Some 143 million patients worldwide are diagnosed with diabetes, almost five times more than estimates of ten years ago. Heart disease, often presenting as disease of the heart muscle (cardiomyopathy) is the leading cause of death among patients with diabetes mellitus. Diabetes, in turn, is the disease most associated with heart failure, and adversely affects outcomes of cardiovascular disease.
Nearly 300 years ago, researchers found that almost all cells possess some kind of intrinsic and self-sustained "clocks" allowing perception of the time of day, independent of external influences. These internal clocks provide the selective advantage of anticipation, allowing an organism to prepare for an expected "stimulus" (such as activity, feeding, or light intensity for a photosynthetic organism) at a given time of the day. In order to maintain their advantageous nature, clocks are reset by environmental cues known as "zeitgebers," that enable the organism to become synchronized with its surroundings.
Mammals possess both a central clock numerous peripheral clocks. The former is located in the suprachiasmatic nucleus (SCN) where a central pacemaker perceives light signals via the retina, which resets its mechanism. Peripheral clocks are those clocks located within other regions of the organism besides the SCN. The central and peripheral clocks require synchronization; zeitgebers involved in the process are light for the central clock, and neurohumoral factors for the peripheral clocks. Several candidates have been suggested as the zeitgebers for peripheral clocks, including glucocorticoids, retinoic acid and melatonin.
In diabetes mellitus, the 24-hour cycle of many neurohumoral factors are abnormal. Plasma levels of insulin, leptin, glucocorticoids, growth hormone, thyroid hormone, glucose, and fatty acids, as well as sympathetic activity, are all affected in diabetes. Given that the heart possesses a fully functional clock, potential zeitgebers are altered in diabetes, and the heart's morphology, gene expression, metabolism and contractile performance are all changed in diabetes, researchers investigated whether the clock of the heart is also affected within this environment.
The authors of "Alterations of the Circadian Clock in the Heart by Streptozotocin-Induced Diabetes" are Martin E. Young, Christopher R. Wilson, Peter Razeghi, Patrick H. Guthrie and Heinrich Taegtmeyer, all from the Department of Internal Medicine, Division of Cardiology, University of Texas-Houston Medical School, Houston, TX. The researchers will present their findings in detail during the American Physiological Society's (APS) annual meeting, part of the "Experimental Biology 2002" conference. More than 12,000 attendees will attend the conference being held at the Ernest N. Morial Convention Center, New Orleans, LA from April 20-24, 2002.
Diabetes was induced into 61 male rats by a single tail vein injection of the toxin streptozotocin. Sixty-two control animals were injected with saline only. Four weeks after initial saline or STZ injection, two control and two diabetic animals were sacrificed at specific time points, over three days, to ensure the reproducibility of the gene expression cycles. After administration of pentobarbital, hearts were isolated, freeze clamped in liquid nitrogen and stored at -80oC prior to RNA extraction. RNA extraction and quantitative RT-PCR of samples was performed. Standard RNA was made for all assays by the T7 polymerase method using total RNA isolated from the rat heart.
Immediately prior to heart isolation, one ml of blood was withdrawn from the rats, placed on ice, centrifuged for 10 minutes at maximum speed in a desk top centrifuge, and the resulting plasma was stored at -80oC. Humoral factors indicative of the development of diabetes were determined in these plasma samples. Plasma glucose levels were measured for control and diabetic rats; plasma non-esterified fatty acid (NEFA) levels were measured spectrophotometrically; specimen blanks were prepared for all samples to allow for possible hemolysis.
In control hearts, all of these genes showed dramatic circadian patterns of gene expression, with differences in the phase of the rhythms and amplitudes of induction. Circadian rhythms of expression were also observed for these transcripts in hearts isolated from diabetic rats, although these rhythms were different compared with control hearts. More specifically, a phase shift and/or early upswing in the rhythm (approximately three hours early) was observed for bmal1, the per genes, the cry genes, and the three output genes (dbp, hlf and tef) in the heart during diabetes. This alteration in the circadian clock of the heart are likely due to changes in circulating zeitgebers during diabetes. Whether these alterations of the clock in the heart cause a loss of synchronization in stimulus-response coupling, and play a role in dysfunction of the heart, requires elucidation.
Insulin-dependent diabetes caused alterations in the phase of the circadian clock in the heart. This suggests that either insulin, or a humoral factor which is under control (either directly or indirectly) by insulin, acts as a zeitgeber in the heart. These alterations of the clock in the heart could result in a loss of the synchronization between the stimulus and responsiveness of the system. Whether such loss of synchronization plays a role in the development of contractile dysfunction associated with the heart during diabetes requires further examination.
The present study shows that the clock in the heart is also abnormal during diabetes. The findings of this research suggest that a loss of stimulus-response synchronization due to alterations in the circadian clock in the heart during diabetes prevent anticipation of the heart towards alterations in it environment, such as increased sympathetic activity in the early hours of the morning, when most heart attacks occur. Accordingly, the findings may play a role in preventing heart failure among diabetes patients.
The American Physiological Society (APS) is one of the world's most prestigious organizations for physiological scientists. These researchers specialize in understanding the processes and functions underlying human health and disease. Founded in 1887 the Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals each year.
APS Newsroom: April 20-24, 2002
Morial Convention Center, New Orleans
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