The kidney maintains homeostasis of the extracellular environment by selectively excreting or retaining water and solutes, according to the organism's needs. It is well established that under basal conditions, water and major solutes (Na+, K+, Cl-, PO4-, Ca2+, Mg2+, urea, ..) are excreted in the urine with a regular (circadian) rhythmicity. It has also been shown that this rhythmicity is driven by circadian changes in both glomerular filtration and tubular reabsorption/secretion. Circadian changes in renal filtration/reabsorption/secretion are thought to be mediated by interplay between the humoral circadian stimuli (hormones, food components, food metabolites) and self-sustained/self-autonomous renal circadian timing system. Functionally, this circadian timing system is thought to provide the kidney with an anticipatory advantage of the daily environmental changes (food and water intake, activity, posture, etc.).
Whilst, the hormonal regulation of renal function has been extensively investigated, the studies of intrinsic renal circadian timing system are just beginning. According to a current model, the circadian timing system is driven by transcriptional/translational feedback loops (core oscillators). These core oscillators confer circadian rhythmicity on a set of output genes underlying the tissue-specific functional rhythms. Which of the elements of circadian core oscillators and output genes underlie the diurnal rhythmicity of specific renal functions remains largely unknown.
Figure legend: Renal excretory rhythms : the intrisic renal clock versus external circadian time cues.