[Extracellular calcium modulates the whole cell potassium currents in Deiters cells isolated from guinea pig cochlea]

To study the modulatory effect of extracellular calcium on the whole cell K(+) currents (I(K)) in isolated Deiters cells, the whole cell K(+) currents were recorded when Deiters cells bathed in normal physiological solutions and calcium-free saline, respectively. The electrophysiological characteristics of I(K) currents were then analyzed with the patch clamp technique. Removing extracellular calcium significantly enhanced the amplitude of the I(K) currents, which increased by 70.2% at +50 mV test pulse. The chord conductance, measured at -30 mV test pulse, also significantly increased from (3.31-/+3.08) ns (n=42) in the normal solutions to (10.81-/+6.01) ns (n=42) in the calcium-free solutions, whereas, the zero current potential of the I(K) currents remained unchanged. In calcium-free solutions, the reversal potential of the I(K) currents was shifted to the direction of hyperpolarization, which was very close to the equilibrium K(+) potential based on the Nernst equation. In addition, both the steady state activation curve and the half activation potential, with the averaged value at (-10.13-/+5.64) mV (n=42), were shifted to the negative. However, the tendency for activation (slope conductances) was the same as that in the normal solutions. Interestingly, both the I-V and the G-V functions deduced from the calcium-inhibited K(+) currents in Deiters cells were "S" shape, implying that at least two different kinds of K(+) conductance were involved in this calcium-inhibited K(+) currents. In summary, we hypothesize that there are two mechanisms for this modulation: one is that the I(K) channels in Deiters cells containing a specific calcium sensitive domain, by which extracellular calcium modulates the structure of the K(+) channels and then the I(K) currents; the other is a novel double gated K(+) channel or an ionotropic receptor coupled to K(+) channels or a new subtype of outward K(+) channels. Removing extracellular calcium activates this novel conductance and then modulates the I(K) currents. These results indicate that a decrease in extracellular calcium not only facilitates the efflux of K(+) out of Deiters cells but also accelerates the repolorization by enhancing the I(K) currents, which in turn can effectively buffer the K(+) concentration around the outer hair cells and maintain the resting membrane potential of Deiters cells.