Comparison of aconitine-modified human heart (hH1) and rat skeletal (mu1) muscle Na+ channels: an important role for external Na+ ions.

Neurotoxins such as aconitine (AC) bind to receptor site 2 on voltage-gated sodium channels and modify channel kinetics. Although AC modification typically induces hyperpolarizing shifts in sodium channel activation, the effects on channel inactivation seem to vary depending on the tissue origin of the channel. In the present study, the alpha subunits of human heart (hH1) and rat skeletal muscle (mu1) sodium channels were transiently expressed in human embryonic kidney (HEK293t) cells. Whole-cell currents were examined before and after AC modification of the channels to determine whether the toxin had isoform-specific effects on channel kinetics. The magnitudes of the hyperpolarizing shifts in steady-state current activation and inactivation were similar for AC-modified hH1 and mu1 channels, and AC modification did not alter the voltage dependence of macroscopic current decay of either channel subtype. There were two notable differences between hH1 and mu1 channels after AC modification. First, the steady-state availability of AC-modified mu1 channels decreased by 5-10% after very negative conditioning pulses. Second, AC-modified mu1 channels inactivated completely at all voltages, whereas AC-modified hH1 channels exhibited sustained inward currents at voltages near the threshold of current activation. Interestingly, AC-modified hH1 channels inactivated completely if the external solution did not contain sodium ions. The data demonstrate that AC modification affects the activation of hH1 and mu1 channels similarly but affects inactivation of the two channels distinctly. The results also imply that the reduced inactivation of AC-modified hH1 channels at least partially depends on the presence of extracellular sodium.