title: The delayed rectifier potassium conductance in the sarcolemma and the transverse tubular system membranes of mammalian skeletal muscle fibers.
abstract: A two-microelectrode voltage clamp and optical measurements of membrane potential changes at the transverse tubular system (TTS) were used to characterize delayed rectifier K currents (IK(V)) in murine muscle fibers stained with the potentiometric dye di-8-ANEPPS. In intact fibers, IK(V) displays the canonical hallmarks of K(V) channels: voltage-dependent delayed activation and decay in time. The voltage dependence of the peak conductance (gK(V)) was only accounted for by double Boltzmann fits, suggesting at least two channel contributions to IK(V). Osmotically treated fibers showed significant disconnection of the TTS and displayed smaller IK(V), but with similar voltage dependence and time decays to intact fibers. This suggests that inactivation may be responsible for most of the decay in IK(V) records. A two-channel model that faithfully simulates IK(V) records in osmotically treated fibers comprises a low threshold and steeply voltage-dependent channel (channel A), which contributes ∼31% of gK(V), and a more abundant high threshold channel (channel B), with shallower voltage dependence. Significant expression of the IK(V)1.4 and IK(V)3.4 channels was demonstrated by immunoblotting. Rectangular depolarizing pulses elicited step-like di-8-ANEPPS transients in intact fibers rendered electrically passive. In contrast, activation of IK(V) resulted in time- and voltage-dependent attenuations in optical transients that coincided in time with the peaks of IK(V) records. Normalized peak attenuations showed the same voltage dependence as peak IK(V) plots. A radial cable model including channels A and B and K diffusion in the TTS was used to simulate IK(V) and average TTS voltage changes. Model predictions and experimental data were compared to determine what fraction of gK(V) in the TTS accounted simultaneously for the electrical and optical data. Best predictions suggest that K(V) channels are approximately equally distributed in the sarcolemma and TTS membranes; under these conditions, >70% of IK(V) arises from the TTS.
authors.affiliation: Department of Physiology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, CA 90095, USA.
authors.forename: Marino
authors.initials: M
authors.lastname: DiFranco
authors.forename: Marbella
authors.initials: M
authors.lastname: Quinonez
authors.forename: Julio L
authors.initials: JL
authors.lastname: Vergara
mesh_terms: D000818:Animals; D051668:Delayed Rectifier Potassium Channels / Q000378:metabolism*; D051665:Kv1.4 Potassium Channel / Q000378:metabolism; D008564:Membrane Potentials; D051379:Mice; D008810:Mice, Inbred C57BL; D008954:Models, Biological; D018485:Muscle Fibers, Skeletal / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D009997:Osmotic Pressure; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism; D011726:Pyridinium Compounds; D012508:Sarcolemma / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D051669:Shab Potassium Channels / Q000378:metabolism; D051670:Shaw Potassium Channels / Q000378:metabolism; D056969:Voltage-Sensitive Dye Imaging
chemical_list: C089669:1-(3-sulfonatopropyl)-4-(beta-(2-(di-n-octylamino)-6-naphthyl)vinyl)pyridinium betaine; D051668:Delayed Rectifier Potassium Channels; C496990:Kcnb1 protein, mouse; C115399:Kcnc4 protein, mouse; D051665:Kv1.4 Potassium Channel; D011726:Pyridinium Compounds; D051669:Shab Potassium Channels; D051670:Shaw Potassium Channels; D011188:Potassium