Voltage-gated Na ؉ channels are composed of pore-forming ␣ and auxiliary ␤ subunits. The majority of Na ؉ channels in the heart contain tetrodotoxin (TTX)-insensitive Nav1.5 ␣ subunits, but TTXsensitive brain-type Na ؉ channel ␣ subunits are present and functionally important in the transverse tubules of ventricular myocytes. Sinoatrial (SA) nodal cells were identified in cardiac tissue sections by staining for connexin 43 (which is expressed in atrial tissue but not in SA node), and Na ؉ channel localization was analyzed by immunocytochemical staining with subtype-specific antibodies and confocal microscopy. Brain-type TTX-sensitive Nav1.1 and Nav1.3 ␣ subunits and all four ␤ subunits were present in mouse SA node, but Nav1.5 ␣ subunits were not. Nav1.1 ␣ subunits were also present in rat SA node. Isolated mouse hearts were retrogradely perfused in a Langendorff preparation, and electrocardiograms were recorded. Spontaneous heart rate and cycle length were constant, and heart rate variability was small under control conditions. In contrast, in the presence of 100 nM TTX to block TTX-sensitive Na ؉ channels specifically, we observed a significant reduction in spontaneous heart rate and markedly greater heart rate variability, similar to sick-sinus syndrome in man. We hypothesize that brain-type Na ؉ channels are required because their more positive voltage dependence of inactivation allows them to function at the depolarized membrane potential of SA nodal cells. Our results demonstrate an important contribution of TTX-sensitive brain-type Na ؉ channels to SA nodal automaticity in mouse heart and suggest that they may also contribute to SA nodal function and dysfunction in human heart.

The functionally important effects on the heart of ACh released from vagal nerves are principally mediated by the muscarinic K+ channel. The aim of this study was to determine the abundance and cellular location of the muscarinic K+ channel subunits Kir3.1 and Kir3.4 in different regions of heart. Western blotting showed a very low abundance of Kir3.1 in rat ventricle, although Kir3.1 was undetectable in guinea pig and ferret ventricle. Although immunofluorescence on tissue sections showed no labeling of Kir3.1 in rat, guinea pig, and ferret ventricle and Kir3.4 in rat ventricle, immunofluorescence on single ventricular cells from rat showed labeling in t-tubules of both Kir3.1 and Kir3.4. Kir3.1 was abundant in the atrium of the three species, as shown by Western blotting and immunofluorescence, and Kir3.4 was abundant in the atrium of rat, as shown by immunofluorescence. Immunofluorescence showed Kir3.1 expression in SA node from the three species and Kir3.4 expression in the SA node from rat. The muscarinic K+ channel is activated by ACh via the m2 muscarinic receptor and, in atrium and SA node from ferret, Kir3.1 labeling was co-localized with m2 muscarinic receptor labeling throughout the outer cell membrane.

Background-The electrical activity of the atrioventricular node (AVN) is functionally heterogeneous, but how this relates to distinct cell types and the 3-dimensional structure of the AVN is unknown. To address this, we have studied the expression of Na v 1.5 and other Na ϩ channel isoforms in the AVN. Methods and Results-The rat AVN was identified by Masson's trichrome staining together with immunolabeling of marker proteins: connexin40, connexin43, desmoplakin, atrial natriuretic peptide, and hyperpolarization-activated and cyclic nucleotide-gated channel 4. Na ϩ channel expression was investigated with immunohistochemistry with isoform-specific Na ϩ channel antibodies. Na v 1.1 was distributed in a similar manner to Na v 1.5. Na v 1.2 was not detected. Na v 1.3 labeling was present in nerve fibers and cell bodies (but not myocytes) and was abundant in the penetrating atrioventricular (AV) bundle and the common bundle but was much less abundant in other regions. Na v 1.5 labeling was abundant in the atrial and ventricular myocardium and the left bundle branch. Na v 1.5 labeling was absent in the open node, penetrating AV bundle, AV ring bundle, and common bundle but present at a reduced level in the inferior nodal extension and transitional zone. Na v 1.6 was not detected. Conclusions-Our findings provide molecular evidence of multiple electrophysiological cell types at the AV junction.Impaired AV conduction as a result of mutations in or loss of Na v 1.5 must be the result of impaired conduction in the AVN inputs (inferior nodal extension and transitional zone) or output (bundle branches) rather than the AVN itself (open node and penetrating AV bundle).