Maturation of diluting capacity in loop of Henle of rat superficial nephrons.

The postnatal development of renal diluting capacity was studied by free-flow micropuncture and by microdissection of single superficial loops of Henle in rats kept on a high salt diet. Total renal filtration rate, sodium absorption, total solute excretion, and systemic arterial pressure were monitored during single nephron evaluation at two maturational stages (12-15 days and 27-35 days postnatal). Nephron filtration rates were identical when measured in early distal and late proximal segments of the same nephron. Absolute fluid absorption between these sites increased by a factor of 2.5, whereas loop fractional absorption of the volume load changed from 38.1 +/- 6.9 to 49.8 +/- 4.8% (SD), while proximal volume delivery to the loop increased from 4.33 +/- 1.12 to 7.14 +/- 0.65 nl/min. Simultaneously, the osmolarity of early distal fluid (8.8% of distal length) decreased significantly from 284 + 19.8 to 180.9 +/- 18.2 mosmol/liter during maturation. This study suggests that the ability of the loop of Henle to generate hypotonic fluid is attained only gradually during ontogeny. Volume absorption in the loop of Henle increases disproportionately to the loop volume load.