Circadian rhythms and time course of adaptive sodium and potassium excretion in rats after uninephrectomy

The relationship between renal circadian cyclic excretion and renal compensatory adaptation after uninephrectomy for K, Na, and water was studied. Rats in a 12:12-h light-dark environment were given a liquid diet, and urine was collected for 16 days with consecutive 90-min periods. Days 1-4 were control, 5-10 followed a sham operation, and 11-16 followed uninephrectomy. The major findings were 1) the circadian cycles in excretion were virtually unchanged after sham and uninephrectomy; 2) an adaptive increase in excretion of Na, K, and water by the remaining kidney occurred within 90 min after uninephrectomy; 3) after uninephrectomy the distribution of the 24-h increment in excretion for the remaining kidney closely followed the preexisting pattern of circadian excretion for Na but was evenly distributed between light and dark phases for K; and 4) after uninephrectomy the ratio of excretion to intake was unchanged. This is the first study to document the time course of adaptation to uninephrectomy using consecutive brief collections over several days in unanesthetized and undisturbed rats. Adaptation after uninephrectomy occurred essentially immediately; was maintained unchanged; and preserved Na, K, and water homeostasis. Uninephrectomy did not alter the circadian control of excretion.