb | |||||
| Serum potassium (mmol/L) | 4.06 (0.32) | 4.14 (0.30) | 4.23 (0.34) | 0.0001 | 0.007 |
| Urine potassium excretion (mmol/24 h) | 74.4 (26.4) | 74.1 (24.5) | 73.7 (22.7) | 0.82 | 0.11 |
Values are presented as mean ± standard deviation. P trend values were calculated using linear regression analyses in both univariate and multivariate models.
Adjusted for age, sex, race, diabetes mellitus, BMI, and 24-hour urine sodium.
Interaction analyses verified effect modification by hypertension status in the relationship between stimulated PRA and serum potassium (P = 0.02) as well as between stimulated PRA and urine potassium (P = 0.04).
Serum Potassium and 24-Hour Urine Potassium Excretion by Stimulated Renin Phenotypes and Corresponding Hypothesized MR Activation States
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| All participants | |||||
| Serum potassium (mmol/L) | 4.11 (0.38) | 4.16 (0.31) | 4.17 (0.32) | 0.07 | 0.28 |
| Urine potassium excretion (mmol/24 h) | 76.4 (25.7) | 75.1 (24.9) | 72.9 (23.6) | 0.14 | 0.008 |
| Normotensives onlyb | |||||
| Serum potassium (mmol/L) | 4.18 (0.43) | 4.17 (0.34) | 4.11 (0.30) | 0.26 | 0.28 |
| Urine potassium excretion (mmol/24 h) | 81.1 (24.9) | 75.3 (24.3) | 72.1 (25.9) | 0.02 | 0.01 |
| Hypertensives onlyb | |||||
| Serum potassium (mmol/L) | 4.06 (0.32) | 4.14 (0.30) | 4.23 (0.34) | 0.0001 | 0.007 |
| Urine potassium excretion (mmol/24 h) | 74.4 (26.4) | 74.1 (24.5) | 73.7 (22.7) | 0.82 | 0.11 |
| Variable | “Inappropriate/Excess” MR Activity Lowest Stimulated PRA Tertile | “Intermediate” MR Activity Middle Stimulated PRA Tertile | “Physiologic” MR Activity Highest Stimulated PRA Tertile | P Trend Univariate | P Trend Multivariatea |
|---|---|---|---|---|---|
| All participants | |||||
| Serum potassium (mmol/L) | 4.11 (0.38) | 4.16 (0.31) | 4.17 (0.32) | 0.07 | 0.28 |
| Urine potassium excretion (mmol/24 h) | 76.4 (25.7) | 75.1 (24.9) | 72.9 (23.6) | 0.14 | 0.008 |
| Normotensives onlyb | |||||
| Serum potassium (mmol/L) | 4.18 (0.43) | 4.17 (0.34) | 4.11 (0.30) | 0.26 | 0.28 |
| Urine potassium excretion (mmol/24 h) | 81.1 (24.9) | 75.3 (24.3) | 72.1 (25.9) | 0.02 | 0.01 |
| Hypertensives onlyb | |||||
| Serum potassium (mmol/L) | 4.06 (0.32) | 4.14 (0.30) | 4.23 (0.34) | 0.0001 | 0.007 |
| Urine potassium excretion (mmol/24 h) | 74.4 (26.4) | 74.1 (24.5) | 73.7 (22.7) | 0.82 | 0.11 |
Values are presented as mean ± standard deviation. P trend values were calculated using linear regression analyses in both univariate and multivariate models.
Adjusted for age, sex, race, diabetes mellitus, BMI, and 24-hour urine sodium.
Interaction analyses verified effect modification by hypertension status in the relationship between stimulated PRA and serum potassium (P = 0.02) as well as between stimulated PRA and urine potassium (P = 0.04).
Discussion
The findings of this physiologic study suggest that the inability to adequately stimulate renin reflects increased MR activity and unrecognized autonomous aldosterone secretion below the diagnostic thresholds of current PA guidelines (1). The degree of renin suppression has been previously used to classify a low-renin phenotype and the degree of excess MR activity (35–38). Although our findings show that renin suppression on liberalized sodium intake correlated highly with the inability to stimulate renin when sodium restricted, the range of renin values with liberalized sodium intake (0.01 to 4.95 ng/mL/h) was narrow and characterized by significant overlap between categories. In contrast, the wide physiologic range of renin stimulation with restricted sodium intake (0.10 to 47.10 ng/mL/h) permitted a more refined discrimination of MR activation phenotypes. By maximally stimulating renin via sodium restriction and upright posture, we were able to demonstrate that individuals with lower stimulated renin had greater autonomous aldosterone secretion (higher ARR and urine aldosterone excretion), more evidence of vascular disease in both the systemic (higher BP and salt sensitivity of BP) and renal vasculature (lower RPF and salt sensitivity of RPF), and trends suggestive of increased MR activity (lower serum potassium and higher urine potassium excretion).
One potential histopathologic explanation for this unrecognized MR activity and autonomous aldosterone secretion may be APCCs. APCCs have been observed in more than one-half of morphologically normal adrenal glands and are speculated to represent nonneoplastic precursors of overt PA (8, 10). Although biochemical correlates of the APCC histopathology have not been demonstrated in humans, we hypothesized that the autonomous and nonphysiologic secretion of aldosterone by APCCs would result in chronic and excessive MR activity that could inhibit the secretion of renin in response to physiologic stimuli. Indeed, in overt PA, we previously showed that more severe cases of PA were unable to stimulate renin when sodium restricted, whereas milder cases of overt PA demonstrated the ability to stimulate renin to a much greater degree (11). In the current study of participants who do not meet the diagnostic thresholds of PA, the application of the same principle resulted in the detection of a continued spectrum of autonomous aldosterone secretion with features consistent with excessive MR activation.
It is notable, but not surprising, that many of our observations were more prominent in hypertension. Individuals with a greater inability to stimulate renin had higher BP, were more likely to be hypertensive than normotensive, had greater impairments in renal vascular function, were older, and had higher BMI. Our cross-sectional study was not designed to determine the causal relationship between these coassociated factors. However, one potential interpretation is that the genesis of renin-independent aldosteronism may begin in normotension, as our findings show that features of excessive MR activity were evident even in normotensives. With time, it is plausible that the excessive aldosterone-MR interactions that begin in normotension result in increases in BP and incident hypertension, which, in turn, along with the direct vascular effects of excessive aldosterone, may progressively impair renal vascular function.
A major takeaway of the current study is that renin phenotype may be the most specific marker of vascular disease attributable to autonomous aldosterone secretion and excess MR activity. We previously observed that the severity of hypertensive PA could not be distinguished by focusing on absolute serum or urinary aldosterone concentrations; however, more severe PA could be distinguished from milder PA by recognizing a greater suppression of renin and greater inability to stimulate renin (11). Similarly, physiology and population-based studies in normotensives have been unable to demonstrate any cross-sectional associations between higher absolute aldosterone levels and BP (7, 39); however, the degree of renin suppression (40) and inability to stimulate renin (39) in normotension appeared to provide a more-refined characterization of potential MR overactivation. These prior observations led us to hypothesize that renin phenotype may be the most specific biomarker of MR activity and therefore a more physiological approach to characterizing autonomous aldosterone secretion and consequent vascular dysfunction. This endocrine concept is already well appreciated in other areas; for example, thyrotropin concentration is a more-specific biomarker of thyroid hormone receptor (also an intranuclear receptor) activation than is absolute thyroid hormone concentration. Indeed, prior studies in resistant hypertension have already demonstrated that a suppressed renin phenotype (analogous to our lowest tertile of renin stimulation phenotype) is most responsive to MR antagonist therapy (35, 37, 38). In this regard, the current study findings suggest that perhaps more emphasis should be placed on renin phenotype, rather than absolute aldosterone concentrations, when considering the identification of excessive MR activity and vascular disease attributed to subclinical and unrecognized autonomous aldosterone secretion. This is particularly relevant because the clinical approach to recognizing renin-independent aldosteronism involves nonspecific recommendations for interpreting both renin and aldosterone levels (1).
The observations in potassium regulation may also support the theory of a spectrum of excess MR activity. Normotensives suspected to have high MR activity (failure to adequately stimulate renin) clearly displayed higher urine potassium excretion rates but were able to maintain normokalemia that was comparable to other stimulated renin categories. There may be many potential reasons why a more supportive trend in serum potassium concentrations was not observed in normotensives, but the most obvious is that all participants were studied on fixed doses of dietary potassium intake (100 mmol/d). This study design was instituted to ensure that major confounders of the RAAS were uniform across participants to minimize variability and confounding. However, an inadvertent consequence of this methodology may have been a masking of subtle differences in serum potassium concentrations in normotensives. In contrast, hypertensives suspected to have high MR activity (failure to adequately stimulate renin) had significantly lower serum potassium concentrations as would be expected, but no differences in urine potassium excretion. This observation may be easier to explain and more consistent with our hypothesis. Despite having impaired glomerular and renal vascular function, features that might be expected to result in hyperkalemia and decreased urine potassium excretion, we observed the opposite pattern, which is supportive of excessive MR activity.
Our study has several strengths. First, the design of the HyperPATH study provided an extreme but controlled setting for obtaining measurements of RAAS activity by accounting for known confounders, such as fixing sodium and potassium intake, withdrawing interfering medications, ensuring measurements were taken in standardized protocol with morning collections, and controlling participants’ posture. Additionally, renal vascular function was assessed experimentally using PAH clearance, which correlated with eGFR trends across stimulated PRA categories, despite no detectable difference in serum creatinine.
Our study also has limitations. First, given the strictly controlled nature of our study design, utilization of renin stimulation to detect renin-independent aldosteronism in the general population is not practical, thereby affecting our study’s generalizability. However, the purpose of the current study was not to develop a bedside diagnostic test but rather to demonstrate the existence of an unrecognized hormonal abnormality that may have clinically relevant consequences. Second, the possibility of reverse causation, with renal vascular dysfunction resulting in elevated aldosterone levels, must be considered. However, if this was true, we would also expect renin activity to be similarly elevated; instead, the individuals who appeared to have greater MR activation actually had lower renin activity. Third, we did not have adrenal imaging or histopathology to comment on adrenal morphology or molecular characteristics. Finally, as this was a cross-sectional study, our findings represent associations but do not determine causality. Prospective studies to assess the longitudinal outcomes associated with these phenotypes are needed.
In summary, in participants without overt PA, the inability to stimulate renin was associated with unrecognized autonomous and renin-independent aldosterone secretion and with excessive MR activation as characterized by vascular dysfunction and impaired potassium regulation. These findings suggest that autonomous renin-independent aldosteronism is far more common than currently recognized and may begin as a subtle abnormality in normotension that progresses to hypertension and subclinical vascular dysfunction over time. These findings strongly support future studies to investigate whether the early identification and intervention (e.g., early MR antagonist therapy) of milder renin-independent aldosteronism, which is not currently considered PA, may preferentially mitigate incident cardiovascular risk.
Abbreviations:
- APCC
aldosterone-producing cell cluster
- ARR
aldosterone-to-renin ratio
- BMI
body mass index
- BP
blood pressure
- DBP
diastolic blood pressure
- eGFR
estimated glomerular filtration rate
- MR
mineralocorticoid receptor
- PA
primary aldosteronism
- PAH
para-aminohippurate
- PRA
plasma renin activity
- RAAS
renin-angiotensin-aldosterone system
- RPF
renal plasma flow.
Acknowledgments
We thank our funding sources and research volunteers and staff.
This work was supported by HyperPATH Project Grants U54LM008748 (National Library of Medicine) and UL1RR025758 and M01-RR02635 (National Center for Research Resources), Award T32DK007527-31 (National Institutes of Health—National Institute of Diabetes and Digestive and Kidney Diseases; to G.L.H.), Grants 1150437, 1150327, 1160836, and 1160695 (Fondo Nacional de Desarrollo Científico y Tecnológico; to R.B.), Grant 13CTI-21526-P1 (Corporación de Fomento de la Producción de Chile; to R.B.), Award K24DK091417-06 (National Institutes of Health—National Institute of Diabetes and Digestive and Kidney Diseases; to G.C.), Award R01HL114765 (National Institutes of Health—National Heart, Lung, and Blood Institute; to G.H.W.), Award R01DK107407 (National Institutes of Health—National Institute of Diabetes and Digestive and Kidney Diseases; to A.V.), Award K23HL111771 (National Institutes of Health—National Heart, Lung, and Blood Institute; to A.V.), and Grant 2015085 (Doris Duke Charitable Foundation; to A.V.).
Disclosure Summary: The authors have nothing to disclose.
References
Author notes
Address all correspondence and requests for reprints to: Anand Vaidya, MD, MMSc, Brigham and Women’s Hospital, 221 Longwood Avenue, RFB 287, Boston, Massachusetts 02115. E-mail: [email protected].
