1. Departments of Molecular and Cellular Physiology and of Obstetrics and Gynecology, University of Cincinnati, College of Medicine, 231 Bethesda Avenue, Cincinnati, Ohio 45220, Department of Physiology and Biophysics, Boston University School of Medicine, 715 Albany Street, Boston, Massachusetts 02118, and Department of Biophysical Chemistry, GBB, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands