title: SkM2, a Na+ channel cDNA clone from denervated skeletal muscle, encodes a tetrodotoxin-insensitive Na+ channel.
abstract: Approximately one third of the Na+ channels expressed in denervated or developing skeletal muscle are tetrodotoxin (TTX) insensitive, with a Kd for channel blockade of approximately 1 microM, similar to that found for cardiac Na+ channels. We have recently reported the cloning of a putative Na+ channel subtype that is characteristic of denervated and developing skeletal muscle (SkM2), the deduced amino acid sequence of which is identical to that of a Na+ channel cDNA isolated from heart. We have now examined the functional properties of SkM2 Na+ channels after expression in Xenopus oocytes. We found that the efficiency of expression of constructs containing the SkM2 clone was strongly dependent on the amount of 5'-untranslated region (5'UTR) included. Constructs containing a 206-nucleotide 5'UTR were expressed poorly, whereas constructs from which most of the 5'UTR was removed were expressed well. The channels showed rapid voltage-dependent activation and inactivation. In addition, SkM2 Na+ channels were insensitive to low concentrations of TTX but were ultimately blocked by this toxin, with a Kd of 1.9 microM. The TTX block exhibited use dependence. Finally, SkM2 Na+ channels were not blocked by 100 nM mu-conotoxin, which blocks Na+ channels in innervated skeletal muscle in the low nanomolar concentration range. These data indicate that SkM2 Na+ channels are the TTX-insensitive Na+ channels found in denervated or developing skeletal muscle and are identical to the TTX-insensitive Na+ channels from heart.
authors.affiliation: David Mahoney Institute of Neurological Sciences, University of Pennsylvania School of Medicine, Philadelphia 19104.
authors.forename: M M
authors.initials: MM
authors.lastname: White
authors.forename: L Q
authors.initials: LQ
authors.lastname: Chen
authors.forename: R
authors.initials: R
authors.lastname: Kleinfield
authors.forename: R G
authors.initials: RG
authors.lastname: Kallen
authors.forename: R L
authors.initials: RL
authors.lastname: Barchi
mesh_terms: D000818:Animals; D001483:Base Sequence; D003001:Cloning, Molecular; D020916:Conotoxins; D004247:DNA / Q000235:genetics*; D005260:Female; D008969:Molecular Sequence Data; D009132:Muscles / Q000294:innervation / Q000502:physiology* / Q000648:ultrastructure; D009206:Myocardium / Q000378:metabolism; D009865:Oocytes / Q000502:physiology; D010456:Peptides, Cyclic / Q000494:pharmacology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D015222:Sodium Channels / Q000187:drug effects / Q000235:genetics*; D013779:Tetrodotoxin / Q000494:pharmacology*; D014982:Xenopus laevis
chemical_list: D020916:Conotoxins; D010456:Peptides, Cyclic; D012333:RNA, Messenger; D015222:Sodium Channels; C046101:conotoxin GIII; D013779:Tetrodotoxin; D004247:DNA