Na+ channel kinetics were studied by recording single-channel currents in the cell-attached patch configuration of the patch-clamp technique in single ventricular cells isolated from guinea pig hearts. The inactivation time course of ensemble currents was accelerated, and the peak amplitude increased temporarily and then decreased within a few minutes after the gigaohm seal formation. After reaching a new steady state, the inactivation-voltage relation was found to have shifted to more negative potentials. The potential of half-maximal inactivation was more negative by 20-31 mV from the resting potential or between -96 and -112 mV. The voltage dependency of the channel activation also shifted. Although the cell membrane was depolarized using the whole cell patch-clamp electrode and single-channel currents were recorded with an independent cell-attached electrode, the shift of the inactivation curve was also evident. Complete removal of Ca2+ using 5 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid in the pipette solution failed to prevent the shift. Increasing Ca2+ to 10 mM, however, reduced magnitude of the shift significantly. Involvement of an increased membrane fluidity and surface potential of the glass pipette to the shift is discussed.
Kimitsuki et al. (1990) studied this question.