Background: Extracellular recordings of electrical activity with substrate-integrated microelectrode arrays (MEAs) enable non-invasive long-term monitoring of contracting multicellular cardiac preparations. However, to characterize not only the spread of excitation and the conduction velocity from field potential (FP) recordings, a more rigorous analysis of FPs is necessary. Therefore in this study we aim to characterize intrinsic action potential (AP) parameters by simultaneous recording of APs and FPs. Methods: A MEA consisting of 60 substrate-integrated electrodes is used to record the FP-waveform from multicellular preparations of isolated embryonic mouse cardiomyocytes. Simultaneous current clamp recordings in the vicinity of individual microelectrodes and pharmacological interventions allowed us to correlate FP and AP components and their time course. Results: The experiments revealed a linear relationship between AP rise time and FP rise time as well as a linear relationship between AP duration and FP duration. Furthermore a direct contribution of the voltage dependent Na⁺- and Ca²⁺-current to the FP could be identified. Conclusion: The characterization of the FP allows us for the first time to estimate AP changes and the contribution of individual current components to the AP by the help of non-invasive recording within a multicellular cardiac preparation during long-term culture.

Homomeric gap junction channels are composed solely of one connexin type, whereas heterotypic forms contain two homomeric hemichannels but the six identical connexins of each are different from each other. A heteromeric gap junction channel is one that contains different connexins within either or both hemichannels. The existence of heteromeric forms has been suggested, and many cell types are known to coexpress connexins. To determine if coexpressed connexins would form heteromers, we cotransfected rat connexin43 (rCx43) and human connexin37 (hCx37) into a cell line normally devoid of any connexin expression and used dual whole cell patch clamp to compare the observed gap junction channel activity with that seen in cells transfected only with rCx43 or hCx37. We also cocultured cells transfected with hCx37 or rCx43, in which one population was tagged with a fluorescent marker to monitor heterotypic channel activity. The cotransfected cells possessed channel types unlike the homotypic forms of rCx43 or hCx37 or the heterotypic forms. In addition, the noninstantaneous transjunctional conductance-transjunctional voltage ( G j/ V j) relationship for cotransfected cell pairs showed a large range of variability that was unlike that of the homotypic or heterotypic form. The heterotypic cell pairs displayed asymmetric voltage dependence. The results from the heteromeric cell pairs are inconsistent with summed behavior of two independent homotypic populations or mixed populations of homotypic and heterotypic channels types. The G j/ V jdata imply that the connexin-to-connexin interactions are significantly altered in cotransfected cell pairs relative to the homotypic and heterotypic forms. Heteromeric channels are a population of channels whose characteristics could well impact differently from their homotypic counterparts with regard to multicellular coordinated responses.

Abstract-Heart failure (HF) is associated with reduced cardiac Na ϩ channel (SCN5A) current. We hypothesized that abnormal transcriptional regulation of this ion channel during HF could help explain the reduced current. Using human hearts explanted at the transplantation, we have identified 3 human C-terminal SCN5A mRNA splicing variants predicted to result in truncated, nonfunctional channels. As compared with normal hearts, the explanted ventricles showed an upregulation of 2 of the variants and a downregulation of the full-length mRNA transcript such that the E28A transcript represented only 48.5% (PϽ0.01) of the total SCN5A mRNA. This correlated with a 62.8% (PϽ0.01) reduction in Na ϩ channel protein. Lymphoblasts and skeletal muscle expressing SCN5A also showed identical C-terminal splicing variants. Variants showed reduced membrane protein and no functional current. Transfection of truncation variants into a cell line stably transfected with the full-length Na ϩ channel resulted in dose-dependent reductions in channel mRNA and current. Introduction of a premature truncation in the C-terminal region in a single allele of the mouse SCN5A resulted in embryonic lethality. Embryonic stem cell-derived cardiomyocytes expressing the construct showed reductions in Na ϩ channel-dependent electrophysiological parameters, suggesting that the presence of truncated Na ϩ channel mRNA at levels seen in HF is likely to be physiologically significant. In summary, chronic HF was associated with an increase in 2 truncated SCN5A variants and a decrease in the native mRNA. These splice variations may help explain a loss of Na Key Words: sodium channels Ⅲ transcriptional regulation Ⅲ mRNA splice variations Ⅲ heart failure Ⅲ arrhythmia H uman heart failure (HF) is associated with decreased cardiac voltage-gated sodium channel current. 1,2 Genetically mediated decreases in Na ϩ current have been implicated in the risk for sudden death, [3][4][5] and Na ϩ channel changes may contribute to the increased risk of sudden death in HF. 6,7 Because transcriptional alterations in other ion channels have been noted to contribute to current changes in HF, 8,9 we investigated Na ϩ channel protein and mRNA abundance in hearts explanted during cardiac transplantation to determine whether there were changes that might explain the reduced Na ϩ current previously reported in this tissue. Materials and Methods Detection of Human SCN5A 3UTR Variants by Rapid Amplification of cDNA Ends PCRTotal human RNA from normal fetal and adult whole hearts was purchased from Clontech (Mountain View, Calif). The RNA ligasemediated rapid amplification of cDNA ends (RACE) method was used to characterize the 3Ј ends of the human SCN5A mRNA using the GeneRacer kit (Invitrogen, Carlsbad, Calif). Primary and nested PCR reactions were performed with primers HE26F (on exon 26) and HE27F (on exon 27) specific to the human SCN5A gene and the GeneRacer 3Ј primer for amplifying the 3Ј-end fragment. The nested PCR products were cloned into pCR4-TOPO vector (Invitrogen) and se...

No abstract