The influence of melatonin on the heart rhythm - An in vitro simulation with murine embryonic stem cell derived cardiomyocytes

Melatonin facts: Lack of evidence that melatonin is a radical scavenger in living systems

Melatonin is a small natural compound, so called a neuro-hormone that is synthesized mainly in pineal gland in animals. Its main role is to master the clock of the body, under the surveillance of light. In other words, it transfers the information concerning night and day to the peripheral organs which, without it, could not "know" which part of the circadian rhythm the body is in. Besides its main circadian and circannual rhythms mastering, melatonin is reported to be a radical scavenger and/or an antioxidant. Because radical scavengers are chemical species able to neutralize highly reactive and toxic species such as reactive oxygen species, one would like to transfer this property to living system, despite impossibilities already largely reported in the literature. In the present commentary, we refresh the memory of the readers with this notion of radical scavenger, and review the possible evidence that melatonin could be an in vivo radical scavenger, while we only marginally discuss here the fact that melatonin is a molecular antioxidant, a feature that merits a review on its own. We conclude four things: (i) the evidence that melatonin is a scavenger in acellular systems is overwhelming and could not be doubted; (ii) the transposition of this property in living (animal) systems is (a) theoretically impossible and (b) not proven in any system reported in the literature where most of the time, the delay of the action of melatonin is over several hours, thus signing a probable induction of cellular enzymatic antioxidant defenses; (iii) this last fact needs a confirmation through the discovery of a nuclear factor-a key relay in induction processes-that binds melatonin and is activated by it and (iv) we also gather the very important description of the radical scavenging capacity of melatonin in acellular systems that is now proven and shared by many other double bond-bearing molecules. We finally discussed briefly on the reason-scientific or else-that led this description, and the consequences of this claim, in research, in physiology, in pathology, but most disturbingly in therapeutics where a vast amount of money, hope, and patient bien-être are at stake.

Core body temperature changes before sleep are associated with nocturnal heart rate variability

Core body temperature (CBT) reductions occur before and during the sleep period, with the extent of presleep reductions corresponding to sleep onset and quality. Presleep reductions in CBT coincide with increased cardiac parasympathetic activity measured via heart rate variability (HRV), and while this appears to persist into the sleep period, individual differences in presleep CBT decline and nocturnal HRV remain unexplored. The purpose of the current study was to assess the relationship between individual differences in presleep CBT reductions and nocturnal heart rate (HR) and HRV in a population of 15 objectively poor sleeping adults [10 males, 5 females; age, 33 ± 4 yr; body mass index (BMI) 27 ± 1 kg/m2] with the hypothesis that blunted CBT rate of decline would be associated with elevated HR and reduced nocturnal HRV. Following an adaptation night, all participants underwent an overnight, in-laboratory sleep study with simultaneous recording of polysomnographic sleep including electrocardiography (ECG) and CBT recording. Correlations between CBT rate of change before sleep and nocturnal HRV were assessed. Blunted rate of CBT decline was significantly associated with increased heart rate (HR) in stage 2 (N2; R = 0.754, P = 0.001), stage 3 (N3; R = 0.748, P = 0.001), and rapid-eye movement (REM; R = 0.735, P = 0.002). Similarly, blunted rate of CBT decline before sleep was associated with reduced HRV across sleep stages. These findings indicate a relationship between individual differences in presleep thermoregulatory processes and nocturnal cardiac autonomic function in poor sleeping adults.NEW & NOTEWORTHY Core body temperature (CBT) reductions before sleep onset coincide with increases in heart rate variability (HRV) that persist throughout the sleep period. However, the relationship between individual differences in the efficiency of presleep core temperature regulation and nocturnal heart rate variability remains equivocal. The present study reports an association between the magnitude of presleep core body temperature changes and nocturnal parasympathetic activity, highlighting overlap between thermoregulatory processes before sleep and nocturnal cardiac autonomic function.

Melatonin Prevents NaAsO2-Induced Developmental Cardiotoxicity in Zebrafish through Regulating Oxidative Stress and Apoptosis

Melatonin is an indoleamine hormone secreted by the pineal gland. It has antioxidation and anti-apoptosis effects and a clear protective effect against cardiovascular diseases. Our previous studies demonstrated that embryonic exposure to sodium arsenite (NaAsO₂) can lead to an abnormal cardiac development. The aim of this study was to determine whether melatonin could protect against NaAsO₂-induced generation of reactive oxygen species (ROS), oxidative stress, apoptosis, and abnormal cardiac development in a zebrafish (Danio rerio) model. We found that melatonin decreased NaAsO₂-induced zebrafish embryonic heart malformations and abnormal heart rates at a melatonin concentration as low as 10⁻⁹ mol/L. The NaAsO₂-induced oxidative stress was counteracted by melatonin supplementation. Melatonin blunted the NaAsO₂-induced overproduction of ROS, the upregulation of oxidative stress-related genes (sod2, cat, gpx, nrf2, ho-1), and the production of antioxidant enzymes (Total SOD, SOD1, SOD2, CAT). Melatonin attenuated the NaAsO₂-induced oxidative damage, DNA damage, and apoptosis, based on malonaldehyde and 8-OHdG levels and apoptosis-related gene expression (caspase-3, bax, bcl-2), respectively. Melatonin also maintained the control levels of heart development-related genes (nkx2.5, sox9b) affected by NaAsO₂. In conclusion, melatonin protected against NaAsO₂-induced heart malformations by inhibiting the oxidative stress and apoptosis in zebrafish.

The influence of light on the beat rate variability of murine embryonic stem cell derived cardiomyocytes

BACKGROUND

The human heart rhythm can be quantified by analyzing the heart rate variability (HRV). A major influencing factor of the HRV is the circadian rhythm. The ocular light and the hormone melatonin play decisive roles in the circadian rhythm. The beat rate variability (BRV) is considered to be the in vitro equivalent of the HRV. Previous studies have demonstrated the influence of melatonin on cardiomyocytes. Also, the influence of light on cardiomyocytes has been described before. Nevertheless, the effect of light on the BRV of cardiomyocytes has not yet been examined.

MATERIAL AND METHODS

The BRV of spontaneously beating cardiomyocytes was measured with microelectrode arrays over a time period of 30 min. The experiments were either performed with light exposure (with and without an infrared filter) or in complete darkness.

RESULTS

The BRV was higher and the beating frequency was lower when the cardiomyocytes were exposed to the full spectrum of light, compared to the measurements in darkness as well as to the measurements with an infrared filter. In contrast, the differences of BRV between the measurements in darkness and the measurements with an infrared filter were not as distinct.

CONCLUSIONS

This is the first study demonstrating the influence of light on the beating rhythm of heart tissue in vitro. The results indicate that especially the infrared spectrum of light alters the BRV. These findings could be of interest for clinical applications such as the field of optical pacing as well as in neonatal patient care.