Expression pattern of neuronal and skeletal muscle voltage-gated Na+ channels in the developing mouse heart

in the mammalian heart, a variety of voltage-gated Na+ channel transcripts and proteins have been detected. However, little quantitative information is available on the abundance of each transcript during development, or the contribution of TTX-sensitive Na+ channels to the cardiac sodium current (I-Na)- Using competitive and real-time RT-PCR we investigated the transcription of six Na+ channels (Na(v)1.1-Na(v)1.6) and the beta 1 subunit during mouse heart development. Na(v)1.5 was predominantly expressed in the adult heart, whereas the splice variant Na(v)1.5a was the major Na+ channel isoform in embryonic hearts. The TTX-resistant Na+ channel transcripts (Na(v)1.5 and Na(v)1.5a) increased 1.7-fold during postnatal development. Transcripts encoding TTX-sensitive Na+ channels (Na(v)1. 1-Na(v)1.4) and the,31 subunit gradually increased up to fourfold from postnatal day (P)1 to P126, while the Na(v)1.6 transcript level remained low and constant over the same period. In adults, TTX-sensitive channel mRNA accounted for 30-40% of the channel pool in whole-heart preparations (Na-v 1.3 > Na-v 1.4 > Na-v 1.2 >> Na-v 1. 1 similar to Na-v 1.6), and 16% in mRNA from isolated cardiomyocytes (Na(v)1.4 > Na(v)1.3 > Nav1.2 > Na(v)1.1 > Na(v)1.6). Confocal immunofluorescence on ventricular myocytes suggested that Na(v)1.1 and Na(v)1.2 were localized at the intercalated disks and in the t tubules. Na(v)1.3 labelling predominantly produced a diffuse but strong intracellular signal. Na(v)1.6 fluorescence was detected only along the Z lines. Electrophysiological recordings showed that TTX-sensitive and TTX-resistant Na+ channels, respectively, accounted for 8% and 92% of the I-Na in adult ventricular cardiomyocytes. Our data suggest that neuronal and skeletal muscle Na+ channels contribute to the action potential of cardiomyocytes in the adult mammalian heart.