Jenifer M. Brown

Human Interventions to Characterize Novel Relationships Between the Renin–Angiotensin–Aldosterone System and Parathyroid Hormone

Observational studies in primary hyperaldosteronism suggest a positive relationship between aldosterone and parathyroid hormone (PTH); however, interventions to better characterize the physiological relationship between the renin–angiotensin–aldosterone system (RAAS) and PTH are needed. We evaluated the effect of individual RAAS components on PTH using 4 interventions in humans without primary hyperaldosteronism. PTH was measured before and after study (1) low-dose angiotensin II (Ang II) infusion (1 ng/kg per minute) and captopril administration (25 mg×1); study (2) high-dose Ang II infusion (3 ng/kg per minute); study (3) blinded crossover randomization to aldosterone infusion (0.7 µg/kg per hour) and vehicle; and study (4) blinded randomization to spironolactone (50 mg/daily) or placebo for 6 weeks. Infusion of Ang II at 1 ng/kg per minute acutely increased aldosterone (+148%) and PTH (+10.3%), whereas Ang II at 3 ng/kg per minute induced larger incremental changes in aldosterone (+241%) and PTH (+36%; P<0.01). Captopril acutely decreased aldosterone (−12%) and PTH (−9.7%; P<0.01). In contrast, aldosterone infusion robustly raised serum aldosterone (+892%) without modifying PTH. However, spironolactone therapy during 6 weeks modestly lowered PTH when compared with placebo (P<0.05). In vitro studies revealed the presence of Ang II type I and mineralocorticoid receptor mRNA and protein expression in normal and adenomatous human parathyroid tissues. We observed novel pleiotropic relationships between RAAS components and the regulation of PTH in individuals without primary hyperaldosteronism: the acute modulation of PTH by the RAAS seems to be mediated by Ang II, whereas the long-term influence of the RAAS on PTH may involve aldosterone. Future studies to evaluate the impact of RAAS inhibitors in treating PTH-mediated disorders are warranted.

The renin–angiotensin–aldosterone system and calcium-regulatory hormones

There is increasing evidence of a clinically relevant interplay between the renin–angiotensin–aldosterone system and calcium-regulatory systems. Classically, the former is considered a key regulator of sodium and volume homeostasis, while the latter is most often associated with skeletal health. However, emerging evidence suggests an overlap in regulatory control. Hyperaldosteronism and hyperparathyroidism represent pathophysiologic conditions that may contribute to or perpetuate each other; aldosterone regulates parathyroid hormone and associates with adverse skeletal complications, and parathyroid hormone regulates aldosterone and associates with adverse cardiovascular complications. As dysregulation in both systems is linked to poor cardiovascular and skeletal health, it is increasingly important to fully characterize how they interact to more precisely understand their impact on human health and potential therapies to modulate these interactions. This review describes the known clinical interactions between these two systems including observational and interventional studies. Specifically, we review studies describing the inhibition of renin activity by calcium and vitamin D, and a potentially bidirectional and stimulatory relationship between aldosterone and parathyroid hormone. Deciphering these relationships might clarify variability in outcomes research, inform the design of future intervention studies and provide insight into the results of prior and ongoing intervention studies. However, before these opportunities can be addressed, more effort must be placed on shifting observational data to the proof of concept phase. This will require reallocation of resources to conduct interventional studies and secure the necessary talent.

Aldosterone, Parathyroid Hormone, and the Use of Renin-Angiotensin-Aldosterone System Inhibitors: The Multi-Ethnic Study of Atherosclerosis

CONTEXT Aldosterone and PTH are implicated in the pathogenesis of cardiovascular and skeletal diseases. An expanding body of evidence supports a bidirectional and positive physiologic relationship between aldosterone and PTH. Large population-based studies confirming this relationship, and whether it may be targeted as a potential method to mitigate the clinical consequences associated with excess aldosterone and PTH, are needed. OBJECTIVE We hypothesized that higher aldosterone levels would associate with higher PTH, and that the use of renin-angiotensin-aldosterone system (RAAS) inhibitors would predict lower PTH in a large, multi-ethnic, community-based cohort. DESIGN, SETTING, PARTICIPANTS We conducted cross-sectional analyses of participants in the Multi-Ethnic Study of Atherosclerosis without apparent primary hyperparathyroidism or chronic kidney disease (n = 5668). We evaluated associations of RAAS inhibitor use with PTH concentration among 1888 treated hypertensive participants. We also tested associations of serum aldosterone concentration with PTH concentration among 1547 participants with these measurements. OUTCOME Serum PTH concentration. RESULTS Higher aldosterone associated with higher PTH (β = 0.19 pg/ml per 1 ng/dl of aldosterone, P < .0001), and this finding was most pronounced among those with a primary hyperaldosteronism-like phenotype. There was a stepwise increment in PTH when comparing untreated normotensives, hypertensives using RAAS inhibitors, untreated hypertensives, and treated hypertensives using non-RAAS inhibitors (40.8, 45.0, 46.2, 47.1 pg/ml, respectively). The use of any RAAS inhibitor independently associated with lower PTH (β = -2.327 pg/ml per use of RAAS inhibitor, P = .006), when compared with the use of any non-RAAS inhibitor medication. CONCLUSIONS Higher serum aldosterone concentration is associated with higher serum PTH concentration, and the use of RAAS inhibitors is associated with lower PTH concentration. These results extend prior evidence from observational and intervention studies suggesting a potentially important and modifiable relationship between the RAAS and PTH in humans.

Aldosterone Dysregulation With Aging Predicts Renal Vascular Function and Cardiovascular Risk

Aging and abnormal aldosterone regulation are both associated with vascular disease. We hypothesized that aldosterone dysregulation influences the age-related risk of renal vascular and cardiovascular disease. We conducted an analysis of 562 subjects who underwent detailed investigations under conditions of liberal and restricted dietary sodium intake (1124 visits) in the General 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 infusion. Cardiovascular risk was calculated using the Framingham Risk Score. In univariate linear regression, older age (&bgr;=−4.60; P<0.0001) and higher SASSI (&bgr;=−58.63; P=0.001) predicted lower RPF and a blunted RPF response to sodium loading and angiotensin II 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.

Interactions between adrenal-regulatory and calcium-regulatory hormones in human health.

Purpose of reviewTo summarize the evidence characterizing the interactions between adrenal-regulating and calcium-regulating hormones, and the relevance of these interactions to human cardiovascular and skeletal health. Recent findingsHuman studies support the regulation of parathyroid hormone (PTH) by the renin–angiotensin–aldosterone system (RAAS): angiotensin II may stimulate PTH secretion via an acute and direct mechanism, whereas aldosterone may exert a chronic stimulation of PTH secretion. Studies in primary aldosteronism, congestive heart failure, and chronic kidney disease have identified associations between hyperaldosteronism, hyperparathyroidism, and bone loss, which appear to improve when inhibiting the RAAS. Conversely, elevated PTH and insufficient vitamin D status have been associated with adverse cardiovascular outcomes, which may be mediated by the RAAS. Studies of primary hyperparathyroidism implicate PTH-mediated stimulation of the RAAS, and recent evidence shows that the vitamin D–vitamin D receptor complex may negatively regulate renin expression and RAAS activity. Ongoing human interventional studies are evaluating the influence of RAAS inhibition on PTH and the influence of vitamin D receptor agonists on RAAS activity. SummaryAlthough previously considered independent endocrine systems, emerging evidence supports a complex web of interactions between adrenal-regulating and calcium-regulating hormones, with implications for human cardiovascular and skeletal health.

Responsiveness to a Physiological Regimen of GnRH Therapy and Relation to Genotype in Women With Isolated Hypogonadotropic Hypogonadism

CONTEXT Isolated hypogonadotropic hypogonadism (IHH) is caused by defective GnRH secretion or action resulting in absent or incomplete pubertal development and infertility. Most women with IHH ovulate with physiological GnRH replacement, implicating GnRH deficiency as the etiology. However, a subset does not respond normally, suggesting the presence of defects at the pituitary or ovary. OBJECTIVES The objective of the study was to unmask pituitary or ovarian defects in IHH women using a physiological regimen of GnRH replacement, relating these responses to genes known to cause IHH. DESIGN, SETTING, AND SUBJECTS This study is a retrospective analysis of 37 IHH women treated with iv pulsatile GnRH (75 ng/kg per bolus). MAIN OUTCOME MEASURES Serum gonadotropin and sex steroid levels were measured, and 14 genes implicated in IHH were sequenced. RESULTS During their first cycle of GnRH replacement, normal cycles were recreated in 60% (22 of 37) of IHH women. Thirty percent of women (12 of 37) demonstrated an attenuated gonadotropin response, indicating pituitary resistance, and 10% (3 of 37) exhibited an exaggerated FSH response, consistent with ovarian resistance. Mutations in CHD7, FGFR1, KAL1, TAC3, and TACR3 were documented in IHH women with normal cycles, whereas mutations were identified in GNRHR, PROKR2, and FGFR1 in those with pituitary resistance. Women with ovarian resistance were mutation negative. CONCLUSIONS Although physiological replacement with GnRH recreates normal menstrual cycle dynamics in most IHH women, hypogonadotropic responses in the first week of treatment identify a subset of women with pituitary dysfunction, only some of whom have mutations in GNRHR. IHH women with hypergonadotropic responses to GnRH replacement, consistent with an additional ovarian defect, did not have mutations in genes known to cause IHH, similar to our findings in a subset of IHH men with evidence of an additional testicular defect.

Continuum of Renin-Independent Aldosteronism in Normotension

Primary aldosteronism is a severe form of autonomous aldosteronism. Milder forms of autonomous and renin-independent aldosteronism may be common, even in normotension. We characterized aldosterone secretion in 210 normotensives who had suppressed plasma renin activity (<1.0 ng/mL per hour), completed an oral sodium suppression test, received an infusion of angiotensin II, and had measurements of blood pressure and renal plasma flow. Continuous associations between urinary aldosterone excretion rate, renin, and potassium handling were investigated. Severe autonomous aldosterone secretion that was consistent with confirmed primary aldosteronism was defined based on accepted criteria of an aldosterone excretion rate >12 &mgr;g/24 hours with urinary sodium excretion >200 mmol/24 hours. Across the population, there were strong and significant associations between higher aldosterone excretion rate and higher urinary potassium excretion, higher angiotensin II–stimulated aldosterone, and lower plasma renin activity, suggesting a continuum of renin-independent aldosteronism and mineralocorticoid receptor activity. Autonomous aldosterone secretion that fulfilled confirmatory criteria for primary aldosteronism was detected in 29 participants (14%). Normotensives with evidence suggestive of confirmed primary aldosteronism had higher 24-hour urinary aldosterone excretion rate (20.2±12.2 versus 6.2±2.9 &mgr;g/24 hours; P<0.001) as expected, but also higher angiotensin II–stimulated aldosterone (12.4±8.6 versus 6.6±4.3 ng/dL; P<0.001) and lower 24-hour urinary sodium-to-potassium excretion (2.69±0.65 versus 3.69±1.50 mmol/mmol; P=0.001); however, there were no differences in age, aldosterone-to-renin ratio, blood pressure, or renal plasma flow between the 2 groups. These findings indicate a continuum of renin-independent aldosteronism and mineralocorticoid receptor activity in normotension that ranges from subtle to overtly dysregulated and autonomous. Longitudinal studies are needed to determine whether this spectrum of autonomous aldosterone secretion contributes to hypertension and cardiovascular disease.

The Spectrum of Subclinical Primary Aldosteronism and Incident Hypertension: A Cohort Study

With an estimated prevalence of 5% to 20% among patients with hypertension, primary aldosteronism (PA) is the most common and modifiable form of secondary hypertension (17). The disorder is characterized by autonomous secretion of aldosterone, independent of renin, which results in excessive activation of the mineralocorticoid receptor (MR). Excessive stimulation of the renal and extrarenal MR in PA has been associated with hypertension and cardiovascular disease, independent of blood pressure (814), highlighting the important role of MR antagonists in mitigating the systemic sequelae of renin-independent aldosteronism. Although PA is usually described as a clinical phenotype of severe hypertension and hypokalemia caused by adrenal neoplasia, recent evidence points to another potentially prevalent cause of autonomous aldosterone secretion by abnormal cell clusters within morphologically normal adrenal glands: aldosterone-producing cell clusters (1518). Furthermore, recent physiology studies have challenged the notion that PA is a categorical disease by showing a continuous spectrum of renin-independent aldosteronism in normotension, ranging from subtle to overtly autonomous (19). In this regard, the overt PA that we currently recognize in severe hypertension (20) may be only the tip of the iceberg in the spectrum of renin-independent aldosteronism and excessive MR activation. Recognizing a potentially milder and expanded continuum of renin-independent aldosteronism, one that originates in normotension and is associated with inappropriate MR activation, may allow mitigation of MR-mediated cardiovascular disease at an earlier stage. We conducted a longitudinal cohort study that used physiologic phenotypes of autonomous aldosterone secretion and MR activity. We investigated untreated normotensive participants enrolled in MESA (Multi-Ethnic Study of Atherosclerosis) to test the hypothesis that those with higher serum aldosterone levels in the context of renin suppression (renin-independent aldosterone secretion) would have a higher risk for hypertension than normotensive participants without renin suppression. Furthermore, we investigated whether MR activity corresponded with these renin and aldosterone phenotypes. Methods Study Population MESA is a multicenter cohort study of 6814 community-dwelling adults aged 45 to 84 years, established to study subclinical cardiovascular disease risk and progression (21). Participants without evidence of clinical cardiovascular disease were recruited between August 2000 and July 2002 from 6 U.S. study sites and examined every 2 to 3 years over approximately 10 years through December 2011, when examination 5 was completed (22). All participants provided informed consent, and the study was approved by institutional review boards at all participating sites. For a random subset of 1960 participants, serum aldosterone and plasma renin activity (PRA) were measured at either examination 2 (between September 2002 and February 2004) or 3 (between March 2004 and September 2005), as previously described (23). We included only those who had aldosterone and PRA assessments, were normotensive (systolic blood pressure <140 mm Hg and diastolic blood pressure <90 mm Hg), and did not use any antihypertensive medications (24) at the time of serum aldosterone and PRA measurement (n= 850) (Appendix Figure). Appendix Figure. Study flow diagram. Determination of the final study population, and which study examination variables were assessed for the longitudinal and cross-sectional analyses. MESA = Multi-Ethnic Study of Atherosclerosis; MR = mineralocorticoid receptor. Assessment of Aldosterone Levels in the Context of Renin Activity Aldosterone was measured by competition-based radioimmunoassay (DiaSorin) (intra-assay coefficients of variation between 6.30% and 8.87%) and PRA by radioimmunoassay (DiaSorin) (interassay coefficients of variation between 6.89% and 18.38%) (23). Both aldosterone and PRA were measured in duplicate and averaged. We created a priori phenotypic categories based on commonly seen and accepted thresholds to reflect hypothesized reninangiotensinaldosterone system and mineralocorticoid receptor (MR) activation physiology. Participants with PRA of 0.50 g/L per hour or less (n= 392) were classified as having a suppressed renin phenotype that could reflect a state of suppressed reninangiotensinaldosterone activity or inappropriate renin-independent aldosterone secretion and MR activation, depending on the corresponding aldosterone levels. Participants with PRA of at least 1.0 g/L per hour (n= 187) were classified as having an unsuppressed renin phenotype, or a state of potentially appropriate MR activation in the setting of physiologic renin-dependent secretion of aldosterone. Participants with PRA between 0.51 and 0.99 g/L per hour (n= 271) were classified as having an indeterminate renin phenotype. Assessment of Incident Hypertension Blood pressure was measured in triplicate using a Dinamap PRO 100 automated oscillometric sphygmomanometer (GE Medical Systems) after 5 minutes of rest in a seated position, as described previously (23). The last 2 of 3 measurements of systolic and diastolic blood pressure were averaged for the analysis. Antihypertensive medication use was determined by medication inventory: At each study examination, participants brought in all medications used in the preceding 2 weeks and study staff transcribed the name, dose, and frequency of each from its container. Staff then asked participants about their adherence to each medication over the past 2 weeks. Demographic and Laboratory Characterization of the Study Participants At each study visit, participants completed self-administered questionnaires; had standardized interviews to evaluate demographics, medical history, medication use, and substance use; and had body mass index measured by trained study staff (25). Fasting venous blood samples were obtained after 12 hours of overnight fasting and at least 5 minutes of seated posture, and a random urine sample was collected. Specimens were immediately flash frozen, processed, and stored at 80C and were then thawed for analysis (23). Estimated glomerular filtration rate was calculated from serum creatinine measurements using the Chronic Kidney Disease Epidemiology Collaboration equation (26). Serum and urinary sodium, potassium, creatinine, and albumin were measured as previously described (22), and these values were used to calculate the urinary fractional excretion of potassium and predicted 24-hour urinary sodium excretion using the INTERSALT (International Cooperative Study on Salt, Other Factors, and Blood Pressure) equation (27, 28). Serum and urinary concentrations of sodium and potassium were measured only at examination 1. Statistical Analysis Our analytic approach was 2-fold (Appendix Figure). We did a longitudinal analysis to investigate the hypothesis that persons with the suppressed renin phenotype were more likely to have subclinical renin-independent aldosteronism and an increased risk for incident hypertension. We then did a cross-sectional analysis to investigate the hypothesis that those with the suppressed renin phenotype would have increased MR activity. We used multivariable discrete Cox proportional hazard models (PROC PHREG procedure with discrete ties in SAS) to evaluate the association between serum aldosterone and incident hypertension by renin phenotypes and report the results as hazard ratios (29). We computed standardized marginal risk differences for renin and aldosterone phenotypes using weighted, adjusted, discrete hazard models (30). Incident hypertension events were assessed only at each follow-up examination and were defined as development of systolic blood pressure of at least 140 mm Hg, development of diastolic blood pressure of at least 90 mm Hg, or initiation of any antihypertensive medication at a follow-up visit. Participants who did not develop hypertension were censored at their final follow-up examination. Covariates included baseline age, sex, race, body mass index, low-density lipoprotein cholesterol level, fasting blood glucose level, smoking status, alcohol consumption, medication use, physical activity, educational attainment, income, and insurance status. Exploratory models further included systolic blood pressure, estimated glomerular filtration rate, and proteinuria. We tested the interactions between serum aldosterone and phenotypes of renin and incident hypertension in adjusted interaction models using multiplicative terms in the regression models with Wald tests to evaluate significance. Multivariable linear regression models were used to assess associations between serum aldosterone and biomarkers of MR activity in each renin phenotype, adjusting for sodium balance using serum sodium and predicted 24-hour urinary sodium excretion in addition to other potential confounding factors. We used adjusted interaction models to assess whether serum aldosterone and renin phenotypes were associated with serum and urinary potassium. Analyses were done using SAS, version 9.4 (SAS Institute), and Stata, version 15 (StataCorp). A P value less than 0.05 was considered to be statistically significant for all analyses. Role of the Funding Source The funding sources had no role in the collection, analysis, or interpretation of the data; writing of the manuscript; or the decision to submit for publication. Results Participant Characteristics Baseline characteristics of the study population at the time of serum aldosterone and PRA measurements are shown in Table 1. Among 850 participants, 46% (n= 392) displayed a suppressed renin phenotype. When compared with participants with higher renin activity, those with a suppressed renin phenotype were older, were more likely to be female and African American, and had higher systolic blood pressure. Of note, although these participants had the lowest serum aldosterone concentrations,

Fibroblast Growth Factor-23, Mineral Metabolism, and Adiposity in Normal Kidney Function.

Context Obesity is associated with poor bone mineralization and quality. Fibroblast growth factor 23 (FGF23) plays an important role in skeletal physiology. Objective To test hypothesis that greater adiposity results in higher FGF23 levels among individuals with normal estimated glomerular filtration rate (eGFR). Design, Setting, Participants Cross-sectional analyses among participants with eGFR ≥60 mL/min/1.73m2. We assessed the association between crude [body mass index (BMI), waist circumference (WC), and waist-to-hip ratio (WHR); n = 5610] and refined (abdominal adipose tissue area by computed tomography; n = 1313) measures of adiposity and FGF23 using multivariable linear regression. Main Outcome Measure Serum FGF23. Results FGF23 was higher across BMI categories (BMI <25: 37.7; BMI 25 to 29.99: 38.7; BMI 30 to 39.99: 39.8; BMI ≥40: 40.9 pg/mL, unadjusted P trend < 0.0001). The association between BMI and FGF23 was independent of known confounders of FGF23 (adjusted β = +7.2% higher FGF23 per 10 kg/m2; P < 0.0001). Similar results were observed using WC and WHR. Abdominal adipose tissue area was also independently associated with higher FGF23 (P < 0.01). Notably, the positive associations between FGF23 and adiposity were observed despite the fact that eGFR did not decline and serum phosphate levels did not increase with adiposity. Conclusion In a large cohort with normal kidney function, adiposity was associated with higher FGF23 levels independent of known confounders, including eGFR and phosphate. Further studies are needed to evaluate the causes of higher FGF23 in settings of greater adiposity and the potential impact on skeletal health.

Parathyroid Hormone and the Use of Diuretics and Calcium-Channel Blockers

Thiazide diuretic (TZ) use is associated with higher bone mineral density, whereas loop diuretic (LD) use is associated with lower bone density and incident fracture. Dihydropyridine‐sensitive calcium channels are expressed on parathyroid cells and may play a role in parathyroid hormone (PTH) regulation. The potential for diuretics and calcium‐channel blockers (CCBs) to modulate PTH and calcium homeostasis may represent a mechanism by which they influence skeletal outcomes. We hypothesized that the use of LD and dihydropyridine CCBs is associated with higher PTH, and TZ use is associated with lower PTH. We conducted cross‐sectional analyses of participants treated for hypertension in the Multi‐Ethnic Study of Atherosclerosis who did not have primary hyperparathyroidism or chronic kidney disease (n = 1888). We used adjusted regression models to evaluate the independent association between TZ, LD, and CCB medication classes and PTH. TZ use was associated with lower PTH when compared with non‐TZ use (44.4 versus 46.9 pg/mL, p = 0.02), whereas the use of LD and CCBs was associated with higher PTH when compared with non‐users of each medication class (LD: 60.7 versus 45.5 pg/mL, p < 0.0001; CCB: 49.5 versus. 44.4 pg/mL, p < 0.0001). Adjusted regression models confirmed independent associations between TZ use and lower PTH (β = –3.2 pg/mL, p = 0.0007), and LD or CCB use and higher PTH (LD: β = +12.0 pg/mL, p < 0.0001; CCB: +3.7 pg/mL, p < 0.0001). Among CCB users, the use of dihydropyridines was independently associated with higher PTH (β = +5.0 pg/mL, p < 0.0001), whereas non‐dihydropyridine use was not (β = +0.58 pg/mL, p = 0.68). We conclude that in a large community‐based cohort with normal kidney function, TZ use is associated with lower PTH, whereas LD and dihydropyridine CCB use is associated with higher PTH. These associations may provide a mechanistic explanation linking use of these medications to the development of skeletal outcomes.