1. Department of Medicine, Harvard Medical School, Brigham and Women's Hospital, and Harvard Digestive Diseases Center, 75 Francis Street, Boston, Massachusetts 02115, Brockton/West Roxbury Department of Veterans Affairs Medical Center, 1400 VFW Parkway, Boston, Massachusetts 02132, and Laboratory of Epithelial Cell Biology, Renal-Electrolyte Division, University of Pittsburgh Medical Center and Oakland Department of Veterans Affairs Medical Center, Pittsburgh, Pennsylvania 15213