1. From the Cardiovascular Research Center (H.O., S.H., H.S., H.N., K.E., S.E.) and Departments of Physiology (H.O., S.H., H.S., H.N., K.E., S.E.) and Pharmacology (E.B.), Temple University School of Medicine. Philadelphia, Pa; the Departments of Pediatrics and Molecular and Integrative Physiology (P.J.D.), University of Michigan, Ann Arbor, Mich; and the Department of Biochemistry (G.D.F.), Vanderbilt University School of Medicine, Nashville, Tenn.