Urinary concentrating ability during dehydration in the absence of vasopressin

Despite the apparent absence of vasopressin (ADH), Brattleboro homozygotes [diabetes insipidus (DI) rats] can concentrate their urine when deprived of drinking water. Since other investigators have shown that reducing glomerular filtration rate (GFR) improves the concentrating ability of water-loaded dogs, the present studies were undertaken to quantify the magnitude and time course of changes in GFR during dehydration. Clearance experiments were performed in 10 conscious DI rats before and following 3, 6, 9, 12, 15, and 24 h of dehydration. Urine osmolality increased from 155.0 +/- 12.6 (SE) to 696.7 +/- 8.4 mosmol/kg H2O after 24 h. GFR averaged 984.3 +/- 79.6 microliters . min-1 . 100 g body wt-1 in the control phase, fell to about 80% of this value over the first 12 h of dehydration, and then declined to 27% at 24 h. The rats lost 20% of their body weight over the 24 h. The osmolality of the papillary tip averaged 896 +/- 44 mosmol/kg H2O at 24 h compared to a control value of 493 +/- 28. The lack of osmotic equilibration between urine and papillary interstitium suggests that dehydration did not appreciably increase the water permeability of the distal nephron. These experiments clearly show a progressive decline in GFR as urine becomes concentrated during dehydration in the absence of ADH; these events may or may not be causally related.