Mbrane, activated by person action potentials in mammalian skeletal muscleBradley S. Launikonis1,2 , D. George Stephenson3 and Oliver Friedrich1School of Biomedical Sciences, University of Queensland, Brisbane, Queensland, Australia Section of Cellular Signaling, Division of Molecular Biophysics and Physiology, Rush University Healthcare Center, Chicago, IL, USA 3 Division of Zoology, La Trobe University, Melbourne, Victoria, AustraliaPeriods of low frequency stimulation are identified to enhance the net Ca2 uptake in skeletal muscle but the mechanism responsible for this Ca2 entry isn’t known. In this study a novel highresolution fluorescence microscopy approach permitted the detection of an action potentialinduced Ca2 flux across the tubular (t) system of rat extensor digitorum longus muscle fibres that seems to be accountable for the net uptake of Ca2 in working muscle. Action potentials were triggered within the tsystem of mechanically skinned fibres from rat by brief field stimulation and tsystem [Ca2 ] ([Ca2 ] tsys ) and cytoplasmic [Ca2 ] ([Ca2 ] cyto ) had been simultaneously resolved on a confocal microscope. When initial [Ca2 ] tsys was 0.2 mm a Ca2 flux from tsystem for the cytoplasm was observed following a single action possible. The action potentialinduced Ca2 flux and related tsystem Ca2 permeability decayed exponentially and displayed inactivation characteristics such that additional Ca2 entry across the tsystem could not be observed following two action potentials at ten Hz stimulation rate. When [Ca2 ] tsys was closer to 0.1 mm, a transient rise in [Ca2 ] tsys was observed virtually concurrently with the raise in [Ca2 ] cyto following the action possible. The transform in direction of Ca2 flux was consistent with alterations in the direction of the driving force for Ca2 . This can be the very first demonstration of a rapid tsystem Ca2 flux associated having a single action possible in mammalian skeletal muscle. The properties of this Phytosphingosine In stock channel are inconsistent using a flux via the Ltype Ca2 channel suggesting that an as but unidentified tsystem protein is conducting this existing. This action potentialactivated Ca2 flux delivers an explanation for the previously described Ca2 entry and accumulation observed with prolonged, intermittent muscle activity.(Received 6 January 2009; accepted after revision 25 March 2009; very first published on line 30 March 2009) Corresponding author B. S. Launikonis: College of Biomedical Sciences, University of Queensland, Brisbane, Qld 4072, Australia. Email: [email protected] Web site: http://profiles.bacs.uq.edu.au/Bradley.Launikonis.htmlCa2 entry into cells is a basic process to regulate cytoplasmic [Ca2 ], [Ca2 ] in intracellular shops and quite a few Ca2 dependent intracellular processes from gene expression to muscle contraction (Berchtold et al. 2000). Cardiac cells have an absolute requirement for Ca2 entry via the Ltype Ca2 channel upon excitation to induce Ca2 release from the sarcoplasmic reticulum (SR) and consequently activate the contractile apparatus. A further isoform in the Ltype Ca2 channel also exists in skeletal muscle ( 1s on the dihydropyridine receptor (DHPR)) however the duration of membrane depolarization in the course of a single action prospective in skeletal fibres is as well brief (2 ms), when compared with that in cardiomyocytes (10050 ms), to activate this channel to any detectable degree. Instead,Cthe 1s subunit from the Ltype Ca2 channel in skeletal muscle acts as a voltage sensor, which straight activates Ca2 rele.
