1. Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892-1603;
2. The Water and Salt Institute, University of Aarhus, DK-8000 Aarhus C, Denmark; and
3. Department of Internal Medicine, University of Iowa, and Veterans Affairs Medical Center, Iowa City, Iowa 52242