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This study examined the diversity of Na' channel gene expression in intact cardiac tissue and purified myocardial cells. The screening of neonatal rat myocardial cell cDNA libraries with a conserved rat brain Na' channel cDNA probe, resulted in the isolation and characterization of a putative rat cardiac Na' channel cDNA probe (pCSC-1). The deduced amino acid sequence of pCSC-1 displayed a striking degree of homology with the eel, rat brain-i, and rat brain-2 Na' channel, thereby identifying pCSC-1 as a related member of the family of Na' channel genes. Northern blot analysis revealed the expression of a 7-kb CSC-1 transcript in rat cardiac tissue and purified myocardial cells, but little or no detectable expression of CSC-1 in rat brain, skeletal muscle, denervated skeletal muscle, or liver. Using RNase protection and Northern blot hybridization with specific rat brain Na' channel gene probes, expression of the rat brain-i Na' channel was observed in rat myocardium, but no detectable expression of the rat brain-2 gene was found. This study provides evidence for the expression of diverse Na' channel mRNAs in rat myocardium and presents the initial characterization of a new, related member of the family of Na' channel genes, which appears to be expressed in a cardiac-specific manner.

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