Kidney aquaporin-2 expression during escape from antidiuresis is not related to plasma or tissue osmolality.

Recent results indicate that renal escape from vasopressin-induced antidiuresis is accompanied by a marked downregulation of whole kidney aquaporin-2 (AQP-2) protein and mRNA expression. However, in those studies, the escaped animals were also markedly hypo-osmolar compared to controls as a result of water loading during antidiuresis. The present studies evaluated whether systemic or local osmolality contributes to the downregulation of AQP-2 expression in this model. In the first study, two groups of 1-deamino-[8-D-arginine]-vasopressin (dDAVP)-infused rats were water-loaded; after establishment of escape, one group was then water-restricted for 4 d to reverse the escape, whereas the other group continued daily water loading. Whole kidney AQP-2 protein was measured by Western blotting, and inner medulla AQP-2 mRNA was determined by Northern blotting. Results were compared to dDAVP-infused rats fed solid chow. After 4 d of water restriction, urine volume decreased to the same level as in the rats on solid chow; however, plasma sodium concentrations and plasma osmolality remained low. Despite maintenance of significant hypo-osmolality, rats in which escape was subsequently reversed by water restriction reestablished high dDAVP-stimulated kidney levels of AQP-2 after 4 d of water restriction. In the second study, AQP-2 expression was evaluated in different regions of kidneys from water-loaded rats undergoing escape from antidiuresis. Despite markedly different interstitial osmolalities, significant downregulation of AQP-2 expression compared to dDAVP-infused control rats was seen in the inner medulla, outer medulla, and cortex. Thus, neither systemic nor interstitial osmolality appears to appreciably be correlated with downregulation of kidney AQP-2 expression during escape from antidiuresis. These results therefore suggest that additional vasopressin- and osmolality-independent factors, likely related to the effects of extracellular fluid volume expansion, also regulate kidney AQP-2 expression in rats.