1. From the Cardiovascular Center (Z.-Y.T., Y.L., C.A.W., M.W.C., F.M.A.) and Departments of Internal Medicine (Z.-Y.T., Y.L., C.A.W., M.W.C., F.M.A.) and Molecular Physiology and Biophysics (M.W.C., F.M.A.), University of Iowa, Iowa City; Department of Physiology (A.E.S.), University of Melbourne, Victoria, Australia; Department of Physiology and Pharmacology (J.F.R.P.), School of Medical Sciences, University of Bristol, United Kingdom; and Veterans Affairs Medical Center (M.W.C.), Iowa City, Iowa.