Abstract 171: Localization and Function of Sodium Channel Subtypes in Human Heart
Introduction: Voltage-gated sodium channels are responsible for the rising phase of the action potential in cardiac muscle. They are composed of pore-forming α- and auxiliary β-subunits. Different α-subunit isoforms have distinct pharmacological properties and subcellular localization in the heart. The puffer fish toxin, tetrodotoxin (TTX), is a specific sodium channel blocker with different affinity for α-subunit isoforms. Nav1.5 is blocked by micromolar concentrations whereas CNS and skeletal muscle α-subunit isoforms are blocked by nanomolar concentrations of TTX.
Methods: We studied acutely isolated human atrial cells with the patch-clamp technique and studied expression and localization by immunoblotting and immunocytochemistry.
Results: We show that both TTX-resistant NaV1.5 channels and TTX-sensitive isoforms are expressed in human atrial cells, as assessed by immunoblotting with specific antibodies. At intercalated disks, we find NaV1.2 in association with β1 and β3 subunits by immunocytochemistry. In the cell surface at z-lines, we find NaV1.2, NaV1.4, and NaV1.5 channels in association with β2 subunits. In addition, NaV1.1, NaV1.3, and NaV1.6 channels are localized in scattered punctate clusters on the cell surface in association with β3 and β4 subunits. NaV1.5 immunostaining comprises approximately 88% of total. Our patch-clamp studies suggest that TTX-sensitive isoforms conduct approximately 15% of total sodium current whereas TTX-resistant isoforms, primarily NaV1.5, are responsible for approximately 85%. Specific blockade of TTX-sensitive isoforms with low concentrations of TTX leads to a reduction of myocardial contractility.
Conclusions: We show that both TTX-sensitive and cardiac, TTX-resistant sodium channels are functionally expressed and are differentially localized in subcellular compartments in atrial myocytes in human heart. TTX-sensitive sodium channels are required for normal contractility.