ARTICLES Increased Abundance of Distal Sodium Transporters in Rat Kidney during Vasopressin Escape CAROLYN A. ECELBARGER,* MARK A. KNEPPER, † and JOSEPH G. VERBALIS* *Department of Medicine, Division of Endocrinology and Metabolism, Georgetown University, Washington, D.C.; and † Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland. Abstract. Hyponatremia is associated with inappropriately el- evated vasopressin levels. A brisk natriuresis precedes the escape from this antidiuresis. Thus, the hypothesis was that the abundance of one or more of the sodium transporters of the distal tubule (a site for fine tuning of sodium balance) would be altered during vasopressin escape. Semiquantitative immuno- blotting was used to examine the regulation of abundance of several sodium transporters/channels of the thick ascending limb through the collecting duct in the rat model. Osmotic minipumps to infuse dDAVP, the V2-selective vasopressin agonist (5 ng/h) for the entire experiment, were implanted in Male Sprague-Dawley rats. After 4 d, rats were divided into a control (dry AIN-76 diet/ad libitum water) or a water-loaded (gelled-agar-AIN-76 diet/ad libitum water) group. Rats were killed after 1, 2, 3, or 7 additional days. The water-loaded rats were hyponatremic (plasma Na + , 98 to 122 mmol/L) and manifested the expected early natriuresis and diuresis of vaso- pressin escape. Water loading (with dDAVP infusion) resulted in increased whole-kidney abundances of the thiazide-sensitive Na-Cl co-transporter, the -subunit of the epithelial sodium channel (ENaC), and the 70-kD dimer of the -subunit of ENaC. No changes were observed for the -subunit of ENaC. Similar protein changes have recently been associated with elevated aldosterone levels in rats. However, plasma aldoste- rone levels were significantly suppressed in this model. These data suggest that several distal sodium reabsorptive mecha- nisms are upregulated during vasopressin escape; this may help to attenuate the developing hyponatremia resulting from water loading when vasopressin levels are inappropriately elevated. Vasopressin is the major hormonal regulator of body water homeostasis. In certain clinical circumstances, e.g., the syn- drome of inappropriate antidiuretic hormone secretion (SI- ADH), vasopressin levels are inappropriately high, resulting in free-water retention and hyponatremia. The degree of hypona- tremia is limited, however, by a process that counters the water-retaining action of vasopressin. This process is the so- called “vasopressin escape” phenomenon. In animal models of vasopressin escape (1–3), continued administration of vasopressin and water typically results in transient free-water retention followed by natriuresis, which is thought to contribute to the hyponatremia. However, after 1 to 3 d, urinary sodium excretion returns to baseline while urinary volume increases dramatically. Previously (4), we showed that this diuretic phase correlates very well with the downregulation of the apical water channel aquaporin-2 in the kidney collect- ing duct (CD) principal cells. Furthermore, the decreased abun- dance of aquaporin-2 protein was associated with an approxi- mate twofold reduction in osmotic water permeability of perfused inner medullary CD (5). Thus, reduction in CD water reabsorption is most likely the predominant mechanism by which the diuresis is achieved. This reduction in water perme- ability of the CD would be expected not only to enhance diuresis but also to attenuate the developing hyponatremia and its associated symptoms. Fine tuning of sodium balance in the body is achieved primarily by regulated sodium reabsorption in the distal con- voluted tubule (DCT) through the CD. The thiazide-sensitive NaCl co-transporter (NCC) of the DCT and the epithelial sodium channel (ENaC) of the cortical and outer medullary CD principal cells are important apical sodium transport pathways in these segments. In the thick ascending limb (TAL), transport of sodium chloride across the apical plasma membrane occurs via secondary active transport through the bumetanide-sensi- tive Na-K-2Cl co-transporter (NKCC2). This protein is thought to be more important in water than in sodium balance. In tandem with decreased water permeability, one would predict that increased sodium chloride reabsorption in the distal seg- ments would also assist in reducing the developing hyponatre- mia during vasopressin-induced water retention. Little is understood about the regulation of sodium reabsorp- tive pathways during vasopressin escape at the molecular level. Aldosterone, a major hormonal regulator of sodium reabsorp- tion in the distal nephron, increases the abundance of NCC (6) Received January 31, 2000. Accepted July 5, 2000. Dr. Jeff M. Sands served as guest editor and supervised the review and final disposition of this manuscript. Correspondence to Dr. Carolyn A. Ecelbarger, Bulding D, Room 232, George- town University, 4000 Reservoir Road NW, Washington, DC. Phone: 202- 687-0453; Fax: 202-687-2040; E-mail: ecelbarc@gunet.georgetown.edu 1046-6673/1202-0207 Journal of the American Society of Nephrology Copyright © 2001 by the American Society of Nephrology J Am Soc Nephrol 12: 207–217, 2001