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1 Neurology and GI CEDD
2 Genetics Research, GlaxoSmithKline Research and Development Ltd, New Frontiers Science Park (North), Harlow, Essex CM19 5AW, UK
The rhabdomyosarcoma cell line (SJ-RH30) exhibits both ultrastructural and electrophysiological hallmarks of mammalian skeletal muscle. We have used patch-clamp electrophysiology to study acid-gated currents in these cells. At a holding potential of –60 mV, rapid application of extracellular solutions of pH 6.5 produced inward current responses in 
85% of cells. The amplitude of these responses exhibited a marked pH dependence. In addition, the kinetics of both activation and desensitization were faster at more acidic pH, whereas the deactivation rate was pH independent. Repeated applications of a pH 6.0 solution produced a tachyphalaxis that could be substantially attenuated by reducing the duration of the acid challenge and increasing the interstimulus interval. The current–voltage relationship of the acid-induced currents was linear at positive potentials but an area of negative slope conductance was observed in the negative potential range. This was not eliminated by removal of extracellular Ca2+, a manoeuvre which did, however, substantially increase current amplitude. Changing the transmembrane Na+ gradient altered the current–voltage relationship in a fashion commensurate with an underlying conductance predominantly permeable to Na+. Pharmacologically, acid-induced currents were blocked 84.4 ± 1.2% by 30 µM amiloride and 91.8 ± 3.0% by a 1 : 1000 dilution of Psalmopoeus
cambridgei venom. Inhibition by both agents could be reversed by a short period of compound washout. By contrast, APETx2 had no significant effect on acid-evoked currents. These observations strongly suggest the acid-induced current is mediated by ASIC1 channels. In agreement with this, current responses of SJ-RH30 cells to a pH 6.0 challenge were greatly enhanced by extracellular lactate. These data demonstrate the presence of ASIC1 channels in a cell line with skeletal muscle characteristics. In addition, significant levels of ASIC1 and ASIC3 mRNA were found in skeletal muscle tissue samples. These findings are discussed with regard to the role such a conductance might play if present in skeletal muscle in vivo.
(Received 23 November 2004;
accepted after revision 26 November 2004;
first published online 2 December 2004)
Corresponding author D. P. Gitterman: Neurology and GI CEDD, GlaxoSmithKline Research and Development Ltd, New Frontiers Science Park (North), Harlow, Essex CM19 5AW, UK. Email: daniel_p_gitterman{at}gsk.com
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