Voltage-dependent open-state inactivation of cardiac sodium channels: gating current studies with Anthopleurin-A toxin.
The gating charge and voltage dependence of the open state to the inactivated state (O-->I) transition was measured for the voltage-dependent mammalian cardiac Na channel. Using the site 3 toxin, Anthopleurin-A (Ap-A), which selectively modifies the O-->I transition (see Hanck, D. A., and M. F. Sheets. 1995. Journal of General Physiology. 106:601-616), we studied Na channel gating currents (Ig) in voltage-clamped single canine cardiac Purkinje cells at approximately 12 degrees C. Comparison of Ig recorded in response to step depolarizations before and after modification by Ap-A toxin showed that toxin-modified gating currents decayed faster and had decreased initial amplitudes. The predominate change in the charge-voltage (Q-V) relationship was a reduction in gating charge at positive potentials such that Qmax was reduced by 33%, and the difference between charge measured in Ap-A toxin and in control represented the gating charge associated with Na channels undergoing inactivation by O-->I. By comparing the time course of channel activation (represented by the gating charge measured in Ap-A toxin) and gating charge associated with the O-->I transition (difference between control and Ap-A charge), the influence of activation on the time course of inactivation could be accounted for and the inherent voltage dependence of the O-->I transition determined. The O-->I transition for cardiac Na channels had a valence of 0.75 e-. The total charge of the cardiac voltage-gated Na channel was estimated to be 5 e-. Because charge is concentrated near the opening transition for this isoform of the channel, the time constant of the O-->I transition at 0 mV could also be estimated (0.53 ms, approximately 12 degrees C). Prediction of the mean channel open time-voltage relationship based upon the magnitude and valence of the O-->C and O-->I rate constants from INa and Ig data matched data previously reported from single Na channel studies in heart at the same temperature.
Sheets
M F
MF
Department of Medicine, Northwestern University Medical School, Chicago, Illinois 60611, USA.
Hanck
D A
DA
D000818:Animals; D002316:Cardiotonic Agents / Q000494:pharmacology*; D004285:Dogs; D004594:Electrophysiology; D036341:Intercellular Signaling Peptides and Proteins; D015640:Ion Channel Gating / Q000187:drug effects* / Q000502:physiology; D018408:Patch-Clamp Techniques; D010455:Peptides / Q000494:pharmacology*; D011690:Purkinje Fibers / Q000187:drug effects* / Q000378:metabolism; D015222:Sodium Channels / Q000187:drug effects* / Q000378:metabolism
D002316:Cardiotonic Agents; D036341:Intercellular Signaling Peptides and Proteins; D010455:Peptides; D015222:Sodium Channels; C012746:anthopleurin-A