The sodium permeability of voltage-gated hair cell calcium channels of the frog labyrinth

The selectivity of the Ca channels was studied in isolated frog semicircular canal hair cells recorded under whole-cell or perforated-patch conditions. As found previously, about 60% of the cells exhibited a steady Ca current generated by L- and R-type (termed R2) components, while the remaining ones exhibited an additional R-type current (termed R1) which inactivated in a Ca-dependent manner. Upon reducing external Ca (10 ms) of the R1 component, 10 microM nifedipine accelerated inactivation by a factor 1.6, indicating a partial block of the open R1 channel. When Na was the charge carrier, nifedipine was able to partially block also the open R2 channel, the block being more effective at negative potentials, whereas no block was observed in normal Ca solution. In conclusion, in low Ca solution, all three channel types lose their Ca selectivity and they were all affected by nifedipine, although to a different exten