1. Department of Cellular and Integrative Physiology, Center for Heart and Vascular Research, University of Nebraska Medical Center, Omaha, NE, U.S.A.
2. Department of Physiology and Biophysics, University of Mississippi Medical Center, Jackson, MS, U.S.A.
3. Research Service, Nebraska-Western Iowa Health Care System, Omaha, NE, U.S.A.