1. Institute of Anatomy, University of Zurich, CH-8057 Zurich;
2. Department of Cellular and Molecular Physiology, Yale University, New Haven, Connecticut 06520;
3. Institut de Pharmacologie et de Toxicologie, Université de Lausanne, CH-1005 Lausanne, Switzerland; and
4. Department of Cell Physiology, University Medical Centre, 6500-HB Nijmegen, The Netherlands