1. Integrated Physiology Research Laboratories, Boston University School of Medicine, Cambridge, Massachusetts 02138;
2. Whitaker Cardiovascular Institute, Boston, Massachusetts 02118;
3. Cardiac Unit, Massachusetts General Hospital, Boston, Massachusetts 02114;
4. Division of Cardiology, University of Colorado, Denver, Colorado 80262; and
5. Third Division, Department of Internal Medicine, Kyoto University, Kyoto 606, Japan