Voltage-gated sodium channels composed of a pore-forming α subunit and auxiliary β subunits are responsible for the upstroke of the action potential in cardiac muscle. cell patch-clamp recording and measurements of contractility in human atrial cells and tissue showed that TTX-sensitive (non-Nav1.5) α subunit isoforms account for up to 27% of total sodium current in human atrium and are required for maximal contractility. Overall our results show that multiple sodium channel α and β subunits are differentially localized in subcellular compartments in human atrial myocytes suggesting that they play distinct functions in initiation and conduction of the action potential and in excitation-contraction coupling. TTX-sensitive sodium channel isoforms even though expressed at low levels relative to TTX-sensitive Nav1. 5 contribute substantially to total Caudatin cardiac sodium current and are required for normal contractility. This article is usually a part of a Special Caudatin Issue entitled “Na+ Regulation in Cardiac Myocytes”. Keywords: Sodium channels Myocardium Immunocytochemistry Contractility 1 Introduction Cardiac action potentials are generated and propagated through the coordinated activity of multiple types Rabbit Polyclonal to SFRS11. of ion channels. Voltage-gated sodium channels generate Caudatin the upstroke of the action potential and their activation and inactivation set the conduction velocity through cardiac tissue and the refractory period between conducted action potentials. Mutations in genes encoding voltage-gated sodium channels Caudatin are known to cause arrhythmias  and to be involved in cardiomyopathy [2-4]. Differential expression and localization of sodium channel subunits are likely to be important determinants of electric excitability of cardiac myocytes. This study defines the subcellular localization of sodium channel subunits in human atrial myocardium. Voltage-gated sodium channels are composed of a pore-forming α subunit with one or two auxiliary β subunits . Ten different genes encoding sodium channel α subunits have been identified and nine have been functionally expressed [5 6 The different α subunit isoforms have distinct patterns of development and localization in the nervous system skeletal and cardiac muscle and they have different pharmacological properties. Isoforms preferentially expressed in the central nervous system (Nav1.1 1.2 1.3 1.6 are inhibited by nanomolar concentrations of the puffer fish toxin tetrodotoxin (TTX) a highly specific sodium channel blocker. The isoform present in adult skeletal muscle (Nav1.4) is also blocked by nanomolar TTX-concentrations. In contrast the primary cardiac Caudatin isoform (Nav1.5) requires micromolar concentrations of TTX for inhibition due to substitution of a cysteine for an aromatic residue in the pore region [6 7 Four genes encoding different β-subunits-β1 β2 β3 and β4-have been identified [8 9 β1 and β3 are noncovalently associated with α subunits whereas β2 and β4 are disulfide-linked to α. The β subunits modulate channel gating interact with extracellular matrix cytoskeleton and cell adhesion molecules play a role in adhesive interactions and influence cell surface localization of sodium channels . Our previous work showed that sodium channel β subunits are differentially localized in the transverse tubules surface membrane and intercalated disks of mouse ventricular myocytes . Nav1.5 has often been termed the “cardiac” sodium channel. However we showed previously that this “brain” sodium channels Nav1.1 and Nav1.3 are also expressed in mouse heart and have distinct distributions and functions from Nav1.5 [11-13] and other groups have extended these findings [14-18]. Multiple sodium channel isoforms are also expressed in human Caudatin atrial myocytes . 2 Materials and methods 2.1 Tissue samples Samples of human atrial tissue were obtained from patients undergoing elective cardiac surgery for multiple indications. Tissue from patients with congestive heart failure or atrial rhythm disorders including atrial fibrillation was excluded to avoid structural and/or electrophysiological alterations in the right atrial myocardium. A detailed description of tissue isolation and preparation is provided in Supplemental Material. All procedures conformed to the principles layed out in the Declaration of Helsinki and were in agreement with.