Structural Basis for Pore Blockade of the Human Cardiac Sodium Channel Na_v1.5 by the Antiarrhythmic Drug Quinidine**

Na_v1.5, the primary voltage-gated Na⁺ (Na_v) channel in heart, is a major target for class I antiarrhythmic agents. Here we present the cryo-EM structure of full-length human Na_v1.5 bound to quinidine, a class Ia antiarrhythmic drug, at 3.3 Å resolution. Quinidine is positioned right beneath the selectivity filter in the pore domain and coordinated by residues from repeats I, III, and IV. Pore blockade by quinidine is achieved through both direct obstruction of the ion permeation path and induced rotation of an invariant Tyr residue that tightens the intracellular gate. Structural comparison with a truncated rat Na_v1.5 in the presence of flecainide, a class Ic agent, reveals distinct binding poses for the two antiarrhythmics within the pore domain. Our work reported here, along with previous studies, reveals the molecular basis for the mechanism of action of class I antiarrhythmic drugs.