Circadian Rhythms of Renal Hemodynamics in Unanesthetized,Unrestrained Rats

Catheters were placed in the jugular vein and femoral artery of male Sprague-Dawley rats and connected to a specially designed perfusor for continuous constant infusion of 0.9% NaCl and a syringe to perform simultaneous and intermittent blood collections. This permitted continuous 24-h study of renal hemodynamics, estimated by inulin (C_in) and p-amino-hippuric acid (C_PAH) clearances; C_in represents glomerular filtration rate and C_PAH renal plasma flow. Animals were individually housed in metabolism cages in a controlled environment with light/dark 12:12 h. Urine was collected every 4 h (12:00, 16:00, 20:00, 24:00, 04:00, and 08:00) and blood sampled at the midpoint of urine collection periods. Urine and plasma sodium, potassium, inulin, and PAH were spectrophotometrically assessed. During continuous infusion of isotonic saline, C_in exhibited circadian changes with large decrease between 12:00 and 20:00 h (0.9 ± 0.2 ml/min) and acrophase at 00:30 h. Rhythmicity in C_PAH was similar with the minimum between 16:00 and 20:00 h (2.5 ± 0.3 ml/min) and peak between 00:00 and 04:00 h (acrophase at 00:25 h). Water and electrolyte excretion were also circadian rhythmic with a similar nighttime enhancement and daytime minimum. Such circadian changes persisted during continuous 0.9% NaCl infusion for several consecutive days. The unanesthetized, unrestrained rat model enables investigations in renal chronopharmacology and chronotoxicology.