1. Department of Pharmacology (M.A.), The University of Iowa, Iowa City, Iowa 52242-1109
2. Dipartimento di Chimica (F.F.), Universitá di Modena e Reggio Emilia, Via Campi 183 41100 Modena, Italy
3. Departments of Physiology and Biophysics (D.L.S.), The University of Iowa, Iowa City, Iowa 52242-1109