Impaired slow inactivation due to a polymorphism and substitutions of Ser-906 in the II-III loop of the human Nav1.4 channel

The loop connecting domains II and III of the sodium channel alpha-subunit is not known to have a major effect on channel gating. Recently mutations in the II-III loop of various sodium channel isoforms have been reported to cause channelopathies suggesting the functional importance of this region. In the II-III loop of the skeletal muscle isoform Na(v)1.4, we found a Ser-to-Thr substitution at position 906 in 5% of patients with dyskalemic periodic paralysis but also in 4% of healthy human individuals. To investigate whether this position is important for channel gating, we characterized the following amino acids at 906 by whole-cell patch-clamp experiments: Gln, Ser, Thr, Cys, Pro, Val, ordered according to their hydrophobicity. All substitutions mainly affected slow inactivation. For example, Gln caused a +13-mV right-shift of the steady-state slow inactivation curve, and entry into slow inactivation was 6 times slower compared with Ser, leading to a destabilization of the slow inactivated state; in contrast, Val, at the other end of the hydrophobicity spectrum, shifted the steady-state slow inactivation curve by -6 mV and slowed the recovery from the slow inactivated state threefold compared with Ser, resulting in an enhancement of slow inactivation. Recovery from the slow inactivated state was also slowed by Pro, Cys and Thr. Our results suggest that (1) a benign polymorphism affects channel function, (2) the II-III loop is important for slow inactivation, and (3) the effects on slow inactivation may depend on the hydrophobicity of the residue at position 906.