1. Department of Pharmaceutical Molecular Biology, Faculty of Pharmaceutical Sciences, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai 980-77, Japan, Department of Cardiovascular Diseases, Medical Research Institute, Tokyo Medical and Dental University, Yushima 1-5-45, Bunkyo-ku, Tokyo 113, Japan, and Department of Radiopharmaceutical Chemistry, Faculty of Pharmaceutical Sciences, Kumamoto University, Ohe-Honmachi 5-1, Kumamoto 862, Japan