Key Points
- To determine whether mutations in the conserved III-IV cytoplasmic linker of human heart (hH1a) Na+ channels produce functional effects on fast inactivation gating analogous to those in neuronal channels.
- Generated site-directed mutations in the cytoplasmic III-IV linker (F1485Q and the triple mutation I1484Q/F1485Q/M1486Q) of human cardiac Na+ channels.
- Expressed wild-type and mutant channels heterologously in Xenopus oocytes and analyzed macroscopic and single-channel currents using voltage-clamp recordings.
- The F1485Q mutation inhibited fast inactivation by < 50% and increased mean single-channel open time twofold, unlike the homologous brain channel mutation (F1489Q) which caused ≥ 85% inhibition.
- Residual fast inactivation in F1485Q channels followed a monoexponential time course (tau = 2 milliseconds), lacked null tracings between -140 and -120 mV, and showed steady-state inactivation overlapping activation.
- Simultaneous mutation of the entire IFM motif (I1484Q, F1485Q, and M1486Q) completely abolished fast inactivation gating.
Structured PICO
PPopulationXenopus oocytes expressing human heart (hH1a) Na+ channels
IInterventionF1485Q mutation and simultaneous mutations of I1484Q, F1485Q, and M1486Q in the III-IV linker
CComparatorWild-type (WT) human heart Na+ channels
OOutcomeFast inactivation gating characteristics (time course, extent of inhibition, single-channel open time)surrogate
In human heart Na+ channels, the IFM cluster controls the stability of both open- and closed-channel inactivation, with structural differences likely explaining distinct gating patterns compared to brain channels.