1. From the Departments of Pharmacology (T.Z., S.M., J.H.B.) and Medicine (N.D.D., J.R.), University of California, San Diego, La Jolla, Calif; and the Department of Physiology (L.S.M., D.M.B.), Loyola University, Chicago, Ill. Present address for L.S.M. is the Department of Cardiology, Georg-August Universitaet Goettingen, Germany.