1. From the School of Physiology and Pharmacology, Bristol Heart Institute, University of Bristol, Bristol, United Kingdom (E.C.H., F.D.M., L.E.K.R., L.Q.S., J.F.R.P.); and Bristol Heart Institute, Bristol Royal Infirmary, University Hospitals Bristol NHS Foundation Trust, Bristol, United Kingdom (A.E.B., A.B., A.N.).