Comparison of creatinine and inulin clearances in male and female rats

Creatinine and inulin clearances were compared in anesthetized male and female rats. Continuous intravenous infusion, midpoint arterial blood sampling, and ureteral catheterization aided accurate measurements. Average inulin clearances were 1.0 ml/min per 100 g. In the control male rat the creatinine clearance persistently exceeded that of inulin. The elevated CCr: CIn ratio could be reduced to 1 by probenecid, PAH, mercury, or high plasma levels of creatinine. In females a single intramuscular dose of testosterone elevated the ratio to that seen in male rats. This effect declined within 6 hr. Manipulations with steroids other than testosterone did not affect results in either sex. It is suggested that male rats secrete creatinine under the influence of androgens and that inulin is therefore a better measurement of the glomerular filtration rate in the male rat.