1. From the Cardiac Muscle Research Laboratory, Whitaker Cardiovascular Institute, Department of Medicine (R.L., M.J., L.C., S.N.), Boston University School of Medicine, and NMR Laboratory for Physiological Chemistry, Division of Cardiovascular Medicine (R.L., J.D.A., F.A., I.L., R.T.), Brigham and Women’s Hospital and Harvard Medical School, Boston, Mass; and Department of Physiology and Medicine (R.M.M.), University of Michigan, Ann Arbor, Mich.