Hair cell Ca channel selectivity of frog labyrinth hair cells

The selectivity of Ca channels was studied in whole-cell recorded hair cells isolated from frog semicircular canals. Ca current in about 60% of the cells was formed by L- and R-type (R2) components, while the remaining cells exhibited an additional R-type fraction (R1) inactivating in a Ca-dependent manner. Nifedipine (10 microM) blocked about the 61% of the maximal steady current in cells exhibiting the L and R2 components, while it blocked about the 69% and about the 78% of the peak and of the plateau of the maximal Ca current, respectively, in cells possessing also the R1 component, and accelerated about 1.6-fold the R1 inactivation time constant (tauinac>10 ms). This indicates that the latter cells endowed a larger L-type component than the former, and a partial nifedipine block of the open R1 channel. The Ca current activation was fitted by a single exponential (tauon about 0.7 ms), whereas deactivation was bi-exponential (tauoff1 about 1 ms, tauoff2 about 10 ms). Upon reducing external Ca (<10 nM), all cells exhibited a steady current, carried by Na, 5-6 times larger in amplitude than the one in control Ca solution; the I-V curve peaked at -50 mV. In 10 nM Ca, tauon and tauoff1 accelerated by about a factor of 2 and 3, respectively, whereas tauoff2 was unaffected. This indicates that the L- (and possibly the R1), but not the R2-type channel gating is affected by the ion species permeating the channel. When Na was the charge carrier, nifedipine partially blocked the open R1 and R2 channels, the block being more effective at more negative potentials, whereas such blockade was not observed when Ca was the current carrier. This indicates that the open state of R1 and R2 channels has a larger affinity for nifedipine in the absence of Ca.