Voltage-gated ionic channels in cultured rabbit articular chondrocytes

The membrane properties of cultured cells of rabbit articular chondrocytes were studied using the whole-cell patch clamp technique. The average cell capacitance was 37.9 +/- 9.0 pF (n = 13), and the cell resting potential was -41.0 +/- 7.0 mV (n = 11). We were unable to induce an action potential by applying a depolarizing current. Upon step depolarization, under voltage clamp conditions, one kind of inward and two kinds of outward currents were elicited. The inward current was initially observed at around -30 mV, peaked at 0 mV, and reversed at around +90 mV. Tetrodotoxin (TTX; 1 microM) was shown to completely block this inward current. At steady state, the inward current was half-inactivated at -51 mV, with a slope factor of 6.3 mV. Two outward currents were determined from measurements of activation threshold, reversal potential, and pharmacological responses. One was observed at around -30 mV, and its amplitude increased with membrane depolarization. Extracellularly applied 4-aminopyridine (4 AP) (1 mM) and tetraethyl ammonium chloride (TEA) (5 mM) blocked this current. The other outward current was observed at around +10 mV, and its direction reversed at a potential close to that predicted by the Nernst equation for a Cl- selective channel. This current fluctuated markedly, and the fluctuation did not decline throughout the 100 ms of the step pulse. Extracellularly applied 4-acetamido-4'-isothiocyanostilbenezene-2,2-disulfonic acid (SITS) (0.25 mM) blocked this current, but the same dose of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) had little effect. These results suggest the presence of TTX-sensitive Na+, 4-AP- and TEA-sensitive K+, and SITS-sensitive Cl- channels in rabbit articular chondrocyte membrane. The functional significance of these channels is discussed.