A method for accurate measurement of GFR in conscious, spontaneously voiding rats

Renal function measurement by clearance methods relies on accurately timed urine collection. In small experimental animals, renal function measurement is usually performed under anesthesia and/or with the application of bladder catheters to ensure accurate urine collection. To avoid both anesthesia and the need for bladder catheters we developed a method to measure glomerular filtration rate (GFR) in spontaneously voiding conscious rats. GFR was measured as the urinary clearance of constantly infused 125I-iothalamate. To correct for incomplete bladder emptying urinary clearance of 125I-iothalamate was multiplied by the ratio of plasma and urinary clearance of simultaneously infused 131I-hippuran, a correction method that has been previously validated in humans. Reproducibility of the technique was evaluated by analysis of the results of four consecutive clearance periods during the day (intra-assay variation) in a group of 17 rats and of two consecutive clearance periods on two or three separate days in a group of 20 rats (inter-assay variation), all with normal renal function. Application of the correction method reduced the intra-assay coefficient of variation (mean +/- SD) from 37.4 +/- 14.3 to 5.4 +/- 2.3% (P < 0.05). The mean inter-assay coefficient of variation fell slightly from 23.4 +/- 10.3 to 11.0 +/- 7.2% (P < 0.10). In rats with moderately impaired renal function (N = 8) the intra-assay variation fell from 27.9 +/- 20.7 to 2.7 +/- 1.6% (P < 0.05). Our data show that this correction method is a useful technique to assess renal function in conscious, spontaneously voiding rats.