1. Howard Hughes Medical Institute; University of California San Diego; La Jolla 92093-0634 California
2. Department of Physiology and Biophysics; University of Colorado; Denver Colorado
3. Cardiology Department; Papworth Hospital; Papworth Everard CB23 3RE Cambridge U.K
4. Institute of Neuroscience; Newcastle University; Newcastle upon Tyne NE2 4HH U.K