Early polyuria and urinary concentrating defect in potassium deprivation

Abstract

The time course of the onset of nephrogenic diabetes insipidus and its relationship to aquaporin-2 (AQP2) expression in K+ deprivation (KD) remains unknown. Rats were fed a K+-free diet and killed after 12 h, 1, 2, 3, 6, or 21 days. Serum K+ concentration was decreased only after, but not before, 3 days of a K+-free diet. Urine osmolality, however, decreased as early as 12 h of KD (1,061 ± 26 vs. 1,487 ± 102 mosmol/kgH2O in control, P < 0.01). It decreased further at 24 h (to 858 ± 162 mosmol/kgH2O in KD, P < 0.004) and remained low at 21 days of KD (436 ± 58 mosmol/kgH2O, P < 0.0001 compared with baseline). Water intake decreased at 12 h ( P < 0.002) but increased at 24 h ( P < 0.05) and remained elevated at 21 days of KD. Urine volume increased at 24 h of KD (8 ± 2 to 15 ± 2 ml/24 h, P < 0.05) and remained elevated at 21 days. Immunoblot analysis demonstrated that AQP2 protein abundance in the outer medulla remained unchanged at 12 h ( P > 0.05), decreased at 24 h (∼44%, P < 0.001), and remained suppressed (∼52%, P < 0.03) at 21 days of KD. In the inner medulla the AQP2 protein abundance remained unchanged at both 12 and 24 h of KD. AQP2 protein abundance in the cortex, however, decreased at 12 h (∼47%, P < 0.01) and remained suppressed at 24 h (∼77%, P < 0.001) of KD. Northern blot analysis showed that AQP2 mRNA decreased as early as 12 h of KD in both cortex ( P < 0.02) and outer medulla ( P < 0.01) and remained suppressed afterward. In conclusion, the urinary concentrating defect in KD is an early event and precedes the onset of hypokalemia. These studies further suggest that the very early urinary concentrating defect in KD (after 12 but before 24 h) results primarily from the suppression of cortical AQP2, whereas the later onset of a urinary concentrating defect (after 24 h) also involves a downregulation of medullary AQP2.