Melatonin secretion is impaired in women with preeclampsia and an abnormal circadian blood pressure rhythm

Abstract

Non-dipping circadian blood pressure (BP) is a common finding in preeclampsia, accompanied by adverse outcomes. Melatonin plays pivotal role in biological circadian rhythms. This study investigated the relationship between melatonin secretion and circadian BP rhythm in preeclampsia. Cases were women with preeclampsia treated between January 2006 and June 2007 in the University Hospital of Larissa. Volunteers with normal pregnancy, matched for chronological and gestational age, served as controls. Twenty-four hour ambulatory BP monitoring was applied. Serum melatonin and urine 6-sulfatoxymelatonin levels were determined in day and night time samples by enzyme-linked immunoassays. Measurements were repeated 2 months after delivery. Thirty-one women with preeclampsia and 20 controls were included. Twenty-one of the 31 women with preeclampsia were non-dippers. Compared to normal pregnancy, in preeclampsia there were significantly lower night time melatonin (48.4 ± 24.7 vs. 85.4 ± 26.9 pg/mL, p < 0.001) levels. Adjustment for circadian BP rhythm status ascribed this finding exclusively to non-dippers (p < 0.01). Two months after delivery, in 11 of the 21 non-dippers both circadian BP and melatonin secretion rhythm reappeared. In contrast, in cases with retained non-dipping status (n = 10) melatonin secretion rhythm remained impaired: daytime versus night time melatonin (33.5 ± 13.0 vs. 28.0 ± 13.8 pg/mL, p = 0.386). Urinary 6-sulfatoxymelatonin levels were, overall, similar to serum melatonin. Circadian BP and melatonin secretion rhythm follow parallel course in preeclampsia, both during pregnancy and, at least 2 months after delivery. Our findings may be not sufficient to implicate a putative therapeutic effect of melatonin, however, they clearly emphasize that its involvement in the pathogenesis of a non-dipping BP in preeclampsia needs intensive further investigation.

• Blood pressure
• circadian rhythm
• melatonin
• preeclampsia

Introduction

Blood pressure (BP) in healthy individuals follows a characteristic circadian pattern with a nocturnal decline of 10–20% followed by an increase early in the morning.1,2 In some patients with hypertension or chronic kidney disease however, BP fails to dip during night and these individuals have been called “non-dippers”, in contrast with those with a normal nocturnal dip, who are called “dippers”.2,3 The clinical relevance of establishing a non-dipping BP pattern lies in its proven association with more severe hypertensive target organ damage and increased cardiovascular risk both in hypertensive and normotensive subjects.4–6 Left ventricular hypertrophy, carotid intima-media thickening, microalbuminuria and cerebrovascular disease are more prevalent in non-dippers than in dippers.7,8

In normal pregnancy, the circadian rhythm of BP is similar to that in the non-pregnant state.9 Patients with preeclampsia are frequently characterized by abnormal circadian BP rhythm with elevated BP during sleep.10,11 An interesting observation is that the reduction in nocturnal BP fall has a high prevalence among pregnant women with severe preeclampsia and that these women have a greater frequency of adverse maternal and fetal outcomes.12 Although the reasons for this nocturnal hypertension in preeclampsia are poorly understood, there is evidence that abnormalities in sympathetic nervous system activity could play a pivotal role.13 Furthermore, the mechanisms underlying the link between non-dipping pattern of nocturnal BP and worse pregnancy outcomes, although still unclear, may be related to endothelial dysfunction.14,15

Melatonin, a hormone produced by the pineal gland at night, plays an important role in the regulation of biological circadian rhythms, including BP and sleep.16,17 The influence of melatonin in the control of cardiovascular rhythmicity is supported by animal studies. Pinealectomy of laboratory rats results in hypertension,18 while the hypertensive effect of pinealectomy is blocked by administration of exogenous melatonin.19 In humans, administration of exogenous melatonin decreases BP in normotensive patients,20,21 in patients with essential hypertension22,23 and adolescents with diabetes mellitus type 1.24 Recent reports have suggested that melatonin is also a significant modulator of immune response25 and a physiologically important antioxidant.26 In addition, some of the antioxidant actions of melatonin are a consequence of its metabolites.27 As a result, melatonin may be involved in the pathophysiology of many diseases and its concentrations have been studied in various clinical populations. The changes of melatonin secretion and their role in the pathogenesis, the clinical features and the prognosis of preeclampsia have not been fully clarified.

The aim of the present study was to evaluate whether melatonin is associated with the regulation of the circadian rhythm of BP in pregnant women with preeclampsia and further to investigate whether this putative association changes in this group after delivery.

Methods

Participants and study design

All pregnant women, hospitalized between January 2006 and June 2007 in the University Hospital of Larissa, were screened for participation in the study. Inclusion criterion was the diagnosis of preeclampsia according to the definition of the National High Blood Pressure Education Program (NHBPEP): Working Group for hypertension in pregnancy.28 In all cases, recruitment and study sampling was before the administration of any antihypertensive medication. Exclusion criteria were pre-gestational hypertension, diabetes, chronic renal, liver or heart disease, cancer and depression. Healthy volunteers with normal pregnancy, matched to cases with preeclampsia for chronological and gestational age, served as controls. The study protocol was approved by the ethics committee of the University Hospital of Larissa. All women gave written informed consent to participate in the study.

Each subject underwent a thorough physical examination and laboratory parameters, that is, creatinine (mg/dL), urea (mg/dL) and uric acid (mg/dL) serum levels as well as proteinuria (mg/d), were registered as determined by common methodology applied in the routine central laboratory of the hospital. Body mass index (BMI; kg/m²) was calculated from the body weight (W) and height (H) according to Quetelet’s formula: BMI = W/H² (the weight in kilograms divided by the square of the height in meters).29

According to NHBPEP, preeclampsia was defined as hypertension (systolic BP ≥140 mmHg and diastolic BP ≥90 mmHg) and proteinuria (≥0.3 g in a 24-h urine collection sample) presenting after 20 weeks of gestation.28 Preeclampsia was classified as severe on the basis of diastolic BP ≥110 mmHg or severe proteinuria (≥2 g in a 24-h urine collection) or the presence of headache, visual disturbances, upper abdominal pain, oliguria, thrombocytopenia, elevated liver enzymes, convulsions or pulmonary edema.

Twenty-four hour blood pressure monitoring

BP was measured in hourly intervals for 24 h with an ambulatory BP monitor (ABPM; Tenso24®, Welch Allyn Speidel & Keller, Jungingen, Germany) and analyzed by specialized software (QTrack 98®, Welch Allyn, Skaneateles Falls, NY). BP monitoring was considered successful if all readings were technically accepted. A non-dipping pattern was defined as a difference in the mean arterial pressure (MAP) of less than 10% between the daytime (6:00 AM to 9:00 PM) and night time hours (9:00 PM to 6:00 AM). MAP was calculated using the standard formula: MAP = Diastolic BP + 1/3(Systolic BP − Diastolic BP).

Twenty-four-hour BP was recorded in all women during pregnancy (gestational age 35.5 ± 2.5 weeks) and 2 months after delivery. In most women with preeclampsia hypertension recedes within 3 months after delivery.30 In our study, we controlled women only 2 months postpartum. This was to some degree a convenience time-point. We anticipated that many participants, especially healthy pregnant women, would have been lost to follow-up if they were examined at a later time-point.

Assessment of 24 h BP was done under hospitalization in all participants in the study (both cases and controls). The reassessment 2 months after delivery was also done during an in-hospital stay. In accordance with previous reports,31,32 measurements were performed with the appropriate size of cuff, which was placed in the non-dominant arm. The monitoring device was attached by one of the medical doctors participating in the study and was detached by the same person in the next morning. Absence of sleep disturbances was ascertained by direct questioning by the medical doctor after detachment of the device. All study participants were instructed to be ambulatory during the day of the measurement and to keep their arm still at the time of BP measurements. Furthermore, they were asked to go to bed no later than 9.00 pm and not to arise before 6.00 am. This way we tried to eliminate any effect of differences in activity between the two groups (cases and controls) and between the two assessments in the group of women with preeclampsia.33

Melatonin secretion measurements

Serum melatonin concentrations were measured in the middle of the dark and light phase, for example, at 14.00 and 24.00 hour. During nocturnal blood sampling the subjects were in recumbent position on a bed without light from 23.00 hour till the end of night sampling. After centrifugation, the blood serum was frozen at −80 °C until the measurement of melatonin.

Serum melatonin concentrations were measured by commercially available enzyme-linked immunoassay (ELISA; IBL, Hamburg, Germany). The lower detection limit was 1.6 pg/mL, and intra-assay and inter-assay coefficients of variance were 3% and 6.4%, respectively.

Although serum melatonin profile faithfully reflects the pineal activity, there is evidence that melatonin secretion is pulsatile.34 In addition, serum melatonin has a very short half-life (2–20 min).35 Taking this information into account, we also measured urinary 6-sulfatoxymelatonin (6-SMT; the main melatonin metabolite), which closely parallels the pineal activity, in order to enhance the reliability of melatonin detection.

Urine samples were collected during daytime (07.00–22.00) and during night time (22.00–7.00). Total urine volume was recorded for each collection and urine samples were frozen at −20 °C until measurements. Excretion of 6-SMT was determined by commercially available ELISA (IBL, Hamburg, Germany). The lower detection limit was 1 ng/mL, and intra-assay and inter-assay coefficients of variance were 5.8% and 5.1%, respectively.

In all cases, serum melatonin and urinary 6-sulfatoxymelatonin (6-SMT) were measured in a different day from that when 24 h BP was assessed because BP registration during night time could disturb sleep and thus alter melatonin secretion. All women were reassessed with the same methodology 2 months after delivery.

Statistical analysis

Results were expressed as a mean value and standard deviation (mean ± SD). Normality of continuous variables was tested by the Kolmogorov–Smirnov test. Pair-wise comparisons of continuous variables were performed by either the t-test or the Wilcoxon signed rank test for paired and the Mann–Whitney U test for unpaired data, as appropriate. General linear model was applied for inter-group (i.e., dippers and non-dippers) comparison of serum melatonin and urine 6-SMT. Confounding factors, that is, age, gestational age, severity of preeclampsia, existence of BP rhythm (dipping), BMI (kg/m²) and serum creatinine (mg/dL), were inserted as covariates in the model. Categorical variables were compared by means of the Fishers’ exact test.

Potential determinants of the BP dipping status were investigated in a binomial logistic regression model. Covariates, age, gestational age, the BMI, proteinuria, and nocturnal serum melatonin (model 1) or urine 6-STM (model 2) as measures of night time melatonin secretion, were added in the model as appropriate. Dipping status was the dependent variable (outcome) and odds ratio (OR) was calculated as the antilogarithm of the b-coefficient (expb) of each independent variable. The goodness of fit of the logistic model was assessed by the Hosmer–Lemeshow test and colinearity was checked using the colinearity matrix. Overall, p-values <0.05 were considered as statistically significant.

Results

Thirty-one women with preeclampsia and 20 controls (healthy pregnant women) were included in the study. Twenty-four hour ABPM was conducted successfully in all pregnant women (n = 51). Standard clinical and laboratory features of all women included in this study are presented in Table 1. There was primigravidity in 22 women with preeclampsia and in 12 controls. No differences in the maternal age, gestational age at sampling, creatinine and urea between women with normal pregnancy and the preeclampsia group was observed. Gestational age was higher than 32 weeks in all cases. As expected, in the preeclampsia group compared to normal pregnancy, we found significantly higher daytime and night time MAP (p < 0.001), serum uric acid (p < 0.001) and urine protein excretion (p < 0.001). BMI (kg/m²) was also significantly higher in preeclampsia (p < 0.01) (Table 1).

Table 1. Clinical and laboratory features of women with normal pregnancy (NP, n = 20), preeclampsia (PE, n = 31), dippers (n = 10) and non-dippers (n = 21) with preeclampsia, during pregnancy.

	Pregnant women			Pregnant women with preeclampsia (PE)
	NP (n = 20)	PE (n = 31)	p^a	Dippers (n = 10)	Non-dippers (n = 21)	p^a
Age(years)	29.9 ± 5.7	29.1 ± 5.1	0.601	28.2 ± 7.2	29.6 ± 3.9	0.306
Gestational age (weeks)	35.5 ± 2.5	34.8 ± 3	0.558	34.7 ± 3.9	35.1 ± 2.7	0.819
Primigravidity [n (%)]	12 (60)	22 (71)	0.545^b	6 (60)	16 (76.2)	0.417^b
Severe PE [n (%)]	n.a.	10 (32.3)	n.a.	1 (10)	9 (42.9)	0.106^b
BMI(kg/m^2)	26 ± 1	30 ± 2.3	0.000	28.3 ± 1.5	30 ± 2.3	0.002
Daytime MAP (mmHg)	84.3 ± 6.9	110.5 ± 4.1	0.000	108.5 ± 3.4	111.5 ± 4.1	0.048
Night time MAP (mmHg)	73.6 ± 6	104.8 ± 7.5	0.000	96 ± 4	109 ± 4.5	0.000
Creatinine (mg/dL)	0.74 ± 0.13	0.81 ± 0.17	0.135	0.82 ± 1.7	0.81 ± 1.8	0.849
Urea (mg/dL)	19.7 ± 5.7	23.6 ± 7.5	0.093	21.2 ± 7.7	24.9 ± 7.3	0.204
Uric acid (mg/dL)	3.9 ± 0.78	6.5 ± 1.28	0.000	5.9 ± 1.1	6.8 ± 1.3	0.072
Proteinuria (mg/d)	70 ± 56	2022 ± 2286	0.000	956 ± 607	2530 ± 2614	0.079

Note: Data are mean ± SD. Serum creatinine in mg/dL may be converted to μmol/L by multiplying by 88.4; urea in mg/dL to mmol/L by multiplying by 0.357; uric acid in mg/dL to μmol/L by multiplying by 59.48.

^aMann–Whitney U test was used for assessing the differences between the two groups.

^bFor categorical data a Fisher’s exact test was performed.

According to ABPM, all healthy pregnant women, that constituted the control group, were classified as dippers, that is, their daytime MAP was significantly higher than their night time MAP (84.3 ± 6.9 vs. 73.6 ± 6 mmHg; p < 0.001). In addition, there was a physiological rhythm of melatonin secretion, that is, night time were significantly higher than daytime serum melatonin levels (85.4 ± 2.9 vs. 28.3 ± 10 pg/mL, p < 0.01). On general linear model analysis no significant relationship (p = 0.623) was observed between the night time MAP (dependent variable) and night time serum melatonin (covariate) in this group. The circadian BP rhythm was physiological in all normal pregnancies and was thus not added as a covariate in the model.

Among pregnant women with preeclampsia (n = 31) studied, 21 (68%) were classified as non-dippers and 10 (32%) as dippers. Daytime serum melatonin did not differ between women with preeclampsia and healthy pregnant women (29.8 ± 13.6 vs. 28.3 ± 10 pg/mL, p = 0.847). In contrast, night time levels were significantly lower in preeclampsia compared to normal pregnancy (48.4 ± 24.7 vs. 85.4 ± 2.9 pg/mL, p < 0.01). After the addition of circadian BP rhythm status as a covariate in a general linear model, this difference became non-significant (p = 0.108) and could be ascribed to the non-dipping status of women with preeclampsia (p < 0.001).

The comparison between non-dippers and dippers with preeclampsia indicated a statistically significant higher BMI, daytime MAP and night time MAP in non-dippers (Table 1). Mean levels of serum uric acid and proteinuria were increased in the non-dippers group, however difference did not reach statistical significance (p = 0.072 and p = 0.079, respectively). Severe preeclampsia was present in nine cases (9/21, 42.8%) of the non-dippers group and in one case (1/10, 10 %) of the dippers group (p = 0.106). The above findings are not statistically significant (Table 1) but clearly implicate that the non-dippers group in the study includes more cases with severe preeclampsia than the dippers group. In order to account for these differences, comparisons of melatonin levels in non-dippers and dippers with preeclampsia were performed by general linear model using the severity of preeclampsia and proteinuria as covariates in the model.

Daytime and night time melatonin secretion in pregnant women with preeclampsia separated according to the dipping profile of BP is presented in Figure 1. The day–night rhythm of melatonin secretion was disturbed in the group of non-dippers with preeclampsia and their night time melatonin levels were significantly lower compared to dippers (37.5 ± 19.8 vs. 71.2 ± 17.5 pg/mL, p < 0.01). In contrast, daytime melatonin concentrations in non-dippers were similar to that in dippers (32.2 ± 13.6 vs. 24.8 ± 13 pg/mL, p = 0.096) (Figure 1a).

Figure 1. Daytime and night time levels of melatonin (A; pg/mL) and 6-Sulfatoxymelatonin (B; 6-SMT; ng/mL) in the two groups of women with preeclampsia, that is, dippers (n = 10) and non-dippers (n = 21), during pregnancy. Note: * represents a statistically significant difference between the night time and daytime levels within the respective group (p < 0.01) and the # a statistically significant difference of the respective day or night time levels between the two groups (dippers and non-dippers; p < 0.01).

On binomial logistic regression, night time serum melatonin was an independent predictor of the BP dipping status in women with preeclampsia (Cox and Snell R² 0.519; Nagelkerke R² 0.725; p = 0.004). Age, gestational age, BMI (kg/m²), proteinuria (mg/d) and night time melatonin levels (pg/mL) were the independent variables (factors or covariates) in this logistic model (model 1; Table 2). The goodness-of-fit analysis by the Hosmer–Lemeshow test indicated adequacy of the model in predicting the non-dipping status (p = 0.647).

Table 2. Binomial logistic regression analysis showed that night time serum melatonin (model 1) and 6-sulfatoxymelatonin (model 2) as measures of melatonin secretion were independent predictors of blood pressure dipping status in women with preeclampsia (n = 31).

Binomial logistic regression
Model 1					Model 2
	OR	p	95% CI for OR			OR	p	95% CI for OR
Melatonin (pg/mL)	0.911	0.035*	0.835	0.993	6-SMT (ng/mL)	0.866	0.026*	0.762	0.983
Age (years)	1.180	0.238	0.896	1.554	Age (years)	1.239	0.233	0.872	1.760
Gestational age (weeks)	1.592	0.176	0.811	3.122	Gestational age (weeks)	1.854	0.210	0.707	4.861
BMI (kg/m^2)	2.699	0.143	0.714	10.203	BMI (kg/m^2)	6.825	0.074	0.833	55.921
Log proteinuria (mg/d)	0.157	0.460	0.010	2.141	log Proteinuria (mg/d)	0.865	0.958	0.040	19.513
Constant	0.0	0.124			Constant	0.0	0.071

Note: Dipping status was the outcome variable and independent variables (covariates) were the BMI (kg/m²), proteinuria (mg/d) and serum melatonin (pg/mL; model 1) or 6-sulfatoxymelatonin (6-SMT ng/mL; model 2). Proteinuria was not following a normal distribution and therefore it was inserted after logarithmical transformation in order to increase stability of the model. Odds ratio (OR) was calculated as the antilogarithm of the b-coefficient of each independent variable (expb), (*p < 0.05).

Two months after delivery, 10 from 21 non-dippers retained the non-dipping status (non-converters; 10/21, 47.6%) whereas in 11 a normalization of the circadian BP rhythm (converters; 11/21, 52.4%) was observed. Thus, among women with a history of preeclampsia (n = 31) 10 were non-dippers and 21 dippers after pregnancy. Again, daytime melatonin levels were comparable between the two groups (non-dippers and dippers; 33.5 ± 13 vs. 26.5 ± 10.1 pg/mL, p = 0.268), while the non-dippers exhibited a significantly lower nocturnal melatonin secretion in comparison to dippers (28 ± 13.8 vs. 60.5 ± 30.2 pg/mL, p < 0.01) (Figure 2a). Moreover, in the group with normalization of the BP rhythm (converters) there was now a normal day–night rhythm of melatonin secretion, as well, that is, nocturnal melatonin levels were significantly higher than daytime levels (44.9 ± 11.7 vs. 26.3 ± 11.1 pg/mL, p = 0.01).

Figure 2. Daytime and night time levels of melatonin (A; pg/mL) and 6-sulfatoxymelatonin (B; 6-SMT; ng/mL) in the two groups of women with preeclampsia, that is, dippers (n = 21) and non-dippers (n = 10), 2 months after delivery. Note: * represents a statistically significant difference between the night time and daytime levels within the respective group (p < 0.01) and the # a statistically significant difference of the respective day or night time levels between the two groups (dippers and non-dippers; p < 0.01).

Overall, urinary 6-SMT levels followed a very similar pattern with the serum levels of melatonin. Findings concerning 6-SMT levels of women with preeclampsia during and after pregnancy are presented on Figures 1(b) and 2(b) and the respective logistic regression analysis (model 2) on Table 2.

Discussion

Since the development and clinical application of the ABPM, various studies have shown that the circadian BP rhythm is lost in up to 70% of patients with preeclampsia.11,36,37 In the present study, the prevalence of sleep hypertension among 31 women with preeclampsia was 68%. Furthermore, we found that non-dippers and dippers with preeclampsia differ not only in their hemodynamic response during night, but also in their physiological response to darkness, as shown by the blunted nocturnal melatonin secretion in non-dippers.

More specifically, daytime serum melatonin did not differ between women with preeclampsia and healthy pregnant women. Night time melatonin secretion was disturbed, not in all women with preeclampsia, but merely in those with a non-dipping BP rhythm. Thus, in preeclampsia an impaired rhythm of melatonin secretion was present only in women with a non-dipping BP. Our findings suggest that melatonin secretion might closely interact with the circadian rhythm of BP in preeclampsia. The exact pathogenetic link is not yet clarified however a hypothesis for a certain causative interrelationship could be justified.

Melatonin influences rhythmic changes in BP by affecting the master clock localized in the suprachiasmatic nucleus of the hypothalamus and subsequently the tone of sympathetic and parasympathetic nervous systems.38 The physiological nocturnal fall in BP is probably associated with the nocturnal decrease in adrenergic activity and the increase in parasympathetic activity.39

In addition, several investigators have shown that nocturnal surge of endogenous melatonin may contribute directly to arterial vasodilatation that leads in turn to nocturnal BP fall in normotensives and dippers with essential hypertension.40–42 Especially, Jonas et al. 42 reported that while the dippers with essential hypertension presented the physiological nocturnal increase in urinary 6-SMT, this surge of melatonin production was missing in the non-dippers with essential hypertension in whom nocturnal urinary 6-SMT concentrations were not significantly different from daily levels. The study presented here shows that above findings are also true in pregnant women with preeclampsia. Moreover, in animal models melatonin has been proven as a pathogenetic link in the development of stress induced and metabolic syndrome-related hypertension.43,44 Therefore, it is probable that the significant association of melatonin with circadian rhythm of BP is observed in any forms of hypertension.

Furthermore, there is considerable piece of evidence in a number of studies that administration of melatonin reduces nocturnal BP.20–24 These observations, of a direct effect of melatonin on BP, strongly support the notion that the impaired melatonin secretion favors predisposition to an abnormal circadian BP rhythm and is a cause of the non-dipping pattern of BP.

During pregnancy the pineal gland is not the exclusive source of melatonin, which is also secreted from the placenta. In addition, according to Lanoix et al. (2012), preeclampsia leads to depressed melatonin levels in the placenta.45 Reduced melatonin levels in preeclampsia, showed in this study, might be in part due to reduced placental secretion. Studies in animal models of hypertension in pregnancy suggest that shallow placentation and reduction in uteroplacental perfusion during pregnancy triggers the release of biologically active factors which affect the sympathetic tone and lead to generalized vasoconstriction, hypertension and disturbance of the circadian BP rhythm.46 Dysfunction of the autonomic nervous system is suspected to be a major contributor to the increase in peripheral vascular resistance in preeclampsia. Schobel et al. 47 using the microneurographic technique for obtaining direct intraneural recordings of postganglionic sympathetic nerve activity, clearly showed that preeclampsia is a state of sympathetic overactivity. This overactivity has been incriminated in the loss of nocturnal BP dip.48 Impaired nocturnal melatonin secretion, shown in this study, might be an additional probable contributor of the autonomic nervous system dysfunction and thus indirectly of nocturnal hypertension in preeclampsia.

Moreover, endothelial dysfunction is a key component in preeclampsia15 and its adverse fetal and maternal outcomes. It has been suggested that administration of melatonin may lead to prevention of endothelial structural alterations.49

Melatonin, a potent free radical scavenger of the highly toxic hydroxyl radical and other reactive oxygen species, increases the levels of several antioxidative enzymes and inhibits the pro-oxidative enzyme nitric oxide synthase.26 Preeclampsia is an oxidative stress related condition. Melatonin levels may be depressed because melatonin, as a radical scavenger, is being consumed very rapidly by the enhanced reactive oxygen species in this disorder. Current evidence shows that decreased endogenous antioxidative defence related to impaired melatonin secretion could reinforce and facilitate the cascade of events related to systematic inflammation and endothelial dysfunction which characterized rheumatoid arthritis or metabolic syndrome.26,49 Furthermore, previous studies have showed a decreased nocturnal serum melatonin in coronary heart disease and during acute myocardial infarction.50,51 This approach raises new questions regarding the mechanism of endothelial dysfunction in preeclampsia and suggests that melatonin might be somehow involved.

These important observations showing that melatonin is associated with various mechanisms inducing hypertension in preeclampsia are in full agreement with the hypothesis that altered melatonin secretion is not a result of a non-dipping pattern of BP, but rather one of the causes of this abnormal circadian rhythm in preeclampsia. Our findings are also important in light of the on-going clinical trial related to the use of melatonin to treat preeclampsia.52 There is ample evidence that melatonin reduces arterial BP and being non-toxic during pregnancy it may be also useful in reducing severity of preeclampsia.

Furthermore, enhanced BMI was found in pregnant women with preeclampsia and in particular in non-dippers in comparison with dippers. However, in binomial regression BMI was not an independent predictor of dipping status in preeclampsia. Nevertheless, an involvement of BMI and obesity in pathogenesis of preeclampsia needs also further study.

Finally, we found that in women with history of preeclampsia the circadian BP rhythm and the melatonin rhythm follow a parallel course, at least 2 months after pregnancy. The reappearance of the BP rhythm and the melatonin rhythm is simultaneous, while in those who remain non-dippers the melatonin rhythm is still altered. From this finding it becomes more obvious first, that both circadian rhythm abnormalities observed, namely the non-dipping BP and the impairment of nocturnal melatonin secretion, are consequences of preeclampsia and not vice versa, and second, that between them there is probably a very close etiopathogenic link. Whether these abnormalities are also part of the vicious circle that causes severe preeclampsia or even eclampsia was not meant to be clarified by this study. A previous study53 concerning also women with history of preeclampsia (n = 8) demonstrated that, although the circadian rhythm of BP normalizes, the melatonin rhythm remains altered 6–12 months after pregnancy. In this study, in contrast to our findings, Tranquilli et al.53 showed that women with preeclampsia and a disturbed circadian BP rhythm have higher melatonin levels than healthy (normotensive) pregnant women. These different results cannot be explained but they may reflect different methods in melatonin determination. Measurements of 6-SMT provide additional credibility to our findings. Moreover, Tranquili et al.53 included only non-dippers with preeclampsia and did not comment on the severity of preeclampsia in their group.

Epidemiological evidence also demonstrate that women with a history of preeclampsia exhibit a greater risk of heart disease later in life compared with women who had normal pregnancies.51 In addition, the non-dipping pattern of circadian BP has been associated with a large increase in cardiovascular morbidity, reflecting an increased risk of target organ damage.54 Our results showed a decreased melatonin secretion in women with a recent history of preeclampsia and nocturnal hypertension. This finding suggests that melatonin rhythm may be involved with the outcome of women with a history of preeclampsia and nocturnal hypertension.

One weak point of our study, and probably of many other studies concerning hypertensive disorders in pregnancy, is the fact that the exact status before pregnancy (existence of essential hypertension, disturbed BP rhythm) remains principally unknown. Furthermore, in our study no investigations were performed on a later time-point than the 2 months. Re-examination of all patients and controls independent of pregnancy would serve as a valuable additional validation of our findings.

In conclusion, we have provided evidence that nocturnal melatonin secretion is impaired in preeclamptic women with non-dipping pattern of circadian BP compared to preeclamptic women with normal nocturnal BP dip and healthy pregnant women. We also found that after pregnancy women with history of preeclampsia who retain their nocturnal hypertension continue to exhibit an impaired melatonin secretion. We thus propose that melatonin might play a role in the altered circadian rhythm of BP in women with preeclampsia. A possible involvement of melatonin in the pathogenesis of preeclampsia needs to be further investigated in prospective studies.

Declaration of interest

The authors report no conflicts of interest.