Measurement of renal function in unrestrained conscious rats

1. Inulin clearance and lithium clearance (used as a marker of end-proximal fluid delivery) were measured in unrestrained conscious rats without the use of catheters. Lithium was added to the food and [14C]inulin was administered via an osmotic minipump implanted in the peritoneal cavity; blood was sampled from the tail. 2. Urinary excretion rates and body weight fell during the 24 h after implantation of the minipump. Within 4 days, however, excretion rates and the rate of gain of body weight were indistinguishable from pre-implantation values. 3. During the rats' active period (i.e. the hours of darkness) urinary excretion rates of sodium, potassium and water were greater than during the inactive period. Inulin clearance, lithium clearance and fractional lithium excretion were also greater during the active period. Fractional distal reabsorptive rates of sodium and water (calculated using lithium clearance) did not differ significantly between the two phases of the light-dark cycle. 4. The experimental procedures used in this study provide an assessment of renal function which involves minimal disturbance to the animal. The results suggest that circadian variations in urinary excretion are caused largely by variations in glomerular filtration rate, accompanied by minor changes in fractional proximal fluid reabsorption.