Effect of Moxonidine on the Aldosterone/Renin Ratio in Healthy Male Volunteers

Abstract

Background:

The most popular screening test for primary aldosteronism is the plasma aldosterone/renin ratio (ARR). Medications, dietary sodium, posture, and time of day all affect renin and aldosterone levels and can result in false-negative or false-positive ARRs if not controlled. Most antihypertensive medications affect the ARR and can interfere with interpretation of results. To our knowledge, no study has been undertaken to evaluate the effects of moxonidine on the ARR.

Methods:

Normotensive, nonmedicated male volunteers (n = 20) underwent measurement (seated, midmorning) of plasma aldosterone (by high-performance liquid chromatography–tandem mass spectrometry), direct renin concentration (DRC), plasma renin activity (PRA), cortisol, electrolytes and creatinine; and urinary aldosterone, cortisol, electrolytes and creatinine at baseline and after 1 week of moxonidine at 0.2 mg/d and a further 5 weeks at 0.4 mg/d.

Results:

Compared with baseline, despite the expected significant falls in both systolic and diastolic blood pressure, levels of plasma aldosterone [median, 134 (range, 90 to 535) pmol/L], DRC [20 (10 to 37) mU/L], PRA [2.2 (1.0–3.8) ng/mL/h], and ARR using either DRC [8.0 (4.4 to 14.4)] or PRA [73 (36 to 218)] were not significantly changed after either 1 [135 (98–550) pmol/L, 20 (11–35) mU/L, 2.0 (1.2–4.1) ng/mL/h, 8.8 (4.2 to 15.9), and 73 (32–194), respectively] or 6 weeks [130 (90–500) pmol/L, 22 (8 to 40) mU/L, 2.1 (1.0 to 3.2) ng/mL/h, 7.7 (4.3 to 22.4), and 84 (32 to 192), respectively] of moxonidine. There were no changes in any urinary measurements.

Conclusion:

Moxonidine was associated with no significant change in the ARR and may therefore be a good option for maintaining control of hypertension when screening for primary aldosteronism.

Until recently thought to be rare, primary aldosteronism (PA) is currently considered the most common specifically treatable and potentially curable form of hypertension, accounting for 5% to 13% of patients (1–3). Detection of PA is important as it is associated with excessive cardiovascular and renal morbidity relative to the degree of hypertension (4), and this excess is ameliorated and quality of life substantially improved by the institution of specific surgical or medical treatment (5, 6).

Excessive and autonomous aldosterone production in PA results in sodium retention and hypertension and, if severe and prolonged enough, may induce sufficient kaliuresis to induce hypokalemia (7). However, most patients with PA are normokalemic (1–3), and plasma potassium therefore lacks sensitivity as a means of detection.

Currently, the most popular screening test for PA is the plasma aldosterone/renin ratio (ARR). Although considered the most reliable available screening approach, the ARR is not without false positives or negatives. Factors that can affect secretion of aldosterone and/or renin and confound the interpretation of ARR include certain medications (8–11), potassium levels (12, 13), dietary sodium intake (12, 13), coexisting diseases such as chronic renal failure or renal artery stenosis (14, 15), and some physiological factors such as posture, time of the day (16, 17), sex, and phase of the menstrual cycle (18). Because most antihypertensive medications are known to affect the ARR, it is often necessary to withdraw such interfering agents to enable confident interpretation of results. During this process, maintenance of hypertension control can be achieved by using alternative agents that have minimal or no appreciable effects on aldosterone and renin levels. However, only a few medications are known to fall into this category, thus limiting the treatment options available, which can prove challenging in some patients (e.g., those with particularly severe hypertension requiring multiple agents for control or who are intolerant of one or more of the ARR-inert drugs). The identification of additional medications that are free of appreciable effects on the ARR would therefore be very worthwhile. To our knowledge, there are no published reports examining the effects of moxonidine, a selective I1-imadazoline agonist that acts centrally to lower blood pressure, on the ARR.

Although aldosterone is most commonly measured in clinical laboratories by radioimmunoassay or automated immunometric methods, concerns over the accuracy of these techniques have led to the recent development of very precise and specific aldosterone assays using mass spectroscopy (19, 20), including one from our laboratory (21, 22). Plasma renin is currently most commonly measured as either 1) plasma renin activity (PRA), by radioimmunoassay of angiotensin I generated by the action of endogenous renin on endogenous substrate (angiotensinogen), or 2) as direct renin concentration (DRC), by immunometric assay of active renin. These are the methods favored for ARR measurement in the recently published US Endocrine Society clinical practice guideline on case detection, diagnosis, and management of PA (23). Some groups claim superiority of PRA in terms of assay performance, whereas others favor DRC because it can be automated and is therefore less labor intensive.

To avoid the potential for cyclical estrogen and progesterone changes to confound results, we evaluated the effect of moxonidine on ARR in healthy male volunteers. We used high-performance liquid chromatography–tandem mass spectrometry (22) to measure aldosterone. We compared the ARR calculated using DRC with that calculated using PRA.

Materials and Methods

Participants

This study was performed with the approval of the Princess Alexandra Hospital and the University of Queensland Human Ethics Review Committees. Informed consent was obtained from all participants. Twenty-five healthy men were included according to the following inclusion criteria: consenting, healthy participants without evidence of renal, liver, or cardiovascular diseases; not hypertensive; not receiving any medications within the previous two months; and no anticipated requirement for any medication during the period of the study. Instructions were given to participants to maintain their usual sodium intake during the period of study, and compliance was assessed by collecting urine specimens to measure urinary sodium excretion at each visit. Two volunteers were excluded after developing dizziness and mild hypotension during the first few days of receiving the study drug. Another three participants withdrew for personal reasons, leaving 20 men (mean ± standard deviation age, 33 ± 5 years) who completed the study.

Medication and sampling times

Moxonidine (Physiotens; Abbott) was administered once daily in a dose of 0.2 mg for 1 week and then 0.4 mg for a further 5 weeks. Nonfasting blood samples were collected into EDTA tubes between 9 and 10 am after sitting for 5 to 15 minutes at baseline and 1 and 6 weeks after commencement of moxonidine for the measurement of plasma aldosterone, DRC, PRA, cortisol, sodium, potassium, and creatinine and centrifuged immediately at 2500 rpm for 10 minutes. Plasma was separated and snap frozen in dry ice and stored at –20°C pending assay.

Fasting spot urine samples were collected on the same days for measurement of sodium, potassium, creatinine, and cortisol. Blood pressure and heart rate were recorded during each visit after sitting for 10 to 15 minutes.

Analytic methods

Plasma aldosterone and cortisol were measured by high-performance liquid chromatography–tandem mass spectrometry, using a method recently validated in our laboratory (22). For aldosterone, the intra-assay coefficient of variation was 7.3% at 238 pmol/L and 4.3% at 1344 pmol/L. The interassay coefficient of variation was 9.3% at 242 pmol/L and 6.0% at 1321 pmol/L. The intra-assay coefficient of variation for cortisol was 2.1% at 55 ng/mL and 1.6% at 472 ng/mL. The interassay coefficient of variation was 3.7% at 56 ng/mL and 1.8% at 462 ng/mL. DRC was assayed by chemiluminescent immunoassay technology (Liaison; DiaSorin). The intra-assay coefficient of variation was 3.7% at 15 mU/L, 2.8% at 34 mU/L, 2.0% at 82 mU/L, and 1.2% at 258 mU/L. The interassay coefficient of variation was 7.4% at 27 mU/L and 6.0% at 107 mU/L. PRA was assayed by GammaCoat radioimmunoassay (DiaSorin). The intra-assay coefficient of variation was 5.6% at 1.6 ng/mL/h, 4.6% at 6.2 ng/mL/h, and 6.8% at 15.2 ng/mL/h. The interassay coefficient of variation was 10.0% at 1.6 ng/mL/h, 7.6% at 10.7 ng/mL/h, and 9.4% at 17.9 ng/mL/h. Urinary cortisol was measured by high-performance liquid chromatography and urinary aldosterone by radioimmunoassay (Siemens DPC).

Statistical analysis

SPSS version 24 for Windows (SPSS, Inc.) was used to analyze the data. Because data were not normally distributed, group data are presented as median (range) unless otherwise stated. Nonparametric testing (Friedman test) was used for multiple comparisons. A P value of less than 0.05 was considered statistically significant.

Results

Results of measured biochemical and hemodynamic parameters at baseline and after 1 week and 6 weeks of moxonidine are shown in Table 1. Compared with baseline, despite the expected significant falls in both systolic and diastolic blood pressure, levels of plasma aldosterone [median, 134 (range 90 to 535) pmol/L], DRC [20 (10 to 37) mU/L], PRA [2.2 (1.0 to 3.8) ng/mL/h], and ARR using either DRC [8.0 (4.4 to 14.4)] or PRA [73 (36–218)] were not significantly changed after either 1 [135 (98 to 550) pmol/L, 20 (11 to 35) mU/L, 2.0 (1.2 to 4.1) ng/mL/h, 8.8 (4.2 to 15.9), and 73 (32 to 194), respectively] or 6 weeks [130 (90 to 500) pmol/L, 22 (8 to 40) mU/L, 2.1 (1.0 to 3.2) ng/mL/h, 7.7 (4.3 to 22.4), and 84 (32 to 192), respectively] of moxonidine.

Table 1.

Effects of Moxonidine After 1 and 6 Weeks in 20 Healthy Men

Characteristic	Baseline	1 Week	6 Weeks	P Value (Friedman Test)
Aldosterone (pmol/L)	134 (90–535)	135 (98–550)	130 (90–500)	0.09 (NS)
DRC (mU/L)	20 (10–37)	20 (11–35)	22 (8–40)	0.70 (NS)
PRA, ng/mL/h	2.2 (1.0–3.8)	2.0 (1.2–4.1)	2.1 (1.0–3.2)	0.19 (NS)
ARR using DRC, (pmol/L)/(mU/L)	8.0 (4.4–14.4)	8.8 (4.2–15.9)	7.7 (4.3–22.4)	0.52 (NS)
ARR using PRA, (pmol/L)/(ng/mL/h)	73 (36–218)	73(32–194)	84 (32–192)	0.165 (NS)
Plasma cortisol, ng/mL	77 (55–159)	99 (52–161)	92 (64–141)	0.046
Plasma Na+, mmol/L	139 (135–141)	140 (136–142)	138 (137–141)	0.68 (NS)
Plasma K+, mmol/L	4.0 (3.5–4.1)	3.9 (3.8–4.8)	3.9 (3.5–4.1)	0.27 (NS)
Plasma creatinine, μmol/L	59 (46–85)	60 (46–78)	61 (51–76)	0.03
Systolic blood pressure, mm Hg	128 (120–134)	124 (118–132)	120 (116–128)	<0.001
Diastolic blood pressure, mm Hg	84 (80–88)	81 (76–86)	80 (76–84)	<0.001
Heart rate, beats/min	87 (78–92)	74 (65–83)	63 (58–78)	<0.001

Values are presented as median (range). P values are the significance levels of multiple comparisons between the three collection time points as determined by Friedman testing. NS, not significant.

Results of measured urinary parameters (corrected for creatinine) are presented in Table 2. There were no significant differences between levels at baseline and those after 1 or 6 weeks of treatment of urinary potassium, sodium, aldosterone, or cortisol levels.

Table 2.

Measured Urinary Parameters (Corrected for Creatinine) in 20 Healthy Men

Characteristic	Baseline	1 Week	6 Weeks	P Value (Friedman Test)
Aldosterone, nmol/mmol	2.8 (1.0–7.8)	3.0 (1.0–10.2)	3.3 (1.1–16.7)	0.54 (NS)
Sodium, mmol/mmol	16.2 (4.0–39.6)	12.1 (2.7–28.1)	13.0 (4.0–31.5)	0.54 (NS)
Potassium, mmol/mmol	6.6 (2.1–23.1)	6.4 (2.8–16.2)	6.7 (2.3–19.1)	0.63 (NS)
Cortisol, nmol/mmol	7.9 (1.3–51.7)	7.2 (1.0–25.1)	7.5 (1.03–32.2)	0.24 (NS)

Values are presented as median (range). P values are the significance levels of multiple comparisons between the three collection time points as determined by Friedman testing. NS, not significant.

Discussion

Moxonidine is a centrally acting imidazoline receptor agonist. Compared with older central-acting antihypertensives such as clonidine, moxonidine binds with much greater affinity to the imidazoline I₁-receptor than to the α₂-receptor. Because moxonidine treatment leads to reduced sympathetic nerve activity (24), it may be expected to reduce renin levels, leading to false-positive ARR values. Very few reported data address this hypothesis. Two studies on acute effects of moxonidine following a single dose reported significant falls in PRA but not aldosterone (measured in only one study) (25, 26). To our knowledge, no studies have been undertaken to evaluate the effects of chronically administered moxonidine on aldosterone and renin levels. In our current study, following administration of moxonidine to healthy normotensive males for 6 weeks, we found no significant changes in ARR levels calculated using either PRA or DRC. Because the young men in this study were normotensive, we introduced moxonidine at the low dose of 0.2 mg/d for 1 week before increasing to 0.4 mg/d for 5 weeks to avoid symptoms due to significant lowering of blood pressure. The results of this study suggest that moxonidine has no effect on ARR and can be used to control blood pressure during screening for PA to minimize the risk of false results.

A strength of this study is that aldosterone levels were measured by a recently described accurate method (22) rather than by immunoassay methods, which are less precise (20). Restricting the study to male participants excluded effects of changing levels of estrogen and progesterone during the menstrual cycle (27, 28). However, involving only males will limit the generalizability of the results. To our knowledge, no previous study has compared the use of PRA vs DRC for calculating the ARR during administration of moxonidine.

Limitations of this study include a relatively small number of participants and use of healthy volunteers rather than hypertensive patients with normal baseline ARR. However, this approach enabled us to avoid the potentially confounding effects of other antihypertensive medications and of restricted sodium intake. Study participants had normal blood pressure, imposing a safety-based requirement for doses to have a modest pharmacodynamic effect (i.e., on blood pressure). In contrast, in the clinical scenario being modeled, patients with suspected PA are likely to have high blood pressures and to require a robust pharmacodynamic effect for effective control pending their screening aldosterone/renin measurement. Any pharmacodynamics effect of moxonidine on aldosterone and/or renin might be expected to have some proportionality with the more overt pharmacodynamics effect on blood pressure. Thus, this dose-constrained model might underestimate moxonidine’s impact on screening.

It is important to study the effects of moxonidine in patients with confirmed PA, and we intend to do this as a next step. Our current study was intended to be an “initial look.” We deliberately chose normotensive, unmedicated participants as this approach avoided the potential confounding issues of antihypertensive medications. We anticipated that moxonidine might suppress renin and cause false-positive and not false-negative results.

In conclusion, moxonidine therapy appears to be associated with no changes in ARR values and therefore may be a good option to use during screening for PA.

Abbreviations:

 
• ARR

  aldosterone/renin ratio

 
• DRC

  direct renin concentration

 
• PA

  primary aldosteronism

 
• PRA

  plasma renin activity.

Acknowledgments

Disclosure Summary: The authors have nothing to disclose.

Author notes

Address all correspondence and requests for reprints to: Michael Stowasser, MBBS, FRACP, PhD, Endocrine Hypertension Research Centre, University of Queensland School of Medicine, Princess Alexandra Hospital, Ipswich Road, Woolloongabba, Brisbane 4102, Australia. E-mail: .