Ca channels of hair cells of frog sernicircular canal

Ca channels of frog semicircular canal haìr cells were studied by using the whole cell patch-clamp technique in mechanically isolated cells. Depolarization to -20, mV activated two Ca currents (4 mM Caout). One (A) had a mean activation time constant (T) of 0.5 ms and an amplitude of 130 pA; the other one (B) had an activation T smaller than or similar to A, peaked at 30 pA and inactivated with a T of 10 ms. The activation threshord was -60 mV for A and -40 mV for B; recovery from inactivation for B had a T of 100 ms at -120 mV and, 300 ms at -70 mV. B run down faster than A during whole-cell recording; the protesse inhibitor calpastatin prevented the run-down of both channels. The current-to-voltage relationship of A peaked at -20 mV, with a Vrev of +40 mV. The small value of Vrev was due to an outward Cs current flowing throngh A at positive potentials. In some cells the A current amplitude increased more than two fold on repeating the stimulating protocol. This increase was probably due to the activation of silent As through a run-up mechanism not involving a cGMP or cAMP dependent phosphorylation. Theoretical considerations demonstrated the impossibility to attain a control, by means of the patch pipette, over the temporal and spatial changes of Cai produced by the Ca inflow. Thus, it was impossible to assess the Ca-dependency of B inactivation and the current run-up mechanisms. A, may sustain the spontaneous activity, and its modulation may be important in the response to slow and prolonged stimoli, whereas B may be important in producing fast transmitter release in response to strong stimuli.