Plasma vasopressin and cortical nephron function in aging rats

The role of vasopressin and Henle's loop transport in age-related polyuria and decrease in urine osmolality was investigated in female WAG/Rij rats free of kidney disease. In these animals, urine osmolality dropped from 2000 mosmol/kg H2O to 1000-1200 mosmol/kg H2O between 10 and 30 months, and urinary volume increased in proportion. Vasopressin concentration measured in plasma withdrawn from conscious, unrestrained, chronically catheterized rats was not significantly different in 10, 20 and 30-month-old animals (mean values 2.5 +/- 0.7, 2.2 +/- 0.2 and 2.0 +/- 0.3 pg/ml (n = 8), respectively). This suggests an impaired responsiveness of old kidney to antidiuretic hormone. The possible involvement of Henle's loop in this defect was studied by micropuncture. Paired collections of tubular fluid were done in the early distal and late proximal convolutions of the same cortical nephrons. Single nephron filtration rates did not significantly differ with age. Tubular fluid osmolalities in the early distal convolution were 165 +/- 13, 178 +/- 9 and 160 +/- 11 (n = 14) mosmol/kg H2O in 10-, 20- and 30-month-old rats, indicating similar diluting capacity of the cortical thick ascending limb. The amount of sodium transported from lumen to peritubular space by Henle's loop was also unchanged with age as were water, calcium, magnesium and potassium reabsorptions. These data indicate that the age-related decrease in urine osmolality is not related to either a significant reduced vasopressin plasma concentration or an increased single glomerular filtration rate or a reduced transport capacity of Henle's loop of the cortical nephron. Rather they suggest an impaired response to vasopressin of other segments of the nephron that is, the medullary thick ascending limb of Henle's loop and/or the collecting duct.