Cell therapy is emerging as a promising strategy for myocardial repair. This approach is hampered, however, by the lack of sources for human cardiac tissue and by the absence of direct evidence for functional integration of donor cells into host tissues. Here we investigate whether cells derived from human embryonic stem (hES) cells can restore myocardial electromechanical properties. Cardiomyocyte cell grafts were generated from hES cells in vitro using the embryoid body differentiating system. This tissue formed structural and electromechanical connections with cultured rat cardiomyocytes. In vivo integration was shown in a large-animal model of slow heart rate. The transplanted hES cell-derived cardiomyocytes paced the hearts of swine with complete atrioventricular block, as assessed by detailed three-dimensional electrophysiological mapping and histopathological examination. These results demonstrate the potential of hES-cell cardiomyocytes to act as a rate-responsive biological pacemaker and for future myocardial regeneration strategies.

Voltage-activated Ca2+ channels exist as multigene families that share common structural features. Different Ca2+ channels are distinguished by their electrophysiology and pharmacology and can be classified as either low or high voltage-activated channels. Six alpha1 subunit genes cloned previously code for high voltage-activated Ca2+ channels; therefore, we have used a database search strategy to identify new Ca2+ channel genes, possibly including low voltage-activated (T-type) channels. A novel expressed sequence-tagged cDNA clone of alpha1G was used to screen a cDNA library, and in the present study, we report the cloning of alpha1H (or CavT.2), a low voltage-activated Ca2+ channel from human heart. Northern blots of human mRNA detected more alpha1H expression in peripheral tissues, such as kidney and heart, than in brain. We mapped the gene, CACNA1H, to human chromosome 16p13.3 and mouse chromosome 17. Expression of alpha1H in HEK-293 cells resulted in Ca2+ channel currents displaying voltage dependence, kinetics, and unitary conductance characteristic of native T-type Ca2+ channels. The alpha1H channel is sensitive to mibefradil, a nondihydropyridine Ca2+ channel blocker, with an IC50 of 1.4 micromol/L, consistent with the reported potency of mibefradil for T-type Ca2+ channels. Together with alpha1G, a rat brain T-type Ca2+ channel also cloned in our laboratory, these genes define a unique family of Ca2+ channels.

The cardiac sodium channel alpha subunit (RHI) is less sensitive to tetrodotoxin (TTX) and saxitoxin (STX) and more sensitive to cadmium than brain and skeletal muscle (microliter) isoforms. An RHI mutant, with Tyr substituted for Cys at position 374 (as in microliter) confers three properties of TTX-sensitive channels: (i) greater sensitivity to TTX (730-fold); (ii) lower sensitivity to cadmium (28-fold); and (iii) altered additional block by toxin upon repetitive stimulation. Thus, the primary determinant of high-affinity TTX-STX binding is a critical aromatic residue at position 374, and the interaction may take place possibly through an ionized hydrogen bond. This finding requires revision of the sodium channel pore structure that has been previously suggested by homology with the potassium channel.