Expression of circadian clock and melatonin receptors within cultured rat cardiomyocytes.

Melatonin, the pineal gland hormone, provides entrainment of many circadian rhythms to the ambient light/dark cycle. Recently, cardiovascular studies have demonstrated melatonin interactions with many physiological processes and diseases, such as hypertension and cardiopathologies. Although membrane melatonin receptors (MT1, MT2) and the transcriptional factor RORα have been reported to be expressed in the heart, there is no evidence of the cell-type expressing receptors as well as the possible role of melatonin on the expression of the circadian clock of cardiomyocytes, which play an important role in cardiac metabolism and function. Therefore, the aim of this study was to evaluate the mRNA and protein expressions of MT1, MT2, and RORα and to determine whether melatonin directly influences expression of circadian clocks within cultured rat cardiomyocytes. Adult rat cardiomyocyte cultures were created, and the cells were stimulated with 1 nM melatonin or vehicle. Gene expressions were assayed by real-time polymerase chain reaction (PCR). The mRNA and protein expressions of membrane melatonin receptors and RORα were established within adult rat cardiomyocytes. Two hours of melatonin stimulation did not alter the expression pattern of the analyzed genes. However, given at the proper time, melatonin kept Rev-erbα expression chronically high, specifically 12 h after melatonin treatment, avoiding the rhythmic decline of Rev-erbα mRNA. The blockage of MT1 and MT2 by luzindole did not alter the observed melatonin-induced expression of Rev-erbα mRNA, suggesting the nonparticipation of MT1 and MT2 on the melatonin effect within cardiomyocytes. It is possible to speculate that melatonin, in adult rat cardiomyocytes, may play an important role in the light signal transduction to peripheral organs, such as the heart, modulating its intrinsic rhythmicity.

Body weight, metabolism and clock genes.

Biological rhythms are present in the lives of almost all organisms ranging from plants to more evolved creatures. These oscillations allow the anticipation of many physiological and behavioral mechanisms thus enabling coordination of rhythms in a timely manner, adaption to environmental changes and more efficient organization of the cellular processes responsible for survival of both the individual and the species. Many components of energy homeostasis exhibit circadian rhythms, which are regulated by central (suprachiasmatic nucleus) and peripheral (located in other tissues) circadian clocks. Adipocyte plays an important role in the regulation of energy homeostasis, the signaling of satiety and cellular differentiation and proliferation. Also, the adipocyte circadian clock is probably involved in the control of many of these functions. Thus, circadian clocks are implicated in the control of energy balance, feeding behavior and consequently in the regulation of body weight. In this regard, alterations in clock genes and rhythms can interfere with the complex mechanism of metabolic and hormonal anticipation, contributing to multifactorial diseases such as obesity and diabetes. The aim of this review was to define circadian clocks by describing their functioning and role in the whole body and in adipocyte metabolism, as well as their influence on body weight control and the development of obesity.

Calorie restriction reduces pinealectomy-induced insulin resistance by improving GLUT4 gene expression and its translocation to the plasma membrane.

The present study aimed to investigate insulin sensitivity and GLUT4 expression protein in pinealectomized rats, as well as to determining the effects of melatonin and calorie restriction on the changes induced by pinealectomy. Wistar rats were pinealectomized (Pinx) or sham operated (Sham), and studied 30 days later. Melatonin replacement treatment (50 g/100 g body weight) was continued for 30 days after pinealectomy. Calorie restriction was performed by offering 60% of the standard food intake. In vivo insulin sensitivity was evaluated using the glucose disappearance constant (kITT) during an insulin tolerance test, and GLUT4 mRNA and protein were assessed by Northern and Western blotting, respectively. The in vitro effect of melatonin on GLUT4 protein content in plasma membrane was investigated in adipocytes isolated from intact rats. Compared with Sham rats, Pinx rats showed decreased kITT (40%), GLUT4 expression in white adipose tissue (WAT, approximately 70%), and unchanged GLUT4 expression in skeletal muscle. Melatonin treatment in Pinx rats restored the kITT and GLUT4 protein to control values. No in vitro effects of melatonin (10-9 m) upon GLUT4 protein were observed. Calorie restriction of Pinx rats increased their kITT value ( approximately 40%), total GLUT4 protein content ( approximately 240%) and its translocation to the plasma membrane ( approximately 80%) in WAT. The results show that pinealectomy, for lack of melatonin, decreased insulin sensitivity as well as GLUT4 gene expression. Calorie restriction improved insulin sensitivity in Pinx rats, and this was related to increased GLUT4 gene expression and insulin-induced GLUT4 translocation to the plasma membrane in WAT.