A voltage- and Ca2+-dependent big conductance K channel in cochlear spiral ligament fibrocytes.

Evidence is accruing that spiral ligament fibrocytes (SLFs) play an important role in cochlear K+ homeostasis, but little direct physiological data is available to support this concept. Here we report the presence and characterization of a voltage- and Ca2+-dependent big-conductance K (BK) channel in type I SLFs cultured from the gerbil cochlea. A single-channel conductance of 298±5.6 pS (n=28) was measured under symmetrical K+. Membrane potentials for half-maximal open probability (P o) were −67, −45 and 85 mV with cytosolic free-Ca2+ levels of 0.7 mM, 10 μM and 1 μM, respectively (n=8–14). The Hill coefficient for Ca2+ affinity was 1.9 at a membrane potential of 60 mV (n=6). The BK channel showed very low activity (P o=0.0019, n=5) under normal physiological conditions, suggesting a low resting intracellular free [Ca2+]. Pharmacological results fit well with the profile of classic BK channels. The estimated half-maximal inhibitory concentration and Hill coefficient for tetraethylammonium were 0.086±0.021 mM and 0.99, respectively (n=4–9). In whole cell recordings, the voltage-activated outward K current was inhibited 85.7±4.5% (n=6) by 0.1 μM iberiotoxin. A steady-state kinetic model with two open and two closed stages best described the BK gating process (τo1 0.23±0.08 ms, τo2 1.40±0.32 ms; τc1 0.26±0.09 ms, τc2 3.10±1.2 ms; n=11). RT-PCR analyses revealed a splice variant of the BK channel α subunit in cultured type I SLFs and freshly isolated spiral ligament tissues. The BK channel is likely to play a major role in regulating the membrane potential of type I SLFs, which may in turn influence K+ recycling dynamics in the mammalian cochlea.