title: Voltage-dependent Ca2+ fluxes in skeletal myotubes determined using a removal model analysis.
abstract: The purpose of this study was to quantify the Ca2+ fluxes underlying Ca2+ transients and their voltage dependence in myotubes by using the "removal model fit" approach. Myotubes obtained from the mouse C2C12 muscle cell line were voltage-clamped and loaded with a solution containing the fluorescent indicator dye fura-2 (200 microM) and a high concentration of EGTA (15 mM). Ca2+ inward currents and intracellular ratiometric fluorescence transients were recorded in parallel. The decaying phases of Ca2+-dependent fluorescence signals after repolarization were fitted by theoretical curves obtained from a model that included the indicator dye, a slow Ca2+ buffer (to represent EGTA), and a sequestration mechanism as Ca2+ removal components. For each cell, the rate constants of slow buffer and transport and the off rate constant of fura-2 were determined in the fit. The resulting characterization of the removal properties was used to extract the Ca2+ input fluxes from the measured Ca2+ transients during depolarizing pulses. In most experiments, intracellular Ca2+ release dominated the Ca2+ input flux. In these experiments, the Ca2+ flux was characterized by an initial peak followed by a lower tonic phase. The voltage dependence of peak and tonic phase could be described by sigmoidal curves that reached half-maximal activation at -16 and -20 mV, respectively, compared with -2 mV for the activation of Ca2+ conductance. The ratio of the peak to tonic phase (flux ratio) showed a gradual increase with voltage as in rat muscle fibers indicating the similarity to EC coupling in mature mammalian muscle. In a subgroup of myotubes exhibiting small fluorescence signals and in cells treated with 30 microM of the SERCA pump inhibitor cyclopiazonic acid (CPA) and 10 mM caffeine, the calculated Ca2+ input flux closely resembled the L-type Ca2+ current, consistent with the absence of SR Ca2+ release under these conditions and in support of a valid determination of the time course of myoplasmic Ca2+ input flux based on the optical indicator measurements.
authors.affiliation: Universität Ulm, Abteilung für Angewandte Physiologie Albert-Einstein-Allee 11, Germany.
authors.forename: R P
authors.initials: RP
authors.lastname: Schuhmeier
authors.forename: W
authors.initials: W
authors.lastname: Melzer
mesh_terms: D000818:Animals; D002118:Calcium / Q000493:pharmacokinetics*; D015220:Calcium Channels / Q000502:physiology*; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D004594:Electrophysiology; D005456:Fluorescent Dyes / Q000493:pharmacokinetics; D016257:Fura-2 / Q000493:pharmacokinetics; D007700:Kinetics; D051379:Mice; D008962:Models, Theoretical; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D018408:Patch-Clamp Techniques; D051381:Rats
chemical_list: D015220:Calcium Channels; D002614:Chelating Agents; D005456:Fluorescent Dyes; D004533:Egtazic Acid; D002118:Calcium; D016257:Fura-2