Circadian regulation of renal function

The kidneys regulate many vital functions that require precise control throughout the day. These functions, such as maintaining sodium balance or regulating arterial pressure, rely on an intrinsic clock mechanism that was commonly believed to be controlled by the central nervous system. Mounting evidence in recent years has unveiled previously underappreciated depth of influence by circadian rhythms and clock genes on renal function, at the molecular and physiological level, independent of other external factors. The impact of circadian rhythms in the kidney also affects individuals from a clinical standpoint, as the loss of rhythmic activity or clock gene expression have been documented in various cardiovascular diseases. Fortunately, the prognostic value of examining circadian rhythms may prove useful in determining the progression of a kidney-related disease, and chronotherapy is a clinical intervention that requires consideration of circadian and diurnal rhythms in the kidney. In this review, we discuss evidence of circadian regulation in the kidney from basic and clinical research in order to provide a foundation on which a great deal of future research is needed to expand our understanding of circadian relevant biology.

Circadian Clocks in the Hematologic System

The hematologic system performs a number of essential functions, including oxygen transport, the execution of the immune response against tumor cells and invading pathogens, and hemostasis (blood clotting). These roles are performed by erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets), respectively. Critically, circadian rhythms are evident in the function of all 3 cell types. In this review, we describe these oscillations, explore their mechanistic bases, and highlight their key implications. Since erythrocytes are anucleate, circadian rhythms in these cells testify to the existence of a nontranscriptional circadian clock. From a clinical perspective, leukocyte rhythms could underlie daily variation in the severity of allergic reactions, the symptoms of chronic inflammatory diseases, and the body’s response to infection, while the rhythmic properties of thrombocytes may explain daily fluctuations in the incidence of heart attack and stroke. Consequently, the efficacy of treatments for these conditions is likely to depend on the timing of their administration. Last, we outline preliminary evidence that circadian disruption in the hematologic system could contribute to the deleterious effects of poor diet, shift work, and alcohol abuse on human health.

Circadian influences on myocardial infarction

Components of circadian rhythm maintenance, or "clock genes," are endogenous entrainable oscillations of about 24 h that regulate biological processes and are found in the suprachaismatic nucleus (SCN) and many peripheral tissues, including the heart. They are influenced by external cues, or Zeitgebers, such as light and heat, and can influence such diverse phenomena as cytokine expression immune cells, metabolic activity of cardiac myocytes, and vasodilator regulation by vascular endothelial cells. While it is known that the central master clock in the SCN synchronizes peripheral physiologic rhythms, the mechanisms by which the information is transmitted are complex and may include hormonal, metabolic, and neuronal inputs. Whether circadian patterns are causally related to the observed periodicity of events, or whether they are simply epi-phenomena is not well established, but a few studies suggest that the circadian effects likely are real in their impact on myocardial infarct incidence. Cycle disturbances may be harbingers of predisposition and subsequent response to acute and chronic cardiac injury, and identifying the complex interactions of circadian rhythms and myocardial infarction may provide insights into possible preventative and therapeutic strategies for susceptible populations.

Molecular time: an often overlooked dimension to cardiovascular disease

Diurnal rhythms influence cardiovascular physiology such as heart rate and blood pressure and the incidence of adverse cardiac events such as heart attack and stroke. For example, shift workers and patients with sleep disturbances, such as obstructive sleep apnea, have an increased risk of heart attack, stroke, and sudden death. Diurnal variation is also evident at the molecular level, as gene expression in the heart and blood vessels is remarkably different in the day as compared to the night. Much of the evidence presented here indicates that growth and renewal (structural remodeling) are highly dependent on processes that occur during the subjective night. Myocardial metabolism is also dynamic with substrate preference also differing day from night. The risk/benefit ratio of some therapeutic strategies and the appearance of biomarkers also vary across the 24-hour diurnal cycle. Synchrony between external and internal diurnal rhythms and harmony among the molecular rhythms within the cell is essential for normal organ biology. Cell physiology is 4 dimensional; the substrate and enzymatic components of a given metabolic pathway must be present not only in the right compartmental space within the cell but also at the right time. As a corollary, we show disrupting this integral relationship has devastating effects on cardiovascular, renal and possibly other organ systems. Harmony between our biology and our environment is vital to good health.

Aldosterone induces circadian gene expression of clock genes in H9c2 cardiomyoblasts

We examined mRNA expression of the clock genes (Per1, Per2, and Bmal1) and PAI-1 (plasminogen activator inhibitor-1) after aldosterone treatment every 4 h up to 48 h in H9c2 cardiomyoblasts by reverse transcription-polymerase chain reaction. To block the MR (mineralocorticoid receptor), the MR antagonist, spironolactone, was added to the medium 1 h before aldosterone treatment. Aldosterone induced an initial increase and rhythmic expression of Per1, while spironolactone attenuated the acute increase in Per1 mRNA induced by aldosterone. On the other hand, aldosterone did not increase the Per2 mRNA in the acute phase, but thereafter induced a rhythmic expression of Per2. Aldosterone also induced rhythmic expression of Bmal1, a positive element of the clock genes. The rhythm of Bmal1 mRNA was anti-phase of that of Per2 mRNA. Aldosterone induced an acute increase in PAI-1 mRNA, but did not induce rhythmic expression of PAI-1. The present study demonstrated first that aldosterone regulates expression of the clock genes Per1, Per2, and Bmal1, and increases PAI-1 expression in H9c2 cardiomyoblasts. Second, an acute increase in Per1 mRNA after aldosterone treatment is mediated through MR. Third, clock genes are not related to PAI-1 expression in H9c2 cardiomyoblasts.