Evidence for functional sodium and calcium ion channels in the membrane of cultured cardiomyocytes of the adult rat

HSP90 and eNOS partially co-localize and change cellular localization in relation to different ECM components in 2D and 3D cultures of adult rat cardiomyocytes

BACKGROUND INFORMATION

Cultivation techniques promoting three-dimensional organization of mammalian cells are of increasing interest, since they confer key functionalities of the native ECM (extracellular matrix) with a power for regenerative medicine applications. Since ECM compliance influences a number of cell functions, Matrigel-based gels have become attractive tools, because of the ease with which their mechanical properties can be controlled. In the present study, we took advantage of the chemical and mechanical tunability of commonly used cell culture substrates, and co-cultures to evaluate, on both two- and three-dimensional cultivated adult rat cardiomyocytes, the impact of ECM chemistry and mechanics on the cellular localization of two interacting signalling proteins: HSP90 (heat-shock protein of 90 kDa) and eNOS (endothelial nitric oxide synthase).

RESULTS

Freshly isolated rat cardiomyocytes were cultured on fibronectin, Matrigel gel or laminin, or in co-culture with cardiac fibroblasts, and tested for both integrity and viability. As validation criteria, integrity of both plasma membrane and mitochondria was evaluated by transmission electron microscopy. Cell sensitivity to microenvironmental stimuli was monitored by immunofluorescence and confocal microscopy. We found that HSP90 and eNOS expression and localization are affected by changes in ECM composition. Elaboration of the images revealed, on Matrigel-cultured cardiomyocytes, areas of high co-localization between HSP90 and eNOS and co-localization coefficients, which indicated the highest correlation with respect to the other substrates.

CONCLUSIONS

Our three-dimensional adult cardiomyocyte cultures are suitable for both analysing cell-ECM interactions at electron and confocal microscopy levels and monitoring micro-environment impact on cardiomyocyte phenotype.

Action potentials, ion channel currents and transverse tubule density in adult rabbit ventricular myocytes maintained for 6 days in cell culture

Adult rabbit ventricular myocytes were cultured in a basic medium (Medium 199) for up to 6 days to assess preservation of morphology and ion channel currents. In culture, cells remained rod shaped and striated but their ends became progressively rounded. Cell cross-sectional area declined slightly (by 14%) over the first 24 h, in contrast, whole-cell capacitance (which reflects external surface membrane plus membrane infoldings) decreased by 42% over the same time. Using whole-cell patch-clamp, we observed that the typical "N" shape steady-state current-voltage (I-V) relation became flattened after 24 h in culture. L-type Ca channel density was assessed as barium flux (IBa,L) via the channel. IBa,L (normalised to cell capacitance) declined by 50% after 24 h and recovered partially by days 4 and 6. The density of inward rectifier K current declined by 54% after 24 h and showed no recovery subsequently. In contrast, there was no significant decline in the density of transient outward K current after 24 h, but it declined subsequently by 65% after 6 days. We speculate that the time course of change in each ion channel density may reflect a change in pattern of ion channel expression, or differential membrane loss since the density of transverse tubules decreased by 57% after 6 days in culture. These results suggest that even by 24 h in culture, ion channel density in myocytes has changed substantially from the acutely isolated state.