Background-Mutations in the gene encoding the human cardiac Na+ channel alp
ha-subunit (hH1) are responsible for chromosome 3-linked congenital long-QT
syndrome (LQT3) and idiopathic ventricular fibrillation (IVF). An auxiliar
y beta(1)-subunit, widely expressed in excitable tissues, shifts the voltag
e dependence of steady-state inactivation toward more negative potentials a
nd restores normal gating kinetics of brain and skeletal muscle Na+ channel
s expressed in Xenopus oocytes but has little if any functional effect on t
he cardiac isoform. Here, we characterize the altered effects of a human be
ta(1)-subunit (h beta(1)) on the heterologously expressed hH1 mutation (T16
20M) previously associated with IVF.
Methods and Results-When expressed alone in Xenopus oocytes, T1620M exhibit
ed no persistent currents, in contrast to the LQT3 mutant channels, but the
midpoint of steady-state inactivation (V-1/2) was significantly shifted to
ward more positive potentials than for wild-type hH1. Coexpression of h bet
a 1 did not significantly alter current decay or recovery from inactivation
of wild-type hH1; however, it further shifted the V-1/2 and accelerated th
e recovery from inactivation of T1620M. Oocyte macropatch analysis revealed
that the activation kinetics of T1620M were normal.
Conclusions-It is suggested that coexpression of h beta(1) exposes a more s
evere functional defect that results in a greater overlap in the relationsh
ip between channel inactivation and activation (window current) in T1620M,
which is proposed to be a potential pathophysiological mechanism of IVF in
vivo. One possible explanation for our finding is an altered alpha-/beta(1)
-subunit association in the mutant.