Developmental pattern of water and electrolyte transport in rat superficial nephrons.

Micropuncture samples were collected from late proximal and early distal sites of the same nephron in nondiuretic young rats aged 13-15, 19-21, and 30-39 days. Plasma ultrafiltrate (UF), tubular fluid (TF), and final urine were analyzed for Cl, Na, K, Mg, P, and Ca concentrations by electron probe. Between days 13 and 39, the proximal convoluted tubule reabsorbed a constant fraction of continuously increasing filtered loads of water, sodium, and potassium. The mean TF/UF concentration ratio for Cl in the late proximal tubule was 0.98 +/- 0.03 in the 13- to 15-day age group and increased to 1.20 +/- 0.03 in the 30- to 39-day age group. The mean TF/UF concentration ratio for Mg increased from 1.17 +/- 0.07 at 13-15 days to 2.07 +/- 0.22 at 30-39 days. The mean TF/UF concentration ratio for Ca at the end of the proximal tubule was found to be significantly lower than unity in the two youngest groups and higher in the oldest. Evidence was also found for maturation of the loop of Henle. The amounts of Cl, Na, K, and Ca reabsorbed by the loop, expressed in percent of the amount delivered, rose significantly with age. In contrast, mean Mg fractional reabsorption was already maximal in the youngest group. Phosphate reabsorption in the loop was limited in all three groups. It is concluded that in the 13- to 15-day rats 1) the fraction of the Na, K, and H2O filtered loads reabsorbed along the proximal tubule is already the same as in adult animals; 2) the proximal tubule epithelium is not able to create or maintain a transepithelial concentration gradient for Cl; and 3) the fractional reabsorption of Mg is higher than in the adult animal. It is also concluded that Cl, Na, K, and Ca transport along the loop of Henle all have the same maturation pattern but that the pattern for Mg is different.