RELATIONSHIP BETWEEN CIRCADIAN CHANGES IN RENAL HEMODYNAMICS AND CIRCADIAN CHANGES IN URINARY GLYCOSAMINOGLYCAN EXCRETION IN NORMAL RATS

Circadian changes in renal hemodynamics and urinary glycosaminoglycan (GAG) excretion were studied in normal Sprague-Dawley rats to further investigate rhythms in kidney function. Urinary water, protein, and GA G excretion, as well as glomerular filtration rate (GFR) and renal pla sma flow (RPF), were determined every 4h over the 24h cycle in an atte mpt to characterize any temporal changes. Urinary flow rate and protei nuria peaked during the dark activity period of the animals, consisten tly at the same hour, whereas the lowest values were detected during t he resting phase. GAG are mucopolysaccharides entering the constitutio n of the glomerular basement membrane (GBM), which is the key componen t in the process of glomerular filtration. Similarly, the urinary excr etion rate of GAG showed a circadian rhythmicity in phase with urinary water and protein excretion, with markedly increased values observed during the nocturnal phase of the animals. Moreover, GFR and RPF were demonstrated to exhibit large circadian variations in phase with renal excretory rhythmicity, showing nighttime values significantly greater compared to daytime ones. Strong correlations were found between GFR and RPF rhythms, as well as between GAG and GFR, and GAG and RPF rhyth ms, although the latter were not statistically significant. This patte rn suggests that the circadian rhythmicity in urinary excretion rate o f GAG in physiological conditions could presumably be secondary to the temporal changes in renal hemodynamics. In this respect, knowledge of renal chronobiology helpfully contributes to increase our understandi ng of renal physiology.