1. From the Cardiovascular Research Center (F.M.A., S.C.H., M.W.C.), Departments of Internal Medicine (F.M.A., S.C.H., M.W.C.) and Molecular Physiology and Biophysics (F.M.A., M.W.C.), Carver College of Medicine, University of Iowa, Iowa City, IA; Veterans Affairs Medical Center (H.W.C.), Iowa City, IA.