1. From the Cardiovascular Center and Departments of Internal Medicine (Y.C., S.I., D.D.L., R.M.W., G.F.D., Y.W., F.M.F., D.D.H.), Pharmacology (F.M.F., D.D.H.), and Physiology (G.F.D.), University of Iowa Roy J. and Lucille A. Carver College of Medicine, and VA Medical Center (D.D.L., R.M.W., G.F.D., D.D.H.), Iowa City, Iowa.