Concentration of urine in the absence of ADH with minimal or no decrease in GFR

It is known that large (more than 50%) reductions in glomerular filtration rate (GFR) can lead to formation of hyperosmotic urine in the absence to depend critically upon reduced delivery of luid to the loops of Henle. In the present study, we tried to determine whether a much lesser decrease in GFR could also result in hyperosmotic urine when ADH is absent. The following mean values (control vs. 3 h of partial aortic constriction) were obtained in 21 conscious diabetes insipidus rats: GRF, 909 +/- 35 (SE) vs. 835 +/- 49 microliter . min-1 . 100 g body wt-1 (8% decrease; P less than 0.02); urinary osmolality (Uosmol), 125 +/- 6 vs. 309 +/- 14 mosmol/kg H2O (P less than 0.001; peak Uosmol 350 +/- 22). Analysis of individual responses revealed that Uosmol increased as much when there was no measurable decrease in GFR as when such decrease occurred. Neither GFR nor filtration fraction bore any systematic relationship to Uosmol or to each other. We conclude that in the absence of ADH Uosmol can increase with minimal or no change in GFR. Changes in filtration fraction--a potential mediator of reduced delivery to the loops--could not explain the increased Uosmol, even in those instances in which GFR remained unchanged.