1. Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, Virginia 23507, MRC National Institute for Medical Research, Mill Hill, London, NW7 1AA, United Kingdom, Laboratory of Molecular Physiology, National Heart, Lung and Blood Institute, Bethesda, Maryland 20892-1762, and Department of Biochemistry, Eötvös University, Pázmány stny.1/c, H-1117 Budapest, Hungary