THE EQUINE PERIODIC PARALYSIS NA-TRANSITIONS BETWEEN THE OPEN AND INACTIVATED STATES( CHANNEL MUTATION ALTERS MOLECULAR)

1. The Na+ channel mutation associated with equine hyperkalaemic perio dic paralysis (HPP) affects it highly conserved phenylalanine residue in an unexplored region of the alpha-subunit. This mutation was introd uced into the rat skeletal muscle Na+ channel gene at the correspondin g location (i.e. F1412L) for functional expression and characterizatio n in Xenopus oocytes. 2. In comparison with wild-type (WT) channels, e quine HPP channels showed clear evidence for disruption of inactivatio n: increased time-to-peak current, slowed rates of whole-cell current decay, significant increases in sustained current, rightward shifts in the steady-state inactivation curve by 9.5 mV, a 6-fold acceleration in the rate of recovery from inactivation at -80 mV, decreased number of blank single-channel sweeps, repetitive opening of single channels throughout depolarizing steps, increased open probability per sweep, a nd an increased mean open time. 3. The observed disruption of inactiva tion in HPP occurred without measurable changes in steady-state activa tion and first latency kinetics of channel opening. 4. Kinetic modelli ng demonstrates that the equine HPP phenotype can be simulated by alte ring the rate constants for transitions entering and leaving the inact ivated states resulting from an energetic destabilization of the inact ivated state. 5. These results suggest that the highly conserved cytop lasmic end of the third transmembrane segment (S3) in the fourth inter nal repeat domain (domain IV) plays a critical role in Na+ channel ina ctivation.