Hypokalemic periodic paralysis and
normokalemic periodic paralysis are caused by mutations of the gating charge-carrying
arginine residues in skeletal muscle Na(V)1.4 channels, which induce gating pore current through the mutant voltage sensor domains. Inward
sodium currents through the gating pore of mutant R666G are only approximately 1% of central pore current, but substitution of
guanidine for
sodium in the extracellular
solution increases their size by 13- +/- 2-fold. Ethylguanidine is permeant through the R666G gating pore at physiological membrane potentials but blocks the gating pore at hyperpolarized potentials.
Guanidine is also highly permeant through the
proton-selective gating pore formed by the mutant R666H. Gating pore current conducted by the R666G mutant is blocked by
divalent cations such as
Ba(2+) and Zn(2+) in a voltage-dependent manner. The affinity for voltage-dependent block of gating pore current by
Ba(2+) and Zn(2+) is increased at more negative holding potentials. The apparent dissociation constant (K(d)) values for Zn(2+) block for test pulses to -160 mV are 650 +/- 150 microM, 360 +/- 70 microM, and 95.6 +/- 11 microM at holding potentials of 0 mV, -80 mV, and -120 mV, respectively. Gating pore current is blocked by trivalent
cations, but in a nearly voltage-independent manner, with an apparent K(d) for
Gd(3+) of 238 +/- 14 microM at -80 mV. To test whether these periodic
paralyses might be treated by blocking gating pore current, we screened several aromatic and aliphatic
guanidine derivatives and found that 1-(2,4-xylyl)guanidinium can block gating pore current in the millimolar concentration range without affecting normal Na(V)1.4 channel function. Together, our results demonstrate unique permeability of
guanidine through Na(V)1.4 gating pores, define voltage-dependent and voltage-independent block by divalent and trivalent
cations, respectively, and provide initial support for the concept that
guanidine-based gating pore blockers could be therapeutically useful.