A micropuncture study of the development of renal function in the young rat.

Total kidney function and function of individual surface nephrons were measured under hypotonic saline load conditions in young rats 22-, 30- and 42-days old. The highest values of urine flow rate and GFR were found in 30-day-old rats. The water reabsorption was higher in 42-day-old rats than in two younger groups. This decreased fractional reabsorption of water in younger animals was detectable already in the late proximal tubule and in the early distal tubule. Fractional reabsorption of sodium was significantly lower at the age of 22 and 30 days than at the age of 42 days in both late proximal tubule and in the early distal one.