1. Division of Nephrology, Hypertension, and Transplantation, University of Florida, and Gainesville Veterans Affairs Medical Center, Gainesville, Florida 32610-0224;
2. Department of Cell Biology, Georgetown University, Washington, District of Columbia 20007; and
3. Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892