1. From the Cardiovascular Disease and Muscle Research Laboratories and the Department of Cellular and Molecular Physiology, Harvard Medical School (R.L., J.K.G.), the Charles A. Dana Research Institute and the Harvard-Thorndike Laboratory, Beth Israel Hospital (R.L., J.K.G.), and the NMR Laboratory for Physiological Chemistry, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School (L.N., J.F., J.S.I.), Boston, Mass.