Influence of Connexin Expression/Co-Expression Levels on Electrical Impulse Propagation Investigated in the HL-1 Cell Model

In the myocardium action potentials are transmitted from cell-to-cell through gap junctions. These specialised junctions play a pivotal role in regulating the speed and safety of impulse propagation by controlling the amount of depolarised current that is passed from excited to non-excited regions of the heart. In mammalian hearts gap junction proteins connexin43, connexin40 and connexin45 are co-expressed in distinctive combinations and relative quantities in functionally specialised subsets of cardiac myocyte. The functional consequences of these connexin expression/co-expression patterns in modulating impulse propagation are poorly understood. To study the relative importance of membrane excitability and electrical coupling in relation to propagation velocities, clones of the HL-1 mouse atrial myocyte tumour line were used as an in vitro cell model. Five clones were characterised for expression of myocytic markers, calcium handling proteins and connexins, two of which (#2 and #6) displayed large differences in conduction velocities using microelectrode arrays. To ascertain which factor(s) were the main determinants of speed of conduction, the membrane excitability (voltage-gated channels) and electrical coupling (gap junctions) between the two clones were compared. Sodium, L- and T-type calcium channels were present in both clones but no significant differences were found in the current densities. However, large differences were seen in expression levels of connexin43, connexin40 and connexin45. RNA interference combined with microelectrode arrays was employed to establish the relative importance of each connexin in impulse propagation. The results indicate that electrical coupling by gap junctions is a major determinant of conduction velocities in HL-1 cell lines. Further experiments using RNA interference to suppress the expression of proteins thought to play a role in the action potential parameters should help in defining the part played by either the active or passive electrical properties in action potential propagation