Voltage-gated K+ currents in mouse articular chondrocytes regulate membrane potential

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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 Kv1.6, and immunological methods confirmed the expression of Kv1.6 a-subunit proteins. These chondrocytes expressed a large time- and potential-dependent, Ca2+-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 -35.0 mV. The current was 50% blocked by 1.48 mM tetraethylammonium, 0.66 mM 4-aminopyridine, and 20.6 nM a-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 oC) 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 (tauin) was 37.9 s, and the 'steady-state' current level was 19% of that at a holding potential of -90 mV; at -40 mV, tauin 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 Ca2+ entry. Potassium currents are required to maintain the resting membrane potential.