Heterogeneous functional expression of the sustained inward Na current in guinea pig sinoatrial node cells.

The sustained inward Na current (I ) identified in the sinoatrial node (SAN) cell has been suggested to play a pivotal role in cardiac pacemaking. However, the composition of cells in the SAN is heterogeneous and cell-to-cell variability in the magnitude of I remains to be fully characterized. The present study investigated the current density of I in morphologically different types of pacemaker cells dissociated from guinea pig SAN. I was preferentially detected in spontaneously active spindle or spider-shaped cells, but was less well expressed in larger-sized elongated spindle-type cells and practically absent in clearly striated atrial-like cells, despite clear expression of the funny current (I ). The current density of I in spindle and spider cells varied from 0.7 to 1.6 pA pF and was significantly reduced in non-beating cells with similar morphologies. By linear regression analysis, we identified a positive correlation between the current densities of I and the L-type Ca current (I ), which was specifically observed in spindle and spider cells. These cells exhibited a more negative voltage for half maximal I activation than atrial-like cells, suggesting a variable ratio between Ca1.2- and Ca1.3-mediated I in SAN cells. Consistent single-cell transcript measurements confirmed a higher relative expression of Ca1.3, which activates at more negative potentials, in spindle cells than in atrial-like cells. Taken together, these results can be interpreted as indicating that I plays a specific role in primary pacemaker cells and that its presence is closely correlated with functional levels of Ca1.3-mediated I .