1. Biomedical Imaging Group, Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, MA 01605, USA.
2. Department of Physiology, University of Arizona, Tucson, AZ 85274, USA.
3. Department of Physiology, University of British Columbia, Vancouver, British Columbia V6T 1 Z3, Canada.
4. Department of Physiology, Martin-Luther-Universität, Halle-Wittenberg, Medizinische Fakultat, Julius-Bernstein-lnstitut für Physiologie, 06097 Halle, Germany.