Relationship between vasopressin and renal concentrating ability in aging rats

The relationship between arginine vasopressin (AVP) secretion and the age-related change in renal concentrating ability was studied in 10-, 20-, and 30-mo-old conscious rats. In control condition, urine osmolality (Uosmol) was 2,358 +/- 99, 1,919 +/- 87, and 1,135 +/- 173 mosmol/kgH2O (mean +/- SE) and the corresponding plasma AVP concentration 3.1 +/- 1.2, 2.8 +/- 0.7, and 3.3 +/- 0.7 pg/ml at 10, 20, and 30 mo. Urinary AVP excretion and AVP content in the hypothalamus were comparable in the 3 age groups, while the basal AVP pituitary content was significantly higher at 10 than at 20 or 30 mo. Three days of dehydration induced 1) a similar increase in plasma concentration and urinary excretion of AVP in the 3 groups, even though the maximal Uosmol reached by the oldest animals was significantly reduced (3,988 +/- 218, 3,652 +/- 273, and 2,826 +/- 197 mosmol/kgH2O at 10, 20, and 30 mo, respectively) and 2) a similar AVP depletion of the pituitary at 10, 20, and 30 mo and an increase of the AVP content in the hypothalamus at 10 mo but not at 20 and 30 mo. These results suggest that the decrease in renal concentrating ability reported in aging rats is not due to an inappropriate secretion of AVP along the hypothalamo-neurohypophysial axis but is rather related to an impaired responsiveness of the kidney to the antidiuretic hormone.