Exposure of rats to continuous light attenuates melatonin production and results in hypertension development. This study investigated whether hypertension induced by continuous light (24 hours/day) exposure induces heart and aorta remodelling and if these alterations are prevented by melatonin or angiotensin converting enzyme inhibitor captopril. Four groups of 3-month-old male Wistar rats (10 per group) were treated as follows for six weeks: untreated controls, exposed to continuous light, light-exposed, and treated with either captopril (100 mg/kg/day) or melatonin (10 mg/kg/day). Exposure to continuous light led to hypertension, left ventricular (LV) hypertrophy and fibrosis, and enhancement of the oxidative load in the LV and aorta. Increase in systolic blood pressure by continuous light exposure was prevented completely by captopril and partially by melatonin. Both captopril and melatonin reduced the wall thickness and cross-sectional area of the aorta and reduced the level of oxidative stress. However, only captopril reduced LV hypertrophy development and only melatonin reduced LV hydroxyproline concentration in insoluble and total collagen in rats exposed to continuous light. In conclusion, captopril prevented LV hypertrophy development in the continuous light-induced hypertension model, while only melatonin significantly reduced fibrosis. This antifibrotic action of melatonin may be protective in hypertensive heart disease.
Left ventricular (LV) hypertrophy, although representing an adaptation to hemodynamic overload, is associated with increased cardiovascular risk [
Melatonin (N-acetyl-5-methoxytryptamine), the most abundant secretory product of the vertebrate pineal gland that controls biological rhythms [
This study investigated whether continuous light-induced hypertension results in the pathological growth of the heart and the aorta and whether melatonin can modify these potential alterations. Moreover, the effect of melatonin was compared with the angiotensin converting enzyme (ACE) inhibitor, captopril. Since ACE-inhibitors are antihypertensives with well-established antihypertrophic effect, which is also based on inhibition of angiotensin II production and interference with aldosterone, catecholamines, or endothelin production [
Male adult three-month-old Wistar rats (Dobra Voda, Slovak Republic) were randomly divided into four groups (
Systolic blood pressure (SBP) was measured each week by noninvasive tail-cuff plethysmography (Hugo-Sachs Elektronic, Freiburg, Germany). After six weeks, rats were decapitated and body weight (BW), heart weight (HW), and LV and right ventricle weights (LVW, RVW) were determined and their relative weights (LVW/BW and RVW/BW ratio) were calculated. Left ventricle samples were frozen at −80°C and later used for the determination of hydroxyproline concentrations and oxidative stress parameters.
Thoracic aorta samples were fixed for 24 hours in 4% formaldehyde, embedded in paraffin, cut in serial 5
Collagenous proteins were isolated according to Pelouch et al. [
Tissue samples were homogenized in 1 mL phosphate buffered saline and centrifuged at 19320 g (4°C) for 5 minutes and the supernatant was used for further biochemical analyses.
Advanced glycation end-products (AGEs) as markers of carbonyl stress were measured using the characteristic fluorescence (
Advanced oxidation protein products (AOPP) were determined spectrophotometrically [
Concentrations of CoQ9ox, CoQ10ox, and gamma- and alpha-tocopherol were determined by HPLC method according to Lang et al. [
Results are expressed as mean ± S.E.M. One-way, two-tailed analysis of variance (ANOVA) and the Bonferroni test were used for statistical analysis. Differences were considered significant if the
SBP was
The body weight (BW), right ventricle weight (RVW), relative RVW (RVW/BW), and left ventricle weight (LVW) in control rats (c), rats exposed to 24 h/day continuous light without treatment (24), and rats treated with captopril (24C) and melatonin (24M).
BW (g) | RVW (mg) | RVW/BW (mg/g) | LVW (mg) | |
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Values are mean ± SEM,
The influence of captopril (24C) and melatonin (24M) on blood pressure (a) and relative left ventricle (LV) weight (LVW/BW) (b) in 24 hours/day continuous light exposure-induced hypertension (24). c: Wistar controls.
The WT of aorta was
The influence of captopril (24C) and melatonin (24M) on wall thickness (a) and cross-sectional area (b) of the aorta in 24 hours/day continuous light exposure-induced hypertension (24). c: Wistar controls.
Hydroxyproline concentration in the soluble collagenous protein was
The influence of captopril (24C) and melatonin (24M) on hydroxyproline concentration in soluble (a) and insoluble (b) collagen proteins and on total hydroxyproline concentration (c) in 24 hours/day continuous light exposure-induced hypertension (24). c: Wistar controls.
Hydroxyproline concentration in the insoluble collagen was
Total hydroxyproline concentration was
The LV AOPP was 5.01 ± 0.32
The influence of captopril (24C) and melatonin (24M) on advanced oxidation protein products (AOPP) in the LV (a) and aorta (b) and on advanced glycation end-products (AGEs) in the LV (c) and in the aorta (d) in 24 hours/day continuous light exposure-induced hypertension (24). c: Wistar controls.
The aortic AGEs were
The left ventricular CoQ9ox, CoQ10ox, and alpha-tocopherol levels were not changed in either group. Gamma-tocopherol concentration in the LV was decreased in the continuous light-group and neither captopril nor melatonin modified it (Table
The alpha- and gamma-tocopherol, oxidized coenzyme Q9 (CoQ9ox), and oxidized coenzyme Q10 (CoQ10ox) in control rats (c), rats exposed to 24 h/day light without treatment (24), and rats treated with captopril (24C) or melatonin (24M).
Alpha-tocopherol in the LV (nmol/g) | Gamma-tocopherol in the LV (nmol/g) | CoQ9ox in the LV (nmol/g) | CoQ10ox in the LV (nmol/g) | |
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Values are mean ± SEM,
Under physiological conditions in humans, melatonin concentration is low during the day, but it is much higher at night and is accompanied by lowering of the nocturnal blood pressure [
In rats exposed to constant lighting, hypertension development was associated with a rise in the LV weight, whereas no changes in the wall thickness and cross-sectional area of the aorta were observed. Interestingly, captopril inhibited the rise in blood pressure completely, reduced the LV weight even bellow the control values, similarly as was shown in SHR model [
The influence of captopril (24C) and melatonin (24M) on wall thickness of aorta stained by hematoxylin-eosin in 24 hours/day continuous light exposure-induced hypertension (24). c: Wistar controls.
Fibrotic LV remodelling after six-week exposure to constant light was characterized by elevated hydroxyproline levels in the insoluble (maturated, more cross-linked) and in total collagen (the sum of soluble and insoluble collagens). Only melatonin significantly reduced hydroxyproline concentration in both insoluble and total collagen. In a previous experiment with a combination of continuous light plus L-NAME model of hypertension analogical protection by melatonin was achieved [
The influence of captopril (24C) and melatonin (24M) on the concentration of collagen I (a), III (b), and the sum of collagens I and III (c) in the aortic media in 24 hours/day continuous light exposure-induced hypertension (24). c: Wistar controls.
The influence of captopril (24C) and melatonin (24M) on the concentration of collagen in the aortic media in 24 hours/day continuous light exposure-induced hypertension (24). Specimens are stained with picrosirius red and evaluated in polarized light: red to yellow color represents collagen I and green color represents collagen III. c: Wistar controls (according to [
Different impacts of melatonin and captopril treatment on the LV hypertrophy and LV fibrosis need to be addressed. It is generally accepted that LV hypertrophy development or its regression is mainly determined by the level of haemodynamic burden. Thus, the fact that only captopril reduced LV weight may be driven by a more prominent reduction in blood pressure by captopril than by melatonin. The distinct effects of captopril and melatonin on the LV fibrosis may be related to the complexity of hypertension development in this particular model and by the pleiotropic actions of both drugs. Fibrosis development is mainly dependent on the humoral disbalance between oxidative load, pro-proteosynthetic, and pro-proliferative factors such as angiotensin II, aldosterone, endothelin, or catecholamines on the one side and substances with growth attenuating potential, such as nitric oxide (NO), prostacyclin, or atrial natriuretic peptide [
The important question is the dose of captopril and melatonin that should be chosen to counteract pathological alterations of the continuous light-induced hypertension and LVH. 100 mg/kg/day of captopril [
The choice of captopril from the large group of accessible ACE-inhibitors was determined by three points. First, captopril is considered to be the gold standard, since it was the first ACE-inhibitor approved by Food and Drug Administration in 1993 as an effective treatment of heart remodeling and failure. Second, captopril contains SH group with potentially beneficial effect against oxidative stress. Third, in our previous experiments captopril was used [
It should be emphasized that in this experiment three-month-old rats (which may be considered to be adult) were used in a “preventive” experiment. However, when substantially younger rats with developing LVH and fibrosis were used, the results could be different.
In conclusion, captopril prevented LV hypertrophy development in the continuous light-induced hypertension model but only melatonin significantly reduced fibrosis. This antifibrotic action of melatonin may be protective in hypertensive heart disease.
In the model of continuous light exposure it would be of benefit to assess blood pressure more accurately by using telemetry system or by making measurements every 12 hours by tail-cuff plethysmography to disclose circadian rhythms of blood pressure. Moreover, it would be useful to investigate the group with continuous light and combination of captopril + melatonin treatment to show the potential complementary protection of both drugs. Furthermore, it may be useful to compare biochemical determination of the collagen level in the LV with the histochemical method, similarly as to know the level of
The authors declare that they have no conflict of interests.
This work was supported by the Scientific Grant Agency of the Ministry of Education (VEGA 1/0227/12, 2/00183/12), the Agency for Science and Technique (APVV-0742-10) and by the program PRVOUK P37/5.