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1 Brown Foundation Institute of Molecular Medicine, University of Texas Health Science Center at Houston, Houston, Texas, USA
2 Division of Endocrinology, Metabolism and Diabetes, University of Utah, Salt Lake City, Utah, USA
3 Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas, USA
4 USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, Texas, USA
5 Department of Biochemistry, University of Houston, Houston, Texas, USA
* To whom correspondence should be addressed. E-mail: Martin.E.Young{at}uth.tmc.edu.
Circadian clocks are intracellular molecular mechanisms that allow the cell to anticipate the time of day. We have previously reported that the intact rat heart expresses the major components of the circadian clock, whose rhythmic expression in vivo is consistent with the operation of a fully functional clock mechanism. The present study exposes oscillations of circadian clock genes (bmal1, rev-erba
, per2, dbp) for isolated adult rat cardiomyocytes in culture. Acute (2 hours) and/or chronic (continuous) treatment of cardiomyocytes with fetal calf serum (50% and 2.5% respectively) results in rhythmic expression of circadian clock genes with periodicities of 20-24 hours. In contrast, cardiomyocytes cultured in the absence of serum exhibit dramatically dampened oscillations in bmal1 and dbp only. Zeitgebers (time-keepers) are factors that influence the timing of the circadian clock. Glucose, which has been shown previously to reactivate circadian clock gene oscillations in fibroblasts, has no effect on the expression of circadian clock genes in adult rat cardiomyocytes, either in the absence or presence of serum. Exposure of adult rat cardiomyocytes to the sympathetic neurotransmitter norephinephrine (10µM) for 2 hours re-initiates rhythmic expression of circadian clock genes, in a serum-independent manner. Oscillations in circadian clock genes were associated with 24 hour oscillations in the metabolic genes pdk4 and ucp3. In conclusion, these data suggest that the circadian clock operates within the myocytes of the heart, and that this molecular mechanism persists under standard cell culture conditions (i.e. 2.5% serum). Furthermore, our data suggest that norepinephrine, unlike glucose, influences the timing of the circadian clock within the heart, and that the circadian clock may be a novel mechanism regulating myocardial metabolism.
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