title: Voltage-gated K+ currents in mouse articular chondrocytes regulate membrane potential.
abstract: Membrane currents and resting potential of isolated primary mouse articular chondrocytes maintained in monolayer cell culture for 1-9 days were recorded using patch clamp methods. Quantitative RT-PCR showed that the most abundantly expressed transcript of voltage-gated K(+) channels was for K(V)1.6, and immunological methods confirmed the expression of K(V)1.6 α-subunit proteins. These chondrocytes expressed a large time- and potential-dependent, Ca(2+)-independent 'delayed rectifier' K(+) current. Steady-state activation was well-fit by a Boltzmann function with a threshold near -50 mV, and a half-activation potential of -34.5 mV. The current was 50% blocked by 1.48 mM tetraethylammonium, 0.66 mM 4-aminopyridine and 20.6 nM α-dendrotoxin. The current inactivated very slowly at membrane potentials in the range of the resting potential of the chondrocytes. Resting membrane potential of the chondrocytes at room temperature (19-21°C) and in 5 mM external K(+) was -46.4 ± 1.3 mV (mean ± s.e.m; n = 23), near the 'foot' of the activation curve of this K(+) current. Resting potential was depolarized by an average of 4.2 ± 0.8 mV by 25 mM TEA, which blocked about 95% of the K(+) current. At a membrane potential of -50 mV, the apparent time constant of inactivation (tau(in)) was 37.9 s, and the 'steady-state' current level was 19% of that at a holding potential of -90 mV; at -40 mV, tau(in) was 20.3 s, and 'steady-state' current was 5% of that at -90 mV. These results demonstrate that in these primary cultured, mouse articular chondrocytes steady-state activation of a voltage-gated K(+) current contributes to resting membrane potential. However, this current is also likely to have a significant physiological role in repolarizing the chondrocyte following depolarizing stimuli that might occur in conditions of membrane stretch. For example, activation of TRP('transient receptor potential') non-specific cation channels in these cells during cyclic loading and unloading of the joint cartilage, or in response to hypertonic challenge is expected to result in depolarization and Ca(2+) entry. Potassium currents are required to maintain the resting membrane potential.
authors.affiliation: Faculty of Kinesiology, University of Calgary, AB, Canada.
authors.forename: Robert B
authors.initials: RB
authors.lastname: Clark
authors.forename: Noriyuki
authors.initials: N
authors.lastname: Hatano
authors.forename: Colleen
authors.initials: C
authors.lastname: Kondo
authors.forename: Darrell D
authors.initials: DD
authors.lastname: Belke
authors.forename: Barry S
authors.initials: BS
authors.lastname: Brown
authors.forename: Sanjay
authors.initials: S
authors.lastname: Kumar
authors.forename: Bartholomew J
authors.initials: BJ
authors.lastname: Votta
authors.forename: Wayne R
authors.initials: WR
authors.lastname: Giles
mesh_terms: D000818:Animals; D002358:Cartilage, Articular / Q000166:cytology*; D002478:Cells, Cultured; D019902:Chondrocytes / Q000502:physiology*; D051667:Kv1.6 Potassium Channel / Q000235:genetics / Q000502:physiology; D008564:Membrane Potentials; D051379:Mice; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism / Q000502:physiology; D024642:Potassium Channels, Voltage-Gated / Q000235:genetics / Q000502:physiology*; D012333:RNA, Messenger / Q000032:analysis
chemical_list: D051667:Kv1.6 Potassium Channel; D024642:Potassium Channels, Voltage-Gated; D012333:RNA, Messenger; D011188:Potassium