Submitted to: American Journal of Physiology - Heart and Circulatory Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/28/2005
Publication Date: 6/3/2005
Citation: Durgan, D.J., Hotze, M.A., Tomlin, T.M., Egbejimi, O., Graveleau, C., Abel, E.D., Shaw, C.A., Bray, M.S., Hardin, P.E., Young, M.E. 2005. The intrinsic circadian clock within the cardiomyocyte. American Journal of Physiology - Heart and Circulatory Physiology. 289:H1530-H1541. Interpretive Summary: Circadian clocks are intracellular "molecular machines" that allow the cell to anticipate the time of day. The present study was designed to investigate the rhythms of circadian clock genes and to determine the factors that regulate these rhythms. Glucose, the simplest form of sugar, had no effect on the expression of circadian clock genes in adult rat heart cells, either in the absence or presence of other nutrients. Another factor in the blood, norephinephrine, which stimulates cell activity, re-starts rhythmic patterns of circadian clock genes. These results suggest that the circadian clock operates within the cells of the heart and that this molecular machine persists under standard cell conditions. 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 mechanism regulating heart metabolism.
Technical Abstract: 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, of which its rhythmic expression in vivo is consistent with the operation of a fully functional clock mechanism. The present study exposes oscillations of circadian clock genes [brain and arylhydrocarbon receptor nuclear translocator-like protein 1 (bmal1), reverse strand of the c-erbaalpha gene (rev-erbaalpha), period 2 (per2), albumin D-element binding protein (dbp)] for isolated adult rat cardiomyocytes in culture. Acute (2 h) and/or chronic (continuous) treatment of cardiomyocytes with FCS (50% and 2.5%, respectively) results in rhythmic expression of circadian clock genes with periodicities of 20-24 h. In contrast, cardiomyocytes cultured in the absence of serum exhibit dramatically dampened oscillations in bmal1 and dbp only. Zeitgebers (timekeepers) are factors that influence the timing of the circadian clock. Glucose, which has been previously shown 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 (10 microM) for 2 h reinitiates rhythmic expression of circadian clock genes in a serum-independent manner. Oscillations in circadian clock genes were associated with 24-h oscillations in the metabolic genes pyruvate dehydrogenase kinase 4 (pdk4) and uncoupling protein 3 (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.