Circadian Variation in Renal Function of the Obese Rat

Abstract

The influence of food and water intake on renal function was assessed by comparisons between the hyperphagic Zucker obese rat and its lean littermate, which demonstrates nocturnal dominance in activity. Serum creatinine and cortisol levels, creatine kinase activities, creatinine and urine clearances, and sodium and potassium excretion rates were measured over a 24-hour period in both lean and obese rats (n = 24 each). Six rats in each group were studied every 8 h to permit characterization over a 12-hour light/dark cycle at 2-hour intervals. Urine and creatinine clearances were increased in lean rats during the dark phase coincident with onset of eating. Similarly, renal sodium and potassium excretion rates were markedly increased during the dark cycle, despite relatively constant serum potassium and sodium levels over the 24-hour period. In contrast, no circadian patterns in urine and creatinine clearances were found in the obese rat, which exhibits continuous feeding habits throughout the 24-hour period. Moreover, renal electrolyte excretion in the obese rat was modestly increased during the dark cycle, unlike the significant differences over time observed in lean rats. Serum creatinine levels were increased during the dark cycle in both rat groups. Creatine kinase activity, a measure of ambulatory activity, was constant in lean rats during the study period. Although creatine kinase activity was increased in obese rats during the dark cycle, no correlations with renal functional parameters were found. These results indicate that differences in food and water intake are significant determinants in diurnal cyclic changes in renal function.

Circadian Rhythms of Renal Hemodynamics in Unanesthetized, Unrestrained Rats

“…This model is particularly well adapted to chronophysiological investigations because it permits, without external surgical disturbance, the continuous determination of circadian changes in inulin and PAH clearance. Overall, it makes possible a better evaluation of 24-h variations in renal hemodynamics than does the method involving endogenous creatinine clearance as previously used in normal and obese rats (9,22). The present study documented circadian rhythmicity in GFR and RPF in chronically catheterized, unanesthetized, and freely moving rats, sampled repeatedly at 4-h intervals during the 24 h. Reproducible circadian changes were noted between animals and in the same animal assessed on consecutive days.…”

Circadian rhythm of plasma sodium is disrupted in spontaneously hypertensive rats fed a high-NaCl diet

“…The circadian rhythm of plasma sodium is synchronous with the rhythms of circulating hormones, such as aldosterone (9,17,21), vasopressin, and oxytocin (31,34,51) and the renin-angiotensin system [i.e., angiotensin II (17,21,40), angiotensin-converting enzyme (44,47), and plasma renin activity (17,19,21,26)]. All of these rhythms are inverse to the rhythms of renal hemodynamics and excretion (18,29,38,39). Third, feeding and drinking patterns and the kinetics of gastrointestinal sodium absorption likely influence the plasma sodium rhythm.…”

A review of the renal system and diurnal variations of renal activity in livestock

“…Many investigators have reported that renal activity indices, like sympathetic activity, increase significantly during the day and decrease during the night; parasympathetic activity does the opposite [33,11,17]. Studies on renal activity diurnal variation have been carried out mainly on humans [13,3,31,5,15] and laboratory animals [32,1,43]. Few studies have been carried out on livestock.…”

“…The circadian rhythm of GFR in livestock is most likely affected by autonomic nervous system activity and also by circadian fluctuations of the following systems: the hypothalamus-hypophysis axis; vasoactive peptides; and, the renin-angiotensin-aldosterone complex, as studies on humans suggest [31,61]. Feeding time and diet (particularly the protein content in food) can modify renal activity, which results in hyperperfusion and then plasma hyperfiltration in renal glomeruli [58,32,52]. However, Muszczynski [35], in his studies on goats, did not observe an influence of feeding time on GFR diurnal variation.…”

“…Changes in the tubular resorption of potassium are the opposite - resorption is higher at night, which results in decreased potassium excretion with urine. Tubular resorption and secretion and electrolyte excretion with urine reach a maximum during the day and a minimum at night in other species of mammals, including dogs [10], rats [58,32] and humans [13,3,31,5]. Koopman et al [31] observed a positive correlation between sodium excretion and GFR rhythm acrophases in humans.…”