1. From the Departments of Internal Medicine, Divisions of Hypertension (A.R.C., J.D.K., L.O.L.) and Cardiovascular Diseases (M.R.-P., A.L.), Physiology and Biophysics (J.C.R.), and Laboratory Medicine and Pathology (J.P.G.), Mayo Clinic, Rochester, Minn; the Department of Medicine, University of Naples, Italy (C.N.); and the Department of Medicine, University of California, San Diego, Calif (C.N.).