Voltage and ATP regulation of Ca2 channels of vestibular hair cells

The regulation of the basolateral voltage-gated Ca2+ channels of frog vestibular hair cells was studied in whole-cell using 40 ms pulses to -20 mV. With 1 mM ATP in the pipette solution, about 60% of the cells displayed a Ca2+ current formed an L and a drug-resistant (R2) component, while the remaining 40% exhibited an additional drug resistant fraction (R1) showing a Ca2+-dependent inactivation. When ATP was raised to 10 mM, the R1 component was progressively enhanced as intracellular ATP equilibrated with the pipette solution. and became apparent in all recordings. In cells having the R1 component, 10 mM ATP increased the R1 and L fraction by about 280% and 70%, respectively. Cells initially lacking the R1 current had a similar increase in the L component, while the R1 fraction raised from 0 to about 25pA. Irrespective of the ATP levels, the addition of 1 mM cGMP or cAMP to the pipette solution did not cause any further current increase. thus excluding a channel modulation via a cyclic nucleotide dependent phosphorylation. Depolarizations longer than 1 s produced a progressive reversible current decay to a plateau level, usually outward for positive potentials. This decay was greatly reduced upon substituting internal Cs with NMG, showing that it was generated by a Cs outflow. The repetitive, fast application of 200 microM Cd during the current decay did not completely cancelled the current, but only a fraction whose envelope had a waveform similar to the current recorded in NMG. These results show that the decay was generateted by the progressive activation of a Cs current flowing through a channel different from the Ca2+ one, possibly a Ca2+-activated K- channel. Consistently, the size of the outward current was proportional to the size of the Ca2+ current.