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This Article Full Text Full Text (PDF) All Versions of this Article: 563/1/83    most recent jphysiol.2004.077644v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Sheets, M. F Articles by Hanck, D. A Search for Related Content PubMed PubMed Citation Articles by Sheets, M. F Articles by Hanck, D. A

¹ The Nora Eccles Harrison Cardiovascular Research & Training Institute and Department of Internal Medicine, University of Utah, Salt Lake City, UT 84112, USA
² Department of Medicine, University of Chicago, Chicago, IL 60637, USA

Recovery from fast inactivation in voltage-dependent Na⁺ channels is associated with a slow component in the time course of gating charge during repolarization (i.e. charge immobilization), which results from the slow movement of the S4 segments in domains III and IV (S4-DIII and S4-DIV). Previous studies have shown that the non-specific removal of fast inactivation by the proteolytic enzyme pronase eliminated charge immobilization, while the specific removal of fast inactivation (by intracellular MTSET modification of a cysteine substituted for the phenylalanine in the IFM motif, ICM_MTSET, in the inactivation particle formed by the linker between domains III and IV) only reduced the amount of charge immobilization by nearly one-half. To investigate the molecular origin of the remaining slow component of charge immobilization we studied the human cardiac Na⁺ channel (hH1a) in which the outermost arginine in the S4-DIV, which contributes 20% to total gating charge (Q_max), was mutated to a cysteine (R1C-DIV). Gating charge could be fully restored in R1C-DIV by exposure to extracellular MTSEA, a positively charged methanethiosulphonate reagent. The RIC-DIV mutation was combined with ICM_MTSET to remove fast inactivation, and the gating currents of R1C-DIV-ICM_MTSET were recorded before and after modification with MTSEA_o. Prior to MTSEA_o, the time course of the gating charge during repolarization (OFF-charge) was best described by a single fast time constant. After MTSEA, the OFF-charge had both fast and slow components, with the slow component accounting for nearly 35% of Q_max. These results demonstrate that the slow movement of the S4-DIV during repolarization is not dependent upon the normal binding of the inactivation particle.

(Received 18 October 2004; accepted after revision 1 December 2004; first published online 2 December 2004)
Corresponding author M. F. Sheets: CVRTI, Bldg 500, 95 South 2000 East, University of Utah, Salt Lake City, UT 84112, USA. Email: michael{at}cvrti.utah.edu

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