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 nM), all cells exhibited a steady current, carried by Na, whose maximal amplitude was 5¬6 times larger than the one in normal Ca solution. The I-V curves shifted to the left, the peak Na current being attained at -50 mV. The current activation in Ca was fitted by a single exponential (t1 = 0.7 +/- 0.1 ms), whereas the deactivation was bi-exponential (t2 = 1.2 +/- 0.2, t3 = 9.1 +/-1.1 ms; n = 22). In 10 nM Ca, both t1 and t2 accelerated by a factor 1.7, whereas t3 was unaffected. In cells exhibiting slow inactivation time constant (>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 extent
The sodium permeability of voltage-gated hair cell calcium channels of the frog labyrinth
MARTINI, Marta;RISPOLI, Giorgio;FARINELLI, Federica;GONZALEZ, Edgardo Maximiliano;ROSSI, Marialisa
2003
Abstract
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 nM), all cells exhibited a steady current, carried by Na, whose maximal amplitude was 5¬6 times larger than the one in normal Ca solution. The I-V curves shifted to the left, the peak Na current being attained at -50 mV. The current activation in Ca was fitted by a single exponential (t1 = 0.7 +/- 0.1 ms), whereas the deactivation was bi-exponential (t2 = 1.2 +/- 0.2, t3 = 9.1 +/-1.1 ms; n = 22). In 10 nM Ca, both t1 and t2 accelerated by a factor 1.7, whereas t3 was unaffected. In cells exhibiting slow inactivation time constant (>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 extentI documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
