The α-subunit of both the human heart (hH1) and human skeletal muscle (hSkM1) sodium channels were expressed in a mammalian expression system. The channels displayed slow (hH1) and fast (hSkM1) current decay kinetics similar to those seen in native tissues. Hence, the aim of this study was to identify the region on the α-subunit involved in the differences of these current-decay kinetics. A series of hH1/hSkM1 chimeric sodium channels were constructed with the focus on the C-terminal region. Sodium currents of chimeric channels were recorded using the patch-clamp technique in whole-cell configuration. Chimeras where the C-terminal region had been exchanged between hH1 and hSkM1 revealed that this region contains the elements that cause differences in current decay kinetics between these sodium channel isoforms. Other biophysical characteristics (steady-state activation and inactivation and recovery from inactivation) were similar to the phenotype of the parent channel. This indicates that the C-terminus is exclusively implicated in the differences of current decay kinetics. Several other chimeras were constructed to identify a specific region of the C-terminus causing this difference. Our results showed that the first 100-amino-acid stretch of the C-terminal region contains constituents that could cause the differences in current decay between the heart and skeletal muscle sodium channels.

This study has uncovered a direct relationship between the C-terminal region and the current-decay of sodium channels. These findings support the premise that a novel regulatory component exists for fast inactivation of voltage-gated sodium channels.