We characterized the hypokalemic periodic paralysis mutation R1129Q in domain III of the voltage-gated sodium channel of skeletal muscle. This mutation was constructed in plasmid sP64T containing SCN4A and in the same construct incorporating the 1303IFM/QQQ, which prohibits fast inactivation of the channel. Messenger RNA was injected in Xenopus oocytes for characterization with electrophysiology. We used the IFM/QQQ background to isolate activation, to determine deactivation kinetics, and in measurements of slow inactivation. Biophysical parameters of equilibrium and kinetics were obtained in the cut-open oocyte configuration to characterize mutational perturbation. Steady-state and kinetic gating parameters for activation fast inactivation and slow inactivation were determined. R1129Q produced loss of function effects through its impact on sodium channel inactivation. The mutation produced a hyperpolarizing shift in the midpoints of the steady-state fast and slow inactivation curves, and decreased recovery from each of these forms of inactivation. Activation probability and kinetics were less impacted by R1129Q or R1129Q/QQQ. Leak current was increased at hyperpolarized potentials. This work was supported by an Institutional Development Award (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health under Grant #P20GM103408, and by an ISU HERCFY24 award to JRG.
James R. Groome (Sun,) studied this question.